This whole conversation has been rather interesting. I'm not sure about the P-factor in a motionless aircraft. I think it may actually still exist. Besides the MU-2, my other plane is a 180hp Husky with extended Atlee-Dodge gear on 31" tundra tires (high deck angle when sitting on its tailwheel). This weekend, I had my son try a takeoff for the first time and even though we discussed applying right rudder, when he advanced the power, we repeatedly (and immediately) turned 30 to 60 degrees left before I cut the throttle and took the plane. Among the left turning tendencies, torque, with a bit more pressure on the left tire might have an effect, but it's on grass and the tire only has 10psi, I doubt it would do much. Gyroscopic precession would not yet come into play. Spiraling slip stream certainly has a role, and may be dominant (but I doubt it). P-factor is still likely in play. As the throttle is advanced, all the air in front of the plane is drawn back along the ground, through the propeller and behind the plane, almost like a big vacuum cleaner. My guess is that this air ends up moving approximately parallel to the ground and then gets "shot out" behind and under the plane. Very soon after applying throttle, even with the plane stationary, I'd guess the descending blade is already producing quite a bit more force than the ascending blade in a conventional gear plane. Whatever the reason, I do know that my Husky darts for the left side of the runway if left to its own devices when the throttle is advanced from a standing start (much to my son's chagrin).
When I took my initial training in the MU-2, I was screwing up the takeoffs, coming off the ground uncoordinated with the "ball" (it's not really a ball any more) offset, and the plane yawing right; I didn't know what was happening and thought I had made a terrible mistake in buying the Mits. It took a short while to figure it out, but I had been habitually applying right rudder like in my tailwheel and it was far too much. When I just let the MU-2 takeoff with my feet lightly resting on the rudder pedals but applying little corrective force, things have gone far better. It's amazing to me how little rudder pressure the MU-2 actaully needs (until an engine fails). Different planes, different techniques.

Spiraling prop wash over the fin/rudder.

Creates an immediate force on the fin to the right, plane goes left.

Applying right rudder essentially zeroes out the fin AOA to counter the force.


This is what happens when you have put in the right rudder trim. It is slightly out of neutral for takeoff, then it comes out as speed builds and is essentially neutral in cruise.


The tail of the MU2 is not greatly affected by prop wash, being a twin.

Mike C.

No aircraft movement and there is no difference in relative wind between either side of the prop. It’s a circle - things are pretty even across it. Up vs down is irrelevant.

Evidence suggests your theory is wrong. The local airflow around the prop follows the prop axis, not the ground.

Mike C.

Is that prop blast and indicative of the local airflow or more indicative of a low pressure vapor trail showing the prop’s high speed tip path through the humid air?

Evidence suggests your theory is wrong. The local airflow around the prop follows the prop axis, not the ground.

Mike C.

Yes.

The vapor trail is descending. It can't move contrary to the air it is in since it is the air itself.


The top tip of the prop is always the same height above the runway, the vapor trail is lowering as the plane goes by.

When the tail comes up, the votex pattern goes flatter. It stays aligned with the prop rotational axis.

[youtube]https://youtu.be/8OCWXbFbWME[/youtube]

Mike C.

Correct, the inflow is a much wider and slower movement of air than the outflow.

But that's doesn't say anything about the flow being angled to the prop axis.


I have provided ample evidence that is false.

Another way to test this is to put an airspeed sensor (say pitot tube) behind the prop disc on the left and right side and do a static runup. If you get the same airspeed on both sides, then both sides had the same thrust from the prop and thus the same blade AOA.

If the AOA was higher on one side, then the thrust and resulting airspeed would be different. It won't be.

Mike C.

Uh, no. The wind created will be in the prop’s line when the aircraft itself is not moving.

For another example, consider a helicopter with no lateral movement. All the prop wash is in the line of the prop, i.e. down.

As Mike said, he has shown you ample visual evidence. Plus physics suggests the same. So... methinks you’re fighting an uphill battle on this one.

I suppose you could also take a bare fuselage taildragger with no tail for the slipstream to hit, feet off the rudders, hit the power and see if it turns; I think it would. This doesn’t seem to be a resolved issue in Aeronautical Science...lots of opinions as illustrated in this thread. Some say P-factor plays no roll on initial power application in a taildragger, others say it is the dominant force. Here’s her another source that did an Interesting experiment with inverted climbs.
https://www.flyingmag.com/technique/pro ... actor-real
I’m not sure the final answer can be known. But as long as I step on the right rudder pedal along with adding throttle in the Husky and don’t do that in the Mits...it’s all good!

See, that's the thing... As with many facts based on scientific principles, I think the final answer can be (and is!) known. Just because some refuse to believe it does not mean it is not known.

Jon, I absolutely agree with you that there must be ONE right answer to this question (and most any question that is not related to faith). But I'm just noting that in reviewing the literature out there, capable and competent folks who are apparently experts in their field are looking at the same aerodynamic science and aircraft behavior data and coming to different conclusions about the relative contribution of spiraling slipstream vs. p-factor. It might be difficult to come to a conclusion with any degree of certainty.
I'm thinking about building a rubber-band airplane that's a taildragger, making the tail just a round wooden dowel, rolling it straight to see if it stays straight, then winding it up and letting it go...if it turns, there's p-factor, if it only turns left after adding a vertical stabilizer to the tailboom, it's slipstream. But even with this simulation, it might not answer definitively as there are lots of other factors in a real plane that might not be accurately replicated in a simple model.

I like your experiment idea. I think it could be improved by using a stationary electric motor with a prop and the ability to change pitch. Add some smoke in the airflow and use an iPhone slow-mo camera to capture the results.

I just love that I am reading about this stuff on a Saturday afternoon. I love the internet.

Ahh,- the finer points!

Are we flying yet?