Subject: P-factor with relation to aircraft propellers

A fella sent in a question regarding P-factor.

Your answer was a good description of prop subsonic design and lift 
characteristics, but that is not what the term P-factor directly refers to, and 
the fella is probably still confused.  P-factor is especially important on prop-
driven, multi-engine aircraft - it is a factor in determining the "critical" 
engine, and the basis for air minimum control speeds.

Having flown and instructed props for years, I will try to put it in simple 
terms; since there are other factors such as torque/gyroscopic force, etc. 
along with the asymmetrical lift associated with a blown-wing design (causing 
greater lift on wing of the operating engine).

P-factor has to do with the angle of incidence of the prop blade in respect to 
the relative wind.  For an aircraft with nose-high pitch, the descending blade 
effectively takes a bigger bite of air than the ascending blade.  The 
greater "bite" of the descending prop is directly translated into a twisting 
moment and increases the lateral torque on the aircraft about the longitudinal 
axis, or roll.  For an engine with clockwise rotating props, this produces left 
roll.  The critical engine would be the outboard, left engine.

Combined with other factors, such as engine torque, asymmetric lift from engine 
failure, these forces can affect the pilot's ability to safely maneuver an 
aircraft with available flight control surface deflection.

Hope this helps.

Re: P-factor with relation to aircraft propellers