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I have read somewhere that propellers have an effect on the pitching moment of the aircraft. Having said that, I have a couple of questions.

  • Is this a significant effect in model aircraft? If so, are there major differences between pusher and tractor configurations?

  • Finally, how would one calculate this effect?

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    $\begingroup$ The most obvious reason for pitching moment is the propeller thrust vector is not aligned with the center of gravity. But there are other origins, explained in this article, starting page 53. $\endgroup$
    – mins
    Commented Sep 17, 2017 at 9:21

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The propeller causes pitching (and yawing) moments. There are two main reasons for that:

1) flow change around the fuselage (and central wing and tail) 2) thrust change.

Thrust change is simpler to grasp and estimate. The thrust vector will cause a moment, because (90% of the time) it's a bit tilted and does not pass through the CG of the aircraft.

Several model aircraft have intentionally tilted propeller / motors. The perfect amount of tilt is found through flight tests, because its simpler and more accurate to try and feel the effect.

A pusher configuration has quite similar concerns related to thrust vector, however the flow change effect is much less than a tractor configuration. This is because for the pusher configuration the accelerated flow does not effect the air vehicle any more.

Calculation of these effects range from hand calculations (spreadsheets) to CFD and finally to flight tests (where you'd do controlled tests to derive the model parameters).

For RC aircraft I'd suggest following the rule of thumbs that have been established in the RC communities.

Finally, tractor / pusher configuration selection should be based on several other factors and thrust moment is probably not the most important one.

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Yes there are several effects that the propeller has:

  • Gyroscopic precession: a pitching moment is transformed into a yawing moment and vice versa, due to the gyroscopic effects of the spinning propeller.

  • P-factor, which is similar but due to differences in blade angle of attack and only at high A0A of the aircraft. From the wiki:

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller,1 that is responsible for the asymmetrical relocation of the propeller's center of thrust when an aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.

These are effects that pilots of single engine propeller aircraft must cater for, and they scale down to model aircraft as well.

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