After yesterdays XB-53 free flight tunnel test model post, I looked a little into how a “free flight” wind tunnel is done. The 12-foot Low Speed Tunnel at NASA-Langley (and before at NACA-Langley… built in 1938) turns out to be a remarkably clever structure, and a love-lived one. The tunnel section itself is relatively short and stubby, and is mounted on a pivot that allows it to pitch up and down. In order to have a proper tunnel air recirculation system, the short length of tunnel is mounted at the center of a sphere. The result is that no matter how the test section is tilted, the flow path for the air recirculatiuon remains identical… without having to actually move the whole structure.
By tilting the test section, the test engineers can turn the downward force of gravity into, essentially, a partially forward force of thrust. There’s no such thing as an object that will hover without either bouyancy (like a balloon) or lift generated by either aerodynamic forces or thrust from lift-jets or the like. So a model airplane, in order to fly, needs to have lift. Placed in a wind tunnel, that lift is generated by air flowing over the wings. But if the wind tunnel is horizontal and the model is not constrained in some way (like a kite), then the wind will pick it up, to be sure, but will also blow it backwards. As the model is blown along, it will of course lose speed with respect to the wind, and thus lose lift and drop back down. But if the wind tunnel is inclined at the right angle, then the model will be in a constant state of trying to glide forward and downward… and if its downward glide angle is the same as the angle of the tunnel… it’ll hover. You just need to tinker with the angle to get it right.
Exterior photo:
Cutaway art:
And here’s a YouTube video (embedding probably disabled, but click on through to it) that shows a scale cutaway model of the structure. Presumably at one point there was, or was supposed to be, audio, but it’s silent. There are several minutes of static shots, but eventually it does get around to showing how the test section pitches up and down. I especially liked the very last scene… apparently in 1939 engineers not only had to wear suits every damned day at pretty much all times no matter what silly-assed thing they were doing (like flying a tiny toy airplane witha stick), but hats, too.
[youtube ousrJAj4itw]
Me, I’d think that today it might be easier and cheaper to just actually *fly* the models than to build this rather sizable structure. But that is due in no small part to the fact that today we have dirt-cheap, lightweight radio control systems. Without such systems, a tunnel like this would seem to be a pretty spiffy way to get long-duration test data for flying scale models.
