I've seen this phenomenon a lot of times but I can't give it a physically explanation. Someone has an idea?

  • $\begingroup$ Might better better answers on Aviation SE $\endgroup$ – StephenG Sep 12 '18 at 20:20

The wheels are not spinning while the plane is in the air. At the moment that they first make contact with the ground, they skid a bit before they start rolling at a rotational rate consistent with the ground-speed of the plane. That skid generates heat and burns off a little rubber. (This also accounts for the black smears on the runway at the point where planes typically first touch down.)

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  • $\begingroup$ But the wheels skid because the pilot uses the brake at the start of the landing? I mean they should start rolling from the first instant... $\endgroup$ – Landau Sep 12 '18 at 17:04
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    $\begingroup$ The wheels have inertia and cannot start spinning instantly, even in theory. Their ability to rotate "instantly" is probably further limited by a variety of practical factors, for example that the initial contact may be "light" while majority of the weight of the aircraft is still held by the wings and that may not create enough torque to get them rolling right away. $\endgroup$ – Brick Sep 12 '18 at 17:06
  • $\begingroup$ Sorry but I'm not totally convinced about your argomentation. If I do a jump with by bike, for example, when the front wheel touches the ground it starts spinning instantly and doesn't skid on the ground. You can see on Youtube a lot of people who jump on bike on slow motion and you will notice that the wheel starts spinning from the first instant. Obviusly this happens only if the ground is rough enough. $\endgroup$ – Landau Sep 13 '18 at 13:03
  • $\begingroup$ The masses and speeds between your bike example and the aircraft are vastly different. I don't think you can draw intuition between them. Also the bike comes down with its full force immediately, so its tires get maximum torque almost immediately. The aircraft glides down, supporting part of its weight on its wings, but may hit "hard" or graze for a moment causing different dynamics on the tires and different torques. Finally, in a bike jump, the wheels are spinning while the bike is in the air because they where spinning before the jump and their angular momentum tends to keep them spinning. $\endgroup$ – Brick Sep 13 '18 at 14:57
  • $\begingroup$ My previous comment is not meant to endorse the idea that the bike wheel does spin instantly - it also has inertia - but it might seem so to you because its inertial is less. (Also, if it was already spinning some because the wheel was in contact with the ground right before the jump, the inertia will tend to keep it spinning and therefore there would be less skid.) $\endgroup$ – Brick Sep 22 '18 at 2:42

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