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Many years ago I had an interesting discussion with a friend (while drinking). We were discussing what would happen if a helicopter was placed into a sphere of some size (slightly larger than the helicopter, a few times larger than the helicopter, etc.). Assume the sphere is filled with air at standard temperature and pressure, and there is standard 1G gravity pulling in one direction.

Can the helicopter maintain flight?

I believe it might, for a very short time, until the air starts circulating in a (relatively) stable manner, causing the "natural flow" of air to be already descending when it encounters the rotor disc, therefore reducing lift to nearly nothing.

I'm curious what an expert might think. If this is off-topic, mark it as such and I'll delete the question. Adding more useful tags I don't know about might also be helpful. :)

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    $\begingroup$ I suppose this might also apply to a helicopter trying to fly within a hanger. $\endgroup$ – Steve Dec 17 '15 at 18:08
  • $\begingroup$ No, it won't fly unless the sphere is so large that recirculation of the air is not a problem since, as you rightly say, it will fly for a short time until the air above the rotor is already descending and therefore, the angle of attack is reduced such that not enough lift to maintain flight is produced. I don't have the maths required to figure out how big the sphere must be for a given helicopter but some masochist might posit one to gain a few points :) $\endgroup$ – Simon Dec 17 '15 at 18:24
  • $\begingroup$ The other question remains, would you starve the pilot and engines of breathable/combustable oxygen before the airflow patters became an issue? $\endgroup$ – Dave Dec 17 '15 at 18:25
  • $\begingroup$ @Simon: So the answer is the rotor blades will stall? $\endgroup$ – mins Dec 17 '15 at 18:30
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    $\begingroup$ Speaking of treadmill debates: the Mythbuster episode 'Birds in a truck' is quite relevant. I think they tested it with a drone, and the result is yes, you can fly something in an enclosed space if it's relatively large compared to the thing you're flying. Furthermore, the weight of this space+heli will not change when it takes off due to the pressure differential you create. $\endgroup$ – Sanchises Dec 17 '15 at 19:24
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A helicopter develops lift by accelerating air downwards through its rotors. This relies on the air above the rotors being slow enough for the rotors to accelerate that air enough to provide sufficient lift. When hovering in still air, the incoming air has zero velocity relative to the helicopter. While of course the air is conserved, in a large volume of air, the downwash is able to diffuse much more, and much of it does not immediately circulate back up above the helicopter.

If the helicopter were enclosed in a sphere, the smaller the sphere, the less the air would be able to diffuse. The downwash would circulate back to the top of the sphere, where the rotors would take that air in again. The rotors would be less effective at accelerating the air that is coming in with significant velocity, just as a propeller develops less thrust with increasing forward speed. Successful hover would depend on the helicopter being able to develop lift at a rate of circulation where the air can slow down enough for the rotors to be effective.

This seems like a experiment manageable enough that someone could try it at home...

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  • $\begingroup$ So this comment is correct? $\endgroup$ – user12485 Dec 18 '15 at 22:09
  • $\begingroup$ @Thesis yes, I would also add Simon's qualification of unless the sphere is sufficiently large that recirculation does not occur. There will be some sphere large enough that the losses equal the energy added by the helicopter. $\endgroup$ – fooot Dec 18 '15 at 22:12
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A serious danger to helicopter flight is flying in a vortex ring state, causing a stall.

Essentially, the helicopter descends into its own downwash. When the condition arises, increasing the rotor power merely feeds the vortex motion without generating additional lift

Several helicopters have crashed as a result.

This state would be very quickly established for a helicopter in a confined container.

enter image description here

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  • $\begingroup$ I think I disagree. In vortex ring state, the helicopter descends into it's own downwash which reduces the velocity of the air accelerated downwards, causes an upflow through the hub and therefore reduces lift as the blade stalls from the root. Recirculation is a different phenomena. With recirculation, the problem is that the inflow from above accelerates downwards. The incipient condition of VRS is an upflow through the hub. $\endgroup$ – Simon Dec 17 '15 at 21:03
  • $\begingroup$ Think of it a different way, VRS can ONLY happen when the helicopter descends. Recirculation can, and does, occur in descent, climb or hover, e.g. when hovering near a building. $\endgroup$ – Simon Dec 17 '15 at 21:05
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    $\begingroup$ I'd love to accept this answer, because it makes sense (maybe confirmation bias on my part? :) and because of the cool graphic (attribution?). I'm going to let this one ride for a day or two, though, to see if anything else interesting is posted. Also, hopefully my question doesn't get closed - 2 votes at the moment! heh... $\endgroup$ – Steve Dec 17 '15 at 23:49
  • $\begingroup$ @Steve: the image was released into the Public Domain by Xmnemonic (James Cho) according to WikiMedia. $\endgroup$ – RedGrittyBrick Dec 17 '15 at 23:57
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Indoor model helicopters would not be able to fly if the answer were no.

Of course it can, but the enclosure must be big or small enough to avoid a toroidal vortex from developing around the rotor. To summarize my answer, three conditions make it possible:

  1. A very big enclosure which allows enough forward speed so the helicopter never flies in its own wake,
  2. a very low ceiling or
  3. a very narrow enclosure.

Case 1 is trivial, so I dwell here on case 2: If you have flown an indoor model helicopter, you probably know that it is not advisable to come too close to the ceiling: The helicopter will become unstable and will be sucked right into it. This is caused by the ground effect (or better ceiling effect in this case): The efficiency of the blades increases the closer they are to a horizontal surface and the helicopter needs less torque for the same lift. Once the rotor head is touching the ceiling, you need to cut power almost to zero to free it again. The same effect can now be used to fly in the constrained space because it will prevent the toroidal vortex of the vortex ring state from developing.

Now the explanation to case 3: If you insist on not touching any surface, the vortex will soon develop and reduce lift. How long you can hover depends on the air volume, and there is a minimum when the diameter of the enclosure is approximately twice the rotor diameter. Once the diameter becomes smaller, the vortex will again be inhibited, and when the enclosure is small enough the rotor will look like a fan in a duct. Now again less energy is needed for lift - just enough to maintain the necessary pressure difference through the rotor disc.

I should add that this solution will best work with a co-axial rotor and no tail sticking out at one end.

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  • $\begingroup$ Interesting. I never considered the 3rd case where the helicopter essentially behaves as a pump. Yes it should work because we know some axial compressor designs work (with some leakage). The narrowness essentially stops recirculation and the disc essentially becomes a very inefficient piston. :D $\endgroup$ – slebetman May 10 '16 at 4:44

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