A bit of physics

[llondel]

When I came down to do my shift with the puppies (and work, of course), my wife had the TV on, one of the Mission Impossible films. It was near the end and they managed to get a helicopter flying along in a tunnel behind a high speed train. I failed to suspend my disbelief enough and started wondering how well a helicopter can fly in an enclosed space with limited headroom, quite assuming the pilot had enough skill not to collide with anything, and how the airflow would work in a tunnel. Then add in the wake vortex of the train to disturb the stability of the system...

I also wondered about the size of tunnel required to fit the rotor blades.

[FD2]

That's an interesting question llondel - neatly phrased.

I'm a bit busy for the next week or two but if you ask on TOP you will have about 100 replies and the same number of theories, as well as two or three arguments, followed by personal insults and abuse.

[TheGreenGoblin]

Fools rush in where angels fear to tread but what the hell, here is a first pass at this nifty thought experiment.

I would suggest that the key to answering whether or not a helicopter would fly and be controllable in the pipe, or circular tunnel, is the diameter of the pipe. Too small and the helicopter's rotors simply won't fit. A just fit would just result in turbulence under the aircraft, with possible destructive resonance as well, and little or no recirculation beyond the tips which would impede the the downwards flow severely, restricting recirculation of the downwash, making control difficult, if not impossible, and impeding lift despite the ground effect. A recipe for disaster in other words.

A larger pipe diameter, would result in improving circulation, which if increased would lead to a point where the aircraft would fly and if increased to a certain width greater than the diameter of the rotors would result in enhanced or optimal circulation and more clearance and aircraft controllability. After that stage, and if increased further to a diameter approaching infinity, the helicopter would effectively be hovering stationary or flying forwards, out of ground effect,with all the usual parameters and aerodynamic considerations pertaining to the kind of fight regime being undertaken.

I have not seen the film but would, knowing the wild imaginations that make these films, would suggest that what was depicted was highly unlikely to be emulated successfully in the real world. CGI makes the impossible possible.

[Capetonian]

I know nothing about flying helicopters and very little about physics but but I once briefly met Marc Wolff who apparently is one of the world's foremost helicopter stunt pilots and stunt scene coordinators.

Marc has managed to turn his flying experience in Vietnam into a successful career of flying for films – you’ve probably seen his work in everything from the Star Wars films to the latest James Bond, SKYFALL, and if you’re watching the opening ceremonies of the Olympics this summer, you’ll catch his involvement in that, too.

https://en.m.wikipedia.org/wiki/Marc_Wolff

It may be possible to contact him through LinkedIn or something and ask the question directly.

I hope that is of some help

[Boac]

CGI makes the impossible possible.

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