Does anyone know of any real aeroplane or wind-tunnel tests that quantifies the effects of asymmetric propeller blade loading (P factor) versus the Spiralling Slipstream effect in creating yaw.
Which effect is greater?
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That depends on Angle of Attack (AOA), as well as other factors, including aircraft design. P-factor is caused by the angle between the free-stream airflow in front of the propellor and the propellor rotation axis. That changes as AOA changes.
Slipstream effect is caused by the impact of the corkscrew airflow created by the propellor impacting the vertical stabilizer.
So, for example, in my Long EZ, where the propellor is at the back, in the downwash from the main wing, P-factor is minimal at most AOAs. And the slipstream effect is nonexistent, since the corkscrew airflow from the prop is all behind the aircraft.