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.
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To do this would require some type of motor on each set of wheels.
This would add weight, cost, complexity, maintenance and one more system that could fail and increase risk.
More frequent replacement of tires is simpler, cheaper, lighter and safer.
If the wheels were powered and spinning, they would add to the speed of the aircraft once it hit the ground.
There is no engine attached to the wheels. There is generally no need for additional complexity of an engine to drive the wheels. The landing gear’s main purposes are to absorb the shock of the landing (to mitigate stress on the body of the plane) and to help slow the plane down (using brakes).
The landing gear isn’t used a lot so avoiding friction to the ground and wear on the tires is not a big issue. It would be far more complex, costly and problem causing to try to drive the landing gear wheels than to just replace the tires more often.
Might be able to put some little vanes on them so the airflow as they’re deployed gets them spinning the right direction. That would be one of those fun but expensive research projects that only the government could afford to do.
The wheels have to stop eventually…. and there’s going to be a crapload of friction regardless of if they’re already spinning or not.
Here’s a video of airplane brakes being tested:
There’s a loooooooooot of engineering that goes into airplane landing gear….. like there are people that wrote entire PhD dissertations on the subject…..
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.
I’ve read that in the days of barnstorming the pilots would sometimes touch their wheels to the roof of a barn to get them spinning before landing, supposedly to make a smoother landing, though it was possibly just a stunt.
As others have said the motors required would add extra weight, however there would also be no benefit. When landing you’re trying to get rid of energy. Brakes and reverse thrust are the most obvious ways of doing this, but the friction of the tyres, the energy of the wheels spinning up and the impact on to the runway absorbed by the suspension all take energy away from the forward momentum slowing the aircraft
Putting motors on the wheels would just be problematic. Unless the wheels were spinning at exactly the right speed. The sudden and inconsistent acceleration of the wheels hitting the ground would very likely overwhelm any motor and cause it to either stall, become damaged, or at the very least, act either as brakes, if the wheels are too slow, or if the wheels are too fast, thrust, which may destabilize the plane, if the motor was powerful enough (but more realistically it would stall the motor).
All these problems are way to complicated and expensive to deal with, when you could just replace tires every now and then and not have to maintain a far more expensive and complicated motor, which would be used for only one purpose, to reduce tire wear.
Note: this is all speculation; I don’t actually know anything about plane engineering.
I always wondered why not add wind vanes to the wheels to make the wheels turn in forward direction. No motors or anything needed.
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