Whenever I hear anyone talking about P-Factor, (whether it be single-engine left turning tendencies or multi-engine loss of directional control scenarios), someone always brings up the fact that the descending blade of a propeller generates more thrust than the ascending blade. I'm wondering why that's the case.
What is P-Factor and why does it occur?
For simplicity, assume a single-engine plane with a two-blade propeller. Just imagine an aircraft is perfectly level and moving forward through the air. The angle of attack on both blades of the propeller would be the same. Now imagine the plane pitches up a bit. One blade's angle of attack will increase and other will decrease.
(source: AOPA.org)
Within reason, the greater the angle of attack, the greater the thrust generated. So one blade (or, to be more precise, a blade on one side of the aircraft) will generate more thrust than the other.
Rather than try to do a full ground school session, I'm going to point you at the section on P-Factor in the awesome (free) textbook See How It Flies.
To attempt to paraphrase, it occurs when your aircraft is operating at high angles of attack. Just like the advancing blade on a helicopter produces more thrust because it's heading upwind, the descending blade of the propeller produces more thrust because it has a higher angle of attack relative to the aircraft's motion through the air.
Angle of attack is defined as the angle between the chord line of an airfoil and the relative wind. Now the relative wind is the apparent wind flow opposite the direction of that airfoil movement. The relative wind depicted in these diagrams is for the airfoil we call the wing! Put the relative wind vector for the rotating propeller blades opposite their movement and we have a minimal change in angle of attack change between the ascending or descending blades.
