Free-fall Autorotate?

[7777]

Looks like a job for Mythbusters.

 

[sardaddy]

I lost faith in Mythbusters when it comes to anything aviation related when they tried to prove whether an airplane would take off if it was on a treadmill going the same speed and in theory keeping the aircraft stationary.

The answer is obviously no as it doesn't matter how fast the wheels are turning, it matters how fast the plane is moving so that lift is created over the wings. If the plane is stationary, there is no lift. Anyway, they declared it plausible despite a poorly designed test bed.

So if they can't do that simple test, I would hate to see what they would do with this.

 

[imacdog]

Some RC heli guys are doing blade-stop autorotations now. If they wait too long to get the blades going after they stop, and it doesn't take much, the heli will tumble out of control and then it's all over.

 

[phr8dawg]

In defense of Mythbusters, that prop-driven "jetpack" very nearly left the ground. Maybe if they inhaled enough helium......

 

[VNugget]

I lost faith in Mythbusters when it comes to anything aviation related when they tried to prove whether an airplane would take off if it was on a treadmill going the same speed and in theory keeping the aircraft stationary.

The answer is obviously no as it doesn't matter how fast the wheels are turning, it matters how fast the plane is moving so that lift is created over the wings. If the plane is stationary, there is no lift. Anyway, they declared it plausible despite a poorly designed test bed.

So if they can't do that simple test, I would hate to see what they would do with this.

Of course the airplane wouldn't fly without airspeed. Nobody but a moron would argue otherwise. But the whole point is that the treadmill can't stop the plane from moving.

What test bed would you propose?

It's a flawed question.

 

[sardaddy]

What test bed would you propose?

It's a flawed question.

Here is the question posed in one spot:

"A plane is standing on a large treadmill or conveyor belt. The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"

Here it is posed a bit differently somewhere else:

"Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"


You are absolutely right it is a flawed question and one that has two answers depending exactly how one looks at it. If the wheels and the treadmill are going the exact speed as posed in the first part of the question, then the answer is no it cannot takeoff, as the plane is obviously not moving forward. But if the question is "can the plane overcome the force of the treadmill, move forward and take off," the answer is yes because the plane's thrust can easily push past the minimal friction caused by the treadmill on the wheels since the wheels don't drive the airplane. That is why an airplane can take of on ice easily. But for this to be true the wheels on the aircraft are moving faster than the treadmill (or else there is no forward motion) so the wheels and the treadmill are no longer equal.

That is why I lost faith in their experiment. They only looked at one point of view despite the fact that the question was flawed.

Now back to our physics/bad guy defying question.

 

[VNugget]

Now back to our physics/bad guy defying question.

Allright, anybody wanna design and build a rotor-wing glider? Paging Otto Sikorsky...

Just make sure they stay in high tow...

 

[Andy Neill]

Allright, anybody wanna design and build a rotor-wing glider? Paging Otto Sikorsky...

Just make sure they stay in high tow...

Isn't that what a propless autogyro is?
http://www.youtube.com/watch?v=gP6VnbeWXSY

 

[nosehair]

The rotor blades would cone up and break before they got up enough speed to stay rigid...

 

[sardaddy]

I thought about that but the rotor blades normally cone when a large collective load is applied. If the aircraft is flat pitch, I agree that they will cone up but depending on the rotor system, fully articulated vs rigid vs whatever else, they may not cone to the point where they would break since they are in free fall and not working to lift the helicopter.

There is no science to these ramblings by the way, just thoughts.

 

[nosehair]

they may not cone to the point where they would break since they are in free fall and not working to lift the helicopter.

Probably true at the beginning when the blades are not turning, but when the start is attempted and the blades begin to turn and lift....

I think they would reach the same point of coning as they tun up as we know they will when turning down...

 

[Andy Neill]

Probably true at the beginning when the blades are not turning, but when the start is attempted and the blades begin to turn and lift....

I think they would reach the same point of coning as they tun up as we know they will when turning down...

nosehair,

I think you are giving too much credit for the amount of lift going on at flat pitch. The only portion of the baldes that will have positive pitch will be very near the root with the vast majority of the disc being driven with negative pitch due to blade twist.

I'm staying in the non cone-heads camp.

 

[sardaddy]

Rotors cone when they are spooling down because there is usually and armpit full of collective applied. I want to verify you mean coning and not drooping on that point.

The rotors don't cone in an autorotation even when the blades do spool down to about 1/3 of their normal rotation speeds if the operator screws it up. So, I don't think the dynamics applied at the bottom of a maneuver can be directly applied to the dynamics at the top but it is a good discussion.

 

[flyjetspeed]

The biggest problem with getting the rotor head moving would be stability. As soon as you went inverted everything that was working for you is now working against you. So you would need a way to keep the aircraft upright for the time needed to begin spooling up the rotor system. I would suggest a detachable cable from the rotor hub with a large parachute system like the old mercury capsules had and spin her up at flat pitch until in the green arc, then jettison. After that it's a walk in the park.

 

[Andy Neill]

I would suggest a detachable cable from the rotor hub with a large parachute system like the old mercury capsules had and spin her up at flat pitch until in the green arc, then jettison. After that it's a walk in the park.

I like it!

 

[phr8dawg]

I'll ride shotgun.