aircraft propellors - variable speed - virtues and drawbacks

[rproctor]

I have become embroiled in a discussion on the possibilities of using a continuously variable speed propellor on a light aircraft.

My concept is to couple a a fixed pitch prop to a Honda CVT trans with the computer feed back programmed to make the prop turn slower as air speed increases to give optimum bite. i.e. vary prop speed instead of pitch, using the manual over ride for take-off.

I am getting some heavy ridicule over this concept but to me it has some logic especially as I have using a diesel engine in mind which develops max paower @ 4,000 rpm and max torque @ 2,250 rpm

here's hoping someone can offer more than insults.

regards

Richard Proctor

 

[Blkscribe61671]

the name of the game is weight to rpm ratios most rpm to lowest weight. i think the answer may well be sport bike engines that dont weigh much more then 150lbs. and can maintain safe operational rpms up to 12k depending on blade and housing design that should be more then enough if your using carbon fiber materials for most of the craft.

 

[jsummerfield]

Clarify your definition of a continuously variable speed propeller.

A fixed pitch propeller directly connected to the engine shaft is a . . . continuously variable speed propeller. This is the normal low-end aircraft design. You increase engine speed and the propeller speed increases. Increasing the throttle applies the power to increase engine speed.

A variable-pitch/constant speed propeller maintains a constant RPM as the engine speed changes. Manifold pressure increases as you apply more throttle.


John

 

[rproctor]

In a CSU or variable pitch propellor unit the pitch of the propellor may be altered to compensate for the forward speed of the a/c and to reduce the load on the engine.

By continuously variable I mean a system whereby the pitch could remain stable and the rotation speed on the propellor continuously variable by means of a continuously variable gearbox as opposed to a fixed ration geared propellor.

This could allow a slow moving corse pitch prop to get optimum 'bite' and reduce the air drag of the prop.

Or so I wonder......

 

[jsummerfield]

Varying the pitch optimizes the bite as you describe. When slow during takeoff and landing the prop is flat and the power is maximum - working like first gear and full throttle (as required to enter the I-70 on-ramp at the Missippi River bridge from the Illinois side toward St. Louis.)

As the plane levels and the velocity increases for cruise, the pilot adjusts the power setting to 65-75%. This also requires adjusting to a more coarse prop angle to bite more air with the same RPM - working as a higher gear in an automobile.

I describe the variable pitch prop as hydraulically operated. An aircraft mechanic can clarify lots about the details. The variable pitch propeller is a normal arrangement for transportation class propeller airplanes, one step up from the basic trainers.

The down sides to a variable pitch prop are added complexity and weight.

You may want to post your question about the prop in a pilot forum such as iPilot.com/forum. Start with the single engine or the experimental topic.

John

 

[karlbamforth]

I agree with jsummerfield in what he says about fine pitch being like low gear and course pitch being high gear as used in a cruising car.
The problem with a prop fixed in coarse pitch is how do you turn it at low forward velocity ?
It would amount to trying to pull away with your car in top gear and would require a lot of horsepower and therefore a large engine using a lot of fuel.
A variable pitch prop is better explained as a constant speed prop. The pitch of the prop varies to maintain a constant engine speed, keeping it at its optimum power and/or economic RPM.
The pilot selects fine pitch and full power for takeoff, this will allow the engine to just reach its RPM "red line". As the aircraft accelerates, the prop will tend to unload and if left unchecked would allow the engine to overspeed. A variable pitch prop increases pitch (becomes more course)loading the engine and keeping the RPM constant.
Once the aircraft has levelled off the pilot puts both the prop and engine into a cruise mode wich is usually course pitch and approx 75% power.
Constant speed variable pitch requires constant (almost) Hp.
Fixed pitch variable speed would require variable Hp.

Hhhhmmm its late here and having read that a second time it isn't the best edited document I have seen but I hope it helps.

Karl.

 

[karlbamforth]

Oooppps.
Yup it is late and it messes with my spelling coarse/course.
Oh well you know what I meant.

Tiredly
Karl.

 

[DaveKillens]

Variable pitch props didn't realy matter when the aircraft took off at 40 knots, and it's maximum speed was 60 knots. But when high performance aircraft took off at around a hundred knots, and their maximum airspeed was well over 400, then a variable speed prop became essential. That is probably one of the prime reasons we have variable speed props. A prop that would work at 400 knots would just plain suck at takeoff speeds, and vice versa.
Getting back to the original question. Yes, you can have a variable pitch prop that can be regulated so that the blades are at the proper angles for all flight regimes. But this system will add a lot of weight, and complexity.That's why you rarely see any on ultra-lights and smaller general aviation aircraft. Just not practical for smaller and slower aircraft.

 

[lengould]

Appears confusion reigns.

1) Variable pitch props are designed to maintain (close to) fixed engine speed with a fairly narrow variation of engine loads. Designed to keep the engine turning near it's optimum rpm in all load conditions.

2) Engine load increases with the cube of rpm in a propellor drive application, unlike ground vehicles where the ratio is (less than) squared. A CVT may need to be specially designed for the purpose with finer control near the higher rpm range.

3) You need to watch out carefully to never allow the prop tip speed to approach the speed of sound under the conditions of atmospheric pressure / altitude you are operating. This requirement may require fitting a less than optimal prop diameter compared to variable pitch, though if you can live with a fixed pitch set to the most agressive pitch of the variables, you should be able to match their performance.

4) Piles of money to get approved for mounting anywhere but Xperimental. Not likely ever to make sense commercially.


Pechez les vaches.

 

[aerodog]

lengould,

Don't discount the size of the experimental market, it is quite substantial.

Visit the Oshkosh Air Show and you will find some really well engineered products neither certified nor TSO'd. Of course there is some junk there that probably should not be in the air.

But I was impressed with some really nifty low cost autopilots and cockpit displays. If these devices went thru any qualification process, they would no longer be low cost. And the certification process tends to stagnate development and product improvement...it is hard to be cutting edge and certified at the same time...it is hard to be low cost and certified.