Re: torque on airplanes.

["Waggoner, Bill" <BWaggoner@METROPO.MCCNEB.EDU>]

Thanks John,

By the way the student was a her.

-----Original Message-----
From: John S. Denker [mailto:jsd@MONMOUTH.COM]
Sent: Thursday, April 12, 2001 2:36 PM
To: PHYS-L@lists.nau.edu
Subject: Re: torque on airplanes.


At 01:40 PM 4/12/01 -0500, Waggoner, Bill wrote:
> A student asked about the following during a discussion on torque and
> rotation. Evidently the student is taking flying lessons,  and they were
> told there was a limit to the acceleration a prop driven craft could have
on
> take off after which point the plane spins off the runway.  What force
> creates this torque? Why doesn't it exist when the plane is in the air?

There are about a dozen torques that act on the airplane.  They act on the
plane on the ground and in the air as well (with the obvious exception of
the torque due to force of the runway acting on the wheels, times the lever
arm thereof).
   http://www.monmouth.com/~jsd/how/htm/yaw.html
   http://www.monmouth.com/~jsd/how/htm/roll.html

Many of these torques are only noticeable at high power and low airspeed
(as opposed to low power and/or normal airspeed), which is why they are
mentioned in connection with takeoff.

The contribution that is far and away most noticeable is the effect of the
helical propwash hitting the tail.
   http://www.monmouth.com/~jsd/how/htm/yaw.html#sec-helical-propwash

The other effects are so small that the typical student pilot can't
perceive them even when the instructor points them out.

Many standard references contain grotesquely wrong explanations of these
effects.  Very commonly they mumble something about P-factor.  That is, by
definition, asymmetric disk loading.  That is significant for
helicopters.  OTOH, for ordinary airplanes, it is not something you need to
worry about.  I've seen P-factor, but it took two flight instructors (with
advanced degrees in engineering and physics) about six hours of planning
and two hours of flying (in a big, powerful twin) to achieve a convincing
demonstration.

============

The fundamental laws of physics do _not_ impose any relevant limitations on
the acceleration.  However, particular designs may have limitations.  I've
never done the experiment, but I'm told that if you mishandle the engine
controls on a P-51 Mustang at very low speeds you can lose control of the
yaw axis.  (It's got a huge turbocharged engine, a huge propeller, and only
modest amounts of rudder authority at low speeds.)

The student has little to worry about;  nobody is going to turn him loose
in a P-51.