Propeller design

I asked Chad more questions about propeller design and the relation with the engine and the Bleriot:

Gert,

People misunderstand the relationship between a propeller design and engine rpm. It is only partly true, and only with clean modern airframes, that an engine should turn it’s maximum rated rpm in full throttle level flight (see below 1). For a fixed pitch prop this means it will run 2-400 rpm less during takeoff and climb when slow airspeed doesn’t allow the prop to unload. The misunderstanding comes from the idea that lack of maximum rpm is “throwing away performance” or that somehow the prop design is “wrong” is false. In the case of very high drag antique airframes this “maximum rpm” concept is even more utterly false.

Performance of propellers on antique airplanes is largely self governing. If the prop rpm is faster it means the primary load, the diameter is less. Lack of diameter is a critical failing in antique prop design because diameter is leverage and leverage is the most important aspect of good design for high drag antiques. If the prop rpm is lower it means that leverage is greater, an ideal situation for high drag airframes. These two scenarios happen to balance themselves perfectly, rpm vs diameter. They do not take into account other important factors such as planform, pitch, blade chord, airfoil and thickness however, and those become real issues of design for best performance. They are commonly referred to collectively as “the black art” of prop design. And I’m not going to go into detail about those here, except to say that they become the cumulative smaller issues that go into making a propeller great compared to having a so-so prop or worse, a club. The club will also fly the airplane by the way, but never well. For antique airplanes and engines, engine maximum rpm or engine over-speed is a critical loss of performance, while engine under-speed, low rpm, is not only acceptable but often results in a much better propeller/airframe/engine combination, provided that other factors (cumulative smaller issues as above) are not in error to begin with.

Lastly, some people may also not have the understanding, and maybe don’t have the experience. They insist on seeing a particular rpm number on their tachometer. They may (wrongly) believe that the perfect accuracy of a given rpm number is very important. It is not at all important if other factors are in error. In general it is correct on a clean modern airframe, but not on a very high drag antique airframes.

Chad

(1) Correct rpm is no indicator of correct prop design. Only that the overall load matches the engine.

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