Subject: Tire burn-out during landings

From: [email-masked] (Brian M. Huey)
Date: 20 Dec 92 16:08:05 CST

I realize this a relatively basic question (I think) nevertheless:

	I am neither a pilot nor an airliner engineer. However,
everytime I watch a airliner land, a cloud of smoke is produced when
the wheels come in contact from the ground due to friction. Couldn't
this cloud and the effect of friction be lessened by inducing a
angular velocity upon the wheels to match the airliners speed with
respect to the runway?

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DISCLAIMER: I can neither confirm nor deny any opinions
expressed in this article directly reflect my own personal or
political views and furthermore, if they did, I would not be at
liberty to yield such an explanation of these alleged opinions.

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From: [email-masked] (MegaZone23)
Date: 21 Dec 92 11:30:44 CST

>angular velocity upon the wheels to match the airliners speed with

Spinning the wheels would indeed cut down on the tire wear, and I believe that
this has been tested. However, there are problems. When you spin a tire it acts
as a gyroscope and will resists having it's course altered. This can make 
handling tricky as you would have 10 or more wheels all spinning on most 
airliners. Have you ever held a spinning bicycle tire in your hands and tried
to move it? Same effect.

You also need to weigh the advantages against the disadvantages. How much does
the tire wear cost the airline? How much would mantenance on a system to spin
the wheels cost?

Plus you will have the initial cost for the system and the added weight. It is
one more system to break down, etc...

All in all it just doesn't appear to be worth it...

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From: [email-masked] (Geoff Miller)
Date: 26 Dec 92 00:01:49 CST

[email-masked] (MegaZone23) writes:

>Spinning the wheels would indeed cut down on the tire wear, and I 
>believe that this has been tested. 

It was tested on the Lockheed Constitution, back in the late Forties
or early Fifties.

>When you spin a tire it acts as a gyroscope and will resists having it's 
>course altered. This can make handling tricky as you would have 10 or more 
>wheels all spinning on most airliners.

Two points:

First, I'd think that the mass of the spinning wheels, and therefore the
magnitude of their gyroscopic effect on handling, would be insignificant
compared to the mass of the rest of the airframe and payload and the 
power of the control surfaces.

Second, assuming that the effect *was* significant, wouldn't it simply be 
a stabilizing influence, i.e., a resistance to changes in attitude about 
the longitudinal (roll) axis?  That doesn't sound all that hazardous.

>Plus you will have the initial cost for the system and the added weight. 
>It is one more system to break down, etc...

I suspect that this is the most likely reason this idea hasn't been 
implemented; tires are simply cheaper to deal with in the long run.

Geoff

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Geoff Miller			+ + + + + + + +        Sun Microsystems
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From: [email-masked] (Gary Benson)
Date: 27 Dec 92 17:34:54 CST

>All in all it just doesn't appear to be worth it...

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.

RE: Maintenance

Maybe there would be NO cost, for example if the tires were designed so that
their tread caught the wind and got their spin from that. Or maybe the hubs
could be fitted with fans.

RE: Initial cost of system

I think this could be done for free, too; particularly if the tread- or hub-
induced spin just mentioned were employed. All you'd need to do is apply the
breaks to keep them from spinning until you were ready. For safety's sake,
you might want to have a "mini-brake" that would be enough to keep the
wheels from spinning but which would easily be overcome if you landed with
it applied. Seems cheap enough.

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?

-- 
Gary Benson   [email-masked]

Stupidity cannot be cured with money, or through education, or by legislation. 
Stupidity is not a sin; the victim can't help being stupid. But stupidity
is the only universal capital crime; the sentence is death, there is no
appeal, and execution is carried out automatically and without pity.
                                                   -Lazarus Long

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From: [email-masked] (MegaZone23)
Date: 28 Dec 92 22:47:43 PST

>power of the control surfaces.

That's probably correct, although I don't have the weight of an average wheel
and tire on hand to compare it seems so. Though a 26" 12-speed tire is enough
to affect my 300lb body, I uspect the ratio of the gear to the aircraft weight
is MUCH less...

I too feel the major reason is cost, both for the hardware and lifetime costs.

>the longitudinal (roll) axis?  That doesn't sound all that hazardous.

Arg,old physics comes back to haunt me...

Making the large assumption that the mass is significant and the effect is as
stated; I can see a few problems in that course correction includes rolling
for turns, although I realize the angles used by airliners are a bit less than
what I use in a C-172....

But that is just speculation, thinking about it now I would agree that the 
mass difference makes the gyroscopic effects negligible.

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From: [email-masked] (Robert Dorsett)
Date: 28 Dec 92 22:47:45 PST

>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.

>you have to spin the tire to get a 10% reduction in wear? 10% of the speed

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
[email-masked]
...cs.utexas.edu!cactus.org!rdd

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From: [email-masked] (Stephen Spackman)
Date: 29 Dec 92 09:43:53 PST

In article <[email-masked]> [email-masked] (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)    [email-masked]
      dfki / stuhlsatzenhausweg 3 / d-w-6600 saarbruecken 11 / germany
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From: [email-masked] (Dennis Chamberlin)
Date: 29 Dec 92 12:39:16 PST

In article <[email-masked]> [email-masked] (Gary Benson) writes:
>cost? 
I recall an old issue of AWST that described the certification flight test
program for the first 747. The flight test budget included a 3 million $
line item for "Wheels, tires, and brakes".

At least one of these tests was known to be destructive. The worst-case
demonstration of a takeoff abort would seem to thoroughly cook all of the 
tires and brakes. I don't know if any of the wheels were savable.

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From: [email-masked] (Lewis E. Wolfgang)
Date: 29 Dec 92 22:53:45 PST

>talking about the rotational momentum set up by a spinning tire, and what to

  (Stuff deleted)
>
> (3) the *additional* wear and tear on the brakes, as they 
>task of slowing down the airplane. 
 
>to landing speed, I can easily see landing distances lengthened considerably.

  (lots more stuff deleted)

Robert, I think you have your mass off by several orders of magnitude.  If the
mass of the rotating tires is insignificant compared to the total mass of the
airframe (to negate the gyroscopic effect) then it will contribute negligibly
to the stopping distance.  

Consider, if you will, when the pilot manages to "grease" one on:  no noticeable
deceleration is observe at the moment of touchdown.  There may be a vertical
"bump",  but not a horizontal one.  (if there is no crosswind component)

IMHO you would see no measurable difference in rollout distance or residual heat 
in the brake system.

					Luck
					Lewie
					[email-masked]

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From: [email-masked] (Norman Yarvin)
Date: 29 Dec 92 22:53:48 PST

[email-masked] (Robert Dorsett) writes:
> [...] (3) the *additional* wear and tear on the brakes, as they 
>task of slowing down the airplane.  [...]

This additional energy is negligible.  Consider just the energy of the wheel
itself.  For a wheel which is rolling along the ground there is the relation:

	(rotational energy) = (some constant) * (energy of forward motion)

where the constant is independent of speed.  I seem to recall that for a
cylindrical wheel of uniform consistency, the constant is 2/7.  At absolute
worst the constant will be 1.  (This would occur if the entire mass of the
wheel were on the tread of the tire.)

Furthermore the energy of forward motion of the wheel is an insignificant
portion of the energy of the entire aircraft.  This goes by weight; if the
airliner weighs 100,000 pounds and a wheel weighs 300, the proportion of
energy in that wheel would be 3/1000 of the aircraft's energy.  Then, using
the 2/7 figure, the spinning energy of the wheel would be 3/1000*2/7 = .08%
of the energy of forward motion of the aircraft.

Assuming constant deceleration force, stopping distance would be lengthened
by that same .08%.  Even the weight of the mechanism required to speed up the
tires might be a bigger factor.

In any case, the practicality of preventing tires from disintegrating
depends on how fast tires presently disintegrate.  How much matter really is
there in that cloud of smoke?  Perhaps a gram per cubic meter of smoke?  And
how much tire is left on the runway?  Do they have to go out and scrape it
off now and then?  (I imagine not.)  Seems to me the loss of tire material
is negligible also.  Compared, that is, with the other costs of running the
airplane.

--
Norman Yarvin						[email-masked]

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From: [email-masked] (Chuck Huffington)
Date: 29 Dec 92 22:53:50 PST

>could be fitted with fans.

This has been done.  I have seen old tires with molded in "scoops" on
the sidewall that spun them up in the airstream.

I'm not sure but I seem to recall the idea was dropped because the
the additional wear on the brakes more than made up for the reduced
wear on the tires.  Apparently a lot of braking action occurs
when the not spinning tires hit the pavement.

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From: [email-masked] (Paul Barnett)
Date: 30 Dec 92 11:20:04 PST

>is negligible also.  Compared, that is, with the other costs of running the

Actually, yes, they do go out and "scrape" it off now and then.
One popular method is very high pressure water jets.

Having said that, I will comment that the most convincing argument I
have heard so far is that the additional weight added by any spin-up
system would negate any reduction in tire wear.
--
Paul Barnett
MPP OS Development     (214)-497-4846
Convex Computer Corp.  Richardson, TX