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Aerospace Commercial aircraft tire skids

  1. Dec 15, 2009 #1

    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?
     
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  3. Dec 15, 2009 #2
    Re: Commercial aircraft tire "skids"

    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!
     
  4. Dec 15, 2009 #3

    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.
     
  5. Dec 15, 2009 #4
    Re: Commercial aircraft tire "skids"

    I've never heard/seen any such thing before. I don't believe this to be the case.
     
  6. Dec 15, 2009 #5

    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.
     
  7. Dec 15, 2009 #6
    Re: Commercial aircraft tire "skids"

    Let me know if you find anything. Also, the space shuttle replaces their tires after every landing. They cost about $30k each tire.
     
  8. Dec 15, 2009 #7

    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
     
  9. Dec 15, 2009 #8

    Mech_Engineer

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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.

    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...
     
  10. Dec 15, 2009 #9

    DaveC426913

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    Re: Commercial aircraft tire "skids"

    Yah, no. I know the cause of the skidding; I'm asking about the rationale for not mitigating it.
     
  11. Dec 15, 2009 #10

    Mech_Engineer

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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... Maybe the tire life isn't limited by the tire skidding when compared to other factors such as thermal cycling and weight carried?
     
  12. Dec 15, 2009 #11

    DaveC426913

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    Re: Commercial aircraft tire "skids"

    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.
     
  13. Dec 15, 2009 #12

    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]
     
    Last edited by a moderator: May 4, 2017
  14. Dec 15, 2009 #13

    Danger

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    Re: Commercial aircraft tire "skids"

    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.
     
  15. Dec 15, 2009 #14
    Re: Commercial aircraft tire "skids"

    Sweet links!
     
    Last edited by a moderator: May 4, 2017
  16. Dec 15, 2009 #15
    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.
     
  17. Dec 15, 2009 #16
    Re: Commercial aircraft tire "skids"

    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.
     
    Last edited: Dec 15, 2009
  18. Dec 15, 2009 #17
    Last edited by a moderator: Apr 24, 2017
  19. Dec 15, 2009 #18
    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?
     
  20. Dec 15, 2009 #19
    Re: Commercial aircraft tire "skids"

    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.

    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.

    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.

    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.

    "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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  21. Dec 16, 2009 #20

    DaveC426913

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    Re: Commercial aircraft tire "skids"

    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.

    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.
     
  22. Dec 16, 2009 #21

    Danger

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    Re: Commercial aircraft tire "skids"

    Mugaliens, it's nice to know that we have a heavy driver on board. We already have civils, fighters, chopper pilots... but you're the first that I've seen who wears the big iron. Your contributions will be very advantageous. :smile:
     
    Last edited: Dec 16, 2009
  23. Dec 16, 2009 #22

    minger

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    Re: Commercial aircraft tire "skids"

    I'm enjoying the discussion, but this is something that I had not thought of. The gyroscopic loading on the bearings/axles may not be significant, but they do exist. Furthermore, rotation rate would be restricted at any point during the wheel spin-up. I am imagining some sort of botched landing that results in the wheel axle just shearing off.
     
  24. Dec 16, 2009 #23
    Re: Commercial aircraft tire "skids"

    Yes. And thanks for your expert input. I only noticed that after watching several YouTube clips. The poor landings usually involved the pilot rotating the nose inline with the runway before touchdown, and a (yaw induced?) roll and overcorrection, recorrection, and so on.
    If you could judge the skid time by the time the tires smoke on the runway, then a half second or even less seems right. But it seems the back wheels would carry the plan to the upwind side of the runway in the remaining second or two before the front wheel touches down. Or do you exepct some amount of sideways motion during touchdown?
     
    Last edited: Dec 17, 2009
  25. Dec 17, 2009 #24
    Re: Commercial aircraft tire "skids"

    I forgot to add, of course, using aileron to hold the wings level throughout, which becomes especially important with some of the lower-slung engines. There's always landing with crossed controls, but it's not well advised... (or thought of).

    And again, I forgot to add that the slots and flaps are only partly for lowering stall speed. One of their major functions is to create drag, which, after pulling the throttles to idle, results in a fairly rapid deceleration of the aircraft. So after the second bounce, er, "when the mains are firmly on the runway," we're usually well below stall speed and will remain there throughout.

    The other function is that to maintain required velocity, we have to keep the engines spooled up considerably higher than we would if we were using less flaps. This may seem counterintuitive, as some would say, "yes, but doesn't this reduce your available power?" The answer is "yes, but we have power to spare. Of more importance is how long it takes that power to spool up to useable levels." By keeping the engines spooled up, useable power is about half as many seconds away as it would be if we were near flight idle throughout our descent.

    While landing in a crab to counter right to left crosswind, and we maintain centerline until touchdown, by the time the mains are spun up, we're slightly left of centerline. The aicraft rapidly begins correcting to the right, but we're correcting with rudder, so it works out fairly even by the time the nosewheel touches down.

    In theory... :yuck:

    But of course! Nice catch. I'd always envisioned them being molded into the tire. A simple vane kit attached to the wheel would indeed be much lighter.

    Well, the velocity loss due to wheel spinup occurs at touchdown, but as I mentioned, it's not much, only a couple of knots at the most. And the landing gear are probably about the strongest item on the aircraft given their weight, simply because we only use them for taxi and take-off, and haul 'em around as luggage the rest of the way!

    So both effort and expense are put in to making them as light as possible while retaining high enough strength to literally skid sideways without failing, to take the entire weight of the aircraft, fully loaded, in a hard landing without breaking, and to full stop in a fully loaded (MGTOW) aircraft after reaching V1.

    In a Boeing 777, that's 288 tons at 180 kts, max braking, with brakes at min allowable thickness! http://www.youtube.com/watch?v=f4LFErD-yls"for the actual test. And then, after stopping, the plane has to be able to taxi, and then either taxi or stand still for five minutes while the brakes are at approximately 3,000 deg C (5,432 deg F). That's for the estimated maximum of five minutes it could take the fire engines to reach the airplane. But the 777 is a newer design, and the plug blows, relieving the pressure. On earlier aircraft you'll see warnings involving staying well clear of the axels, as the whees tended to blow themselves apart parallel to the axels, hence the requirement to "gather off the nose" or "gather off the tail."

    Still, even in much older aircraft (fifty years) the landing gear is a lot stronger than most people would imagine. You'll usually bend the airframe in some way before you break the landing gear. Now this applies to airliners, as a measure of passenger safety. I wouldn't try pranging the gear in a SEL, prop Cessna, Piper, or Beechcraft, as you may very well break the gear. Then again, they usually touch down around 50 kts, not 120!
     
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