Helicopter on a turntable

[Dave-ROR]

Originally Posted by HunterGreene

[VIDEO]http://youtu.be/YORCk1BN7QY[/VIDEO]

/argument

I assume that bad link is mythbusters? I was going to let it continue for a bit before providing video evidence lol

[Rayme]

The helicopter will take off. I love these riddles.

On the same subject...

A closed box the size of a fridge weights 5 lbs. On a scale.

If you put a flying bird inside it..would the scale still register 5 lbs?

[Dave-ROR]

Originally Posted by ottopilot

The helicopter will take off. I love these riddles.

On the same subject...

A closed box the size of a fridge weights 5 lbs. On a scale.

If you put a flying bird inside it..would the scale still register 5 lbs?

The copter one isn't as obvious to me. If we are measuring rotor RPM via the shaft anyways. If we are measuring it compared to a fixed point then sure. If the object generating rotation is spinning at 5 RPM and spins the rotor to 5RPM (which is zero based on the airframe) I'd see the rotor just not spinning at all, relative to the air around it. If the shaft RPM is 0, it spins with the airframe (Assuming it's locked) backwards at 5RPM. In this case the rotation is in the same plane, unlike the airplane riddle.

If on the other hand the 5RPM is measured compared to the ground, then it's spinning at 5RPM anyways.

Oh and I'm assuming a skid based copter, a wheel based one would take off just fine for the same reasons as the plane, assuming the front wheel was turned 90 degrees

That image was a joke on the airplane riddle and likely was never a riddle at all based on the lack of information to make an informed analysis.

[ichitaka05]

...this get me thinking.

Just say airplane jet engines were pushing the airplane 100mph (I know it can do faster, but just for the "example" sake) and treadmill is going 100mph. & I know airplane tires don't do anything w pushing the airplane forward... but if airplane is stuck at same pot (cuz 100mph jet speed vs 100mph treadmill speed = no moving forward nor backward) they don't have enough air ressistance to lift itself up.

So unless airplane jet engine can produce more power to move forward than treadmill, can't lift.

Is that make sense? Or my sense is wrong?

[neutron256]

Originally Posted by ottopilot

The helicopter will take off. I love these riddles.

On the same subject...

A closed box the size of a fridge weights 5 lbs. On a scale.

If you put a flying bird inside it..would the scale still register 5 lbs?

Explain your reasoning why the helicopter will take off.

As for the bird it does increase the weight because it needs to provide downforce equal to its weight to keep itself flying. It may cause relative fluctuations in the measured weight as it flys.

[wbradley]

The helicopter only lifts if there is significant rotor speed relative to the air (or ground for that matter) in this case, so it cannot lift off.

The airplane lifts if the airspeed over the wings is sufficient to create a lower pressure over the wing than under it. If the plane is pushed forward by my hand or thrust forward by motorized wheels or a spinning prop is irrelevant. What is relavant is airspeed over the wings. If th airplane remains stationary on the treadmill there is no airspeed and no lift. There is significant airspeed behind the propeller caused by the push of the angled blades, but that air over the lift surface is unlikely to suffice if the aircraft itself is stationary.
The plane is pushed forward by the prop, and countered equally and opposite by the belt under the wheel. It doesnt matter if thrust is from the prop or motorized wheels.

Now if the plane thrust is greater than the reverse thrust of the treadmill and creates sufficient net forward thrust to achieve V1 airspeed then the plane can rotate and lift. Ditto with the ratio for turntable to rotor speed.

Note: there is friction in the axles of the plane so it must thrust the propeller to remain stationary on the moving treadmill. As long as it remains stationary (and cant vector the air flow off the prop over the wings) there can be no lift. The reason Myth Busters showed the plane eventually take off is because it's virtually impossible to regulate the treadmill speed to exactly counter the forward speed generated by the prop's thrust. Also, there is no treadmill that can achieve OR regulate speeds equal to that generated by an airplane's prop.

[GTB/ZR-1]

Much like helicopter blades, my head spins @ these riddles... Lol

[ichitaka05]

As for helicopter goes, I think it'll fly. Here's my reason. On helicopter's tail part, there's another propel to resist the rotation of the main propel. It may not fully resist the turntable, but prob resist enough to fly up

[Dave-ROR]

Originally Posted by ichitaka05

...this get me thinking.

Just say airplane jet engines were pushing the airplane 100mph (I know it can do faster, but just for the "example" sake) and treadmill is going 100mph. & I know airplane tires don't do anything w pushing the airplane forward... but if airplane is stuck at same pot (cuz 100mph jet speed vs 100mph treadmill speed = no moving forward nor backward) they don't have enough air ressistance to lift itself up.

So unless airplane jet engine can produce more power to move forward than treadmill, can't lift.

Is that make sense? Or my sense is wrong?

If the airplane can take off at 100MPH, it takes off. It's not stuck at the same spot, the wheels are free to move as they wish. An airplane that takes off at 100MPH with zero wind has a wheelspeed (assuming the ground isn't moving) of 100MPH, with a 30mph headwind it takes off at 70MPH ground speed (100MPH relative airspeed) while the wheelspeed is 70MPH. That's an easy way to figure out that the wheelspeed is meaningless.

The airplane thing confuses people because people think of automobiles, which drive the tires. Airplanes don't drive the wheels/tires, they exist to reduce friction, etc but not to move the airplane.

[neutron256]

Originally Posted by ichitaka05

...this get me thinking.

Just say airplane jet engines were pushing the airplane 100mph (I know it can do faster, but just for the "example" sake) and treadmill is going 100mph. & I know airplane tires don't do anything w pushing the airplane forward... but if airplane is stuck at same pot (cuz 100mph jet speed vs 100mph treadmill speed = no moving forward nor backward) they don't have enough air ressistance to lift itself up.

So unless airplane jet engine can produce more power to move forward than treadmill, can't lift.

Is that make sense? Or my sense is wrong?

Your assuming the resistance to the airplanes motion is the ground (treadmill). The majority of resistance is aerodynamic drag not friction from the ground. The only additional resistance caused by the treadmill would be the energy needed to spin the wheels at 200mph instead of 100mph.

[Dave-ROR]

Originally Posted by ichitaka05

As for helicopter goes, I think it'll fly. Here's my reason. On helicopter's tail part, there's another propel to resist the rotation of the main propel. It may not fully resist the turntable, but prob resist enough to fly up

For the sake of the discussion I'm assuming that the copter isn't sliding around on the turntable though.

Regardless the copter will be destroyed well before takeoff RPM anyways lol The friction between the skid and turntable will only hold for so long, then the airframe goes flying off sideways, like any other object placed on a turntable when spun fast enough.

[Dave-ROR]

Originally Posted by wbradley

The helicopter only lifts if there is significant rotor speed relative to the air (or ground for that matter) in this case, so it cannot lift off.

The airplane lifts if the airspeed over the wings is sufficient to create a lower pressure over the wing than under it. If the plane is pushed forward by my hand or thrust forward by motorized wheels or a spinning prop is irrelevant. What is relavant is airspeed over the wings. If th airplane remains stationary on the treadmill there is no airspeed and no lift. There is significant airspeed behind the propeller caused by the push of the angled blades, but that air over the lift surface is unlikely to suffice if the aircraft itself is stationary.
The plane is pushed forward by the prop, and countered equally and opposite by the belt under the wheel. It doesnt matter if thrust is from the prop or motorized wheels.

Now if the plane thrust is greater than the reverse thrust of the props and creates sufficient net forward thrust to achieve V1 airspeed then the plane can rotate and lift. Ditti with the ratio for turntable to rotor speed.

Note: there is friction in the axles of the plane so it must thrust the propeller to remain stationary on the movingtreadmill. As long as it remains stationary (and cant vector the air flow off the prop over the wings) there can be no lift.

But that's the point, it WON'T remain stationary. There is no reason for it to and a spinning prop or turbine as the reason it won't.

As for whether the drive method matters, it absolutely does. In one case it stays stationary (car) and the other case it doesn't (airplane).

If we make it stay stationary then of course it won't fly, even if the treadmill was spinning in the direction we need the plane to go at 1,000,000MPH, the plane still would move 0MPH. (assuming we are forcing it to stay stationary to the ground, not the treadmill).

And of course there would be pull on the airplane from the treadmill, but not that much. Mythbusters could have taken off if that treadmill was moving 4x as fast, it wouldn't have made any difference in the world because the prop still provides more than enough thrust to counter the wheel bearing/tire friction and gain airspeed.

[neutron256]

Originally Posted by ichitaka05

As for helicopter goes, I think it'll fly. Here's my reason. On helicopter's tail part, there's another propel to resist the rotation of the main propel. It may not fully resist the turntable, but prob resist enough to fly up

That would be true if the wheels/skids/whatever landing gear were made so it could rotate freely around the same axis as the main rotor shaft. But I have yet to see that helicopter. If we're bringing the effects of the tail rotor into play its force is dynamic depending on what the pilot is doing with the controls. It could be neutral, work with, or against the motion of the turntable.

[wbradley]

Originally Posted by ichitaka05

As for helicopter goes, I think it'll fly. Here's my reason. On helicopter's tail part, there's another propel to resist the rotation of the main propel. It may not fully resist the turntable, but prob resist enough to fly up

Ah but you forget the tail rotor is designed to counter spin when the chopper is airborne. There would need to be a HUGE powerful tail rotor to counter the friction of the choppers bottom moving against the platter.