Newsgroups: sci.aeronautics.airliners Path: news From: rdd@cactus.org (Robert Dorsett) Subject: Re: Tire burn-out during landings X-Submission-Date: Mon, 28 Dec 92 06:48:40 CST References: Message-ID: Approved: kls@ohare.Chicago.COM Organization: Capital Area Central Texas UNIX Society, Austin, Tx Sender: kls@ohare.Chicago.COM X-Submission-Message-Id: <9212281248.AA13397@cactus.org> Date: 28 Dec 92 22:47:45 PST In article 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@cactus.org ...cs.utexas.edu!cactus.org!rdd