P-Factor and Relative wind!

[mhcasey]

I'll revive another one...

Here's where I'm confused. P-Factor = everything said above (essentially the good ole' oversimplified "the descending blade takes a bigger bite" explanation). But then we have Gyroscopic action...Nose down a taildragger on your takeoff role and expect the plane to yaw left due to the "applied force" at the top of the blade "resulting" in an applied force at the right.

It seems that the "center of thrust" would have to be at the top of the blades rotation to have a resultant force, 90 degrees and in the plane of rotation, the at the right which would just like gyroscopic action, result in excess thrust at that point (relative to the left) and cause a yawing moment to the left. However, center of thrust at the top is just the opposite of what we'd expect at high angles of attack since the ascending blade is retreating from the oncoming wind.

Hope this question makes sense. I'm expecting one of the following scenarios:

1) We all scratch our heads and think "Dang, is this kid on crack?"
2) Somebody (Tgrayson/fish) comes up with a brilliantly simple explanation.
3) Tgrayson and/or fish show up and say "well it's not that simple really.."

 

[tgrayson]

Hope this question makes sense. I'm expecting one of the following scenarios:

Hopefully it's 2 or 3.


First, gyroscopic precession has nothing to do with P-Factor. It would happen with a whirling metal disk which would develop no thrust at all.

Second, even though we create the mental model of applying a force to the top or bottom of the blades during rotation and then moving it 90 degrees in the direction of rotation, that's not a particularly accurate model of what's going on. The only real force is being applied via the crankshaft to the root of the propeller blade.

You may get a vague idea of the nature of gyroscopic action by considering that as you push on the yoke, the top half of the propeller arc would tend to tip forward, and the bottom arc would tend to tip backwards, but the blades which comprise the arc are continually moving around in a circle. The blade which wants to tip forward while on top, wants to yaw left as it moves to the right half of the arc and then wants to tip backwards on the bottom half, etc. The actual net result is the sum total of all these odd behaviors on all the propeller blades.

 

[mhcasey]

Hopefully it's 2 or 3.


First, gyroscopic precession has nothing to do with P-Factor. It would happen with a whirling metal disk which would develop no thrust at all.

Not sure I'm buying it. I agree with the effect being observable with a whirling metal disk, but for P-factor to make any sense, the downward moving blade must be receiving some applied force from its acceleration into the relative wind, while the ascending blade receives a slightly lesser force due to its slight retreat from the wind...both blades receive force from the crankshaft, but the downward moving blade must receive a greater force to get through the stronger wind lest it should separate from the crankshaft or bend...if the blades are similar, momentum has to be conserved somehow, right? (Hey p.s. - I think I remember from physics hearing that momentum is not necessarily conserved between reference frames...I'll have to look that crap up again).

It just so happens that the prop is not just a whirling metal disk, but is generating thrust. The thrust should increase at the point of resulting force (I would think) which is 90 degrees and in the plane of rotation...

Second, even though we create the mental model of applying a force to the top or bottom of the blades during rotation and then moving it 90 degrees in the direction of rotation, that's not a particularly accurate model of what's going on. The only real force is being applied via the crankshaft to the root of the propeller blade.

You may get a vague idea of the nature of gyroscopic action by considering that as you push on the yoke, the top half of the propeller arc would tend to tip forward, and the bottom arc would tend to tip backwards, but the blades which comprise the arc are continually moving around in a circle. The blade which wants to tip forward while on top, wants to yaw left as it moves to the right half of the arc and then wants to tip backwards on the bottom half, etc. The actual net result is the sum total of all these odd behaviors on all the propeller blades.

I think the bottom part is getting closer and is actually a much clearer/simpler way of explaining gyroscopic precession than I've been teaching it...except that if the gyro crap does really have an effect on p-factor, then nose up should produce something other than left-turning...I think my issue is still why p-factor and gyroscopic action are completely unrelated.

 

[tgrayson]

except that if the gyro crap does really have an effect on p-factor, then nose up should produce something other than left-turning

I don't understand what you mean here.

...I think my issue is still why p-factor and gyroscopic action are completely unrelated.

Gyroscopic precession only occurs when the axis of rotation is moved, but P-factor is a yawing moment which always exists; you control it with the rudder. It wants to change the axis of rotation, but you won't let it. If you remove your foot from the rudder, the nose would yaw left, which would change the axis of rotation of the propeller. (Think "crankshaft pivots left".) This would indeed cause the airplane to pitch up due to gyroscopic precession.

This same effect happens if you stomp on the left rudder.