Question posed on Facebook

[StarCruiser]

The lift would have to be in a sealed tube to have a vacuum above it and be sealed around it to that tube. Like a cruiser bonnet, the area would be a low pressure zone wit considerable turbulence.
I think the helicopter could take off but it would have a bit of a rough time doing it.

 

[Chris]

Strewth I'd thought we'd done with this one. The lift might create an area of low pressure above it but certainly not a vacuum. I agree.

Helicopter rotors are wings and work in the same ay by creating a difference in pressure over their surface. Check. However, rotary wing aircraft, at low altitude produce something called ground effect where they essentially ride on a cushion of air that is created by their own downdraught but this is at only low altitude. Helicopters have a limit on what they can lift and that includes their own weight as part of the payload. If the helo is dropping due to gravity and weighs say 5 tonnes, then before it could even hover, it has to be able to overcome the inertia of falling first and that would be huge given gravity's effect.

The answer to whether it COULD is yes, but the answer to whether it WOULD, depends on how much the blades are able to lift. Ie it's a maths calculation for that particular helicopter.

I think Nick has it. Airwolf definitely could but not in stealth mode.

 

[frank rabbets]

I'll settle for low pressure. Does any travelling object have a vacuum behind it ? eg a rifle bullet. I know those buses don't because the cyclists riding close behind don't wear oxygen masks.

 

[moggy1968]

Helicopters are more susceptible to low pressure than fixed wing, hence their lower operating altitudes so it is possible the lift would reduce air pressure enough above it to cause problems with lift from the rotors. It doesn’t have to be a vacuum to make take off impossible

 

[Towpack]

The height of the rotor would also come into play here. The low pressure area would probably be below it but, whatever, there’s going to be an awful lot of turbulence.

 

[Chris]

Low pressure yes, but a vacuum not really. You get an area of low pressure hence being able to slipstream a bus on your peddley bike. In water you get cavitation where a prop sins fast. I've never quite understood that. But no you don't get a vacuum behind a bullet.

 

[Towpack]

Cavitation is just gas (air) bubbles forming in the low pressure area in the water round the prop, similar to the low pressure on the top of an aircraft wing. When you open a bottle of fizzy drink the gas held in the liquid by pressure forms bubbles when the pressure is released. Cavituon gradually eats away at the prop surface when the bubbles collapse but can be minimised by good prop design.

 

[Chris]

Cheers TP. I always wondered where the 'non' water came from.

 

[Shayne]

I don't mind admitting the original question in this thread bugged me because i couldn't think of a practical way
to test the theory , but on about my 10th pint last night the fog in my head cleared and i thought of standing with a toy plane in my hand circling to gather speed for take off on a loop runway surrounding me spinning in the opposite direction at any speed you like .

No point in sharing my thought process as the question is answered but that's how my mind works . I really should get around to building my perpetual motion machine one day