From: stadler@Apple.COM (Andy Stadler)
Newsgroups: rec.aviation.misc,rec.aviation.student
Subject: Re: what is P-factor
Date: 15 Dec 92 03:20:47 GMT
Followup-To: rec.aviation.misc
Distribution: na
Organization: Apple Computer Inc, Cupertino, CA

In article <pvf.724034008@diablo> pvf@alantec.com (Paul Fries) writes:
>In a couple of recent posts, the question has come up as to just what
>is meant by the term "P-factor".
>
>What comes under the heading of "P-factor".


Your description of P-Factor is essentially correct:

>"P-factor" refers to:
>
>    1) asymmetrical thrust from the propeller caused by different
>    angles-of-attack of the up-swinging and down-swinging propeller
>    blades;

Yes.

>    2) these different angles-of-attack usually occur when the pitch
>    angle of the craft is not level, and

Mmm..  Not necessarily.  First, "pitch angle of the craft" is a red herring.
The relevant angles are the thrust angle of the propellor vs. the angle
with which the propellor is moving through the air.  Most (not all!) prop
installations have a thrust line which extends straight through the aircraft,
and in these, "thrust angle" == "craft level".

You can understand P-Factor more easily if you throw away all of the semi-
defined terms and just state it like this:

  P-Factor occurs when the propellor disk is not at a right angle to
  the air flowing through it.

>    3) the magnitude of the asymmetry is proportional to the
>    difference between the pitch angle and level.

Correct idea, wrong terminology.


>My question is whether the slip stream and other yaw producing
>effects are normally called, or considered a part of, "P-factor".

What's going on here is that P-Factor is just one of a group of forces
known as "left turning tendencies".  It turns out there are FIVE (not four)
left turning tendencies, one of which is P-Factor;  All require you to push
on the right rudder, so they are sometimes misidentified as P-Factor.

They've been listed before, and I'll try it again.

  THE FIVE CAUSES OF LEFT-TURNING TENDENCY

1.  P-Factor

P-Factor occurs when the propellor disk is not at a right angle to the
air flowing through it.  It causes a YAW to the left.  Typically occurs
during climbs.  "Reverse" P-Factor, a right-turning tendency, can be noticed
during descents, but is less noticeable because the engine is usually
generating much less thrust at the time.

2.  Torque

Torque is a the equal-opposite reaction to the clockwise-turning crankshaft
and propellor.  It causes a ROLL to the left.

3.  Spiraling or Corkscrew Slipstream

As the air is blown back from the propellor, it has a clockwise "twist"
imparted to it.  When it reaches the tail surfaces, it tends to to "blow" on
the left face of the vertical stabilizer, pushing the tail to the right.
This causes a YAW to the left.

4.  Gyroscopic Precession

Most noticeable during the takeoff roll of a tailwheel aircraft.  Most light
taildraggers have a two-step takeoff:  You accelerate until the tail comes
alive, at which point you pick up the tail.  You then further accelerate to
takeoff speed, rotate slightly, and become airborne.  Gyroscopic Precession
is most problematic during that first tail-lift.  You are generating lots of
power - big gyroscope (the prop).  You are moving slowly - little effective
rudder, and you've just lost your tailwheel steering.  And a quick "pitch
down" from three-point to two-point.  As you rotate the "gyproscope" forwards,
it responds by pulling to the left.  The aircraft, only during that short time
you are rotating, responds with a YAW to the left.

Gyroscopic Precession is interesting because it usually creates the following
scenario.  Student pilot pulls onto runway, applies full power.  Rolls down
a hundred feet or so, and applies forward stick to lift tail.  Nose begins
to swerve left.  Student notices the swerve, and begins to apply right 
rudder, but because of slow reactions, doesn't put it in until just about the
time the tail is up.  With the pitch is no longer changing, gyroscopic force
disappears as quickly as it appeared.  With the right rudder jammed down,
the aircraft now swerves to the right.  Student continues down runway, swerv-
ing to left and right, with requisite curses spilling from instructor....

If you have to react to gyroscopic precession, it's too late.  The real 
solution is more of a pre-emptive "kick" to the right rudder during tailwheel
pickup.  Any later than that and it's too late.

5.  Left Tire Friction

Slightly noticeable, only in taildraggers.  In two-point stance, there is
little directional stability in a taildragger's takeoff roll.  Meanwhile, the
torque from the engine drives the right main slightly harder into the turf
or asphalt, causing slightly greater rolling friction and an ensuing YAW
to the left.


The first reduction of these is to say "why bother, they all turn you to
the left."  But it's important to understand all 3 or 5 (4 and 5 really don't
apply in tricycle gear a/c) because they occur in different situations:

1.  P-Factor                             Climbs or descents
2.  Torque                               High power
3.  Spiraling or Corkscrew Slipstream    High Power
4.  Gyroscopic Precession                Initial tailwheel rotation
5.  Left Tire Friction                   On the runway only



--Andy    stadler@apple.com
Newsgroups: rec.aviation.misc