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For an airfoil, in general, the aerodynamic force is more or less perpendicular to the chord line.

A low-pitched propeller blade has it's chord line more perpendicular to the direction of flight, therefore, more of the force acting on it can have a greater component parallel to the direction of notion, and less perpendicular. If this force is acting forward, then this is efficient. But this may not be possible at high speeds and low rotation velocities. Thus:

A high-pitched blade has it's chord line more parallel to the direction of flight than the above. Therefore, the aerodynamic force acting on it is likely to have a greater component perpendicular and lesser component parallel to the direction of motion. This is always inefficient.

The above manifests itself in the higher-pitched propeller producing more energetic swirl than the lower pitched propeller.

So my conclusion would be that all else being equal, high-pitched propellers are inherently inefficient. Is this correct? Is this what is observed in practice?

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  • $\begingroup$ Are you talking about fixed pitch props? $\endgroup$
    – skipper44
    Commented Dec 4, 2020 at 13:19
  • $\begingroup$ @skipper44 any. $\endgroup$
    – Abdullah is not an Amalekite
    Commented Dec 5, 2020 at 6:21

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Your conclusion is too simplistic. On aircraft, the difference between a cruise propeller and a climb propeller is rather small. However, in other applications, a low pitch propeller gives great acceleration but the engine will reach max RPM faster, which reduces top-end performance. A high pitch propeller will result in slower acceleration but higher top speeds.

This answer is also admittedly very simplistic -- there is a universe of math and engineering that goes into propeller designs. Sensenich, for example, produces more than 100 fixed-pitch propellers for aircraft, airboats and UAV's. McCauley Propellers produces thousands of constant-speed and fixed-pitch configurations for commercial, military, UAV, and general aviation applications.

If you want to get into the nitty-gritty of this, I have been told this book is one of the propeller design bibles for aircraft applications, "Aircraft Propeller Design" by Fred Weick, published in 1930.

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It’s true that there will always be a component of the blade’s thrust that directly opposes the rotation of the propellor, and hopefully a component that propels the aircraft. However, it would be a mistake to think that the former is wasted, on the contrary it is essential because that force is the source of the energy from which forward thrust is derived - if no energy were required to keep the propeller spinning then there could be no thrust. Another thing to consider is that steeper pitch propellers have a smaller ratio of blade speed to aircraft speed, and so will tend to be ineffective rather than inherently inefficient.

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  • $\begingroup$ Mulling this over (and I know here I'm not answering the 'what happens in practice' question), the aerodynamic force generated by each blade can be resolved into two components; one in the plane of the propellor, which applies a load to the engine and one that provides forward thrust to the aircraft. Mathematically, the magnitudes are given by the sine and cosine of the angle of the aerodynamic force (which will be more or less perpendicular to the chord line). The energy in each vector is given by the force multiplied by the airspeed in that direction. Thus a low pitch will result in a... $\endgroup$
    – Frog
    Commented Dec 6, 2020 at 3:10
  • $\begingroup$ small retarding load on the ending but a high rotational speed, which is converted to a large thrust force but at a low speed. Inefficiency would not be caused by a large (or small) ratio of forces; we see that an aerofoil can do this quite efficiently, but more likely by parasitic drag (tip vortices). It is well established that large propellers are more effective and efficient than small ones. Surface drag on the propeller will be greatest when the blade speed is large, which suggests that a slow-rotating propellor could be more efficient, and that would require a steeper pitch. $\endgroup$
    – Frog
    Commented Dec 6, 2020 at 3:17
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Let's imagine a plane fitted with a variable-pitch prop. For a given prop revs and a given forward speed of the airplane, there's an optimum pitch so that the prop blades work with an optimal forward component of the aerodynamic force. The prop revs remain more or less constant, and as the plane forward increases, the pilot has to adjust the pitch to higher and higher values. As the (linear) pitch tends to infinite, the forward component of the blade's aerodynamic force tends to zero. So, yes... The propulsive efficiency of a variable-pitch prop starts to decay beyond a certain pitch value (geometric pitch of about 45º) where it reaches the maximum, and tends to zero as the geometric pitch approaches 90º...

From http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm

Graph from http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm

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