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When an aircraft lands with the tires of the landing gear at rest, as soon as the the aircraft touches down, the landing gear experiences a large amount of wheel-spin up forces (because of the static friction involved between the stationary tires and the relatively moving runway at a very fast speed), which even cause it to vibrate at some frequency. This force is considered to be highly undesirable.

Are airplane manufacturers using any kind of measures to reduce these forces on the landing gear at touchdown? How are they compensated?

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  • $\begingroup$ Edited to remove ambiguity. Thanx for pointing out. $\endgroup$
    – Victor Juliet
    Commented Jul 7, 2015 at 11:40
  • $\begingroup$ There is a thread on motor-driven gear wheels around, but I can´t find it right away. $\endgroup$
    – yankeekilo
    Commented Jul 7, 2015 at 11:41
  • $\begingroup$ My friend is working on motor driven gear wheels, which causes significant reduction in loads on the landing gear while touchdown. But I was curious to know if this has been implemented already $\endgroup$
    – Victor Juliet
    Commented Jul 7, 2015 at 11:42
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    $\begingroup$ Nice - I guess your question differs quite a bit with regard to the motivation for such a thing. Let´s see if someone can contribute... $\endgroup$
    – yankeekilo
    Commented Jul 7, 2015 at 11:48
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    $\begingroup$ Related topic: aviation.stackexchange.com/questions/3702/… $\endgroup$
    – ROIMaison
    Commented Jul 7, 2015 at 14:00

2 Answers 2

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There are lots of forces acting on the gear at the point of landing. The best way to minimize loads on the gear is to land smooth (I know that sounds obvious).

For what it's worth you do not necessarily want to spin up the gear before landing for a few reasons. First off you would need to spin up the gear to precisely (or at least very close to) the speed that the plane was going to touch down at. If you spin the gear up faster you actually will create a problem by accelerating the plane when it touches down. Spinning the gear up can also have a serious gyroscopic effect (since you now have a fast rotating mass) (see here as well) that will affect the handling of the aircraft.

On top of that remember that in aviation it's often about cost. What are the costs to maintain a system that pre-spins the gear vs the cost of tires. Some times its actually cheaper and easier to make a part that wears out some what quickly and replace it often, then design a complex system that require maintenance and up keep.

It appears that some devices for spin up have been looked into but the rotational inertia problem seems to be more of a concern than wheel wear. I would imagine that unintentional side loads on gear are a bigger issue and bigger cause of wear on the landing assembly. Keep in mind that the addition of an assembly like this also adds weight to the aircraft, and no one likes weight that is not needed.

This is a great research paper on the topic that suggests (in their conclusion) a pre spin situation would see 1.07% of the wear of a static situation. They go on to state that that they used a simple linear approximation and only accounted for forces in one direction but it would seem that it is beneficial for the tire.

On a side note this article even suggests the tires can be remolded so even if the are replaced often it seems they are recycled a few times.

an A340 will complete roughly 6,000 take-offs and landings, calling for about 25 sets of replacement tires, each 1.30 m high and weighing about 200 kg. The tires can be remolded several times before they are finally scrapped.

In general, it's the job of the pilot to reduce the forces on the landing gear when landing. One of the best descriptions I have ever heard of landing is that the goal is not to land but to "transition". In other words what you really want to do is transition the load from the wings to the landing gear as smooth as possible, this will reduce the load on the gear. In some cases (short runway heavy plane) your landing may see more force on the gear. I have never flown a 737, but I can tell you that I can put a Piper Warrior down on a 7000-foot strip with out ever feeling that you have touched the ground.

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  • $\begingroup$ ... aka a "greased landing" or "greaser". The primary maneuver used to "transition" from the glide slope to touchdown is called a "flare", and it can be done with a slight pitch-up, a little more engine power, or both, the idea being to curve your flight path upward from the glide slope to level flight, just as your wheels touch, then power to idle, set the nose gear down, thrust reversers or reverse prop pitch (if equipped) coupled with wheel brakes to slow to taxi speeds, retract all control surfaces, and clear the runway. Easy. $\endgroup$
    – KeithS
    Commented Jul 7, 2015 at 14:57
  • $\begingroup$ Comprehensive, but the paragraphs that relate to tire-maintenance and pilotage are a bit side-topic. The question is more focused on the design of airplanes, which can be expanded in this answer a bit more. $\endgroup$
    – Gürkan Çetin
    Commented Jul 7, 2015 at 15:46
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    $\begingroup$ Claiming that you'd need to spin the wheels up to exactly the right speed is an example of the nirvana fallacy: you're rejecting a potential improvement because it doesn't completely eliminate the problem it tries to solve. The acceleration to the plane from hitting the ground with tyres rotating too fast would be negligible. Spinning the wheels at anything greater than zero and less than twice the correct speed would decrease tyre wear from skidding. So we must (and you do!) look for other reasons to reject the idea. $\endgroup$
    – David Richerby
    Commented Feb 7, 2017 at 14:17
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The second part of the question is simpler. Aircraft are designed to handle all operational loads (=limit loads) as defined in the FAR or other regulations. Plus they have reserve strength requirements for larger loads (1.5 times or 1.25 times the limit loads). This latter loads are the ultimate loads, which the aircraft are usually required to stay-together for about 3 seconds.

Reducing wheel spin-up forces is desired, but it is not a winner of the trade-off against the complexity that arises from adding additional systems to spin the wheels up, before touchdown. It takes tremendous efforts to add new functions to existing, proven systems (landing gears in this case), and to prove that they will not fail under normal and foreseen failure conditions. If they will fail, then the structure will neglect the existence of such systems and the benefits of reducing loads would be gone.

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