180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?

[Lawdogg]

I thought this discussion died long ago when several of us stopped the treadmill long enough to board the plane, restarted the treadmill, fired up the plane, started to move forward due to its thrust and free spinning wheels, and then got airborne with ease.

[RandyN]

It is amazing how people think that the contact of wheels and a treadmill have an effect on air speed.

It is quite simple:


The plane just needs the required thrust to overcome the friction of the treadmill and wheels. If the plane required 80 knots of airspeed, it does not matter if the wheels traveled 200 mph in the opposite direction.

Here is a simple question.

Would the same plane take off the treadmill if it was in a wind tunnel?

If you say yes, then you have proven my point.

As a matter fact, the instance the plane's airfoil generates lift, the friction of its wheels decreases as the plane increases lift.

Wake up.....wheel speed has nothing to do with flight.

PS for all you R/C people. Take your Flight sim(G-3 or whatever), set the wind speed to 100 mph. Pick the F-16 turbine, put the flaps down and practice flyng and landing backwards relative to ground speed.

[Lawdogg]

[quote=RandyN;1185689]It is amazing how people think that the contact of wheels and a treadmill have an effect on air speed.

Wake up.....wheel speed has nothing to do with flight.

QUOTE]

Randy, I guess we'll just have to accept the flat earth members on this Forum too.

[RocketSled]

See? We *are* all talking about the same thing now... the fundamental question is whether or not the plane will move forward on the treadmill. I say that in a "theoretical" sense (and this *is* a thought experiment, after all) with a treadmill capable of achieving a sufficiently high speed, the plane will not be able to move forward and so will not fly.

Now, here's my logic:

The wheels of the plane have friction with the treadmill; we all agree on that. There are two types of friction we can discuss, static and rolling. It's easy to comprehend how, if the pilot stood on the brake pedal the engines would need to overcome a heck of a lot more friction in order to move the plane forward on the treadmill. But the key is, friction is friction. It's only a matter of degree. The plane moves forward on the treadmill and the friction at the wheels (brakes on or not) dissipates some of the power being produced by the engines (that there is a fair amount of power going to the wheels in spite of the fact that they're not mechanically driven should be apparent to anyone who's ever seen an airplane tire smoke or catch fire on a take off roll. The heat is generated by friction, sure, but the power that turns that friction in to heat comes from the airplane's engines, there is no other source). It doesn't matter that the "transmission" is the air being pushed by the Prop, rather than a geartrain being turned by a driveshaft. There is friction, there is motion, there is power coming from the engines being lost as heat at the wheels. The laws of thermodynamics demand nothing more or less. Energy cannot be created or destroyed, only redistributed.

Remember that power is force exerted over time, the power dissipated at the wheels of the airplane is a function of how fast those wheels are turning. The faster the treadmill goes, the more power is dissipated per unit time at the wheels. But the power being produced by the airplane's engines stays fixed per unit time (assuming the pilot goes to WOT and stays there). As a result, assuming the treadmill can go fast enough, the power generated by the airplanes engines will be totally consumed at the wheels and the plane will not be able to move forward. Again, the laws of thermodynamics demand this. There is but one source of power, the airplane's engines and it doesn't matter where that power is "consumed", once it's all being consumed it's "game over".

I agree, this might mean the treadmill has to run at a million miles per hour to consume enough power to overcome the engines. And in a practical sense I agree that this is not possible. But, as a thought experiment, it certainly is. No matter how high or low the friction between the wheels and the treadmill, assuming no upper limit to the speed of the treadmill, the treadmill will be able to go fast enough to "keep ahead" of the power coming from the airplanes engines and hold the airplane stationary and thereby prevent it's taking flight.

In reality, the power dissipated at the wheels is so small compared to the power generated by the engines that no treadmill would be capable of going fast enough to matter to any appreciable degree. What's more, if you *could* dissipate the engine's power at the wheels, they'd get so hot the plane would burst in the flames and probably explode, thereby getting "airborne" in at least some sense, for at least a short while...

[11sec_lx]

Wow!

[TechSTL]

Sounds like we need the original wording of riddle. The plane would have to move forward relative to its starting position. IIRC, the riddle says that the plane remains stationary.

Since the friction of the wheels on the surface is irrelevant. How about taking that model airplane and putting it on the ground. Put a board in front of the plane so it can have no forward motion. Try to take off.

[Kens06]

OK, Here I go!!

We have a Ginormus powered treadmill with a 747 sitting on it.
The 747 has numerous optical sensors being monitored by a super computer!!

The computer senses ANY forward movement of the 747 and tells the treadmill to counteract this by moving in the opposite direction turning the wheels to negate movement!!

Much like the 'brake torque' many loaded planes do before rolloff!!
Thrust does not create forward motion in and of itself!!

The treadmill acts as the infinite 'brake torque' and the vehicle never achieves forward motion i.e. zero lift and NO FLY!!

Gravity plays a major role here!!
We aren't talking about a Harrier using vectored thrust....

OK, kill me for keeping this going.................
Ken

[No Doubt]

Perhaps I was not clear enough in my (many) explanations. This answers the question and in addition addresses the notion proposed by RockeSled that friction will play a role in preventing the aircraft from taking off...

"At first, the conveyor will hold the plane still. But only to a certain point, after which, driven by thrust from its engines, the craft will accelerate. The key is in the behavior of friction. Friction is a peculiar force in that it has an upper limit. For instance, push an object on your desk, but not hard enough to move it. Why doesn't it move? Because the friction force exactly balances the force of your push. At some point you push hard enough to set the object in motion. This is the point where friction has topped out and is not capable of growing any larger.

With the airplane and treadmill, there is, at the outset, friction force capable of rotating the tires at the proper speed to keep the plane stationary. However, as the thrust is increased, that force eventually maxes out. (Two separate frictions are at play here, actually, one between the tires and belt, the other between the plane's axles/bearings and its wheels. The first will max out before the second.)

And at that point the wheels no longer roll, they slide, or rather, they roll and slide at the same time. Tire motion is now decoupled from the belt motion. No matter how much you whiz up the treadmill, you won't add any more rotational velocity to the wheels because friction is already doing everything it is capable of. The plane skids toward takeoff -- likely accompanied by much smoke and a powerful rubbery stink."

Dr. Paul J. Camp
Department of Physics
Spelman College

Edit:

Sounds like we need the original wording of riddle.

As you wish. Below is the text taken from the original post on this forum.

Link:

http://www.z06vette.com/forums/showthread.php?p=1016587

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not?