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Re: torque on airplanes.

At 07:38 PM 4/12/01 -0500, brian whatcott wrote:
>
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)

That's true as stated, but not relevant to the question that was asked.

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

That would be a fanciful supposition, not a factual explanation.

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.

OK. (There are also non-airflow effects, such as gyroscopic precession,
but they are only marginally relevant to the question that was asked.)

W = F x V

OK.

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.

The "range of speeds" over which the constant-power notion applies does not
include the speeds of interest. Otherwise there would be infinite
acceleration during the initial takeoff roll.

(... turbojets ... show more of a constant
thrust character with airspeed change)

OK.

F = M x A

OK.

> 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.

False.

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.

True as far as it goes, but not the whole story, and irrelevant in any case.

If not, disaster ensues.

Disaster would ensue long before that. Having enough elevator authority to
"push the nose down" is far from being a sufficient condition for safe flight.

But the design constraint on rudder power is less pressing or obvious.

Less pressing? I would have said equally indispensable.

Less obvious? I don't consider the stability and control requirements
around any of the axes to be obvious. Hefty books have been written on the
subject. I recommend Perkins & Hage (ISBN 0 471 68046 X).

Too much rudder area can be destabilizing (in dutch roll) and the need
decreases (the effectiveness increases) with increasing speeds.

That's not the whole story on rudder effectiveness. That's certainly not
the whole story on Dutch roll, either.

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.

Fine. That's the dominant effect.

This limit is most obviously noticed on a high power take off in a
high cross wind.

OK.

The other situation calling for maximal yawing
moment also occurs at low speed, when the pilot may need to apply
opposite rudder to despin.

(I assume we are no longer talking about the takeoff-roll situation.)

Recovery from spinning flight is irrelevant to the question that was
asked. One can eliminate engine-torque issues by reducing the power to
idle during spin recovery.

Fortunately, FAA decisions limit the need for spin training,

Whether that is fortunate or not is a subject of debate.

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.

I know of no such specification. Perhaps someone can take a look at 14 CFR
23.221 and tell me what I'm missing.
http://www.faa.gov/avr/AFS/FARS/far-23.txt

I'd also be curious to know what "usually ensure" means in this
context. In the regulations with which I am familiar, either something is
required or it is not; there's no "usually" about it.