View Full Version : P Factor Question

13th Jul 2006, 05:17
The accepted definition of Pfactor is that the prop disc of an a/c at high aoa produces asymmetric thrust between the advancing and retreating blades. This means on a prop rotating CW viewing from rear, the right half of the disc produces more thrust causing left yaw of the a/c. My question is, why isn't gyroscopic precession involved? As you know on rotorwing the cyclic input involves precession of the rotor discs, i.e. creating more lift on the right side of the rotor disc will cause a pitch up of the helo, assuming CCW rotorspin viewing from top. So, back to the CW rotating prop disc, why doesn't the asymmetrical thrust occur 90degs later causing a pitch up of the a/c?

13th Jul 2006, 05:27
Gyroscopic factors are quite pronounced in taildraggers when raising the tail, causing an immediate turn to the left with a CW rotating prop(viewed from behind).
Everything including P factor all works in the same direction in the 185, which can make it a handfull.

13th Jul 2006, 10:31
So, on the CW rotating prop disc, why doesn't the asymmetric lift occur 90degs later causing a pitch up of the a/c?

Perhaps gyroscopic forces are not particularly noticeable because the propeller is light and small compared to a helicopter main rotor.

The aircraft will still yaw left in your example because of couples and moments, not gyroscopic forces. At high angle of attack the line of thrust physically moves to the right, while the line of drag doesn't move significantly. It's not a gyroscopic yaw to the left, but a force couple causing a yawing moment about the CG.

With very powerful engines mounted on very light airframes, the effect you describe would be much more noticeable. High powered aerobatic aircraft use gyroscopic forces to do more whacky manoeuvres - the lomcevak (sp?) is one example.


13th Jul 2006, 18:16
P- factor on low performance aircract is mostly negligeable, with the possible exception of taxiing/TO in quatering Xw's. :)