LouW,

Gyroscopic forces are another thing to contend with. Your description is correct but I think not complete. Regarding P effect you stated "A force applied to the rim of a gyroscope always appears as a reaction displaced ninety degrees to the applied force. Thus the left turning force applied by aerodynamic force on the propeller disc produces a pitching moment at the hub. P-factor is real but gives a pitching moment not a yawing moment on the airplane." I think it is more correct to say P factor gives a yawing moment, and if the aircraft is allowed to give into this yawing moment (actually yaw), then and only then does the gyroscopic forces come into play producing the pitching forces you describe. Gyroscopic stuff only happens when the gyro axis is allowed to react to an applied force. If heading is held straight, there is no gyroscopic reaction.

At high AOA such as in steady state level flight at slow speed or in a steep climb like a 45 degree up line, (+) P factor is happening (unless there is down thrust that zeros out the prop disk at this AOA), therefore some additional R rudder or R thrust is needed to hold heading (in addition to that needed for the spiral slip stream effects of course). It is only when sufficient R correction does not exist that the plane actually begins to yaw, and gyroscopic reactions are generated.

Specifically the difference is this. Under your description of how P effect acts, some additional “elevator” trim is needed to fly straight during P factor generating condition (such as slow level flight and climbing). Following my description above, some additional R rudder (or R thrust) must be held to fly straight during P factor generating conditions. So, respectfully, your statement that my explanation of left turning tendencies as seen in many books “simply isn’t true,” just simply isn’t true, in my humble opinion.


To all,

I am sensing a general discounting of P effect. I caution against this. P effect is very real and significant. For example, imagine a variable pitch propeller, which varies in such a way as to allow the pitch to increase only on one side of the shaft and to decrease only on the other side. Essentially this is mechanically introduced P effect. This is known as cyclic pitch control, and is Igor Sikorsky’s great insight which made the modern helicopter possible.

Multiflyer