Re: Tire burn-out during landings

From: (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         28 Dec 92 22:47:45 PST
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In article <airliners.1992.192@royko.Chicago.COM> you write:
>I'm glad this question was asked - I've often wondered the same thing! And I
>think the answer was very thoughtful and all in all, probably states the
>real reason spinnning the wheels is not done. However, if I may opine:
>RE: Gyroscope effect
>It seems that this could be used to advantage. After all, the wheels would
>tend to make the bird retain its current course. If you didn't start
>spinning till you were lined up with the runway, it seems that the spinning
>wheels could conceivably even help counteract sheer forces.

Strictly speaking, I don't see this as a gyroscopic effect.  We're just
talking about the rotational momentum set up by a spinning tire, and what to
do about it.

We need to consider three issues: (1), the means by which the tires get 
"spinning," (2) the actual control benefits by having the tires spinning on 
touch-down, and (3) the *additional* wear and tear on the brakes, as they 
must absorb the spinning energy, in addition to performing their normal 
task of slowing down the airplane.  We could also add a (4), having the
wheel assemblies spinning at high speed for extended periods of flight
(outer marker to completion of roll-out), with the ramifications on the
wheel structure (for one thing, a balancer to stop in-air "wobbling" would 
be needed).

(3) seems the major disqualifier of the idea.  With an inert tire, you'll 
have *minor* control problems ("bump", and that's it), but the energy absorbed 
by the tire in *spinning up*, on landing, in itself helps slow the airplane.  
That smoke's the energy being absorbed by the tire.  If the tire's already up 
to landing speed, I can easily see landing distances lengthened considerably.

In addition, with the excess energy being mopped up by the brakes, you've
got a mandatory "cooling-down" time to consider.  This could lengthen 
stop-over times considerably: an airplane can't take off again with hot 
brakes, since braking efficiency (which one would need for a rejected
takeoff) goes WAY down, not to mention the resulting dangers of tire damage 
or wheel well fires.  

In reality, the issue is distance, not controllability.  Anything to shorten
takeoff and landing distances is to be supported; anything increasing them
had better have some whopping benefits. :-)  The current system is obviously
cost-effective enough to be used.  I don't have stats on tires handy, but
the airlines do get a lot of wear out of them.

>Does anyone have any estimates about the costs using the current "cloud of
>smoke" and friction method of landing? How much does one of those tires
>cost? What is the expected number of landings it can endure? How fast would
>you have to spin the tire to get a 10% reduction in wear? 10% of the speed
>of the aircraft?

How would a "modified" tire design work on wet or snowy runways?  And would 
a 20% increase in landing distance, resulting in a 30% reduction in the number 
of airports the carrier can service, be worth it?  With companies eliminating
movable autothrottles for 20-lb savings, do we really expect them to go for
something with a potentially high number of "unforeseen" variables? :-)  
Landing and takeoff performance is an awesomely complex discipline.  There 
are a lot of variables to consider.

Robert Dorsett!!rdd