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Smart or Shape-memory materials have been known for quite long. What factors/limitations are put on it for applications in aircraft?

For someone who doesn't know much about its material properties, it seems quite good to have deforming wing or other parts and seem to improve aircraft performance. (One example might be: suitable wing geometry deformation according to aircraft's speed regime)

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  • $\begingroup$ As stated by @PeterKämpf, SMAs are only one of many so-termed smart materials (a soft definition in any case). So perhaps you should remove the smart part from your question, as it will be too broad otherwise. $\endgroup$ – yankeekilo Dec 22 '14 at 7:40
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Shape-memory alloys are only a small part of smart materials. Once deformed, they will change back to their old shape when heated or subjected to strong magnetic fields. Of the many alloys with this characteristic, only a few like nickel-titanium would be suitable for primary structures. They are extremely expensive to make and to process, so they have never been an economical alternative.

Also, heating and cooling would be required to switch between shapes, which is very energy intensive when the outer structure of an aircraft is involved. Airflow will ensure intense convective heat transfer, and to maintain the needed temperature will be very uneconomical. Please consider that an aircraft with shape-memory alloys would need to carry extra fuel for heating or cooling over much of the flight time. Hinges and electric or hydraulic actuators will have a clear mass advantage when you consider the system mass, including energy sources.

I guess you agree that creating strong magentical fields in parts of an aircraft is an exceedingly bad idea, so I will rule out magnetic shape-memory alloys.

Shape-memory alloys will only switch between two shapes, and intermediate shapes are easier to realize with hinges and actuators. Sure, a smooth shape is aerodynamically better than one with a contour break at the hinge line, but the small aerodynamic advantage of shape-memory structures is quickly lost when you look at the whole picture.

Another disadvantage is the accumulation of small cracks with every shape change. The low number of cycles will either require frequent replacement of the shape-memory parts or a very limited structural life of the aircraft.

In the end, much of what can be said about other smart materials also applies to shape-memory alloys. Their main function is to justify research grants and to keep the press occupied.

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    $\begingroup$ I vaguely remember hearing somewhere that shape-memory alloys are being used for things like pipe fittings in some aircraft. They create a tight fitting from such alloy, stretch it out, put the pipe in and then make it remember its contracted shape (I think it was stretched at liquid nitrogen temperature or something like that) and it seals tight around the pipe and for a joint that can't shake loose like screwed fitting could. But that's one-time use for manufacturing purpose only and for small parts where the durability is worth the extra cost. $\endgroup$ – Jan Hudec Dec 21 '14 at 22:33
  • $\begingroup$ @JanHudec: Interesting and well-conceived application of shape-memory alloys. But wouldn't heating the fitting for assembly give the same effect due to thermal expansion? It will need much more heat, though. $\endgroup$ – Peter Kämpf Dec 21 '14 at 22:38
  • $\begingroup$ I heard it long ago and don't remember much details. I think the point was that it shrinked so much that grooves in the fitting cut into the pipe and that probably needs larger difference in size than is possible with heating without reaching temperature where the material would loose strength. $\endgroup$ – Jan Hudec Dec 21 '14 at 22:40
  • $\begingroup$ I also remember it was some fighter, so they wanted to make the thing extra durable and especially easy to maintain, so it warranted even the complicated manufacture (involving some liquid nitrogen) and cost. $\endgroup$ – Jan Hudec Dec 21 '14 at 22:43
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SMAs are being used by Boeing in so called chevrons, See this link and this image: Chevrons

The idea is that these chevrons heat up due to the hot flow coming from the engine. This causes them to deform, and bend into the oncoming flow. This causes vortices, which will reduce the engine noise by gradually mixing the hot and cold flows coming from the engine.

It should be noted that this is a very different application than the one OP mentioned, as it is non-critical and not a stress bearing part, however, it does show that SMA's can be used in commerical aircraft

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  • $\begingroup$ Is this a "smart metal" or just bimetallic strips -- like on old thermostats? $\endgroup$ – RoboKaren Jun 18 '15 at 14:51
  • $\begingroup$ I think that the working principle is the same, only that SMA's can be used to deform to a certain shape. As such , to me they are just complex bimetals. $\endgroup$ – ROIMaison Jun 18 '15 at 14:54
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Im not sure this falls under Smart materials but is an interesting side note about shape distortion with heat. The SR-71 stealth plane was subject to such high heat at upper ends of its speed range that the planes panels were actually designed with gaps so that it would not crunch its self to bits when it heated up and expanded. Keeping this in mind you can get an idea of how fast you need to travel to generate enough surface heat to deform the metal substantially (im sure its a bit slower for softer metals).

Back to the topic at hand

Weight is a serious concern as well. Generating heat or substantially cooling a structure at will is no simple task. Such systems would further weigh down an airplane when weight is really the biggest concern in aviation.

Money and well more money is a very real factor in airplane building. FAA type certs are not cheap or easy to come buy and as such commercial aviation tech tends to lag behind what is hot on the market since it takes many years to get an airplane designed, certified and into production. People have been building aluminum airplanes since we had engines powerful enough to make it a reality. Its a light, fairly easy to work with material that has been tried and tested on thousands of airframes flown for millions of hours. We have tooling and shops to make it into what ever we may need. Keep in mind that introducing a new material is not simple. If you would like a current example look at the adoption of carbon fiber which has also been around for some time now and is only just beginning to see use in real commercial aviation on a large scale. Its been used in general aviation for a bit longer since the parts are smaller and a bit simpler to lay up but still more costly than aluminum.

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