# Helicopter on a turntable

yeah this probably sounds somewhat familiar.

If you perfectly centered (so that it doesn’t just get thrown off from centrifugal force) a helicopter on a rotating turntable, and the turntable was rotating at the same speed as the helicopters blades, in the same direction, would it take off?

You sick, sick puppy. You’ve created a monster, you realise…
Now, lessee…

I think the first problem is that the centre of gravity of the chopper would not be the same as the hub of the rotors, due to the big, heavy tail assembly sticking out one side. So that might mess things up somewhat.

Then there is the fact that the tail rotor isn’t spinning (I assume), so you won’t get anything like stable flight.
If we ignore all of these problems, my WAG is that a high enough rotational speed would allow the rotors to lift the helicopter off the turntable, but as soon as it did so it would lose rotatioonal speed and come back down. You might get a couple of feet altitude if you were lucky.

In the real world, the thing will spin out very quickly, making an expensive mess.

The rotors are spinning under their own power too?
Then baring problems of centring, I think it’d take off twice as quickly if the turntable was spinning in the same direction as the rotors. Spinning against them, on the other hand… I suppose it’d just sit there.

Oh yeah, now that you mention it, I hadn’t thought that through well enough. Yeah lets assume its spinning in the opposite direction of the turntable.

Would it just sit there though? I have a sneaking suspicion theres some counter-intuitive logic here very similar to the plane treadmill…

If the rotor blades are stationary with respect to the ground, they aren’t moving any air down, so there is no lift.

Also, the tail rotor is working at regular speed and the torque will probably rip the tail off.

Yep, sorry to say there’s no controversy here. The blades would remain still and the helicopter would probably rip to shreds, but no chance of take off.

[QUOTE=Suda]
yeah this probably sounds somewhat familiar.
I’ll bite. The helicopter will NOT take off. It WILL wobble badly from an out of balance fuselage. It does not matter if the rotor is spinning under power or not, the body will still be rotating with the turntable. A helicopter fuselage is not balanced to spin, more to weather vane in foward flight. I’d say a helicopter on a turntable is doomed to tip over, rotor strike either the tail boom or ground, and beat it’s self to death.

Various Googling suggests that helicopter main rotor RPM is typically 200 to 400. It should be obvious that no real helicopter could deal with that - and the occupants certainly couldn’t.

Assuming the turntable has sufficient torque to overpower the tail rotor, which will be fighting the turntable, the rotor blades will not be moving relative to the surrounding air and will not produce downward thrust. A helicopter can fly normally because only the air is resisting the tail rotor’s torque, nothing is neutralizing or overpowering it in the opposite direction.

The CG is in the rotor’s thrust centerline in stable flight. The cyclic pitch is adjusted to make that happen. Not an issue here.

Oh, lawd, I guess we should have seen this sequal coming. First airplanes on treadmills, now this. What’s next? Aircraft carriers travelling faster than their catapaults?
Somebody pull Cecil’s face out of the eggnog. I think we need him for this one.

At least this would be simple <?> enough to do with a remote-control “toy” helicopter.

yeah surely people have those around! and a record player which I’d think would be relatively fast enough for the purposes of this experiment.

Wheres Scylla?

Pogo stick on a trampoline.

He’s all wet.

The lifting force is created by the relative movement of the main rotor blades with the air.
Everything else is irrelevant in terms of how much upward force is created.
When the lifting force exceeds the weight of the helicopter it will lift off.

As a practical matter, spinning the body of the helicopter itself relative to the air will make it very unstable and also create a great deal of turbulent air, decreasing the efficiency of the rotors. It would wobble and crash immediately.

There is no particular complexity to trying to figure out artificial scenarios around anything below the blades, so to speak–spinning helicopter; counter-rotating turntable–whatever. The lift is created by the relative movement of the blades through the air. Whatever is going on below that may contribute to unstable flight or inefficient lift, but it’s not some sort of tricky deal to decide whether or not the thing will lift off. When the speed of the rotor relative to the air reaches whatever speed it needs to be when the helicopter body is stationary, the helicopter will lift off. Obviously if you are artificially turning the body, the relative speed of the rotor and the body will vary to get the same rotor/air takeoff spin rate.

IIRC this was pretty thoroughly understood thus the reason why carriers turn into the wind. Launching a plane off the back of a moving carrier, especially without wind assistance = large probablility of big expensive splash.

Maybe, I don’t know - I guess that would result in a very ugly crash. But if the turntable was rotating in the opposite direction it probably wouldn’t take off. This is all assuming the forces involved and the erratic winds won’t smash the helicopter to the ground or rip it apart in the first place.

And even if the turntable has sufficient torque, the next question is whether the friction between the turntable and helicopter is enough to counteract the force of the tail rotor. I suspect it doesn’t.

But what if the helicopter is on casters?

What if you put one of these planes on a treadmill and put a helicopter on the plane’s disc thingy, and tried to take both off at the same time??

–FCOD