What would a giant vortex cannon aimed upwards do to a helicopter passing overhead?
I imagine it would be nothing short of epic, but could someone explain the (theoretical) physics behind it please?
Oh, to clarify, I'm asking not from an anarchistic curiosity, but rather I wondered what would happen with the constant downward thrust of the helicopter if it was interrupted with a sudden burst of thrust in the opposite direction.
I don't think it would do anything significant. The vortex turns the wrong way. If it were to hit the helicopter exactly right, perhaps the helicopter would lose a tiny bit of lift for a short while, when the vortex disturbs the air flow at the tips of the rotors.
That is, assuming the vortex has the speed to make it up to the helicopter, against the current of air caused by the helicopter.
The air has a density of 1.2 kg/m^3. A helicopter weighs 750 kg at least. The surface area of the helicopter blades is pi * length of blades ^2 = about 78 m^2.
So the helicopter produces 94 kg of lift per meter it moves the air downward.
750/94 = 7.98 meters per second.
The air moves down 8 meters every second. Many vortex cannons wouldn't be able to move against that fast enough.
So, sadly, no effect.
Send your ideas to hollywood anyway. It would look awesome in Hollywood Physics.
About your edit: the vortex cannon would only affect the helicopter if it did something with the blades. Anything that happens below the blades doesn't matter to the helicopter. The vortex sent out by a vortex cannon has air moving towards or away from the center of a circle. Helicopter blades push air vertical to the circle, and in the direction of movement. Therefore, the two barely interact.
Only near the tips of the blades, where currents of air act weirdly due to the sudden ending of the blades, is there any current in the direction in which the vortex acts. And there is where there would be some effect. Probably a bad effect, since helicopters are designed to be most efficient without a vortex cannon disturbing the air currents.
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