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2 Answers

P factor and airspeed

Asked by: 3689 views ,
Aerodynamics

Hello

Books say P-factor is more strong at low airspeed.

Is it just because usually at lower airspeed airplanes maintain high AOA?

What if two different propellers have a same AOA but different airspeed?

 

thank you for your help! :)

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2 Answers



  1. Dustin on Aug 10, 2017

    “Is it just because usually at lower airspeed airplanes maintain high AOA?” – sort of…

    P-factor is caused by the AOA of the individual blades of the propeller. Not the airplane itself. But the AOA of the prop blades is usually determined by the AOA of the airplane (ie its wing).

    Imagine an airplane in cruise, it’s prop blades would be rotating at 90 degrees to the relative wind, so all blades would have the same AOA. When the airplane is flown at low airspeeds, such as during take-off, the AOA of the wing increases, which pitches the nose of the airplane up and the prop blades into a position that is no longer 90 degrees perpendicular to the relative wind. The downward moving blades on the right side of the prop (in a clockwise rotating prop) have a higher AOA and are producing more thrust, than the upward moving blades on the left side. This additional thrust causes the airplane to turn to the left.

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  2. JDJ on Sep 16, 2017

    The PHAK offers a description describing the fact that the “downswinging” blade attacks the relative wind at a higher velocity. While AOA is certainly a factor in this conversation, velocity of the blade is also a consideration. Think of the propeller as an airfoil (or a wing). If you look in the PHAK, the lift equation is as follows: Lift= Coefficient of lift (Cl) X Density of the air (p) X Area of the wing (S) X Velocity (V) /(divided) by 2. If you look at AOA and a side view of a rotating propeller, you will see that the downswinging blade is “attacking” the air more directly (think incoming relative wind). Because of this, the “airfoil” or propeller has a higher velocity and is producing more “lift” on the side of the downswinging blade. Hope this helps and didn’t create more confusion. Let me know if I need to clear up any statements!

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