I used to watch a lot of airplane landings and take-off. My favourite moment is when airplane tyres touch the ground. Friction produces a lot of heat and hence, smoke.
But i wonder, why not pilot start rotating wheels before landing and match the speed of wheels such that it doesn’t cause any friction.
In: 111
There are a few factors, particularly with respect to larger jet aircraft, and without regard for bad weather, etc.; if ignoring reverse thrust, you’re going to be converting all of that energy into heat at some point.
An aircraft has two main ways of dumping energy after landing:
* Reverse thrust (which consumes extra fuel, and is a mechanically complex system that can fail, possibly causing asymmetric thrust, and will almost never be used alone). Typically < 50% as effective as forward thrust, as it’s difficult to fully reverse the bypass airflow of a turbofan engine while maintaining normal engine operation (the airflow is generally splayed out a bit, and more diffuse than normal).
* Brakes, where they are converting their kinetic energy into heat. These can take full advantage of the weight of the aircraft pressing the tires to the pavement for maximum effect; most of the effort to stop an airplane will almost always be through the brakes.
Also, the tire/wheel assemblies don’t weigh very much, and will spin up from friction very quickly. They are sacrificial, and meant to be easily replaced after aggressive braking (along with the brakes, to a lesser degree).
A slight tangent, the landing typically needs to be fairly firm to e.g. compress the struts enough to trigger sensors that tell the plane it has landed (which among other things tells the plane to activate the automatic brakes, if armed), and to make sure that all main wheels actually do spin up at the same time, for even braking action.
The pilot will be on the brakes, possibly using autobrakes, (typically) as soon as possible after landing anyhow – combined with the firm landing (the time between the tires touching, and airplane rebounding slightly is long enough for a good bit of (edit: extra, basically for free) braking action to occur prior to the roll ), a small bit of tire rubber smokes off as the tires get dragged up to speed. It really looks a lot more violent than it is, as tire rubber smoke is exceptionally dirty.
edit: as a side note, they do actually have a related but nearly opposite problem that is solved through various means – after takeoff, you need to stop the wheels from moving, lest the ~200mph equivalent rotating masses come apart inside of the wheel wells. The nose wheel generally has no brakes and will probably use a snubber which halts the tire rotation via friction against a rubber pad, but the main gear will likely apply some brake pressure to achieve the same.
edit2: to answer the original question more concisely – once an airplane is on the ground, your primary goal is almost always to slow the plane down as quickly as reasonably possible without causing excessive wear, and conventional brakes + reverse thrust are very effective in this respect. Some (very small) amount of effectiveness would be lost by adding extra energy in the form of tire rotation.
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