NASA and the French aerospace agency have conducted research into magnetic suspension and balancing systems (MSBS) to suspend models during wind tunnel tests, like this system in South Korea.

  1. Where did the NASA and ONERA research lead?
  2. If MSBS reduces interference from the springs and arms that usually hold models, why don't all wind tunnels use this technology? What are the disadvantages?

Also, can somebody create a wind-tunnel or wind-tunnel-testing tag?

  • $\begingroup$ Not too sure, but I think magnets can't hold the models steady enough compared to springs and arms. Might also have to do with weight considerations. $\endgroup$ – Nzall Jun 6 '15 at 22:27
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    $\begingroup$ Interesting concept! I would expect that controlling the strength and shape of the magnetic field must allow rapid changes, and historically such control was completely impossible. I would be interested to see the specs of the converters driving those magnets. $\endgroup$ – Peter Kämpf Jun 7 '15 at 0:09
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    $\begingroup$ According to this SAE paper, "previous attempts at magnetic levitation within the aerospace community have been restricted to small scale due to the large magnetic air gap required between the model and the tunnel walls." $\endgroup$ – mins Jun 7 '15 at 20:27
  • $\begingroup$ I think there are many reasons. First you cannot suspend any object with static magnets(There is a proved theorem for this) as it won't be stable. So complex electronics is needed. Magnetic force belongs on distance with square so very powerful magnets will be needed if model is not very close to the tunnel wall. $\endgroup$ – Andrius Jun 8 '15 at 8:06
  • $\begingroup$ @PeterKämpf: Serious question, if we can (almost) land rockets back on earth, why can't we control the electromagnets of a magnetic wind tunnel? $\endgroup$ – techSultan Jun 8 '15 at 13:47

Two reasons:

  1. When a model is suspended in a magnetic field, and then the wind tunnel is switched on, you'll see a change in the force along 'y-axis' (The lift forces) and change in force along 'x-axis' (The drag forces). Hence, you'll need a complex mechanism for sensing this forces, and changing the magnetic field in such a way that the change in forces is compensated by the 'change in magnetic field' so that the model does not translate in any of these directions and stays static. Then you'd measure the change in magnetic field to measure the forces, and hence the system becomes expensive and cumbersome.
  2. For pressure measurements, you'd have to use wireless sensors on the pitot ports, as you dont want tubes dangling form the suspended models. This makes the process more cumbersome and expensive

This is why the magnetic suspension is not used every where, as the cost to accuracy ratio in normal wind tunnels is much less than the ratio in magnetically suspended models of magnetic wind tunnels

  • $\begingroup$ Wouldn't the advantages of better model control during testing outweigh the initial expenses? On a related note, what are the disadvantages of traditional wind tunnel setups? $\endgroup$ – techSultan Jun 8 '15 at 13:54
  • $\begingroup$ @techSultan Who says the model control would be better? We have extremely good model control with the current mounts. Much better than with magnetic, I would guess, since it doesn't have to deal with the problem of the model moving when the wind stream changes. In particular, I would think that precise rotational control would be a pain with magnetic levitation, whereas it's easy with the current mechanical mounts. Also, we don't have to worry about the model flying off at Mach 2 and breaking stuff (or people) if (when) something goes wrong. $\endgroup$ – reirab Jun 8 '15 at 19:49

At least in the case of the U.S., almost all of our wind tunnels were build decades ago before the technology was available to do this. I used to work at the USAF base that has most of the US wind tunnels. Most of them are quite old. One of them was actually taken from NAZI Germany after the war, though I think that one is mothballed.

As far as the reasons they aren't widely used, these come to mind:

  1. It's a lot more complicated to design and build. In engineering, you normally choose the less complicated solution if it's good enough. Apparently the existing ones are good enough because we've been using them quite effectively for several decades.

  2. I'm sure it would require an enormous magnetic field, as those models can be quite large (several meters) and, thus, quite heavy. Given all of the very sensitive sensors in the tunnels, it would likely be a pain to keep the field from interfering with the rest of the electronics. This is especially true in the engine testing tunnels, since the engines themselves are full of electronics nowadays. Almost everything in modern jet engines is electronically controlled.

  • $\begingroup$ Hmm, I hadn't considered the magnetic interference in sensors. You say most wind tunnels were built decades ago. I assume that means no solid state electronics. What if we built an MSBS-controlled wind tunnel with shielded wires and solid state electronics. Would magnetic interference still be an issue? $\endgroup$ – techSultan Jun 9 '15 at 19:49
  • $\begingroup$ @techSultan I can't really speak to any specifics, especially since I can't remember how much of that stuff is classified, but I can't imagine that an enormous magnetic field in the test article area would be very helpful to the electronics. $\endgroup$ – reirab Jun 9 '15 at 20:24
  • $\begingroup$ It all goes back to classification. I guess I'll just ask my school's wind tunnel people. Thanks! $\endgroup$ – techSultan Jun 9 '15 at 20:29

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