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Re: torque on airplanes.



At 13:40 4/12/01 -0500, you 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?


I have a couple of ideas I threw out, but look forward to hearing your
expert opinions.

Thanks in Advance,

Bill Waggoner

As a general rule, increasing the power output of a tractor propeller
airplane is somewhat destabilizing.
(A pusher prop may by contrast provide increased stability)

This effect can be rationalized by supposing that increasing tractor
propeller power provides an increasing proportion of the variability
due to the slipstream.

And it is the variation in airflow with displacement in pitch roll and yaw
that determines the airframe's response which we might wish to act in
opposition to disturbances.

W = F x V
---------
A propeller plane using a reciprocating engine, especially if it is
equiped with a variable pitch prop can couple its available shaft
horsepower reasonably well to the airstream over a range of speeds,
so that the elementary definition of work shows that there is a
fairly constant thrust X airspeed product for constant horsepower.
(This is not the case for turbojets which show more of a constant
thrust character with airspeed change)

F = M x A
----------
During the slow take off phase, this means there is much more thrust
available, so that for a given mass flow through the prop disk, there is
more airflow acceleration than in faster phases of flight.

That's enough about propulsion force. For control force, it is important
to provide enough pitch up elevator moment arm to lift the nose at
takeoff, when loaded to the forward limit, and enough pitch down elevator
moment to push the nose down when loaded to the aft limit.
If not, disaster ensues.

But the design constraint on rudder power is less pressing or obvious.
Too much rudder area can be destabilizing (in dutch roll) and the need
decreases (the effectiveness increases) with increasing speeds.

So the powerful tractor providing a strong helical airstream at
slow speed is countered by the pilot providing right rudder
(on most US planes) or left rudder (on some European planes), and in
the limit, he or she may run out of rudder.

This limit is most obviously noticed on a high power take off in a
high cross wind. The other situation calling for maximal yawing
moment also occurs at low speed, when the pilot may need to apply
opposite rudder to despin.

Fortunately, FAA decisions limit the need for spin training,
and FAA airworthiness specifications also usually ensure that
a spinning light airplane will recover on its own, power-off
with hands and feet removed from the controls, given reasonable
amounts of sky beneath.

(This is probably more than you wanted, and is not quite a rounded
view, but stresses effects with which a physics student might be
familiar.)


brian whatcott <inet@intellisys.net> Altus OK
Eureka!