Helicopter on a turntable

[Rayme]

Originally Posted by Dave-ROR

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.

I will retract my comment that it will lift. I pictured it wrong on my head lol. The blade would indeed be stationnary and i dont think a spinning fuselage provoques lift..

[wbradley]

Originally Posted by Dave-ROR

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.

Ground speed= air speed of aircraft (pitot tube) + tailwind or -headwind

Air speed= speed of aircraft through the air, which prevents stall. Not relative to the ground but relative to the air.

[wbradley]

Originally Posted by Dave-ROR

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.

wont fly off if perfectly balanced on the platter and accelerated gently. However parts WILL probably fly off.

[Dave-ROR]

Originally Posted by wbradley

Ground speed= air speed of aircraft (pitot tube) + tailwing or -headwind

Air speed= speed of aircraft through the air, which prevents stall. Not relative to the ground but relative to the air.

Of course. Too bad this has nothing to do with the riddle

You are focusing on WHEELSPEED which is the only thing the belt is really affecting. It's not preventing the prop from generating thrust which, unless the airframe is strapped to the stationary ground next to the belt, will move the airframe forward. The discussion on headwinds was just to show that WHEELSPEED is meaningless.

Assuming zero wind, a plane travelling 100MPH on a belt will have an airspeed of 100MPH, a ground speed of 100MPH, and a wheelspeed of 200MPH.

Your entire arguement boils down to this: The friction of the tires against the belt, combined with the wheels against the wheel bearing will exactly counteract the THRUST generated by the prop. That is the ONLY way the aircraft won't move forward.

[Dave-ROR]

Originally Posted by wbradley

wont fly off if perfectly balanced on the platter and accelerated gently. However parts WILL probably fly off.

I didn't realize you could prevent centrifugal force by accelerating a rotating mass slowly................................

The friction of the skids will be overcome, just not as violently.

[MmmHamSandwich]

As far as the airplane it depends. Does the conveyor match the groundspeed, or the wheel speed? If the ground speed everything is fine and the plane takes of relatively normally.

If the conveyor matches the wheel speed then within nanoseconds the conveyor accelerates to ludicrous speed in an exponential attempt to constantly outpace the wheels ultimately causing the bearings to explode and unless the conveyor stops quickly the plane is flung rearwards at horrifying speed and spectators shriek "WHAT HAS SCIENCE DONE!!???!"

[ichitaka05]

Originally Posted by Dave-ROR

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.

I know, tire on the airplane doesn't matter. Only thing matter is airplane jet engine force and air cut through the wing. Which means, lift is generated by the velocity of the flow over the wings.

Edit: I did some google and found this quote and pretty convincing to me.

First, assume you have some wheels of zero mass sitting on the treadmill, connected to frictionless bearings. If you accelerate the treadmill, there is no force on the treadmill from the wheels, and nothing to resist its acceleration. In this scenario, you're right. There's nothing the treadmill can do to keep the airplane from moving. If you add bearing friction, now you've got a constant force from a constant treadmill speed. So far so good. This is the point where you and I both agreed (before I had read the more complex scenario Treis put forward).

Now imagine you replace the massless wheels with big heavy ones. At a constant speed, they exert no force on the treadmill, because they're just spinning around on their frictionless bearings. But if the treadmill accelerates, the heavy wheels will resist the acceleration because of their mass. This couples the axle the wheels are spinning on to the treadmill. If they resist spinning, then the force pulling on them is transmitted to the axle, which is now being accererated in the direction of the treadmill. Once the wheels get up to the new speed, the force stops. But any time there is acceleration from the treadmill, the mass of the wheels couples the airplane to the treadmill to some degree.

So, to keep the airplane from taking off, you need a constant acceleration from the treadmill, so that there's a constant force on the axle opposing thrust. The more thrust that is applied, the more acceleration is needed to keep the airplane from moving.

So if treadmill can produce constant force on the axle opposing the thrust, airplane cannot fly

[wbradley]

Originally Posted by Dave-ROR

Of course. Too bad this has nothing to do with the riddle

You are focusing on WHEELSPEED which is the only thing the belt is really affecting. It's not preventing the prop from generating thrust which, unless the airframe is strapped to the stationary ground next to the belt, will move the airframe forward. The discussion on headwinds was just to show that WHEELSPEED is meaningless.

Assuming zero wind, a plane travelling 100MPH on a belt will have an airspeed of 100MPH, a ground speed of 100MPH, and a wheelspeed of 200MPH.

Your entire arguement boils down to this: The friction of the tires against the belt, combined with the wheels against the wheel bearing will exactly counteract the THRUST generated by the prop. That is the ONLY way the aircraft won't move forward.

Dave, my answer is based on a THEOETICAL situation where the treadmill speed is regulated to oppose the forward speed generated by the thrust of the prop. Also, it is assuming there is some friction in the wheel bearings/axle to create SOME reverse motion of the aircraft from the moving treadmill if the props are stationary. However the reality is the friction of the wheel bearings is relatively low and regulation of the treadmill is near impossible and no treadmill can go fast enough to create enough wheel bearing friction to prevent forward airspeed from building.

In theory I am 100% sure I have the physics right.
In the practical world due to the nature of wheel bearings low friction and treadmills low relative speed the plane can take off. I did see that episode of MB BTW.

If you had a treadmill that was servo controlled for speed regulation and was capable of infinite speed the plane could not take off!

[wbradley]

Originally Posted by Dave-ROR

I didn't realize you could prevent centrifugal force by accelerating a rotating mass slowly................................

The friction of the skids will be overcome, just not as violently.

Newtons law (which #?) states that an object tends to movein at a constant speed/direction unless a force is applied thus causing acceleration.

If a solid object is perfectly centered relative to its weight over the centre of the platter it will stay there if it is gently (not violently) accellerated up to speed. Why? If you accelerate to drastically then rotational acceleration > friction between object and platter. In other words you are exceeding the traction so it will break loose. Now, that isnot to say that while it rotates at massive speed there wouldnt be parts flying off in all directions until the thing rips apart.

A perfectly balance metal ball with decent initial stick to the bottom of the platter should be able to be rotated to huge velocity in a vacuum if it is not accelerated violently. Again, if tracton wasnt a concern then you could accelerate violently. If centripedal force wasnt a concern, we'd use a ball of Jello.

The difference here is some people are answering based on a theoretical situation whereas others are basing it on a practical situation.

Incidently, in that case both are right but in this case I thought it was a theoretical question not a practical Myth Busters experiment.

[Dave-ROR]

Originally Posted by wbradley

Dave, my answer is based on a THEOETICAL situation where the treadmill speed is regulated to oppose the forward speed generated by the thrust of the prop. Also, it is assuming there is some friction in the wheel bearings/axle to create SOME reverse motion of the aircraft from the moving treadmill if the props are stationary. However the reality is the friction of the wheel bearings is relatively low and regulation of the treadmill is near impossible and no treadmill can go fast enough to create enough wheel bearing friction to prevent forward airspeed from building.

In theory I am 100% sure I have the physics right.
In the practical world due to the nature of wheel bearings low friction and treadmills low relative speed the plane can take off. I did see that episode of MB BTW.

If you had a treadmill that was servo controlled for speed regulation and was capable of infinite speed the plane could not take off!

Your treadmill would likely have to be spinning at tens of thousands of times faster than the speed of the plane. The tires would shred. The riddle is that if the plane is generating thrust to travel at 100MPH, and the belt is going 100MPH the other way, will it take off? And the answer is yes, because the friction in question is nothing compared to the thrust created.

You can easily test this with an RC plane. Put a treadmill at the highest speed and it takes almost no throttle to move the plane forward on it, full throttle and it smashes into the treadmill That same minor throttle to overcome the wheel friction would basically be moving on solid ground too.

So let's say a Cessna 172 turning 1,500RPM tevels (on solid ground) 50MPH, on a -50MPH treadmill it might only go 49.5... :shrug:

[neutron256]

Originally Posted by wbradley

Dave, my answer is based on a THEOETICAL situation where the treadmill speed is regulated to oppose the forward speed generated by the thrust of the prop. Also, it is assuming there is some friction in the wheel bearings/axle to create SOME reverse motion of the aircraft from the moving treadmill if the props are stationary. However the reality is the friction of the wheel bearings is relatively low and regulation of the treadmill is near impossible and no treadmill can go fast enough to create enough wheel bearing friction to prevent forward airspeed from building.

In theory I am 100% sure I have the physics right.
In the practical world due to the nature of wheel bearings low friction and treadmills low relative speed the plane can take off. I did see that episode of MB BTW.

If you had a treadmill that was servo controlled for speed regulation and was capable of infinite speed the plane could not take off!

How fast do you imagine this treadmill going to generate enough resistance through the wheels to cancel out the thrust of the prop. Make the treadmill go 1000 mph and I'm sure the wheels will seeze and stop spinning all together.

I can see where you might be able to generate resistance through the wheels to cancel out the thrust of the prop, but the usual version of this myth is the plane needs say 60 mph airspeed to take off and the the belt is going 60 mph in the oppisite direction. In that case the plane will still fly. Again Mythbusters did this.

[Dave-ROR]

Originally Posted by wbradley

Newtons law (which #?) states that an object tends to movein at a constant speed/direction unless a force is applied thus causing acceleration.

If a solid object is perfectly centered relative to its weight over the centre of the platter it will stay there if it is gently (not violently) accellerated up to speed. Why? If you accelerate to drastically then rotational acceleration > friction between object and platter. In other words you are exceeding the traction so it will break loose. Now, that isnot to say that while it rotates at massive speed there wouldnt be parts flying off in all directions until the thing rips apart.

A perfectly balance metal ball with decent initial stick to the bottom of the platter should be able to be rotated to huge velocity in a vacuum if it is not accelerated violently. Again, if tracton wasnt a concern then you could accelerate violently. If centripedal force wasnt a concern, we'd use a ball of Jello.

The difference here is some people are answering based on a theoretical situation whereas others are basing it on a practical situation.

Incidently, in that case both are right but in this case I thought it was a theoretical question not a practical Myth Busters experiment.

Pretty sure a helicopter isn't 1) balanced in that direction and 2) not going to be all centered over the center of rotation. It probably would break apart first though.

[wbradley]

Originally Posted by Dave-ROR

Your treadmill would likely have to be spinning at tens of thousands of times faster than the speed of the plane. The tires would shred. The riddle is that if the plane is generating thrust to travel at 100MPH, and the belt is going 100MPH the other way, will it take off? And the answer is yes, because the friction in question is nothing compared to the thrust created.

You can easily test this with an RC plane. Put a treadmill at the highest speed and it takes almost no throttle to move the plane forward on it, full throttle and it smashes into the treadmill That same minor throttle to overcome the wheel friction would basically be moving on solid ground too.

So let's say a Cessna 172 turning 1,500RPM tevels (on solid ground) 50MPH, on a -50MPH treadmill it might only go 49.5... :shrug:

Ahh, I was basing this on the theoretical, so the "treadmill" has no speed limit. And I am sure that if the net resistance from the axle and the resistance to acceleration due to mass of the spinning wheels equalled say 1% of the thrust, we woild need to travel at 10,000 mph for a 100 mph prop speed.

Nobody is wrong here, the experiment is well known. So are the theories of physics.

Please just admit that in theory I am correct. You are correct in practice however.

Somebody please change this thread from "Helicopter on a Turntable" to "Hotdog Down a Hallway" cuz were gettin nowhere fast here.

[Dave-ROR]

Originally Posted by wbradley

Ahh, I was basing this on the theoretical, so the "treadmill" has no speed limit. And I am sure that if the net resistance from the axle and the resistance to acceleration due to mass of the spinning wheels equalled say 1% of the thrust, we woild need to travel at 10,000 mph for a 100 mph prop speed.

Nobody is wrong here, the experiment is well known. So are the theories of physics.

Please just admit that in theory I am correct. You are correct in practice however.

But we are talking about the riddle, not what's possible outside of the riddle. AFAIK we never left the riddle.

Yes, it's absolutely possible to generate enough resitance to prevent the airplane from moving. You could just bolt it to the ground too Both of those solutions are outside the point of the riddle and (I thought) the conversation.