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Comprehensive PROPELLER Text Needed
- Thread starter Aimes
- Start date
Hi Aimes:
If you realy need to know about propeller under complet design point of view(Aerodinamic, Structural analysis, Vibrations), take a look.
Propeller design procedures and data Vol I,II, III
Henry V. Borst and Associates
From : National Technical Information Service
Regards
If you realy need to know about propeller under complet design point of view(Aerodinamic, Structural analysis, Vibrations), take a look.
Propeller design procedures and data Vol I,II, III
Henry V. Borst and Associates
From : National Technical Information Service
Regards
- Thread starter
- #3
I have tried to locate this information from the online service and can't find it. Is it only available hard copy?Hi,
Few months ago, I was searching on internet the same informations. I found on different web sites what you will see in the next rows.
Unfortunatelly, I didn't put any bookmarks about those sites but I'm very sure if you want to find the source you can take a search with Google, given a simple copy and paste from this text and put it in Google search field. Sorry for "teaching style" but I didn't have so much time to put the text in other form. The initial text has also some figures but I didn't find the way to paste also those figures here. Hope this will help you
Propellers
The propeller is a rotating airfoil. It is subject to drag, stalls and other aerodynamic factors that apply to any airfoil. The propeller provides the thrust to pull the aircraft through the air. The cross section near the hub of the propeller is thick, and has a fairly large angle of attack. The angle of attack and the thickness decreases toward the tip of the blade. Since the linear speed at the tip is much faster than at the hub the change in angle of attack provides uniform thrust along the surface of the blade.
The propeller is normally connected directly to the engine crankshaft. Some aircraft, however, employ gear arrangements between the engine and the propeller.
Propellers fall into two main categories.
· Fixed Pitch
· Controllable pitch
Controllable pitch propellers allow the pilot to set the pitch of the blades, either directly or via a governor, to the best angle for the flight condition and performance desired. Usually for takeoff, a fairly “flat” angle of attack and high engine RPN is used to produce maximum horsepower and thrust. As altitude is gained the pilot can reduce RPM and increase pitch for a cruise climb condition. Once cruise altitude is reached the throttle, mixture and propeller pitch can be adjusted for the desired cruise performance.
The pilot has only one method of controlling thrust on fixed pitch propellers; that being adjusting engine RPM. With controllable pitch propellers, the pilot can adjust two controls; these being RPM (throttle) and Manifold Pressure (propeller pitch control). The Tachometer indicates RPM and the Manifold Pressure Gauge indicates the manifold pressure.
On constant speed propellers, a governor automatically adjusts the pitch of the propeller blade whenever the engine throttle setting is changed. Low RPM and High Manifold pressure should be avoided, as this places undue stress on engine components, and can lead to eventual engine failure.
For any given blade angle, the propeller has an ideal geometric pitch. It is designed to travel a certain distance in one revolution. However, due to slippage, the ideal geometric pitch is never attained. Therefore the effective pitch is always less than the geometric pitch. The propeller is never 100% efficient.
Propellers Common Terms
Blade: one arm of a propeller from hub to tip.
Hub: center section of the propeller, which carries the blades and is attached to the engine shaft.
Spinner: a metal cover enclosing the propeller hub, which improves the appearance of the propeller and may also streamline airflow for engine cooling purposes.
Blade tip: the part of the blade furthest from the hub.
Blade root: the section of the blade nearest the hub.
Blade shank: the portion of a blade inside the hub used to retain the blade.
Blade camber surface: the cambered or most-cambered side of a blade (visible from front of the aircraft).
Blade face or thrust surface: the flat side of a blade (normally visible from the cockpit of the aircraft).
Blade leading edge: the forward full ³cutting² edge of the blade that leads in the direction of rotation.
Blade trailing edge: the continuous edge of the blade that trails the leading edge in the direction of rotation.
Governor: a device, generally mounted on and driven by the engine, which senses and controls engine speed (RPM) by hydraulically adjusting the blade angle of the propeller.
Prop diameter: the diameter of the circle circumscribed by the blade tips.
Blade station: one of the designated distances along the blade as measured from the center of the hub.
Blade thickness: the maximum thickness between the cambered surface and the face or thrust surface at a given blade station.
Blade width: the measurement between the leading edge and the trailing edge at a given station.
Chord line: a theoretical straight line (perpendicular to blade length) drawn between the leading and trailing edges of the blade.
Blade angle: the angle between the chord line of a propeller blade section and a plane perpendicular to the axis of propeller rotation.
Blade angle settings: low and high angle settings of a controllable-pitch prop- for feather, reverse, latch and start locks - which are determined by built-in mechanical hard stops.
We can say that the propeller is the action end of an aircraft's reciprocating engine, because it converts the useful energy of the engine into thrust as it spins around and around. The propeller has the general shape of a wing, but the camber and chord (curvature and cross-sectional length) of each section of the propeller are different, as shown here. The wing provides lift upward, while the propeller provides lift forward.
The wing has only one motion which is forward, while the propeller has forward and rotary motion. The path of these two motions is like a corkscrew as the propeller goes through the air .
Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing (see the figure on the right for definitions of blade back and blade face). The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation, as shown here. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.
There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power.
The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.
The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft.
Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
Modern propellers are fabricated from high-strength, heat-treated, aluminum alloy forgings. New composite materials are being used in applications where weight and mass are critical.
Propellers are typically designed with two to six blades. Generally, props with more than three blades are used primarily for twin-engine aircraft. These blades tend to be shorter for increased ground clearance and more fuselage clearance. Multi-blade props also produce higher, less objectionable sound frequency; reduced vibration; greater flywheel effect and improved aircraft performance.
Types of Propellers
Propellers are classified according to pitch configuration. Blade pitch is the angle of the blades with relation to the plane of rotation and is a significant variable affecting the performance of the propeller.
Fixed Pitch: a one-piece prop with a single fixed blade angle. The pitch (blade angle) must be high enough to offer good cruising performance yet low enough to achieve acceptable takeoff and climb characteristics.
Constant Speed: a prop used with a governor, that automatically provides constant RPM by controlling the forces acting on the propeller to change the blade angle within a preset range.
Full-Feathering: a prop which allows blades to be rotated to a high positive angle to stop rotation (windmilling) after an engine is shut down, thereby reducing drag and asymmetric control forces on twin-engine applications.
Reversing: a prop with blades that can be rotated to a position less than the normal positive low blade angle setting until a negative blade angle is obtained, producing a rearward thrust to slow down, stop or move the aircraft backward. Typically provided for turbine installations.
Beta Control: a prop which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever. These types of propellers are installed on turbine engines.
VARIABLE PITCH PROPELLERS
Full-Feathering vs. Constant Speed
A constant-speed (RPM) system permits the pilot to select the propeller and engine speed for any situation and automatically maintain that RPM under varying conditions of aircraft attitude and engine power. Thereby permitting operation of propeller and engine at most efficient RPMs. RPM is controlled by varying the pitch of the propeller blades - that is, the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given RPM setting, aircraft speed and torque on the engine will increase. For economy cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected RPM.
A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or ³windmill,² causing increased drag. To prevent this, the propeller can be ³feathered² (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller.
Changing Pitch
Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.
In a single-acting propeller system, oil pressure supplied by the governor, acting on the piston produces a force that is opposed by the natural centrifugal twisting moment of the blades in constant speed models or counterweights and large springs in full-feathering systems. To increase or decrease the pitch, high pressure oil is directed to the propeller, which moves the piston back. The motion of the piston is transmitted to the blades through actuating pins and links, moving the blades toward either high pitch for constant-speed systems or low pitch for full-feathering systems.
When the opposing forces are equal, oil flow to the propeller stops and the piston also stops. The piston will remain in this position, maintaining the pitch of the blades until oil flow to or from the propeller is again established by the governor.
From this position, pitch is decreased for constant-speed systems or increased for full- feathering systems by allowing oil to flow out of the propeller and return to the engine sump. When the governor initiates this procedure, hydraulic pressure is decreased and the piston moves forward, changing the pitch of the blades. The piston will continue to move forward until the opposing forces are once again equal. Mechanical stops are installed in the propeller to limit travel in both the high and low pitch directions.
PERFORMANCE CONSIDERATIONS
Shape of Propeller Tips
Propeller tips can be rounded, swept or square. Various tips are often used to meet blade vibration resonance or special design conditions. Tip shape is also a function of aesthetics, noise requirements, flight performance, repair ability and ground clearance.
Propeller Diameter
Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds. For example, the diameter of a fixed-pitch propeller is often large to favor low airspeed operation, while the blade size is small to favor higher airspeeds and faster turning at low airspeeds. The diameter and blade size of a constant-speed propeller is often larger (than a fixed-pitch), due to the variability of blade angles.
Engine Horsepower and RPM
For fixed-pitch props, at a fixed throttle setting, propeller and engine RPM increases or decreases with the airspeed. At a constant airspeed, fixed-pitch propeller and engine RPM change if power is increased or decreased. A constant-speed prop uses a governor to provide constant RPM at the selected throttle setting. The blade angle automatically increases or decreases as the RPM setting or engine power changes. With a fixed RPM and power setting, the blade angle automatically changes as airspeed increases or decreases.
De-ice System
After ice has formed, a de-ice system applies electric heat to the blade, melting the ice near the surface of the blade so the ice will be removed by centrifugal force as the prop spins. A de-ice system typically consists of boots, slip rings and brushes. Older technology, anti-ice equipment, prevents the formation of ice by allowing alcohol to flow over the propeller blades.
Synchronizing and Synchrophasing Systems
On twin-engine applications, the benefits of synchronizing and synchrophasing systems are the reduction of noise beats produced by the interaction of the prop and the fuselage. The governing system provides the means for synchronizing and synchrophasing the two propellers on twin-engine aircraft. The synchronizing option adjusts propeller RPM so that both props are turning at the same speed. Tis can be done by installing a pick-up disc on each governor drive shaft, along with a transducer that sends a frequency signal to an electronic control. This control compares the signals from both governors and adjusts one of them to bring it into ³synch² with the other.
Once the props are synchronized, the synchrophaser option allows the pilot to adjust the position of the blades on one propeller with respect to the position of the blades on the second prop for reduced noise and vibration. Synchrophasers are solid-state units that automatically synchronize prop speed combined with a phasing control operated by the pilot. This phasing control allows the pilot to manually adjust the difference between the two propellers to minimize the ³beat² of the props.
Overhauling or Reconditioning Your Propeller
Blade reconditioning covers major or minor blade damage from accident or other causes and includes balancing of the prop. Blades should also be reconditioned if they have been damaged and filed often. This work is performed on an ³as required² basis by an approved propeller repair station. For a one-piece, fixed-pitch prop, reconditioning is equivalent to an overhaul. For other types of props, if damage is major but repairable, an overhaul may be included with the reconditioning.
All props require periodic overhaul to increase safety, prolong propeller life and improve function or operation. The overhaul interval is generally based on hours of service (operating time) as well as a calendar limit. During overhaul, the propeller is disassembled and inspected for wear, cracks, corrosion and other abnormal conditions. Parts may be replaced or reconditioned and refinished. The propeller is then re-assembled and balanced.
CHANGING THE PROPELLER
A propeller is designed to be compatible with a specific engine, in order to achieve maximum thrust or efficiency and reliability from the aircraft. Even though the propeller might fit another engine shaft, only the propeller manufacturer can determine whether it is suitable for use on a particular aircraft. Installation requirements are usually available for all manufacturers. Propellers are generally changed either to upgrade performance or to restore original performance compromised by wear and tear. Whatever the reason, changing propellers deserves careful consideration. The propeller is intimately linked to aircraft performance and operates in partnership with all other components. Many factors can enhance or impair performance.
Four ways to change propellers:
1. OEM Type Certificate
2. One-Time Field Approval
3. Experimental Certificate
4. Supplemental Type Certificate
1.OEM Type Certificate
Any propeller that appears on the Original Equipment Manufacturer¹s (OEM) approved equipment list, on the Aircraft Type Certificate Data Sheet, is automatically approved for that application. No further paperwork is required.
2.One-Time Field Approval
There are only two things for certain about the One-time Field Approval:
€ It requires the endorsement of the aeronautical authority
€ It has to have some degree of technical justification
3.Experimental Certificate
The Experimental Certificate option is available only to experimental aircraft owners or operators..
4.Supplemental Type Certificate (STC)
The aeronautical authority issues an STC for propellers that have passed rigorous and extensive testing but which are not listed on the OEM¹s approved equipment list for a particular aircraft. The STC is the easiest way to modify an existing airplane in the field. Most owners, operators and mechanics who wish to upgrade propeller performance will use STCs.
Single-component STCs involve a specific propeller that has been approved for a specific aircraft. For example, the single-component STC is commonly used to upgrade an aircraft from a two-bladed to a three-bladed propeller. It may also be used by owners or operators who are not satisfied with the performance of their original propeller. The combination STC involves multiple components, such as a propeller and an engine upgrade. Although less common than single-component STCs, the combination STC is gaining popularity because of the integral relationship between propeller and engine.
The STC holder may be the original propeller manufacturer, the original aircraft manufac- turer or an individual. To obtain an STC, the STC applicant often works with the aeronautical authority and the OEM, tests and evaluates the propeller, and pays for flight performance testing and stress surveys. Developing the STC for a simple, one-propeller changeover for a particular aircraft can be a significant expense. However:
? STC holders do not always work with the original propeller manufacturer prior to obtaining STC approval from the aeronautical authority.
? The aeronautical authority usually does not notify the original propeller manufacturer when it grants an STC to someone other than the OEM.
Therefore, always contact the manufacturer of the STC propeller you plan to install, and ask if the OEM is aware of the STC or of any potential problems. Also, contact the STC holder directly to discuss the performance changes you should expect. Request a list of owners who have performed similar installations. Make sure everything is working properly under usual operating conditions before installing any STC conversion. To determine whether or not a problem is propeller related, use the process of elimination, changing one variable at a time. For example, a recently overhauled engine may cause vibration, which could be mistakenly blamed on a new propeller installed at the same time. If you converted from a two-bladed propeller to a three-bladed propeller immediately after an engine overhaul, try out the overhauled engine using the two-bladed propeller. If you experience vibration that was not apparent before the overhaul, you will know that it is an engine problem, not a propeller problem.
The warranty that comes with the STC conversion covers the propeller assembly. Technically, the original propeller manufacturer is responsible only if the propeller is defective. The STC holder is responsible for problems with installation adjustments. However, owners and operators may have adjustment or performance trouble that is not propeller-related, including problems with the engine, engine mounts, cowling configuration or airframe. As a result, performance varies by individual aircraft.
Composite propellers
Composite propellers are seeing more and more use these days. But there are inspection issues that are unique to these propellers. With proper maintenance and inspection, these sturdy propellers can provide a long service life. But neglect can lead to catastrophic failure.
Propellers, both metal and composite, are some of the most critical components of an aircraft. Failure of a propeller can lead to much more than loss of thrust. If a propeller blade is thrown, the result is catastrophic, much more so than an engine failure.
Centrifugal force
There are five operational forces that act on a propeller simultaneously. These are centrifugal force, thrust bending force, torque bending force, aerodynamic twisting moment, and centrifugal twisting moment. Of them all, centrifugal force causes the greatest stress on propellers. This is the force that tends to pull the blades from the propeller hub. It is related to RPM - the higher the RPM, the more centrifugal force the blades are subjected to.
Propellers are subjected to great amounts of centrifugal force. There can typically be 25 tons of centrifugal force at the root end of metal propeller blades, minimally 20 tons. It is an exponential load. In other words, it's not a straight line relationship. As the RPM goes up, the amount of centrifugal force acting on the propeller blades goes up exponentially.
With so much centrifugal force acting on the propeller blades, it is easy to see how the loss of a blade in flight can be catastrophic. The danger is not necessarily all from the unrestrained propeller blade that can impact the aircraft and cause serious damage. Major danger lies in the transfer of energy. It goes back to Newton's Law of momentum conservation. The force that the propeller blade was subjected to is transferred to the system when it departs the aircraft. This can be an enormous amount of force that can rip the engine from its mounts and cause severe damage to the aircraft structure.
Benefits of composite blades
One of the most evident benefits of composite blades is their weight. They offer a substantial weight reduction compared to metal propellers, thereby offering more efficient operation (less horsepower is needed to produce the same thrust).
Another advantage to composite propellers is the fact that they don't shrink dimensionally after rework. As a typical metal blade experiences damage, the damage is blended out according to the repair manual. So, over time more and more material is taken away until eventually the blade reaches its minimum limits. That is not the case with composite blades.
The composite blades never wear out. They may eventually have a shank go bad, or a bearing go out, but the blades themselves never change. They are always reworked to the same size. As you repair them, they go back to the original dimensions, where with metal blades as you keep on taking metal away, sooner or later they are going to go undersize.
Not losing dimensional area is a definite advantage. If a prop blade is undersize, it has a tendency to be susceptible to resonance. Every metal propeller blade is like a tuning fork. If it finds a sympathetic frequency that it can respond to, it's tip can deflect up to 6 inches. This can cause it to fail instantly. When the blade is within dimensional limits, it can't do that.
Prop maintenance
The health of your composite blade begins with inspection. Following the recommended inspection schedule is essential. The main inspection for composite prop blades is the tap test. A metal tool is tapped on the propeller blade surface to look for delaminated areas. Damaged areas sound like dull thuds compared to the light sound of the normal structure. The defect may not be visible, but the tap test will give a definite indication of a flaw.
In addition to the tap test, a good visual inspection of the propeller is in order. Check the condition and security of the leading edge and de-icer. A nickel covering protects the leading edge of the propeller from erosion and impact damage.
The de-ice boots should be checked for any evidence of overheating. Excessive heat can be very damaging to composite prop blades.
If a boot goes bad and burns, blade damage to the structure beneath is assured. Temperatures over 140 degrees can cause the Hartzel composite to 'pop' - basically delaminating on the inside. Hamilton blades will withstand approximately 100 degrees more. Most of the time the prop blades aren't destroyed by this heat damage. They can be fixed, but the cause of the overheating should be addressed.
The paint coating should be examined carefully for eroded areas. Areas where the coating is eroded could allow fluids to enter and saturate the composite. This can be especially damaging to fiberglass prop blades. When oil or grease are allowed to saturate the material, it causes the fiberglass to deteriorate quickly. The Kevlar® type prop blades aren't as susceptible, but they can still be heavily damaged by infiltration. Pay close attention to the blade coating and repair any eroded areas in accordance with the maintenance manual.
Lightning protection
Composites are also susceptible to lightning strikes. Therefore, various methods are employed to protect composite propellers from the damage caused by lightning strikes. This can be in the form of metal spars, erosion sheaths, and/or special metal-based coatings placed over the composite that helps dissipate the electricity from a lightning strike. Whatever the type of protection on your prop blades, make sure that it is intact. With no path to airframe ground for the energy to dissipate, the propeller blade could literally blow apart if struck by lightning. Also, be aware of the signs of a lightning strike during the inspection. A tell-tale sign is a dark, discolored brown or black spot on the outer trailing edge of a blade that looks like an overheated area. If a lightning strike is suspected, the use of a gauss meter to check for residual magnetism in the ferrous attaching parts of the propeller is recommended. If a strike is verified, the propeller should be sent to an authorized repair facility for inspection and disposition.
Overhaul
The overhaul process for composite propellers begins with an incoming inspection. Any areas that may require follow-up are noted.
Next, the paint coating is removed as necessary. This process is usually done by hand to ensure that only the protective paint coating is removed without going into the composite layer.
The propeller blade is then inspected visually and re-tap tested. All damaged areas are repaired as necessary. The leading edge is inspected for security and integrity. The nickel leading edges are sacrificial, protecting the composite structure underneath. Eventually they have to be replaced. To remove edges from Hamilton propeller blades, they are ground down the center line of the blade's leading edge and then each remaining side is pried off the blade using a thin chisel. On the Hartzells, a propane torch is used to flash heat the edge in order to soften up the glue, paying close attention not to overheat the underlying blade. The edge is then removed once the glue has softened. Structure under the edge is inspected and repaired as necessary before installing a new edge.
When ready to install the new edge, a template is first placed over the repaired bare leading edge of the prop. It is basically an edge with witness holes ground in it to ensure that when placed on the leading edge of the blade the mating surface beneath is dimensionally correct. On Hartzell blades, adhesive is applied and the edge is put in place and secured with a vacuum bag until cured. For the Hamiltons, the process is a little more involved. Epoxy is applied to the blade edge, and a special pressure bag is placed around it. This ensures that uniform computer-controlled pressure and temperature is applied during the curing process. Special paint is then re-applied to each prop blade. This involves applying multiple layers of cross coats as required. Each propeller blade is then balanced to a calibrated master, reassembled, documented, and carefully packaged. It is then ready to be shipped back to the customer.
In the end, by following the manufacturer's recommended maintenance and inspection procedures, these sturdy props will last a long time. In fact, neglect is the most common factor leading to higher costs at overhaul and a shortened propeller life. Considering the extreme damage that can be caused by the loss of a propeller blade in flight, there is no other choice but to pay close attention to these critical aircraft components.
High Performance
Lightweight, composite material allows blades to be built with a thicker cross-section dimension, which increases takeoff and climb performance. The capability to construct multi-blade propellers with a smaller diameter eliminates high-speed drag and increases cruise speed. Metal prop blades built to these dimensions would be too heavy. The reduction in diameter also reduces noise while increasing ground clearance.
Light Weights
A lightweight propeller increases the useful load of the aircraft and also reduces the stress on the engine and crankshaft.
Propeller Life
Natural composite propeller blades will not fatigue over time whereas metal propeller blades are life-limited by fatigue and dimension. Natural composite props are unique in that each overhaul returns the blades to their original dimension by adding composite material to the damaged section of the blade. In the event of a ground strike, the blades are often reparable, the hub is re-usable and the risk of internal damage to the engine is significantly reduced. The blades are also protected by a replaceable stainless steel leading edge to prevent blade erosion. Stainless steel is more durable than aluminum and makes these blades especially compatible for flying in the rain. This leading edge is particularly useful in seaplane applications where water erosion is a concern.
De-Ice
Electric propeller de-icing boots are available as an option for use on all hydraulic MT models. They consume half the amperage of standard de-ice boots and therefore allow for the use of a propeller with a greater number of blades while maintaining the same or less amp draw.
Vibration
Natural composite propellers vibration is drastically reduced due to itslight weight, low polar moment of inertia and high harmonic dampening
characteristics. In turn, this helps lengthen equipment life and makes flight more comfortable.
.
Weather
The stainless steel leading edge protects against water damage and erosion. Years of testing in rain, ice and snow as well as tests at the University of Dayton have proven that MT`s natural composite propeller is all-weather durable.
Cost
Because natural composite propellers can be overhauled to return the blades to original factory specifications, costly blade replacements are eliminated. Therefore, these props are more efficient and less expensive to own and increase the resale value of the aircraft.
Propeller manufacturers:
Hartzell Propeller
(937) 778-4379
Hamilton Sundstrand
(860) 654-6000
McCauley Propeller Systems
(937) 890-5246
MT-Propeller Entwicklung GmbH
Flugplatzstr. 1,D-94348 Atting / Germany
Phone: 011-49-9429-94090
Fax: 011-49-9429-8432
Regards
Fernando
Few months ago, I was searching on internet the same informations. I found on different web sites what you will see in the next rows.
Unfortunatelly, I didn't put any bookmarks about those sites but I'm very sure if you want to find the source you can take a search with Google, given a simple copy and paste from this text and put it in Google search field. Sorry for "teaching style" but I didn't have so much time to put the text in other form. The initial text has also some figures but I didn't find the way to paste also those figures here. Hope this will help you
Propellers
The propeller is a rotating airfoil. It is subject to drag, stalls and other aerodynamic factors that apply to any airfoil. The propeller provides the thrust to pull the aircraft through the air. The cross section near the hub of the propeller is thick, and has a fairly large angle of attack. The angle of attack and the thickness decreases toward the tip of the blade. Since the linear speed at the tip is much faster than at the hub the change in angle of attack provides uniform thrust along the surface of the blade.
The propeller is normally connected directly to the engine crankshaft. Some aircraft, however, employ gear arrangements between the engine and the propeller.
Propellers fall into two main categories.
· Fixed Pitch
· Controllable pitch
Controllable pitch propellers allow the pilot to set the pitch of the blades, either directly or via a governor, to the best angle for the flight condition and performance desired. Usually for takeoff, a fairly “flat” angle of attack and high engine RPN is used to produce maximum horsepower and thrust. As altitude is gained the pilot can reduce RPM and increase pitch for a cruise climb condition. Once cruise altitude is reached the throttle, mixture and propeller pitch can be adjusted for the desired cruise performance.
The pilot has only one method of controlling thrust on fixed pitch propellers; that being adjusting engine RPM. With controllable pitch propellers, the pilot can adjust two controls; these being RPM (throttle) and Manifold Pressure (propeller pitch control). The Tachometer indicates RPM and the Manifold Pressure Gauge indicates the manifold pressure.
On constant speed propellers, a governor automatically adjusts the pitch of the propeller blade whenever the engine throttle setting is changed. Low RPM and High Manifold pressure should be avoided, as this places undue stress on engine components, and can lead to eventual engine failure.
For any given blade angle, the propeller has an ideal geometric pitch. It is designed to travel a certain distance in one revolution. However, due to slippage, the ideal geometric pitch is never attained. Therefore the effective pitch is always less than the geometric pitch. The propeller is never 100% efficient.
Propellers Common Terms
Blade: one arm of a propeller from hub to tip.
Hub: center section of the propeller, which carries the blades and is attached to the engine shaft.
Spinner: a metal cover enclosing the propeller hub, which improves the appearance of the propeller and may also streamline airflow for engine cooling purposes.
Blade tip: the part of the blade furthest from the hub.
Blade root: the section of the blade nearest the hub.
Blade shank: the portion of a blade inside the hub used to retain the blade.
Blade camber surface: the cambered or most-cambered side of a blade (visible from front of the aircraft).
Blade face or thrust surface: the flat side of a blade (normally visible from the cockpit of the aircraft).
Blade leading edge: the forward full ³cutting² edge of the blade that leads in the direction of rotation.
Blade trailing edge: the continuous edge of the blade that trails the leading edge in the direction of rotation.
Governor: a device, generally mounted on and driven by the engine, which senses and controls engine speed (RPM) by hydraulically adjusting the blade angle of the propeller.
Prop diameter: the diameter of the circle circumscribed by the blade tips.
Blade station: one of the designated distances along the blade as measured from the center of the hub.
Blade thickness: the maximum thickness between the cambered surface and the face or thrust surface at a given blade station.
Blade width: the measurement between the leading edge and the trailing edge at a given station.
Chord line: a theoretical straight line (perpendicular to blade length) drawn between the leading and trailing edges of the blade.
Blade angle: the angle between the chord line of a propeller blade section and a plane perpendicular to the axis of propeller rotation.
Blade angle settings: low and high angle settings of a controllable-pitch prop- for feather, reverse, latch and start locks - which are determined by built-in mechanical hard stops.
We can say that the propeller is the action end of an aircraft's reciprocating engine, because it converts the useful energy of the engine into thrust as it spins around and around. The propeller has the general shape of a wing, but the camber and chord (curvature and cross-sectional length) of each section of the propeller are different, as shown here. The wing provides lift upward, while the propeller provides lift forward.
The wing has only one motion which is forward, while the propeller has forward and rotary motion. The path of these two motions is like a corkscrew as the propeller goes through the air .
Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing (see the figure on the right for definitions of blade back and blade face). The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation, as shown here. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.
There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power.
The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.
The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft.
Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
Modern propellers are fabricated from high-strength, heat-treated, aluminum alloy forgings. New composite materials are being used in applications where weight and mass are critical.
Propellers are typically designed with two to six blades. Generally, props with more than three blades are used primarily for twin-engine aircraft. These blades tend to be shorter for increased ground clearance and more fuselage clearance. Multi-blade props also produce higher, less objectionable sound frequency; reduced vibration; greater flywheel effect and improved aircraft performance.
Types of Propellers
Propellers are classified according to pitch configuration. Blade pitch is the angle of the blades with relation to the plane of rotation and is a significant variable affecting the performance of the propeller.
Fixed Pitch: a one-piece prop with a single fixed blade angle. The pitch (blade angle) must be high enough to offer good cruising performance yet low enough to achieve acceptable takeoff and climb characteristics.
Constant Speed: a prop used with a governor, that automatically provides constant RPM by controlling the forces acting on the propeller to change the blade angle within a preset range.
Full-Feathering: a prop which allows blades to be rotated to a high positive angle to stop rotation (windmilling) after an engine is shut down, thereby reducing drag and asymmetric control forces on twin-engine applications.
Reversing: a prop with blades that can be rotated to a position less than the normal positive low blade angle setting until a negative blade angle is obtained, producing a rearward thrust to slow down, stop or move the aircraft backward. Typically provided for turbine installations.
Beta Control: a prop which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever. These types of propellers are installed on turbine engines.
VARIABLE PITCH PROPELLERS
Full-Feathering vs. Constant Speed
A constant-speed (RPM) system permits the pilot to select the propeller and engine speed for any situation and automatically maintain that RPM under varying conditions of aircraft attitude and engine power. Thereby permitting operation of propeller and engine at most efficient RPMs. RPM is controlled by varying the pitch of the propeller blades - that is, the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given RPM setting, aircraft speed and torque on the engine will increase. For economy cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected RPM.
A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or ³windmill,² causing increased drag. To prevent this, the propeller can be ³feathered² (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller.
Changing Pitch
Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.
In a single-acting propeller system, oil pressure supplied by the governor, acting on the piston produces a force that is opposed by the natural centrifugal twisting moment of the blades in constant speed models or counterweights and large springs in full-feathering systems. To increase or decrease the pitch, high pressure oil is directed to the propeller, which moves the piston back. The motion of the piston is transmitted to the blades through actuating pins and links, moving the blades toward either high pitch for constant-speed systems or low pitch for full-feathering systems.
When the opposing forces are equal, oil flow to the propeller stops and the piston also stops. The piston will remain in this position, maintaining the pitch of the blades until oil flow to or from the propeller is again established by the governor.
From this position, pitch is decreased for constant-speed systems or increased for full- feathering systems by allowing oil to flow out of the propeller and return to the engine sump. When the governor initiates this procedure, hydraulic pressure is decreased and the piston moves forward, changing the pitch of the blades. The piston will continue to move forward until the opposing forces are once again equal. Mechanical stops are installed in the propeller to limit travel in both the high and low pitch directions.
PERFORMANCE CONSIDERATIONS
Shape of Propeller Tips
Propeller tips can be rounded, swept or square. Various tips are often used to meet blade vibration resonance or special design conditions. Tip shape is also a function of aesthetics, noise requirements, flight performance, repair ability and ground clearance.
Propeller Diameter
Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds. For example, the diameter of a fixed-pitch propeller is often large to favor low airspeed operation, while the blade size is small to favor higher airspeeds and faster turning at low airspeeds. The diameter and blade size of a constant-speed propeller is often larger (than a fixed-pitch), due to the variability of blade angles.
Engine Horsepower and RPM
For fixed-pitch props, at a fixed throttle setting, propeller and engine RPM increases or decreases with the airspeed. At a constant airspeed, fixed-pitch propeller and engine RPM change if power is increased or decreased. A constant-speed prop uses a governor to provide constant RPM at the selected throttle setting. The blade angle automatically increases or decreases as the RPM setting or engine power changes. With a fixed RPM and power setting, the blade angle automatically changes as airspeed increases or decreases.
De-ice System
After ice has formed, a de-ice system applies electric heat to the blade, melting the ice near the surface of the blade so the ice will be removed by centrifugal force as the prop spins. A de-ice system typically consists of boots, slip rings and brushes. Older technology, anti-ice equipment, prevents the formation of ice by allowing alcohol to flow over the propeller blades.
Synchronizing and Synchrophasing Systems
On twin-engine applications, the benefits of synchronizing and synchrophasing systems are the reduction of noise beats produced by the interaction of the prop and the fuselage. The governing system provides the means for synchronizing and synchrophasing the two propellers on twin-engine aircraft. The synchronizing option adjusts propeller RPM so that both props are turning at the same speed. Tis can be done by installing a pick-up disc on each governor drive shaft, along with a transducer that sends a frequency signal to an electronic control. This control compares the signals from both governors and adjusts one of them to bring it into ³synch² with the other.
Once the props are synchronized, the synchrophaser option allows the pilot to adjust the position of the blades on one propeller with respect to the position of the blades on the second prop for reduced noise and vibration. Synchrophasers are solid-state units that automatically synchronize prop speed combined with a phasing control operated by the pilot. This phasing control allows the pilot to manually adjust the difference between the two propellers to minimize the ³beat² of the props.
Overhauling or Reconditioning Your Propeller
Blade reconditioning covers major or minor blade damage from accident or other causes and includes balancing of the prop. Blades should also be reconditioned if they have been damaged and filed often. This work is performed on an ³as required² basis by an approved propeller repair station. For a one-piece, fixed-pitch prop, reconditioning is equivalent to an overhaul. For other types of props, if damage is major but repairable, an overhaul may be included with the reconditioning.
All props require periodic overhaul to increase safety, prolong propeller life and improve function or operation. The overhaul interval is generally based on hours of service (operating time) as well as a calendar limit. During overhaul, the propeller is disassembled and inspected for wear, cracks, corrosion and other abnormal conditions. Parts may be replaced or reconditioned and refinished. The propeller is then re-assembled and balanced.
CHANGING THE PROPELLER
A propeller is designed to be compatible with a specific engine, in order to achieve maximum thrust or efficiency and reliability from the aircraft. Even though the propeller might fit another engine shaft, only the propeller manufacturer can determine whether it is suitable for use on a particular aircraft. Installation requirements are usually available for all manufacturers. Propellers are generally changed either to upgrade performance or to restore original performance compromised by wear and tear. Whatever the reason, changing propellers deserves careful consideration. The propeller is intimately linked to aircraft performance and operates in partnership with all other components. Many factors can enhance or impair performance.
Four ways to change propellers:
1. OEM Type Certificate
2. One-Time Field Approval
3. Experimental Certificate
4. Supplemental Type Certificate
1.OEM Type Certificate
Any propeller that appears on the Original Equipment Manufacturer¹s (OEM) approved equipment list, on the Aircraft Type Certificate Data Sheet, is automatically approved for that application. No further paperwork is required.
2.One-Time Field Approval
There are only two things for certain about the One-time Field Approval:
€ It requires the endorsement of the aeronautical authority
€ It has to have some degree of technical justification
3.Experimental Certificate
The Experimental Certificate option is available only to experimental aircraft owners or operators..
4.Supplemental Type Certificate (STC)
The aeronautical authority issues an STC for propellers that have passed rigorous and extensive testing but which are not listed on the OEM¹s approved equipment list for a particular aircraft. The STC is the easiest way to modify an existing airplane in the field. Most owners, operators and mechanics who wish to upgrade propeller performance will use STCs.
Single-component STCs involve a specific propeller that has been approved for a specific aircraft. For example, the single-component STC is commonly used to upgrade an aircraft from a two-bladed to a three-bladed propeller. It may also be used by owners or operators who are not satisfied with the performance of their original propeller. The combination STC involves multiple components, such as a propeller and an engine upgrade. Although less common than single-component STCs, the combination STC is gaining popularity because of the integral relationship between propeller and engine.
The STC holder may be the original propeller manufacturer, the original aircraft manufac- turer or an individual. To obtain an STC, the STC applicant often works with the aeronautical authority and the OEM, tests and evaluates the propeller, and pays for flight performance testing and stress surveys. Developing the STC for a simple, one-propeller changeover for a particular aircraft can be a significant expense. However:
? STC holders do not always work with the original propeller manufacturer prior to obtaining STC approval from the aeronautical authority.
? The aeronautical authority usually does not notify the original propeller manufacturer when it grants an STC to someone other than the OEM.
Therefore, always contact the manufacturer of the STC propeller you plan to install, and ask if the OEM is aware of the STC or of any potential problems. Also, contact the STC holder directly to discuss the performance changes you should expect. Request a list of owners who have performed similar installations. Make sure everything is working properly under usual operating conditions before installing any STC conversion. To determine whether or not a problem is propeller related, use the process of elimination, changing one variable at a time. For example, a recently overhauled engine may cause vibration, which could be mistakenly blamed on a new propeller installed at the same time. If you converted from a two-bladed propeller to a three-bladed propeller immediately after an engine overhaul, try out the overhauled engine using the two-bladed propeller. If you experience vibration that was not apparent before the overhaul, you will know that it is an engine problem, not a propeller problem.
The warranty that comes with the STC conversion covers the propeller assembly. Technically, the original propeller manufacturer is responsible only if the propeller is defective. The STC holder is responsible for problems with installation adjustments. However, owners and operators may have adjustment or performance trouble that is not propeller-related, including problems with the engine, engine mounts, cowling configuration or airframe. As a result, performance varies by individual aircraft.
Composite propellers
Composite propellers are seeing more and more use these days. But there are inspection issues that are unique to these propellers. With proper maintenance and inspection, these sturdy propellers can provide a long service life. But neglect can lead to catastrophic failure.
Propellers, both metal and composite, are some of the most critical components of an aircraft. Failure of a propeller can lead to much more than loss of thrust. If a propeller blade is thrown, the result is catastrophic, much more so than an engine failure.
Centrifugal force
There are five operational forces that act on a propeller simultaneously. These are centrifugal force, thrust bending force, torque bending force, aerodynamic twisting moment, and centrifugal twisting moment. Of them all, centrifugal force causes the greatest stress on propellers. This is the force that tends to pull the blades from the propeller hub. It is related to RPM - the higher the RPM, the more centrifugal force the blades are subjected to.
Propellers are subjected to great amounts of centrifugal force. There can typically be 25 tons of centrifugal force at the root end of metal propeller blades, minimally 20 tons. It is an exponential load. In other words, it's not a straight line relationship. As the RPM goes up, the amount of centrifugal force acting on the propeller blades goes up exponentially.
With so much centrifugal force acting on the propeller blades, it is easy to see how the loss of a blade in flight can be catastrophic. The danger is not necessarily all from the unrestrained propeller blade that can impact the aircraft and cause serious damage. Major danger lies in the transfer of energy. It goes back to Newton's Law of momentum conservation. The force that the propeller blade was subjected to is transferred to the system when it departs the aircraft. This can be an enormous amount of force that can rip the engine from its mounts and cause severe damage to the aircraft structure.
Benefits of composite blades
One of the most evident benefits of composite blades is their weight. They offer a substantial weight reduction compared to metal propellers, thereby offering more efficient operation (less horsepower is needed to produce the same thrust).
Another advantage to composite propellers is the fact that they don't shrink dimensionally after rework. As a typical metal blade experiences damage, the damage is blended out according to the repair manual. So, over time more and more material is taken away until eventually the blade reaches its minimum limits. That is not the case with composite blades.
The composite blades never wear out. They may eventually have a shank go bad, or a bearing go out, but the blades themselves never change. They are always reworked to the same size. As you repair them, they go back to the original dimensions, where with metal blades as you keep on taking metal away, sooner or later they are going to go undersize.
Not losing dimensional area is a definite advantage. If a prop blade is undersize, it has a tendency to be susceptible to resonance. Every metal propeller blade is like a tuning fork. If it finds a sympathetic frequency that it can respond to, it's tip can deflect up to 6 inches. This can cause it to fail instantly. When the blade is within dimensional limits, it can't do that.
Prop maintenance
The health of your composite blade begins with inspection. Following the recommended inspection schedule is essential. The main inspection for composite prop blades is the tap test. A metal tool is tapped on the propeller blade surface to look for delaminated areas. Damaged areas sound like dull thuds compared to the light sound of the normal structure. The defect may not be visible, but the tap test will give a definite indication of a flaw.
In addition to the tap test, a good visual inspection of the propeller is in order. Check the condition and security of the leading edge and de-icer. A nickel covering protects the leading edge of the propeller from erosion and impact damage.
The de-ice boots should be checked for any evidence of overheating. Excessive heat can be very damaging to composite prop blades.
If a boot goes bad and burns, blade damage to the structure beneath is assured. Temperatures over 140 degrees can cause the Hartzel composite to 'pop' - basically delaminating on the inside. Hamilton blades will withstand approximately 100 degrees more. Most of the time the prop blades aren't destroyed by this heat damage. They can be fixed, but the cause of the overheating should be addressed.
The paint coating should be examined carefully for eroded areas. Areas where the coating is eroded could allow fluids to enter and saturate the composite. This can be especially damaging to fiberglass prop blades. When oil or grease are allowed to saturate the material, it causes the fiberglass to deteriorate quickly. The Kevlar® type prop blades aren't as susceptible, but they can still be heavily damaged by infiltration. Pay close attention to the blade coating and repair any eroded areas in accordance with the maintenance manual.
Lightning protection
Composites are also susceptible to lightning strikes. Therefore, various methods are employed to protect composite propellers from the damage caused by lightning strikes. This can be in the form of metal spars, erosion sheaths, and/or special metal-based coatings placed over the composite that helps dissipate the electricity from a lightning strike. Whatever the type of protection on your prop blades, make sure that it is intact. With no path to airframe ground for the energy to dissipate, the propeller blade could literally blow apart if struck by lightning. Also, be aware of the signs of a lightning strike during the inspection. A tell-tale sign is a dark, discolored brown or black spot on the outer trailing edge of a blade that looks like an overheated area. If a lightning strike is suspected, the use of a gauss meter to check for residual magnetism in the ferrous attaching parts of the propeller is recommended. If a strike is verified, the propeller should be sent to an authorized repair facility for inspection and disposition.
Overhaul
The overhaul process for composite propellers begins with an incoming inspection. Any areas that may require follow-up are noted.
Next, the paint coating is removed as necessary. This process is usually done by hand to ensure that only the protective paint coating is removed without going into the composite layer.
The propeller blade is then inspected visually and re-tap tested. All damaged areas are repaired as necessary. The leading edge is inspected for security and integrity. The nickel leading edges are sacrificial, protecting the composite structure underneath. Eventually they have to be replaced. To remove edges from Hamilton propeller blades, they are ground down the center line of the blade's leading edge and then each remaining side is pried off the blade using a thin chisel. On the Hartzells, a propane torch is used to flash heat the edge in order to soften up the glue, paying close attention not to overheat the underlying blade. The edge is then removed once the glue has softened. Structure under the edge is inspected and repaired as necessary before installing a new edge.
When ready to install the new edge, a template is first placed over the repaired bare leading edge of the prop. It is basically an edge with witness holes ground in it to ensure that when placed on the leading edge of the blade the mating surface beneath is dimensionally correct. On Hartzell blades, adhesive is applied and the edge is put in place and secured with a vacuum bag until cured. For the Hamiltons, the process is a little more involved. Epoxy is applied to the blade edge, and a special pressure bag is placed around it. This ensures that uniform computer-controlled pressure and temperature is applied during the curing process. Special paint is then re-applied to each prop blade. This involves applying multiple layers of cross coats as required. Each propeller blade is then balanced to a calibrated master, reassembled, documented, and carefully packaged. It is then ready to be shipped back to the customer.
In the end, by following the manufacturer's recommended maintenance and inspection procedures, these sturdy props will last a long time. In fact, neglect is the most common factor leading to higher costs at overhaul and a shortened propeller life. Considering the extreme damage that can be caused by the loss of a propeller blade in flight, there is no other choice but to pay close attention to these critical aircraft components.
High Performance
Lightweight, composite material allows blades to be built with a thicker cross-section dimension, which increases takeoff and climb performance. The capability to construct multi-blade propellers with a smaller diameter eliminates high-speed drag and increases cruise speed. Metal prop blades built to these dimensions would be too heavy. The reduction in diameter also reduces noise while increasing ground clearance.
Light Weights
A lightweight propeller increases the useful load of the aircraft and also reduces the stress on the engine and crankshaft.
Propeller Life
Natural composite propeller blades will not fatigue over time whereas metal propeller blades are life-limited by fatigue and dimension. Natural composite props are unique in that each overhaul returns the blades to their original dimension by adding composite material to the damaged section of the blade. In the event of a ground strike, the blades are often reparable, the hub is re-usable and the risk of internal damage to the engine is significantly reduced. The blades are also protected by a replaceable stainless steel leading edge to prevent blade erosion. Stainless steel is more durable than aluminum and makes these blades especially compatible for flying in the rain. This leading edge is particularly useful in seaplane applications where water erosion is a concern.
De-Ice
Electric propeller de-icing boots are available as an option for use on all hydraulic MT models. They consume half the amperage of standard de-ice boots and therefore allow for the use of a propeller with a greater number of blades while maintaining the same or less amp draw.
Vibration
Natural composite propellers vibration is drastically reduced due to itslight weight, low polar moment of inertia and high harmonic dampening
characteristics. In turn, this helps lengthen equipment life and makes flight more comfortable.
.
Weather
The stainless steel leading edge protects against water damage and erosion. Years of testing in rain, ice and snow as well as tests at the University of Dayton have proven that MT`s natural composite propeller is all-weather durable.
Cost
Because natural composite propellers can be overhauled to return the blades to original factory specifications, costly blade replacements are eliminated. Therefore, these props are more efficient and less expensive to own and increase the resale value of the aircraft.
Propeller manufacturers:
Hartzell Propeller
(937) 778-4379
Hamilton Sundstrand
(860) 654-6000
McCauley Propeller Systems
(937) 890-5246
MT-Propeller Entwicklung GmbH
Flugplatzstr. 1,D-94348 Atting / Germany
Phone: 011-49-9429-94090
Fax: 011-49-9429-8432
Regards
Fernando
Hi,
Few months ago, I was searching on internet the same informations. I found on different web sites what you will see in the next rows.
Unfortunatelly, I didn't put any bookmarks about those sites but I'm very sure if you want to find the source, you can take a search with Google, given a simple copy and paste from this text and put it in Google search field. Sorry for "teaching style" but I didn't have so much time to put the text in other form. The initial text has also some figures but I didn't find the way to paste also those figures here. Hope this will help you
Propellers
The propeller is a rotating airfoil. It is subject to drag, stalls and other aerodynamic factors that apply to any airfoil. The propeller provides the thrust to pull the aircraft through the air. The cross section near the hub of the propeller is thick, and has a fairly large angle of attack. The angle of attack and the thickness decreases toward the tip of the blade. Since the linear speed at the tip is much faster than at the hub the change in angle of attack provides uniform thrust along the surface of the blade.
The propeller is normally connected directly to the engine crankshaft. Some aircraft, however, employ gear arrangements between the engine and the propeller.
Propellers fall into two main categories.
· Fixed Pitch
· Controllable pitch
Controllable pitch propellers allow the pilot to set the pitch of the blades, either directly or via a governor, to the best angle for the flight condition and performance desired. Usually for takeoff, a fairly “flat” angle of attack and high engine RPN is used to produce maximum horsepower and thrust. As altitude is gained the pilot can reduce RPM and increase pitch for a cruise climb condition. Once cruise altitude is reached the throttle, mixture and propeller pitch can be adjusted for the desired cruise performance.
The pilot has only one method of controlling thrust on fixed pitch propellers; that being adjusting engine RPM. With controllable pitch propellers, the pilot can adjust two controls; these being RPM (throttle) and Manifold Pressure (propeller pitch control). The Tachometer indicates RPM and the Manifold Pressure Gauge indicates the manifold pressure.
On constant speed propellers, a governor automatically adjusts the pitch of the propeller blade whenever the engine throttle setting is changed. Low RPM and High Manifold pressure should be avoided, as this places undue stress on engine components, and can lead to eventual engine failure.
For any given blade angle, the propeller has an ideal geometric pitch. It is designed to travel a certain distance in one revolution. However, due to slippage, the ideal geometric pitch is never attained. Therefore the effective pitch is always less than the geometric pitch. The propeller is never 100% efficient.
Propellers Common Terms
Blade: one arm of a propeller from hub to tip.
Hub: center section of the propeller, which carries the blades and is attached to the engine shaft.
Spinner: a metal cover enclosing the propeller hub, which improves the appearance of the propeller and may also streamline airflow for engine cooling purposes.
Blade tip: the part of the blade furthest from the hub.
Blade root: the section of the blade nearest the hub.
Blade shank: the portion of a blade inside the hub used to retain the blade.
Blade camber surface: the cambered or most-cambered side of a blade (visible from front of the aircraft).
Blade face or thrust surface: the flat side of a blade (normally visible from the cockpit of the aircraft).
Blade leading edge: the forward full ³cutting² edge of the blade that leads in the direction of rotation.
Blade trailing edge: the continuous edge of the blade that trails the leading edge in the direction of rotation.
Governor: a device, generally mounted on and driven by the engine, which senses and controls engine speed (RPM) by hydraulically adjusting the blade angle of the propeller.
Prop diameter: the diameter of the circle circumscribed by the blade tips.
Blade station: one of the designated distances along the blade as measured from the center of the hub.
Blade thickness: the maximum thickness between the cambered surface and the face or thrust surface at a given blade station.
Blade width: the measurement between the leading edge and the trailing edge at a given station.
Chord line: a theoretical straight line (perpendicular to blade length) drawn between the leading and trailing edges of the blade.
Blade angle: the angle between the chord line of a propeller blade section and a plane perpendicular to the axis of propeller rotation.
Blade angle settings: low and high angle settings of a controllable-pitch prop- for feather, reverse, latch and start locks - which are determined by built-in mechanical hard stops.
We can say that the propeller is the action end of an aircraft's reciprocating engine, because it converts the useful energy of the engine into thrust as it spins around and around. The propeller has the general shape of a wing, but the camber and chord (curvature and cross-sectional length) of each section of the propeller are different, as shown here. The wing provides lift upward, while the propeller provides lift forward.
The wing has only one motion which is forward, while the propeller has forward and rotary motion. The path of these two motions is like a corkscrew as the propeller goes through the air .
Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing (see the figure on the right for definitions of blade back and blade face). The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation, as shown here. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.
There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power.
The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.
The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft.
Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
Modern propellers are fabricated from high-strength, heat-treated, aluminum alloy forgings. New composite materials are being used in applications where weight and mass are critical.
Propellers are typically designed with two to six blades. Generally, props with more than three blades are used primarily for twin-engine aircraft. These blades tend to be shorter for increased ground clearance and more fuselage clearance. Multi-blade props also produce higher, less objectionable sound frequency; reduced vibration; greater flywheel effect and improved aircraft performance.
Types of Propellers
Propellers are classified according to pitch configuration. Blade pitch is the angle of the blades with relation to the plane of rotation and is a significant variable affecting the performance of the propeller.
Fixed Pitch: a one-piece prop with a single fixed blade angle. The pitch (blade angle) must be high enough to offer good cruising performance yet low enough to achieve acceptable takeoff and climb characteristics.
Constant Speed: a prop used with a governor, that automatically provides constant RPM by controlling the forces acting on the propeller to change the blade angle within a preset range.
Full-Feathering: a prop which allows blades to be rotated to a high positive angle to stop rotation (windmilling) after an engine is shut down, thereby reducing drag and asymmetric control forces on twin-engine applications.
Reversing: a prop with blades that can be rotated to a position less than the normal positive low blade angle setting until a negative blade angle is obtained, producing a rearward thrust to slow down, stop or move the aircraft backward. Typically provided for turbine installations.
Beta Control: a prop which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever. These types of propellers are installed on turbine engines.
VARIABLE PITCH PROPELLERS
Full-Feathering vs. Constant Speed
A constant-speed (RPM) system permits the pilot to select the propeller and engine speed for any situation and automatically maintain that RPM under varying conditions of aircraft attitude and engine power. Thereby permitting operation of propeller and engine at most efficient RPMs. RPM is controlled by varying the pitch of the propeller blades - that is, the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given RPM setting, aircraft speed and torque on the engine will increase. For economy cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected RPM.
A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or ³windmill,² causing increased drag. To prevent this, the propeller can be ³feathered² (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller.
Changing Pitch
Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.
In a single-acting propeller system, oil pressure supplied by the governor, acting on the piston produces a force that is opposed by the natural centrifugal twisting moment of the blades in constant speed models or counterweights and large springs in full-feathering systems. To increase or decrease the pitch, high pressure oil is directed to the propeller, which moves the piston back. The motion of the piston is transmitted to the blades through actuating pins and links, moving the blades toward either high pitch for constant-speed systems or low pitch for full-feathering systems.
When the opposing forces are equal, oil flow to the propeller stops and the piston also stops. The piston will remain in this position, maintaining the pitch of the blades until oil flow to or from the propeller is again established by the governor.
From this position, pitch is decreased for constant-speed systems or increased for full- feathering systems by allowing oil to flow out of the propeller and return to the engine sump. When the governor initiates this procedure, hydraulic pressure is decreased and the piston moves forward, changing the pitch of the blades. The piston will continue to move forward until the opposing forces are once again equal. Mechanical stops are installed in the propeller to limit travel in both the high and low pitch directions.
PERFORMANCE CONSIDERATIONS
Shape of Propeller Tips
Propeller tips can be rounded, swept or square. Various tips are often used to meet blade vibration resonance or special design conditions. Tip shape is also a function of aesthetics, noise requirements, flight performance, repair ability and ground clearance.
Propeller Diameter
Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds. For example, the diameter of a fixed-pitch propeller is often large to favor low airspeed operation, while the blade size is small to favor higher airspeeds and faster turning at low airspeeds. The diameter and blade size of a constant-speed propeller is often larger (than a fixed-pitch), due to the variability of blade angles.
Engine Horsepower and RPM
For fixed-pitch props, at a fixed throttle setting, propeller and engine RPM increases or decreases with the airspeed. At a constant airspeed, fixed-pitch propeller and engine RPM change if power is increased or decreased. A constant-speed prop uses a governor to provide constant RPM at the selected throttle setting. The blade angle automatically increases or decreases as the RPM setting or engine power changes. With a fixed RPM and power setting, the blade angle automatically changes as airspeed increases or decreases.
De-ice System
After ice has formed, a de-ice system applies electric heat to the blade, melting the ice near the surface of the blade so the ice will be removed by centrifugal force as the prop spins. A de-ice system typically consists of boots, slip rings and brushes. Older technology, anti-ice equipment, prevents the formation of ice by allowing alcohol to flow over the propeller blades.
Synchronizing and Synchrophasing Systems
On twin-engine applications, the benefits of synchronizing and synchrophasing systems are the reduction of noise beats produced by the interaction of the prop and the fuselage. The governing system provides the means for synchronizing and synchrophasing the two propellers on twin-engine aircraft. The synchronizing option adjusts propeller RPM so that both props are turning at the same speed. Tis can be done by installing a pick-up disc on each governor drive shaft, along with a transducer that sends a frequency signal to an electronic control. This control compares the signals from both governors and adjusts one of them to bring it into ³synch² with the other.
Once the props are synchronized, the synchrophaser option allows the pilot to adjust the position of the blades on one propeller with respect to the position of the blades on the second prop for reduced noise and vibration. Synchrophasers are solid-state units that automatically synchronize prop speed combined with a phasing control operated by the pilot. This phasing control allows the pilot to manually adjust the difference between the two propellers to minimize the ³beat² of the props.
Overhauling or Reconditioning Your Propeller
Blade reconditioning covers major or minor blade damage from accident or other causes and includes balancing of the prop. Blades should also be reconditioned if they have been damaged and filed often. This work is performed on an ³as required² basis by an approved propeller repair station. For a one-piece, fixed-pitch prop, reconditioning is equivalent to an overhaul. For other types of props, if damage is major but repairable, an overhaul may be included with the reconditioning.
All props require periodic overhaul to increase safety, prolong propeller life and improve function or operation. The overhaul interval is generally based on hours of service (operating time) as well as a calendar limit. During overhaul, the propeller is disassembled and inspected for wear, cracks, corrosion and other abnormal conditions. Parts may be replaced or reconditioned and refinished. The propeller is then re-assembled and balanced.
CHANGING THE PROPELLER
A propeller is designed to be compatible with a specific engine, in order to achieve maximum thrust or efficiency and reliability from the aircraft. Even though the propeller might fit another engine shaft, only the propeller manufacturer can determine whether it is suitable for use on a particular aircraft. Installation requirements are usually available for all manufacturers. Propellers are generally changed either to upgrade performance or to restore original performance compromised by wear and tear. Whatever the reason, changing propellers deserves careful consideration. The propeller is intimately linked to aircraft performance and operates in partnership with all other components. Many factors can enhance or impair performance.
Four ways to change propellers:
1. OEM Type Certificate
2. One-Time Field Approval
3. Experimental Certificate
4. Supplemental Type Certificate
1.OEM Type Certificate
Any propeller that appears on the Original Equipment Manufacturer¹s (OEM) approved equipment list, on the Aircraft Type Certificate Data Sheet, is automatically approved for that application. No further paperwork is required.
2.One-Time Field Approval
There are only two things for certain about the One-time Field Approval:
€ It requires the endorsement of the aeronautical authority
€ It has to have some degree of technical justification
3.Experimental Certificate
The Experimental Certificate option is available only to experimental aircraft owners or operators..
4.Supplemental Type Certificate (STC)
The aeronautical authority issues an STC for propellers that have passed rigorous and extensive testing but which are not listed on the OEM¹s approved equipment list for a particular aircraft. The STC is the easiest way to modify an existing airplane in the field. Most owners, operators and mechanics who wish to upgrade propeller performance will use STCs.
Single-component STCs involve a specific propeller that has been approved for a specific aircraft. For example, the single-component STC is commonly used to upgrade an aircraft from a two-bladed to a three-bladed propeller. It may also be used by owners or operators who are not satisfied with the performance of their original propeller. The combination STC involves multiple components, such as a propeller and an engine upgrade. Although less common than single-component STCs, the combination STC is gaining popularity because of the integral relationship between propeller and engine.
The STC holder may be the original propeller manufacturer, the original aircraft manufac- turer or an individual. To obtain an STC, the STC applicant often works with the aeronautical authority and the OEM, tests and evaluates the propeller, and pays for flight performance testing and stress surveys. Developing the STC for a simple, one-propeller changeover for a particular aircraft can be a significant expense. However:
? STC holders do not always work with the original propeller manufacturer prior to obtaining STC approval from the aeronautical authority.
? The aeronautical authority usually does not notify the original propeller manufacturer when it grants an STC to someone other than the OEM.
Therefore, always contact the manufacturer of the STC propeller you plan to install, and ask if the OEM is aware of the STC or of any potential problems. Also, contact the STC holder directly to discuss the performance changes you should expect. Request a list of owners who have performed similar installations. Make sure everything is working properly under usual operating conditions before installing any STC conversion. To determine whether or not a problem is propeller related, use the process of elimination, changing one variable at a time. For example, a recently overhauled engine may cause vibration, which could be mistakenly blamed on a new propeller installed at the same time. If you converted from a two-bladed propeller to a three-bladed propeller immediately after an engine overhaul, try out the overhauled engine using the two-bladed propeller. If you experience vibration that was not apparent before the overhaul, you will know that it is an engine problem, not a propeller problem.
The warranty that comes with the STC conversion covers the propeller assembly. Technically, the original propeller manufacturer is responsible only if the propeller is defective. The STC holder is responsible for problems with installation adjustments. However, owners and operators may have adjustment or performance trouble that is not propeller-related, including problems with the engine, engine mounts, cowling configuration or airframe. As a result, performance varies by individual aircraft.
Composite propellers
Composite propellers are seeing more and more use these days. But there are inspection issues that are unique to these propellers. With proper maintenance and inspection, these sturdy propellers can provide a long service life. But neglect can lead to catastrophic failure.
Propellers, both metal and composite, are some of the most critical components of an aircraft. Failure of a propeller can lead to much more than loss of thrust. If a propeller blade is thrown, the result is catastrophic, much more so than an engine failure.
Centrifugal force
There are five operational forces that act on a propeller simultaneously. These are centrifugal force, thrust bending force, torque bending force, aerodynamic twisting moment, and centrifugal twisting moment. Of them all, centrifugal force causes the greatest stress on propellers. This is the force that tends to pull the blades from the propeller hub. It is related to RPM - the higher the RPM, the more centrifugal force the blades are subjected to.
Propellers are subjected to great amounts of centrifugal force. There can typically be 25 tons of centrifugal force at the root end of metal propeller blades, minimally 20 tons. It is an exponential load. In other words, it's not a straight line relationship. As the RPM goes up, the amount of centrifugal force acting on the propeller blades goes up exponentially.
With so much centrifugal force acting on the propeller blades, it is easy to see how the loss of a blade in flight can be catastrophic. The danger is not necessarily all from the unrestrained propeller blade that can impact the aircraft and cause serious damage. Major danger lies in the transfer of energy. It goes back to Newton's Law of momentum conservation. The force that the propeller blade was subjected to is transferred to the system when it departs the aircraft. This can be an enormous amount of force that can rip the engine from its mounts and cause severe damage to the aircraft structure.
Benefits of composite blades
One of the most evident benefits of composite blades is their weight. They offer a substantial weight reduction compared to metal propellers, thereby offering more efficient operation (less horsepower is needed to produce the same thrust).
Another advantage to composite propellers is the fact that they don't shrink dimensionally after rework. As a typical metal blade experiences damage, the damage is blended out according to the repair manual. So, over time more and more material is taken away until eventually the blade reaches its minimum limits. That is not the case with composite blades.
The composite blades never wear out. They may eventually have a shank go bad, or a bearing go out, but the blades themselves never change. They are always reworked to the same size. As you repair them, they go back to the original dimensions, where with metal blades as you keep on taking metal away, sooner or later they are going to go undersize.
Not losing dimensional area is a definite advantage. If a prop blade is undersize, it has a tendency to be susceptible to resonance. Every metal propeller blade is like a tuning fork. If it finds a sympathetic frequency that it can respond to, it's tip can deflect up to 6 inches. This can cause it to fail instantly. When the blade is within dimensional limits, it can't do that.
Prop maintenance
The health of your composite blade begins with inspection. Following the recommended inspection schedule is essential. The main inspection for composite prop blades is the tap test. A metal tool is tapped on the propeller blade surface to look for delaminated areas. Damaged areas sound like dull thuds compared to the light sound of the normal structure. The defect may not be visible, but the tap test will give a definite indication of a flaw.
In addition to the tap test, a good visual inspection of the propeller is in order. Check the condition and security of the leading edge and de-icer. A nickel covering protects the leading edge of the propeller from erosion and impact damage.
The de-ice boots should be checked for any evidence of overheating. Excessive heat can be very damaging to composite prop blades.
If a boot goes bad and burns, blade damage to the structure beneath is assured. Temperatures over 140 degrees can cause the Hartzel composite to 'pop' - basically delaminating on the inside. Hamilton blades will withstand approximately 100 degrees more. Most of the time the prop blades aren't destroyed by this heat damage. They can be fixed, but the cause of the overheating should be addressed.
The paint coating should be examined carefully for eroded areas. Areas where the coating is eroded could allow fluids to enter and saturate the composite. This can be especially damaging to fiberglass prop blades. When oil or grease are allowed to saturate the material, it causes the fiberglass to deteriorate quickly. The Kevlar® type prop blades aren't as susceptible, but they can still be heavily damaged by infiltration. Pay close attention to the blade coating and repair any eroded areas in accordance with the maintenance manual.
Lightning protection
Composites are also susceptible to lightning strikes. Therefore, various methods are employed to protect composite propellers from the damage caused by lightning strikes. This can be in the form of metal spars, erosion sheaths, and/or special metal-based coatings placed over the composite that helps dissipate the electricity from a lightning strike. Whatever the type of protection on your prop blades, make sure that it is intact. With no path to airframe ground for the energy to dissipate, the propeller blade could literally blow apart if struck by lightning. Also, be aware of the signs of a lightning strike during the inspection. A tell-tale sign is a dark, discolored brown or black spot on the outer trailing edge of a blade that looks like an overheated area. If a lightning strike is suspected, the use of a gauss meter to check for residual magnetism in the ferrous attaching parts of the propeller is recommended. If a strike is verified, the propeller should be sent to an authorized repair facility for inspection and disposition.
Overhaul
The overhaul process for composite propellers begins with an incoming inspection. Any areas that may require follow-up are noted.
Next, the paint coating is removed as necessary. This process is usually done by hand to ensure that only the protective paint coating is removed without going into the composite layer.
The propeller blade is then inspected visually and re-tap tested. All damaged areas are repaired as necessary. The leading edge is inspected for security and integrity. The nickel leading edges are sacrificial, protecting the composite structure underneath. Eventually they have to be replaced. To remove edges from Hamilton propeller blades, they are ground down the center line of the blade's leading edge and then each remaining side is pried off the blade using a thin chisel. On the Hartzells, a propane torch is used to flash heat the edge in order to soften up the glue, paying close attention not to overheat the underlying blade. The edge is then removed once the glue has softened. Structure under the edge is inspected and repaired as necessary before installing a new edge.
When ready to install the new edge, a template is first placed over the repaired bare leading edge of the prop. It is basically an edge with witness holes ground in it to ensure that when placed on the leading edge of the blade the mating surface beneath is dimensionally correct. On Hartzell blades, adhesive is applied and the edge is put in place and secured with a vacuum bag until cured. For the Hamiltons, the process is a little more involved. Epoxy is applied to the blade edge, and a special pressure bag is placed around it. This ensures that uniform computer-controlled pressure and temperature is applied during the curing process. Special paint is then re-applied to each prop blade. This involves applying multiple layers of cross coats as required. Each propeller blade is then balanced to a calibrated master, reassembled, documented, and carefully packaged. It is then ready to be shipped back to the customer.
In the end, by following the manufacturer's recommended maintenance and inspection procedures, these sturdy props will last a long time. In fact, neglect is the most common factor leading to higher costs at overhaul and a shortened propeller life. Considering the extreme damage that can be caused by the loss of a propeller blade in flight, there is no other choice but to pay close attention to these critical aircraft components.
High Performance
Lightweight, composite material allows blades to be built with a thicker cross-section dimension, which increases takeoff and climb performance. The capability to construct multi-blade propellers with a smaller diameter eliminates high-speed drag and increases cruise speed. Metal prop blades built to these dimensions would be too heavy. The reduction in diameter also reduces noise while increasing ground clearance.
Light Weights
A lightweight propeller increases the useful load of the aircraft and also reduces the stress on the engine and crankshaft.
Propeller Life
Natural composite propeller blades will not fatigue over time whereas metal propeller blades are life-limited by fatigue and dimension. Natural composite props are unique in that each overhaul returns the blades to their original dimension by adding composite material to the damaged section of the blade. In the event of a ground strike, the blades are often reparable, the hub is re-usable and the risk of internal damage to the engine is significantly reduced. The blades are also protected by a replaceable stainless steel leading edge to prevent blade erosion. Stainless steel is more durable than aluminum and makes these blades especially compatible for flying in the rain. This leading edge is particularly useful in seaplane applications where water erosion is a concern.
De-Ice
Electric propeller de-icing boots are available as an option for use on all hydraulic MT models. They consume half the amperage of standard de-ice boots and therefore allow for the use of a propeller with a greater number of blades while maintaining the same or less amp draw.
Vibration
Natural composite propellers vibration is drastically reduced due to itslight weight, low polar moment of inertia and high harmonic dampening
characteristics. In turn, this helps lengthen equipment life and makes flight more comfortable.
.
Weather
The stainless steel leading edge protects against water damage and erosion. Years of testing in rain, ice and snow as well as tests at the University of Dayton have proven that MT`s natural composite propeller is all-weather durable.
Cost
Because natural composite propellers can be overhauled to return the blades to original factory specifications, costly blade replacements are eliminated. Therefore, these props are more efficient and less expensive to own and increase the resale value of the aircraft.
Propeller manufacturers:
Hartzell Propeller
(937) 778-4379
Hamilton Sundstrand
(860) 654-6000
McCauley Propeller Systems
(937) 890-5246
MT-Propeller Entwicklung GmbH
Flugplatzstr. 1,D-94348 Atting / Germany
Phone: 011-49-9429-94090
Fax: 011-49-9429-8432
Regards
Fernando
Few months ago, I was searching on internet the same informations. I found on different web sites what you will see in the next rows.
Unfortunatelly, I didn't put any bookmarks about those sites but I'm very sure if you want to find the source, you can take a search with Google, given a simple copy and paste from this text and put it in Google search field. Sorry for "teaching style" but I didn't have so much time to put the text in other form. The initial text has also some figures but I didn't find the way to paste also those figures here. Hope this will help you
Propellers
The propeller is a rotating airfoil. It is subject to drag, stalls and other aerodynamic factors that apply to any airfoil. The propeller provides the thrust to pull the aircraft through the air. The cross section near the hub of the propeller is thick, and has a fairly large angle of attack. The angle of attack and the thickness decreases toward the tip of the blade. Since the linear speed at the tip is much faster than at the hub the change in angle of attack provides uniform thrust along the surface of the blade.
The propeller is normally connected directly to the engine crankshaft. Some aircraft, however, employ gear arrangements between the engine and the propeller.
Propellers fall into two main categories.
· Fixed Pitch
· Controllable pitch
Controllable pitch propellers allow the pilot to set the pitch of the blades, either directly or via a governor, to the best angle for the flight condition and performance desired. Usually for takeoff, a fairly “flat” angle of attack and high engine RPN is used to produce maximum horsepower and thrust. As altitude is gained the pilot can reduce RPM and increase pitch for a cruise climb condition. Once cruise altitude is reached the throttle, mixture and propeller pitch can be adjusted for the desired cruise performance.
The pilot has only one method of controlling thrust on fixed pitch propellers; that being adjusting engine RPM. With controllable pitch propellers, the pilot can adjust two controls; these being RPM (throttle) and Manifold Pressure (propeller pitch control). The Tachometer indicates RPM and the Manifold Pressure Gauge indicates the manifold pressure.
On constant speed propellers, a governor automatically adjusts the pitch of the propeller blade whenever the engine throttle setting is changed. Low RPM and High Manifold pressure should be avoided, as this places undue stress on engine components, and can lead to eventual engine failure.
For any given blade angle, the propeller has an ideal geometric pitch. It is designed to travel a certain distance in one revolution. However, due to slippage, the ideal geometric pitch is never attained. Therefore the effective pitch is always less than the geometric pitch. The propeller is never 100% efficient.
Propellers Common Terms
Blade: one arm of a propeller from hub to tip.
Hub: center section of the propeller, which carries the blades and is attached to the engine shaft.
Spinner: a metal cover enclosing the propeller hub, which improves the appearance of the propeller and may also streamline airflow for engine cooling purposes.
Blade tip: the part of the blade furthest from the hub.
Blade root: the section of the blade nearest the hub.
Blade shank: the portion of a blade inside the hub used to retain the blade.
Blade camber surface: the cambered or most-cambered side of a blade (visible from front of the aircraft).
Blade face or thrust surface: the flat side of a blade (normally visible from the cockpit of the aircraft).
Blade leading edge: the forward full ³cutting² edge of the blade that leads in the direction of rotation.
Blade trailing edge: the continuous edge of the blade that trails the leading edge in the direction of rotation.
Governor: a device, generally mounted on and driven by the engine, which senses and controls engine speed (RPM) by hydraulically adjusting the blade angle of the propeller.
Prop diameter: the diameter of the circle circumscribed by the blade tips.
Blade station: one of the designated distances along the blade as measured from the center of the hub.
Blade thickness: the maximum thickness between the cambered surface and the face or thrust surface at a given blade station.
Blade width: the measurement between the leading edge and the trailing edge at a given station.
Chord line: a theoretical straight line (perpendicular to blade length) drawn between the leading and trailing edges of the blade.
Blade angle: the angle between the chord line of a propeller blade section and a plane perpendicular to the axis of propeller rotation.
Blade angle settings: low and high angle settings of a controllable-pitch prop- for feather, reverse, latch and start locks - which are determined by built-in mechanical hard stops.
We can say that the propeller is the action end of an aircraft's reciprocating engine, because it converts the useful energy of the engine into thrust as it spins around and around. The propeller has the general shape of a wing, but the camber and chord (curvature and cross-sectional length) of each section of the propeller are different, as shown here. The wing provides lift upward, while the propeller provides lift forward.
The wing has only one motion which is forward, while the propeller has forward and rotary motion. The path of these two motions is like a corkscrew as the propeller goes through the air .
Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing (see the figure on the right for definitions of blade back and blade face). The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation, as shown here. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.
There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power.
The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.
The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft.
Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
Modern propellers are fabricated from high-strength, heat-treated, aluminum alloy forgings. New composite materials are being used in applications where weight and mass are critical.
Propellers are typically designed with two to six blades. Generally, props with more than three blades are used primarily for twin-engine aircraft. These blades tend to be shorter for increased ground clearance and more fuselage clearance. Multi-blade props also produce higher, less objectionable sound frequency; reduced vibration; greater flywheel effect and improved aircraft performance.
Types of Propellers
Propellers are classified according to pitch configuration. Blade pitch is the angle of the blades with relation to the plane of rotation and is a significant variable affecting the performance of the propeller.
Fixed Pitch: a one-piece prop with a single fixed blade angle. The pitch (blade angle) must be high enough to offer good cruising performance yet low enough to achieve acceptable takeoff and climb characteristics.
Constant Speed: a prop used with a governor, that automatically provides constant RPM by controlling the forces acting on the propeller to change the blade angle within a preset range.
Full-Feathering: a prop which allows blades to be rotated to a high positive angle to stop rotation (windmilling) after an engine is shut down, thereby reducing drag and asymmetric control forces on twin-engine applications.
Reversing: a prop with blades that can be rotated to a position less than the normal positive low blade angle setting until a negative blade angle is obtained, producing a rearward thrust to slow down, stop or move the aircraft backward. Typically provided for turbine installations.
Beta Control: a prop which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever. These types of propellers are installed on turbine engines.
VARIABLE PITCH PROPELLERS
Full-Feathering vs. Constant Speed
A constant-speed (RPM) system permits the pilot to select the propeller and engine speed for any situation and automatically maintain that RPM under varying conditions of aircraft attitude and engine power. Thereby permitting operation of propeller and engine at most efficient RPMs. RPM is controlled by varying the pitch of the propeller blades - that is, the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given RPM setting, aircraft speed and torque on the engine will increase. For economy cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected RPM.
A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or ³windmill,² causing increased drag. To prevent this, the propeller can be ³feathered² (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller.
Changing Pitch
Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.
In a single-acting propeller system, oil pressure supplied by the governor, acting on the piston produces a force that is opposed by the natural centrifugal twisting moment of the blades in constant speed models or counterweights and large springs in full-feathering systems. To increase or decrease the pitch, high pressure oil is directed to the propeller, which moves the piston back. The motion of the piston is transmitted to the blades through actuating pins and links, moving the blades toward either high pitch for constant-speed systems or low pitch for full-feathering systems.
When the opposing forces are equal, oil flow to the propeller stops and the piston also stops. The piston will remain in this position, maintaining the pitch of the blades until oil flow to or from the propeller is again established by the governor.
From this position, pitch is decreased for constant-speed systems or increased for full- feathering systems by allowing oil to flow out of the propeller and return to the engine sump. When the governor initiates this procedure, hydraulic pressure is decreased and the piston moves forward, changing the pitch of the blades. The piston will continue to move forward until the opposing forces are once again equal. Mechanical stops are installed in the propeller to limit travel in both the high and low pitch directions.
PERFORMANCE CONSIDERATIONS
Shape of Propeller Tips
Propeller tips can be rounded, swept or square. Various tips are often used to meet blade vibration resonance or special design conditions. Tip shape is also a function of aesthetics, noise requirements, flight performance, repair ability and ground clearance.
Propeller Diameter
Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds. For example, the diameter of a fixed-pitch propeller is often large to favor low airspeed operation, while the blade size is small to favor higher airspeeds and faster turning at low airspeeds. The diameter and blade size of a constant-speed propeller is often larger (than a fixed-pitch), due to the variability of blade angles.
Engine Horsepower and RPM
For fixed-pitch props, at a fixed throttle setting, propeller and engine RPM increases or decreases with the airspeed. At a constant airspeed, fixed-pitch propeller and engine RPM change if power is increased or decreased. A constant-speed prop uses a governor to provide constant RPM at the selected throttle setting. The blade angle automatically increases or decreases as the RPM setting or engine power changes. With a fixed RPM and power setting, the blade angle automatically changes as airspeed increases or decreases.
De-ice System
After ice has formed, a de-ice system applies electric heat to the blade, melting the ice near the surface of the blade so the ice will be removed by centrifugal force as the prop spins. A de-ice system typically consists of boots, slip rings and brushes. Older technology, anti-ice equipment, prevents the formation of ice by allowing alcohol to flow over the propeller blades.
Synchronizing and Synchrophasing Systems
On twin-engine applications, the benefits of synchronizing and synchrophasing systems are the reduction of noise beats produced by the interaction of the prop and the fuselage. The governing system provides the means for synchronizing and synchrophasing the two propellers on twin-engine aircraft. The synchronizing option adjusts propeller RPM so that both props are turning at the same speed. Tis can be done by installing a pick-up disc on each governor drive shaft, along with a transducer that sends a frequency signal to an electronic control. This control compares the signals from both governors and adjusts one of them to bring it into ³synch² with the other.
Once the props are synchronized, the synchrophaser option allows the pilot to adjust the position of the blades on one propeller with respect to the position of the blades on the second prop for reduced noise and vibration. Synchrophasers are solid-state units that automatically synchronize prop speed combined with a phasing control operated by the pilot. This phasing control allows the pilot to manually adjust the difference between the two propellers to minimize the ³beat² of the props.
Overhauling or Reconditioning Your Propeller
Blade reconditioning covers major or minor blade damage from accident or other causes and includes balancing of the prop. Blades should also be reconditioned if they have been damaged and filed often. This work is performed on an ³as required² basis by an approved propeller repair station. For a one-piece, fixed-pitch prop, reconditioning is equivalent to an overhaul. For other types of props, if damage is major but repairable, an overhaul may be included with the reconditioning.
All props require periodic overhaul to increase safety, prolong propeller life and improve function or operation. The overhaul interval is generally based on hours of service (operating time) as well as a calendar limit. During overhaul, the propeller is disassembled and inspected for wear, cracks, corrosion and other abnormal conditions. Parts may be replaced or reconditioned and refinished. The propeller is then re-assembled and balanced.
CHANGING THE PROPELLER
A propeller is designed to be compatible with a specific engine, in order to achieve maximum thrust or efficiency and reliability from the aircraft. Even though the propeller might fit another engine shaft, only the propeller manufacturer can determine whether it is suitable for use on a particular aircraft. Installation requirements are usually available for all manufacturers. Propellers are generally changed either to upgrade performance or to restore original performance compromised by wear and tear. Whatever the reason, changing propellers deserves careful consideration. The propeller is intimately linked to aircraft performance and operates in partnership with all other components. Many factors can enhance or impair performance.
Four ways to change propellers:
1. OEM Type Certificate
2. One-Time Field Approval
3. Experimental Certificate
4. Supplemental Type Certificate
1.OEM Type Certificate
Any propeller that appears on the Original Equipment Manufacturer¹s (OEM) approved equipment list, on the Aircraft Type Certificate Data Sheet, is automatically approved for that application. No further paperwork is required.
2.One-Time Field Approval
There are only two things for certain about the One-time Field Approval:
€ It requires the endorsement of the aeronautical authority
€ It has to have some degree of technical justification
3.Experimental Certificate
The Experimental Certificate option is available only to experimental aircraft owners or operators..
4.Supplemental Type Certificate (STC)
The aeronautical authority issues an STC for propellers that have passed rigorous and extensive testing but which are not listed on the OEM¹s approved equipment list for a particular aircraft. The STC is the easiest way to modify an existing airplane in the field. Most owners, operators and mechanics who wish to upgrade propeller performance will use STCs.
Single-component STCs involve a specific propeller that has been approved for a specific aircraft. For example, the single-component STC is commonly used to upgrade an aircraft from a two-bladed to a three-bladed propeller. It may also be used by owners or operators who are not satisfied with the performance of their original propeller. The combination STC involves multiple components, such as a propeller and an engine upgrade. Although less common than single-component STCs, the combination STC is gaining popularity because of the integral relationship between propeller and engine.
The STC holder may be the original propeller manufacturer, the original aircraft manufac- turer or an individual. To obtain an STC, the STC applicant often works with the aeronautical authority and the OEM, tests and evaluates the propeller, and pays for flight performance testing and stress surveys. Developing the STC for a simple, one-propeller changeover for a particular aircraft can be a significant expense. However:
? STC holders do not always work with the original propeller manufacturer prior to obtaining STC approval from the aeronautical authority.
? The aeronautical authority usually does not notify the original propeller manufacturer when it grants an STC to someone other than the OEM.
Therefore, always contact the manufacturer of the STC propeller you plan to install, and ask if the OEM is aware of the STC or of any potential problems. Also, contact the STC holder directly to discuss the performance changes you should expect. Request a list of owners who have performed similar installations. Make sure everything is working properly under usual operating conditions before installing any STC conversion. To determine whether or not a problem is propeller related, use the process of elimination, changing one variable at a time. For example, a recently overhauled engine may cause vibration, which could be mistakenly blamed on a new propeller installed at the same time. If you converted from a two-bladed propeller to a three-bladed propeller immediately after an engine overhaul, try out the overhauled engine using the two-bladed propeller. If you experience vibration that was not apparent before the overhaul, you will know that it is an engine problem, not a propeller problem.
The warranty that comes with the STC conversion covers the propeller assembly. Technically, the original propeller manufacturer is responsible only if the propeller is defective. The STC holder is responsible for problems with installation adjustments. However, owners and operators may have adjustment or performance trouble that is not propeller-related, including problems with the engine, engine mounts, cowling configuration or airframe. As a result, performance varies by individual aircraft.
Composite propellers
Composite propellers are seeing more and more use these days. But there are inspection issues that are unique to these propellers. With proper maintenance and inspection, these sturdy propellers can provide a long service life. But neglect can lead to catastrophic failure.
Propellers, both metal and composite, are some of the most critical components of an aircraft. Failure of a propeller can lead to much more than loss of thrust. If a propeller blade is thrown, the result is catastrophic, much more so than an engine failure.
Centrifugal force
There are five operational forces that act on a propeller simultaneously. These are centrifugal force, thrust bending force, torque bending force, aerodynamic twisting moment, and centrifugal twisting moment. Of them all, centrifugal force causes the greatest stress on propellers. This is the force that tends to pull the blades from the propeller hub. It is related to RPM - the higher the RPM, the more centrifugal force the blades are subjected to.
Propellers are subjected to great amounts of centrifugal force. There can typically be 25 tons of centrifugal force at the root end of metal propeller blades, minimally 20 tons. It is an exponential load. In other words, it's not a straight line relationship. As the RPM goes up, the amount of centrifugal force acting on the propeller blades goes up exponentially.
With so much centrifugal force acting on the propeller blades, it is easy to see how the loss of a blade in flight can be catastrophic. The danger is not necessarily all from the unrestrained propeller blade that can impact the aircraft and cause serious damage. Major danger lies in the transfer of energy. It goes back to Newton's Law of momentum conservation. The force that the propeller blade was subjected to is transferred to the system when it departs the aircraft. This can be an enormous amount of force that can rip the engine from its mounts and cause severe damage to the aircraft structure.
Benefits of composite blades
One of the most evident benefits of composite blades is their weight. They offer a substantial weight reduction compared to metal propellers, thereby offering more efficient operation (less horsepower is needed to produce the same thrust).
Another advantage to composite propellers is the fact that they don't shrink dimensionally after rework. As a typical metal blade experiences damage, the damage is blended out according to the repair manual. So, over time more and more material is taken away until eventually the blade reaches its minimum limits. That is not the case with composite blades.
The composite blades never wear out. They may eventually have a shank go bad, or a bearing go out, but the blades themselves never change. They are always reworked to the same size. As you repair them, they go back to the original dimensions, where with metal blades as you keep on taking metal away, sooner or later they are going to go undersize.
Not losing dimensional area is a definite advantage. If a prop blade is undersize, it has a tendency to be susceptible to resonance. Every metal propeller blade is like a tuning fork. If it finds a sympathetic frequency that it can respond to, it's tip can deflect up to 6 inches. This can cause it to fail instantly. When the blade is within dimensional limits, it can't do that.
Prop maintenance
The health of your composite blade begins with inspection. Following the recommended inspection schedule is essential. The main inspection for composite prop blades is the tap test. A metal tool is tapped on the propeller blade surface to look for delaminated areas. Damaged areas sound like dull thuds compared to the light sound of the normal structure. The defect may not be visible, but the tap test will give a definite indication of a flaw.
In addition to the tap test, a good visual inspection of the propeller is in order. Check the condition and security of the leading edge and de-icer. A nickel covering protects the leading edge of the propeller from erosion and impact damage.
The de-ice boots should be checked for any evidence of overheating. Excessive heat can be very damaging to composite prop blades.
If a boot goes bad and burns, blade damage to the structure beneath is assured. Temperatures over 140 degrees can cause the Hartzel composite to 'pop' - basically delaminating on the inside. Hamilton blades will withstand approximately 100 degrees more. Most of the time the prop blades aren't destroyed by this heat damage. They can be fixed, but the cause of the overheating should be addressed.
The paint coating should be examined carefully for eroded areas. Areas where the coating is eroded could allow fluids to enter and saturate the composite. This can be especially damaging to fiberglass prop blades. When oil or grease are allowed to saturate the material, it causes the fiberglass to deteriorate quickly. The Kevlar® type prop blades aren't as susceptible, but they can still be heavily damaged by infiltration. Pay close attention to the blade coating and repair any eroded areas in accordance with the maintenance manual.
Lightning protection
Composites are also susceptible to lightning strikes. Therefore, various methods are employed to protect composite propellers from the damage caused by lightning strikes. This can be in the form of metal spars, erosion sheaths, and/or special metal-based coatings placed over the composite that helps dissipate the electricity from a lightning strike. Whatever the type of protection on your prop blades, make sure that it is intact. With no path to airframe ground for the energy to dissipate, the propeller blade could literally blow apart if struck by lightning. Also, be aware of the signs of a lightning strike during the inspection. A tell-tale sign is a dark, discolored brown or black spot on the outer trailing edge of a blade that looks like an overheated area. If a lightning strike is suspected, the use of a gauss meter to check for residual magnetism in the ferrous attaching parts of the propeller is recommended. If a strike is verified, the propeller should be sent to an authorized repair facility for inspection and disposition.
Overhaul
The overhaul process for composite propellers begins with an incoming inspection. Any areas that may require follow-up are noted.
Next, the paint coating is removed as necessary. This process is usually done by hand to ensure that only the protective paint coating is removed without going into the composite layer.
The propeller blade is then inspected visually and re-tap tested. All damaged areas are repaired as necessary. The leading edge is inspected for security and integrity. The nickel leading edges are sacrificial, protecting the composite structure underneath. Eventually they have to be replaced. To remove edges from Hamilton propeller blades, they are ground down the center line of the blade's leading edge and then each remaining side is pried off the blade using a thin chisel. On the Hartzells, a propane torch is used to flash heat the edge in order to soften up the glue, paying close attention not to overheat the underlying blade. The edge is then removed once the glue has softened. Structure under the edge is inspected and repaired as necessary before installing a new edge.
When ready to install the new edge, a template is first placed over the repaired bare leading edge of the prop. It is basically an edge with witness holes ground in it to ensure that when placed on the leading edge of the blade the mating surface beneath is dimensionally correct. On Hartzell blades, adhesive is applied and the edge is put in place and secured with a vacuum bag until cured. For the Hamiltons, the process is a little more involved. Epoxy is applied to the blade edge, and a special pressure bag is placed around it. This ensures that uniform computer-controlled pressure and temperature is applied during the curing process. Special paint is then re-applied to each prop blade. This involves applying multiple layers of cross coats as required. Each propeller blade is then balanced to a calibrated master, reassembled, documented, and carefully packaged. It is then ready to be shipped back to the customer.
In the end, by following the manufacturer's recommended maintenance and inspection procedures, these sturdy props will last a long time. In fact, neglect is the most common factor leading to higher costs at overhaul and a shortened propeller life. Considering the extreme damage that can be caused by the loss of a propeller blade in flight, there is no other choice but to pay close attention to these critical aircraft components.
High Performance
Lightweight, composite material allows blades to be built with a thicker cross-section dimension, which increases takeoff and climb performance. The capability to construct multi-blade propellers with a smaller diameter eliminates high-speed drag and increases cruise speed. Metal prop blades built to these dimensions would be too heavy. The reduction in diameter also reduces noise while increasing ground clearance.
Light Weights
A lightweight propeller increases the useful load of the aircraft and also reduces the stress on the engine and crankshaft.
Propeller Life
Natural composite propeller blades will not fatigue over time whereas metal propeller blades are life-limited by fatigue and dimension. Natural composite props are unique in that each overhaul returns the blades to their original dimension by adding composite material to the damaged section of the blade. In the event of a ground strike, the blades are often reparable, the hub is re-usable and the risk of internal damage to the engine is significantly reduced. The blades are also protected by a replaceable stainless steel leading edge to prevent blade erosion. Stainless steel is more durable than aluminum and makes these blades especially compatible for flying in the rain. This leading edge is particularly useful in seaplane applications where water erosion is a concern.
De-Ice
Electric propeller de-icing boots are available as an option for use on all hydraulic MT models. They consume half the amperage of standard de-ice boots and therefore allow for the use of a propeller with a greater number of blades while maintaining the same or less amp draw.
Vibration
Natural composite propellers vibration is drastically reduced due to itslight weight, low polar moment of inertia and high harmonic dampening
characteristics. In turn, this helps lengthen equipment life and makes flight more comfortable.
.
Weather
The stainless steel leading edge protects against water damage and erosion. Years of testing in rain, ice and snow as well as tests at the University of Dayton have proven that MT`s natural composite propeller is all-weather durable.
Cost
Because natural composite propellers can be overhauled to return the blades to original factory specifications, costly blade replacements are eliminated. Therefore, these props are more efficient and less expensive to own and increase the resale value of the aircraft.
Propeller manufacturers:
Hartzell Propeller
(937) 778-4379
Hamilton Sundstrand
(860) 654-6000
McCauley Propeller Systems
(937) 890-5246
MT-Propeller Entwicklung GmbH
Flugplatzstr. 1,D-94348 Atting / Germany
Phone: 011-49-9429-94090
Fax: 011-49-9429-8432
Regards
Fernando
Aimes:
Yes, you must pay for this formidable information about
blade and hub design.
The documents ID on NTIS are:
Vol I = AD-774831
Vol II = AD-774836
Vol III= AD-776989
You coul buy separate volumes.( The 3 vol are 3 inches thick !!)
Vol I = Aerodynamic design and installation
Vol II = Structural analysis and blade design
Vol III=Hub,actuator and control designs.
If you have any question, please let me know.
Regards
Mohr
Yes, you must pay for this formidable information about
blade and hub design.
The documents ID on NTIS are:
Vol I = AD-774831
Vol II = AD-774836
Vol III= AD-776989
You coul buy separate volumes.( The 3 vol are 3 inches thick !!)
Vol I = Aerodynamic design and installation
Vol II = Structural analysis and blade design
Vol III=Hub,actuator and control designs.
If you have any question, please let me know.
Regards
Mohr
Hi all, first of all this is my first post, and I really enjoy this threads... I think I have engage a good target.
I work on maintenance so my point of view is slightly different, but to understand how propeller and related system works and different approachs working in the industry you could find helpful some Jeppessen textbooks (Amazon should work):
1. A&P Airframe (general review of all systems)
2. Aircraft Propellers and Controls by Frank Delp
3. Aircraft governors by Frank Delp
Hopefully could help
Regards
LM
I work on maintenance so my point of view is slightly different, but to understand how propeller and related system works and different approachs working in the industry you could find helpful some Jeppessen textbooks (Amazon should work):
1. A&P Airframe (general review of all systems)
2. Aircraft Propellers and Controls by Frank Delp
3. Aircraft governors by Frank Delp
Hopefully could help
Regards
LM
Interesting literature, maybe old, but nevertheless interesting basic knowledge can be found in NACA reports; see With the search function, you can sort out the propeller related reports,
good luck
good luck
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