Muscle wires actuators are really simple, have a high power density and are reliable. Landing gears on other hand are used only twice per flight while taking off and landing (if everything goes well though). During flight these machines are not useful and there are numerous gear up landings because of landing gear failures.

What is so complex in landing gears?

Why are electric motors/hydraulic actuators used instead of muscle wire actuators in landing gear which are simple, light weight, reliable and cheap? (If there are electric power requirements, that can be achieved by supercapacitors.)

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    $\begingroup$ You may provide some links or reading about muscle wire actuator as I'm not sure how familiar is the aviation.SE community with this kind of actuator and how it is operated. Moreover, you may restrict your question to one kind of aircraft as many GA aircrafts are equipped with fixed landing gear $\endgroup$
    – Manu H
    Apr 20 '19 at 19:55
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    $\begingroup$ > my preferred tech is 100% reliable > all others are not $\endgroup$ Apr 21 '19 at 16:47
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    $\begingroup$ "There are numerous gear up landings". Source? For an airliner, a gear up liner usually means to write the aircraft off. We don't see many of those, by any means. $\endgroup$ Apr 21 '19 at 18:10
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    $\begingroup$ "Why are electric motors/hydraulic actuators used instead of muscle wire actuators in" X could be asked about any field that uses electric motors or hydraulic actuators for any limited range movement - from small printers to earth moving equipment. If muscle wire are ever going to replace current devices in a wide range of fields, I don't expect planes to be among the first instances. $\endgroup$
    – Pere
    Apr 21 '19 at 19:15
  • $\begingroup$ Can you provide any large-scale industrial application of muscle wires at all? To me it appears that muscle wire technology is pretty much useless outside of toys and experiments so far. $\endgroup$
    – Agent_L
    Apr 22 '19 at 11:09

Shape-memory alloys change their shape with temperature, with control usually achieved with electric heating in one direction, and back with convective heat transfer to the environment. There is one specific temperature where the change in shape occurs. There are two huge problems with this:

  1. The operating temperature range for gear actuators is huge. It may easily be below -40°C on a cold arctic night, but on a hot tropical day the sun can easily heat the structure to upward of +60°C. That means the transformation temperature must be pretty high, but that means a lot of energy has to be put to heating it, especially if the ambient temperature happens to be low on the day.

    Note that the actuators are not very energy efficient. A super-capacitor can give you high peak power if you need that, but will not help at all with the total energy. The work the actuator has to do is significant, so the efficiency matters.

  2. Worse, a shape-memory alloy actuator requires constant power to remain at one end of the motion range and spontaneously returns to the other. However, gear must be stable in both end positions without requiring energy, so there would still have to be uplocks and separate actuator for extension or retraction (or maybe extension would be gravity-only, but then you have to ensure it gets reliably locked). Not really simple anymore.

I also don't think they actually scale to the required forces and displacements well. A 2 mm wire bends quite easily, 2 cm rod, not so much. The thicker will of course need more force to bend—which is OK—but it will also break much sooner, because the difference in strain is bigger.

  • $\begingroup$ Point 2 might be removed if engine waste-heat is used. $\endgroup$ Apr 21 '19 at 13:24
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    $\begingroup$ @leftaroundabout Using engine heat at the gear seems to fly in the face of 'light, reliable' mechanisms like the question asker seems to be seeking. $\endgroup$
    – Saiboogu
    Apr 21 '19 at 15:38
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    $\begingroup$ @Saiboogu for a jet plane – definitely. For a GA craft with a single piston engine, this wouldn't require much more than a bit of clever exhaust piping though. $\endgroup$ Apr 21 '19 at 15:53
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    $\begingroup$ @leftaroundabout So if the engine fails, you wouldn't be able to extend landing gear? That's obviously a bad idea. $\endgroup$
    – StephenS
    Apr 21 '19 at 16:03
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    $\begingroup$ @leftaroundabout That could equally be as bad. As a pilot, I want the gear out when I tell it to go out. Just because my engine dies doesn't mean I want a huge drag-inducing thing hanging out of my plane. Let me pick the landing spot, glide there, and extend the gear when I'm assured to make the landing spot. $\endgroup$
    – Ron Beyer
    Apr 21 '19 at 16:13

The weight/complexity of modern landing gear is not in the extension mechanism, that is relatively simple (a hydraulic cylinder typically).

The complexity of the gear comes from the robustness of the design.

enter image description here
Source: Flyertalk

(Not the greatest picture), but the retraction mechanism there is the simplest part of the gear. The complex part is the linkages/hydraulic dampeners (shock absorbers), brake systems, etc. The structure needs to be there because you have a (in the case of an A380) a 1,234,000lb (600 tons) aircraft slamming into the pavement at about 300 feet/min.

There needs to be a hydraulic system for the brakes, so they already have hydraulic systems there. The other nice thing about hydraulics is that they can be extended manually in the event of an electrical failure.

So what would you really gain by introducing a complex system like muscle wires, given the additional failure modes? For example lets say your gear bay is really cold from your trans-atlantic flight at 45,000 feet and your electrical system fails? Is your Ram-Air turbine and batteries going to be able to supply enough current to keep the system extended, or retracted if need be? You don't want your gear hanging out when you are trying to make the airport in a glide.

Hydraulics are used because they are proven cheap, reliable, robust, and easy to work on. Many aircraft systems are hydraulic (including control surfaces) for exactly that reason, as well as muscle wire only being around for a short time.

That isn't to say that shape-memory alloys don't have an application in the aviation world. Many aviation companies are looking at using SMA's in applications such as engine control and wing shape shifting applications, but they are a long way from being used in practice.


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