RCU Forums - View Single Post - Lets discuss P-Factor

02-08-2006 | 12:39 AM

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William Robison

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From: Mary Esther, Florida, FL

RE: Lets discuss P-Factor

DK:

Quote:

...but I think that's more due to the swirling slip stream hitting the rudder and fuse, than to what's described as the P-Factor. And the reason this is so much worse on a tail dragger than a tri gear, is that a trigear has the steering wheel firmly planted on the tarmac until lift off, whereas the tail dragger has the tail flagging in the air, very succeptical to any air stream.

This, sir, is just plain wrong.

Lee and Tom have tried to explain it, simply put P-factor is caused by the off-center angle of air entry into the prop disc.

Here's a note I wrote several years ago, it explains it in yet another way that might help you see whnat it really is.
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P-Factor myths exploded (post # 1)

Gentlemen, and twin builders too:

There are many misconceptions about “P-factor,” and the term is misused more often than not.

First, what it is.

When the airflow enters the propeller disc at any angle other than 90 degrees, we have P-factor effects.

For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch. The majority of the pulling power is developed by the descending blade, giving off center thrust, and that off center thrust is “P-factor.” The effect is zero at zero airspeed, and the effect builds until the tail wheel comes off he ground. This is why you have to gradually add right rudder as the airplane accelerates, and neutralize it when the tail wheel lifts, disregarding torque.

When the tail wheel comes off the ground and the airplane assumes a level position continuing the take off run, the air flow into the propeller disc is then on center, P-factor no longer has any effect, because it just isn’t there anymore.

With tricycle gear, and the plane sitting level at rest, there is NO affect on the plane from P-factor. It does not exist. If the plane sits slightly nose down or nose up, there is a small amount, but it’s so small it can be ignored. This is one of the reasons why a trike is so popular for training. Both in full scale and R/C. They are just easier on take off.

In normal flight P-factor will never affect the airplane, as the airflow, in relation to the airplane, never gets more than one or two degrees off axis. Key word here is “Normal” flight. Most aerobatics are done in a normal controlled flight regime.

When doing aerobatics that depart from normal flight, 3d, gyroscopic maneuvers, harriers, and so forth, p-factor can rear its ugly head.

But 99% of what people call P-factor in normal flight is truly nothing but torque reaction, and that’s another story for another time.

I welcome any and all comments pointing out my errors in this, amplifying my statements, or adding something I have forgotten.

Bill.
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This note, and several others you might find of interest are located at
http://www.rcuniverse.com/forum/tm.asp?m=1063414

Bill.

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