let's say I need 1000 N of thrust for a given aircraft and flight velocity. I have available a fixed pitch propeller with a given shape, and thus given performance to advance ratio curves. Let's say the propeller diameter is 2m.

Now, what would determine if this is the best diameter for the application on hand? What if the blade shapes were scaled up to 4m? Wouldn't this generally be better for efficiency as the RPM would decrease and the air acceleration required would be lower? Other than practical factors like clearances, stability, etc., what aerodynamic reasons are there to stop increasing the diameter at a certain value?


2 Answers 2


Let's start over with the basics.

In any application, the prop tips have to be maintained subsonic or else propulsion efficiency will suffer and noise generation will become extreme. Therefore, for any given prop diameter, there is a do-not-exceed RPM value associated with the sonic limit: big props have to turn slower, smaller ones can turn faster.

(Prop diameters are also limited by ground clearance considerations but for small planes like a Cessna 172 or a Cherokee 160, the need to keep the prop tips out of the grass can be accomodated readily.)

Next you have to match the prop with the best power operating point of the engine turning it. For 4-stroke gasoline engines of typical design this will be around 2500 RPM at which the engine is producing roughly a half horsepower per cubic inch of displacement. You then pick the maximum allowable prop diameter to keep the tips subsonic at 2500 RPM.

Next you assume the minimum number of blades for a start (2) and select the pitch angle at that diameter which will absorb the full output of the engine running at 2500 RPM. If two blades will not load the engine enough to hold it at 2500RPM at full throttle, then you add another blade and repeat the process.

For engines of less than about 180HP, the right answer is a two-bladed propeller of fixed pitch. For engines bigger than that, the right answer is generally a two- or three-bladed propeller with variable pitch.

One way to beat the sonic limit while getting more power out of the engine is to gear the engine down so it is spinning faster, and thereby generating more power, while still maintaining the prop tips subsonic. This also lets you further reduce the prop tip speed and go for a larger diameter prop if you have the ground clearance available, which will generally be more aerodynamically efficient than a smaller one- but running the engine faster shortens its service life and the cost to rebuild a geared engine has to include the cost to rebuild the gearbox itself.

For light aircraft in the homebuilt class or for ultralights, a less-expensive alternative to the geared engine is one with a rubber-belt reduction drive. This lets you use lighter and simpler 2-stroke engines which have a best power point of around 4500 RPM and still maintain the prop tips subsonic.

  • $\begingroup$ Thank you for the design process, the subsonic part was probably closest to what I was looking for. However, how do you determine where "the pitch angle will absorb the full output at 2500 RPM"? I do not exactly understand what you mean by that. Wouldn't the prop always absorb the power input, but it would just have different efficiencies at different RPM and thus convert it into varying amounts of useful power? When would two blades ever not be able to load the engine at a given power? $\endgroup$ Oct 7, 2018 at 16:17
  • $\begingroup$ here is how it works. you bolt on a prop, start the engine, and advance the throttle.if the prop is pitched too deep, then at full throttle the engine is bogged down to, say, 2000RPM and is not putting out its full rated power. alternatively, if the prop is pitched too shallow, then when you advance the throttle you hit 2500RPM before the throttle is wide open and if you open it all the way, the engine goes overspeed and the prop tips go supersonic. The prop manufacturer has performance charts that spell all this out for you in advance. $\endgroup$ Oct 7, 2018 at 18:56

For engines of less than about 180HP, the right answer is a two-bladed propeller of fixed pitch.

There a lot of planes with 180 HP engines (such as Lycoming O-360) with constant speed prop. Redline prop speed seems to be set to keep the tips from going subsonic, so prop length would also be a factor. I don't know the math, but the prop total area has to be sized to absorb the engine power - so shorter length to stay out of the grass would need a wider blade.

I am guessing the 'right answer' was also based on a price point the plane manufacturer was trying to meet. Fixed pitch cost is much less than the combination of constant speed blades, hub, different sized spinner, governor, control cable, manifold pressure gage, and of course an engine that can supply oil pressure for props controlled by oil pressure.


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