Spinning airplane wheels before landing
September 2, 2007 11:33 AM   Subscribe

Why don't commercial jets start the wheels rotating before landing?

When an airplane lands, that burst of smoke is rubber burning off the tires as the wheels go from not rotating at all to rolling without slipping in a very short time. This must wear the tires pretty quickly, adding to the operation cost. There are various patents dating as far back as the 1940's for fins on the wheels or the tires to get the wheels spinning before landing. But it doesn't seem that any of these inventions are actually used. Why?
posted by Killick to Travel & Transportation (24 answers total) 4 users marked this as a favorite
I suspect that the massive downforce on the tires at touchdown would still wear the rubber rapidly even if the tires were already rotating. You also have the issue where added weight and complexity might be more costly in the long run than saving a few bucks on rubber.
posted by Lame_username at 11:52 AM on September 2, 2007

Total guess here, but that friction is also braking, which needs to be done in an optimal time. The cost of rubber is small compared to the cost of one airplane running over the end of the runway.
posted by weapons-grade pandemonium at 11:53 AM on September 2, 2007

I believe it's something to do with having to calculate and then spin the wheels up to the ground speed of the aircraft (both of which are difficult). But IANAP.
posted by gene_machine at 11:53 AM on September 2, 2007

Replacing the tires is probably less expensive than ongoing maintenance, extra fuel cost, etc on the hardware to spin the tires up.
posted by kindall at 11:53 AM on September 2, 2007

Possibly because the cost of repairing and maintaining any mechanism for getting wheels spinning is more than the cost of the rubber and free spinning wheel.
posted by knowles at 11:54 AM on September 2, 2007

It's because installing motors to spin up wheels makes the plane weigh more. More weight = more fuel consumption. The additional cost of fuel costs more than the tires.
posted by deadmessenger at 11:56 AM on September 2, 2007

Some of the patents are for things as simple as C-shaped ribs along the sides of the tires that offer more resistance to the air at the bottom of the tire than when they are backwards at the top, causing air-driven spin (sort of like an anemometer). So I can't imagine them adding much cost or weight.
posted by Killick at 11:59 AM on September 2, 2007

I am a [lapsed] aerospace engineer, and I say that kindall and knowles have it. The cost of the current solution is far less than any proposed new solution.

Also note that airline tires get retreads frequently.

For fun, go to youtube and search for "crosswind landing" sometime.
posted by intermod at 12:00 PM on September 2, 2007 [3 favorites]

> Also note that airline tires get retreads frequently.

I wish you hadn't told me that. Considering all those dead black alligators you see on the Interstate which turn out to be the remains of eighteen-wheeler retreads blowing their tread layer off, I hope the folks who do retreads for aircraft know how to do a more durable job than the outfits who recap truck tires!
posted by jfuller at 12:30 PM on September 2, 2007

A friend of my father told me, many years ago, that someone had invented a cheap(ish) system for getting the wheels to turn before landing based on a similar principle to an anemometer but a major tyre manufacturer (possibly Dunlop) bought the patent and kept the invention from the market.
posted by i_cola at 12:49 PM on September 2, 2007

(FWIW, my father has just confirmed the story and thinks it was in the 60's.)
posted by i_cola at 1:04 PM on September 2, 2007

In which case the design would be in the public domain now.
posted by Mitheral at 1:22 PM on September 2, 2007

there's an article from someone who worked at american airlines saying that it seems to be a weight issue. they also raise the interesting (and probably valid) point that turning also scuffs rubber, so landing may not be the dominant source of wear.

this question has also been addressed on google answers and why not?.

there's also a curious post, that i don't believe, arguing that the wear on brakes is significantly greater (since less energy is lost via spin up).
posted by andrew cooke at 1:38 PM on September 2, 2007

Those crosswind landing clips left me speechless, especially this one.

Good to know that the planes can handle stuff like that, and that there are at least some pilots out there who can make it look easy.
posted by Brian James at 1:56 PM on September 2, 2007

The Cessna Citation business jet actually has a nosewheel spin-up device as an option. This is done not to save the tires but to prevent the spray of gravel up into the engine turbines when landing on gravel airstrips. There are vanes on the rim of the wheel which are spun up by bleeding compressed air from the turbines. It takes about 30 seconds with increased engine speed to spin up the wheel. This wheel is only 18 inches in diameter.

I think you might be underestimating the force necessary to spin up a large aircraft tire. A 747 main wheel is about 50 inches in diameter and probably weighs several hundred pounds. Just little wind vanes wouldn't do the trick. Even for the small Citation wheel it requires compressed air from the engines to get sufficient speed.

Motors to spin up the wheels would be too heavy and expensive. Say a 20 pound motor for each wheel, there are 16 main wheels on a 747 so that is 800 pounds. Each additional pound of weight on an airplane costs about $20 per year for fuel so that's $16,000 per year. Not to speak of the tremendous amount of electrical energy required to spin up several thousand of pounds of wheels shortly before landing.
posted by JackFlash at 1:58 PM on September 2, 2007

A friend of my father told me, many years ago, that someone had invented a cheap(ish) system for getting the wheels to turn before landing based on a similar principle to an anemometer but a major tyre manufacturer (possibly Dunlop) bought the patent and kept the invention from the market.

The patent would have long expired by now, so if this thing actually existed there'd be no reason for manufacturers not to implement it (assuming it's actually better as claimed). Sounds like aero-engineer urban legend to me.
posted by 0xFCAF at 2:09 PM on September 2, 2007

Oops, that should have been 320 pounds and $6,400, but still too heavy and expensive. That's two fewer passengers on hundreds of flights per year.
posted by JackFlash at 2:16 PM on September 2, 2007

A "little puff of smoke" can be created by a surprisingly small amount of rubber. The tyre wear is probably a lot less than you are imagining for the resulting smoke.

Also, I add my support to the 'too expensive/difficult/heavy' argument. Being as ground speed (the speed you want the tyre to go at) varies by so many things for each landing (windspeed, air density etc) that it would only help on a direct approach (ie no crosswind) at perfect speed and blah blah blah. You get the point, it'd only help in a variable manner, and not always at maximum efficiency, etc., etc

You'd remove some of the wear, but not by any means all. Again, the braking of the aircraft wears more rubber off, per landing, I'd wager. So why spend time effort and money trying to reduce 20% (made up number for affect) of the wear when tyres don't cost all that much by comparison?
posted by Brockles at 4:13 PM on September 2, 2007

If the wear was at all significant, it would be warming up the tires, which could be good for the breaking performance (laying a little rubber for the next guy too).. I vote for 'no significant wear, so why worry' though.
posted by Chuckles at 4:49 PM on September 2, 2007

It would only work if the wheel-spinning engines could somehow read the plane's speed, or the speed of the ground underneath, to match their spin to the ground. If they were spinning too slowly it wouldn't make much difference, would it?

Which leads me to wonder: remember that plane that had the front wheel stuck sideways? How would a landing like that work, if the wheel-spinning motor still went off? I'm imagining a plane going tail over teakettle.

(I'm an artist, not any sort of pilot or engineer, and my last experiment with spinning wheels in midair was watching one of the neighbor kids faceplant after trying to jump off a ledge into the street with a bike. Ow.)
posted by cmyk at 5:06 PM on September 2, 2007

The articulated main landing gear of most "heavy" type aircraft (747, A320, MD-11, etc.) already serves to minimize touchdown tire wear, and reduce blowouts, by providing some articulated initial "soft" loading of the tire clusters at touchdown. You can see this working as the rear most tires, "hanging" lowest immediately prior to touchdown, first contact the runway, and the landing gear then is quickly compressed/rotated slightly to bring additional tires, farther forward, into ground contact.
posted by paulsc at 5:26 PM on September 2, 2007

Paul, even more curious are the articulated bogies on the the Boeing 767. The trucks are tilted so that the front wheels touch before the rear. It seems that they are stubbing their toes, but it doesn't seem to make much difference for handling. The reason for this tilt is that the main gear swing slightly forward as they rotate to their stowed position in the belly of the plane. The forward truck tilt is so that the trucks will lie parallel to the keel in order to fit into the wheel wells when retracted. So the tilted bogies have more to do with their storage position in the wheel wells, I think, than their effect on landing.
posted by JackFlash at 6:18 PM on September 2, 2007

Thanks for all the great answers. I've wondered about this for a while, but you all have brought up a lot of aspects of this I hadn't thought of. (It's also humbling to realize how much information is out there that my web search did not turn up.)
posted by Killick at 6:34 PM on September 2, 2007

I was very interested in the above coments.
This happens be a favorite subject of mine.
I firmly believe the answers are out there that can pre- spin wheels in a cost effective manner.
I offer the following for consideration.
Install fan type -pelton wheel covers. change wheel bearing from grease to heated lube oil (planes fly @ -69 degree F) with seal by-pass. Incororate disc pads with high pressure air momentary release (free spin no drag).
This is the basic concept; I have filed for patent pending on this concept & design. I look forward to further comments. Del

posted by dellori3 at 5:48 PM on November 16, 2007

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