Aerospace Commercial aircraft tire skids

DaveC426913

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Commercial aircraft tire "skids"

Why do jetliner tires skid when they touch down? Surely this dramatically shortens the life of the tires. (I know in the ground transport industry they go to great lengths to save on tires because they go through them so fast.)

I've always wondered why there isn't a mechanism to bring the tires up-to-speed before touching down. It could be as sophisticated as a motor that powers them up or as simple as a rotor device that uses the wind to spin them.

Is it simply that they can afford to replace tires as often as needed?
 
2,788
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Re: Commercial aircraft tire "skids"

Why do jetliner tires skid when they touch down? Surely this dramatically shortens the life of the tires. (I know in the ground transport industry they go to great lengths to save on tires because they go through them so fast.)

I've always wondered why there isn't a mechanism to bring the tires up-to-speed before touching down. It could be as sophisticated as a motor that powers them up or as simple as a rotor device that uses the wind to spin them.

Is it simply that they can afford to replace tires as often as needed?
I don't see how it would shorten it all that significantly, as it only happens once per landing and occurs for a half second, at most? I would expect aircraft tires to last a long time, the only real problem being the constant thermal cycles it sees.

Disclaimer: Everything I've said in this post is simply speculation, so actual data is in order!
 

Borek

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Re: Commercial aircraft tire "skids"

I believe wheels ARE spinned up before touch down, at least in some types of planes. Whatever smoke we see is just because of "small" difference in speed.
 
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Re: Commercial aircraft tire "skids"

I believe wheels ARE spinned up before touch down, at least in some types of planes. Whatever smoke we see is just because of "small" difference in speed.
I've never heard/seen any such thing before. I don't believe this to be the case.
 

Borek

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Re: Commercial aircraft tire "skids"

I have googled for a moment, but apart from some discussions on the subject and patents describing such devices (which doesn't mean it was ever implemented) I can't find anything. I am sure I have heard about such a solution - not as an universal approach, but in some specific type of plane. Or perhaps not plane but something higly specialized, like Space Shuttle. It was not a motor, but some kind of aerodynamic device.
 
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Re: Commercial aircraft tire "skids"

I have googled for a moment, but apart from some discussions on the subject and patents describing such devices (which doesn't mean it was ever implemented) I can't find anything. I am sure I have heard about such a solution - not as an universal approach, but in some specific type of plane. Or perhaps not plane but something higly specialized, like Space Shuttle. It was not a motor, but some kind of aerodynamic device.
Let me know if you find anything. Also, the space shuttle replaces their tires after every landing. They cost about $30k each tire.
 

Borek

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Re: Commercial aircraft tire "skids"

I have found a lot of information about why not... They mostly revolved about gyroscopic effects making plane harder to fly close to the ground.

But obviously idea is not new and have been researched: http://archives.sensorsmag.com/articles/0300/14/index.htm
 

Mech_Engineer

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Re: Commercial aircraft tire "skids"

Why do jetliner tires skid when they touch down?
They skid because the tire/rim assemblies are heavy, and go from zero (or at least slow) to 200mph in less than a second on touchdown.

Surely this dramatically shortens the life of the tires. (I know in the ground transport industry they go to great lengths to save on tires because they go through them so fast.)

I've always wondered why there isn't a mechanism to bring the tires up-to-speed before touching down. It could be as sophisticated as a motor that powers them up or as simple as a rotor device that uses the wind to spin them.

Is it simply that they can afford to replace tires as often as needed?
It probably does shorten their life somewhat, but I'm guessing the cost of replacing the tires is less than the cost of engineering and maintenance on a system that is able to spin them up to speed for landing. I do like the idea of a set of vanes that get the wheels spinning when the landing gear are dropped into the wind though...
 

DaveC426913

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Re: Commercial aircraft tire "skids"

They skid because the tire/rim assemblies are heavy, and go from zero (or at least slow) to 200mph in less than a second on touchdown.
Yah, no. I know the cause of the skidding; I'm asking about the rationale for not mitigating it.
 

Mech_Engineer

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Re: Commercial aircraft tire "skids"

I'm asking about the rationale for not mitigating it.
Well, It seems to me that if it was really such a drain the designers would have come up with a solution... Maybe the tire life isn't limited by the tire skidding when compared to other factors such as thermal cycling and weight carried?
 

DaveC426913

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Re: Commercial aircraft tire "skids"

Well, It seems to me that if it was really such a drain the designers would have come up with a solution...
But this is circular logic.

Why do we get spam? If spam were a problem, we would have fixed it.
Why does cancer kill so many? If cancer was such a killer we would have cured it.
 

FredGarvin

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Re: Commercial aircraft tire "skids"

Not all landings are perfectly in line with a runway. Any crosswind and you'll get a skid on the tires. The B-52 is the only aircraft I know of that has rotating main gear to help align with the runway in the event of strong crosswind landings.

I know that the required change out time on a big airliner is not that many landings. However, I think the cost and weight penalties of instituting some kind of wind up system on the gear is enormous. I do remember seeing something on that idea for trying to help this but I don't think it has ever come to be. So Borek, you're not crazy (...or we both are...). That being said, you just have a butt load of weight on those things moving at high speeds. I would think landings are not the only harsh thing they need to handle. I would imagine a fair amount of taxiing with full loads has to take its toll as well.

More than you probably would ever want to know:
http://www.goodyearaviation.com/resources/pdf/effects.pdf [Broken]

Taken from:
http://www.goodyearaviation.com/resources/tirecare_en.html [Broken]
 
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Danger

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Re: Commercial aircraft tire "skids"

Not all landings are perfectly in line with a runway. Any crosswind and you'll get a skid on the tires.
Although I have been grounded for over 30 years, I can honestly say that I have never experienced a landing that didn't involve some degree of side-slip. It didn't make too much difference, tire-wise, with the stuff that I flew (150's, 152's, 172's), but the heavies are a whole different matter. A hundred tonnes or so planting a couple of dozen square centrimetres of rubber on the road is a significant event from the perspective of the rubber.
Aviation has always been a leading area for new technology. Radial tires, disc brakes and seat belts are merely a sampling of the things that we now take for granted in cars but were originally used in aircraft.
 
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Re: Commercial aircraft tire "skids"

Not all landings are perfectly in line with a runway. Any crosswind and you'll get a skid on the tires. The B-52 is the only aircraft I know of that has rotating main gear to help align with the runway in the event of strong crosswind landings.

I know that the required change out time on a big airliner is not that many landings. However, I think the cost and weight penalties of instituting some kind of wind up system on the gear is enormous. I do remember seeing something on that idea for trying to help this but I don't think it has ever come to be. So Borek, you're not crazy (...or we both are...). That being said, you just have a butt load of weight on those things moving at high speeds. I would think landings are not the only harsh thing they need to handle. I would imagine a fair amount of taxiing with full loads has to take its toll as well.

More than you probably would ever want to know:
http://www.goodyearaviation.com/resources/pdf/effects.pdf [Broken]

Taken from:
http://www.goodyearaviation.com/resources/tirecare_en.html [Broken]
Sweet links!
 
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164
0
Re: Commercial aircraft tire "skids"

Yes - very good link!

No, there are no mechanisms currently in existence to bring a tire up to touchdown velocity before actual touchdown. I do know one idea that's been discussed is to mold in sidwall ridges with a flat side facing the wind below the axel, and a streamlined side facing the wind above the axel. Designed right, they could easily bring the tire up to velocity in the couple of minutes the wheels are in the down and locked position.

There are three reasons why this might not be done:

1. It's more expensive than simply accepting the tire wear.

2. Due to the extreme nature of the tire environment, it may require significantly great tire weight beyond the ridges simply to accomodate the ridges.

3. Spinning those tires up to speed requires energy, and that energy results in an initial knot or two slowdown of the aircraft on tounchdown. That's not a lot, but it's something.
 
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Re: Commercial aircraft tire "skids"

I have found a lot of information about why not... They mostly revolved about gyroscopic effects making plane harder to fly close to the ground.

But obviously idea is not new and have been researched: http://archives.sensorsmag.com/articles/0300/14/index.htm
You may have inadvertently stumbled on the reason implementation has not occurred. Someone with more experience may have better comment.

Say, in a high cross-wind landing, the tires are spun-up to, say, 130mph at the radius. Directly before touchdown the pilot rotates the plane. 1) This places a great deal of stress on the axle bearings. Perhaps this is better met by the tires hitting the runway.

2) Spun-up, the tires will meet the runway with rolling friction, not sliding. Again, in a crab landing, I think you would prefer sliding friction to compensate for misalignment.

3) In addition, if implementing pre-spun tire, something has to keep the front steering wheel aligned in the direction the pilot wants it to go. You may not want a stow-lock engaged directly before the time of landing, as you yaw to alignment. This is something you would want to control for last second correction, in an unlocked state. In addition, it may not disengage when it's supposed to, carrying your plane off the side of the runway.

In summation, for crabbed landing conditions, it seems advantageous to have non-rotating and skidding wheels.
 
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Re: Commercial aircraft tire "skids"

How large are the frictional forces on an airplane tire during the initial touchdown? Do they cause any significant deceleration of the plane?
 
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Re: Commercial aircraft tire "skids"

Say, in a high cross-wind landing, the tires are spun-up to, say, 130mph at the radius. Directly before touchdown the pilot rotates the plane.
Actually, for airliners, that's a great way to get blow off the runway. We touch down first, on centerline, in a crab, tires skidding, then use rudder to maintain centerline. It's only about a 1 to 2 second correction.

1) This places a great deal of stress on the axle bearings.
Let's say we have 30 kt crosswind component (serious) and we're touching down with a groundspeed of 130 kts. That gives us 13 degrees of crab. Those tires certainly aren't light weight, but the airplane is a whole lot heavier, and the forces on the bearings just from normal taxi operations far exceed the forces of turning a spinning wheel 13 degrees in one second. Landing forces, particularly in a crab, exceed taxi forces by an order of magnitude or more.

2) Spun-up, the tires will meet the runway with rolling friction, not sliding. Again, in a crab landing, I think you would prefer sliding friction to compensate for misalignment.
Didn't think about that, and it could be somewhat of a problem, but again, it only takes about half a second, at most, for the tires to come up to speed. By that time we're still in a crab, but will begin correcting momentarily.

3) In addition, if implementing tires pre-spun, something has to keep the front steering wheel aligned in the direction the pilot wants it to go.
We don't let the nosewheel down until we're firmly on the ground and tracking down centerline. Nosewheel steering is centered as part of the before landing checklist, and we will predominantly use rudder for maintaining centerline after landing until it's effectiveness deteriorates to the point where we revert to nosewheel steering.

You may not want a stow-lock engaged directly before the time of landing, as you yaw to alignment. This is something you would want to control for last second correction, in an unlocked state. In addition, it may not disengage when it's supposed to, carrying your plane off the side of the runway.
"Stow-lock?" We have http://wiki.answers.com/Q/Function_of_mechanical_downlock_indicator" [Broken], but they don't affect steering. :biggrin:
 
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DaveC426913

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Re: Commercial aircraft tire "skids"

I do know one idea that's been discussed is to mold in sidwall ridges with a flat side facing the wind below the axel, and a streamlined side facing the wind above the axel. Designed right, they could easily bring the tire up to velocity in the couple of minutes the wheels are in the down and locked position.

1. It's more expensive than simply accepting the tire wear.

2. Due to the extreme nature of the tire environment, it may require significantly great tire weight beyond the ridges simply to accomodate the ridges.
I see both points if it's part of the tire. I had envisioned it as part of the rim, thus not needing repalcement with every tire.

3. Spinning those tires up to speed requires energy, and that energy results in an initial knot or two slowdown of the aircraft on tounchdown. That's not a lot, but it's something.
Ooh. I hadn't thought of that. Both loss-in-speed and gyroscopic effects would be something you'd very much like to avoid the moment before touchdown.
 

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