selection of airfoil and sizing of propeller

[shaft10]

Two problem to solve.
1) I have 4 propellers built with different airfoils. I have the graphs of coefficient of thrust and efficiency in function of advance ratio. There is a mathematical method to select which one is the best or the only way is just look at graphs
2)I know the maximum thrust necessary at my aircraft so I need to found the right dimension (diameter) and number of blade for a propellers .
I can calculate the thrust produced by the propeller but this change with the advance ratio and the pitch angle so really i don't know how to select the best diameter and number of blade.

I thank you in advance for the answers

 

[rb1957]

1) looking at the graphs is a good place to start. you'll probably see one is better is one area, but worse in others; depending on advance ratio. pick the one that has the best performance for the advance ratio that's most important to you.

2) different advance ratios mean different speeds mean different thrust required. are the prop's constant pitch ? (then that answers your pitch question

another day in paradise, or is paradise one day closer ?

 

[gurkanc]

Hi, in addition to rb1957, also, make sure that your engine can also supply the minimum and max RPMs and the necessary torque (/power).

The max thrust (and related max power required) is one design point (possiby hi-speed flight or climb), but the low-power (cruise, or descent) regions are also important. So, pick your design criteria (constraints), and your optimum point together.

Blade number is a tricky thing to consider, and it depends on several factors:
1) tip-speed limited or not
2) ground clearance constraints
3) pusher or puller (pusher aircraft probably needs larger diameters to compensate for the fuselage-blockage, limited to ground clearance of course)

 

[shaft10]

Considering the ideal case that I can have always the best engine appropriate for the propeller and a variable pitch angle.

The advance ratio J=V/(nD) is function of speed but also the diameter.
if i change the diameter the best propeller is always the same ?

for the second problem I use the maximum thrust that my aircraft need , this mean also the max thrust but i can't know the exact adavance ratio
because i don't know the diameter and the number of rotation per second.
An other problem is that i have a low advance ratio this give me high thrust but low efficiency around the 50%
it's that a possible value to use ?

 

[tbuelna]

As gurkanc notes, you probably want to optimize your propulsion system efficiency for the condition it spends most of its time operating at. With fixed wing aircraft this is usually cruise.

Another significant factor to consider is the useable speed range available from the powerplant. A recip engine will have a useable speed range of around 50-100%, while a turboshaft engine will have a useable speed range of around 80-100%.

I can't imagine a situation where you would use a prop operating at 50% efficiency. Less than half of the power produced by the engine would be converted to thrust by the prop.

 

[shaft10]

the aircraft is a solar airship with electric motor.
the velocity is less of 30 m/s and at this velocity the advance ratio is low and this mean very low efficiency and if i do the blade smaller I don't have enough thrust.
It's a lop where I don't found a solution for this I am doing some simplification like I can have the best engine for the propeller and considering only the condition where it spend the major part of time.

 

[rb1957]

yes, designing a propeller is tricky ... full of compromises.

but start with V, then you know the thrust required. with the installed thrust, you know the propulsive efficiency you need. now you know the advance ratio you need, so you have a few choices (number of blades tells you the diameter you need ... about three possible designs). you may need to check tip speed to make sure things are ok.

at your low speed I think you'll need a specialised propeller ... the wash-out along the blade should be quite different to a typical airplane propeller.

another day in paradise, or is paradise one day closer ?

 

[shaft10]

I know the speed , i know the drag of airship , so i know the thrust .
I can calculate all the characteristic curve of propeller but there isn't a mathematical method to select the best solution or it is Just look at graphs and see Which One look the best .
The same to select the best airfoil for the blade there isn't some ratio With thrust , efficiency , power that can show the best solution

 

[rb1957]

yes, propeller design is about compromise and sub-optimal design.

as you say there are four choices that will give you the required thrust.

consider, does one propeller give you a higher efficiency than you set as the requirement ? minimum efficiency = req'd thrust/installed thrust. can you get a solution with a higher efficiency (ie using less engine power to achieve the same required thrust ?)

next, you're looking at the advance ratio. well, bigger diameters are generally bad (more weight, more tip speed = more noise) and multiple blades (more than 4) are generally bad (hard to build, but your installation could be different).

next, what about over design performance ? does one propeller look as though it'll perform better (or worse) when the airship is going slower ?

another day in paradise, or is paradise one day closer ?

 

[dgapilot]

For your application, I would suspect the airfoil will change from hub to tip. In order to get the best possible efficiency, the airfoil at the hub will be turning very slow - relative to the tip, and will need a much higher AOA. Slower RPM more often yield more efficiency. If you look at WWI aircraft, they typically had low HP high torque engines turning very slow with long wide blade props. A JN-4 only had a 90 HP engine, but the OX-5 created that 90 HP 1400 RPM. Imagine trying to get an aircraft the size & weight of a JN-4 with a modern (40s vintage) Continental C-90 turning at 2475 rpm? I doubt you could even taxi it. Same HP different efficiency.

 

[shaft10]

this is an example of 4 blade propeller with a diameter of 4 meter.
For my application the advance ratio is between zero and 0.35 if i consider the maximum speed at tip with mach 0.75.
how you can see the thrust is good but the efficiency don't reach the 60% .
I can increase it with a three blade propeller and with a small diameter but still i can have only 65% in the best case and at lower speed than 30 m/s
the efficiency is very low.
It's possible have this value ?

 

[shaft10]

I have a doubt is possible have a propeller with a variable RPM, I mean the change the maximum speed at tip of propeller ( always less than mach 1 ) but changing this not only with a variable pitch angle but directly from the engine . Because in this way at low speed of fly I can use a lower rotational speed to increase the efficiency and at high speed increase the RPM to have more thrust for the increasing of drag .
the variation are like 500 RPM to 1200 RPM , this is something of realistic ?

 

[rb1957]

lets see if I understand your plots right ...

the green plot says that for a J = 0.3, CT = 0.28, so you can calc the thrust produced; and eff = 0.45, so you can calc the installed engine power (req'd to produce the thrust, and Thrust*V = propulsive power). yes?

for my money, just from looking at these plots, black looks to be the best compromise.

does your engine have only one operating speed ? variable pitch propellers change pitch to allow the propeller to work at a higher aerodynamic efficiency over different speed ranges (a fixed pitch prop is designed for maximum efficiency at one speed (aircraft speed, that is) and pretty much sucks at any other. The "normal" design would be to have the engine working away at it's preferred design speed (rotational, that is) and have a gearbox to change the propeller speed.

another day in paradise, or is paradise one day closer ?

 

[shaft10]

Thank you for your help.

 

[tbuelna]

A solar airship powered by an electric motor driving a 13.1 ft diameter 4-blade prop seems a bit unusual. With a large aircraft relying on electrical power produced from conversion of solar energy it would not seem acceptable to have a propeller having an efficiency as low as what you describe. A solar-electric aircraft needs the ultimate efficiency possible from every part of its propulsion system. And a prop operating at 65% efficiency would not be acceptable.