From: spackman@disco-sol.dfki.uni-sb.de (Stephen Spackman)
Subject: Re: Tire burn-out during landings
X-Submission-Date: 29 Dec 1992 11:47:44 GMT
Reply-To: stephen@acm.org
Organization: DFKI Saarbruecken GmbH, D-W 6600 Saarbruecken
X-Submission-Message-Id: <SPACKMAN.92Dec29125212@disco-sol.dfki.uni-sb.de>
Date: 29 Dec 92 09:43:53 PST

In article <airliners.1992.194@ohare.Chicago.COM> rdd@cactus.org (Robert Dorsett) writes:

|>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.

There *is* definitely also a gyroscopic effect, but I agree that it's
hard to imagine that it would be very significant.

|(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.  

We should go carefully here. The auto industry has only recently started
putting anitlock brakes on consumer vehicles because it is "intuitively
obvious" that a sliding tire brakes you better than a rolling one. It's
also completely false - sliding friction is significantly lower than
static friction (and it's static friction, not rolling friction, that's
the right analysis when the brakes are on and you are on a good surface)
- which is part of the reason trucks have used such brakes for much of
my lifetime (the other is to avoid jackknifing. If aircraft have trouble
in bad weather from uneven lateral braking forces causing them to slew
suddenly at touchdown, that argument applies here, too; theoretically it
might also help compensate for crosswinds, but now it's *really* time
for me to defer to an expert). The effect is so pronounced that the
method of *releasing the brakes completely* whenever adhesion drops is
apparently a win (of course, you reapply them as soon as the sliding
stops).

I would guess that in fact having the tyres hit the ground synchronous
would let you apply full mechanical braking force *immediately* without
having to wait for the bumping and sliding to stop, and this should let
you stop sooner.

As to the brake cooling issue (assuming it is real and you can't just
put bigger fins on :-), my computer scientist's instinct (not
necessarily to be followed in the mechanical domain...) is to say, let
the brakes look to themselves: we could put a processor in there that
measures *both* temperature and adhesion and controls brake application
- with respect to measured speed and a reported distance left to roll.
It should be easy enough to programme the thing so that it is kinder to
the brakes when there is lots of room, but knows enough to overstress
them in emergencies. We can easily arrange it so that there is a
mechanical override, if you like, too.

Increased complexity, perhaps, but a more tightly controlled braking
profile would again argue in favour of the non-slip approach.

|Landing and takeoff performance is an awesomely complex discipline.  There 
|are a lot of variables to consider.

Sounds like we may have found another whole batch.

Disclaimer: I'm in programming languages, so what would I know?
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stephen p spackman    +49 681 302 5288(o) 5282(sec)    stephen@acm.org
      dfki / stuhlsatzenhausweg 3 / d-w-6600 saarbruecken 11 / germany
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