View Full Version : P-factor confusion

13th Apr 2004, 12:17
I thought I understood p-factor, until I tried explaining it, and now I've got myself confused.

So, here's what I (think I) know:

- P-factor occurs when the propellor is not facing in the same direction as the aircraft is moving, such as in slow flight, or at the start of the take-off roll in a taildragger.

- In this configuration, the down-going blade moves further than the up-going blade in each half-rotation of the propellor. This is because it moves from the rear-most position of the propellor plane of rotation at the start of the half-rotation, to the front-most position of the plane at the end of the half-rotation, whilst the up-going blade does the opposite.

- Because of this, the down-going blade produces more thrust than the up-going blade

- The extra thrust on one side of the propellor results in yaw.

But then I thought about the down-going blade. According to what I've written above, it creates more thrust because it's moving faster. But my understanding is that increasing the forward speed of a propellor (whilst leaving the blade angle and the RPM unchanged) results in a lower angle of attack, less thrust, and less torque. (Less torque, of course, results in the propellor RPM increasing if the engine power is unaltered, when we're talking about the propellor as a whole.)

So if an increase in forward speed reduces thrust, then why does the extra forward speed of the down-going blade result in more thrust being produced on that side????? :confused:


Mark 1
13th Apr 2004, 13:14
Thinking about it in slightly different terms:

If you have a two blade propeller in the 3 and 9 o'clock position and then pitch it nose up; then the down-going blade has its pitch increased and the up-going blade decreased.

This is because the aircraft pitch axis is parallel to the pitch change axis at this position.

The effect will reduce to zero as the blades reach 6 or 12 o'clock positions where only the apparent blade sweep is different.

Seat Of My Pants
13th Apr 2004, 13:16
Ok, the down going blade travels (relative to airflow) further in the same time so its going faster.

Don't forget that we are changing other factors though - the AoA of the downgoing blade is greater owing to the displacement from "straight ahead".

Does this help ? or am I barking up the wrong tree again ?

13th Apr 2004, 13:21
Guys, I think you are correct - the change in AoA due to the prop being tilted upwards is more significant than the difference in forward speed between the two blades. Will have to go away and draw lots of pictures to get my head around it properly, but I think I've got it - thanks!

(If anyone could explain it such that I don't need to draw loads of pictures, though, that would be even better!)


DJ Mixmaster
13th Apr 2004, 13:39
For propellors: At low airspeeds angle-of-attack is almost the same as the angle between the chord line and the plane of rotation. At high airspeeds, relative wind changes this so that the wind that hits the prop comes more from the direction of movement, and thereby reducing aoa.
This also applies for differencial thrust between upgoing/downgoing prop blades. At low airspeeds: The upgoing has a larger angle between relative wind and plane of rotation than the downgoing, this causes a larger proportion of rel. wind to decrease the aoa. The downgoing has a smaller angle between relative wind and plane of rotation, so the wind hitting the blade is closer to the plane of rotation than on the other side.
Make a drawing, it's quit easy to see!


How about this:

The upgoing blade is moving a bit backwards, the downgoing a bit forward.
To get a descent lift from any profile, you need a positive aoa.
Sitting in a moving car with your hand out in the air, it's quit easy to feel the lift because of forward motion and positive aoa. Consider moving your hand backwards to create lift...it can't be done because you can't get a positive aoa due to the movement of the car.