Mythbusters tackles the plane/treadmill

[83hurstguy]

I'd like to reiterate this point.... let's say you are standing next to this experiment off the conveyor.

In order for the conveyor to be moving, the plane has to be moving forward relative to you. If the plane is not moving relative to you, then it effectively has 0 speed, and the conveyor would not be moving either. So without forward motion of the aircraft, the conveyor is frozen. The airplane has to fly.

Trying to track the planes speed relative to its wheels creates a redundant question and my brain just starts to hurt

 

[83hurstguy]

oh yeah, one more thing... conveyor becomes negligible once the static friction in the wheel bearings is overcome. If the wheels aren't rolling or sliding, the plane will move with the conveyor belt.

However, once they begin to roll, you'll have a certain frictional drag due to the bearings, but the conveyor becomes negligible.

 

[Pitbozz]

I keep seeing people saying the question doesn't state how the planes speed is being measured and again IT DOESN'T MATTER! Measure it any way you want to and multiply it by 500 and the plane will still go right down the runway/treadmill just like it normally would. Because:

The treadmill is irrelevant.
The treadmill is irrelevant.
The treadmill is irrelevant.
The treadmill is irrelevant.
The treadmill is irrelevant.


Okay....let's try it this way. Forget about 'matching' the speed of the treadmill to the airplane. Set the treadmills speed to 300MPH. This is well above takeoff speed for a normal jet powered plane. Correct? Have the airplane ready for take off, 1 foot from the treadmill, all systems go! Pilot applies thrust as he normally would for a routine takeoff. What happens?

Now again, this scenario is barring wheel/bearing/tire failure. But if we can have a runway length treadmill, we can have bearings and tires that will hold up!

 

[Geoff87]

I say that the plane WILL NOT fly, unless there is a hurricane force headwind to induce lift on the wings.

I wonder what would happen if I ran a racecar on a dyno to 200 MPH, then pulled on the parachute?......

 

[imjoesnuffy]

The treadmill in this scenario is the ground.
If what the earth was doing underneath the plane mattered at all there would be a flight instrument for it. Find one for me.

An altimeter not only tells you the distance from the ground. It can also tell you how fast the aircraft is approaching the ground. Probably the most important cockpit instrument there is in my opinion.


I understand what most are arguing in the pro argument. Thrust will over come the speed of the treadmill, creating forward motion. The treadmill in turn would speed up negating the effect of thrust.

Thrust alone will pull SOME air across the wings, it would take an enormous amount of thrust, spread out over an enormous airsurface for this to occur.

 

[Pitbozz]

An altimeter not only tells you the distance from the ground. It can also tell you how fast the aircraft is approaching the ground. Probably the most important cockpit instrument there is in my opinion.


I understand what most are arguing in the pro argument. Thrust will over come the speed of the treadmill, creating forward motion. The treadmill in turn would speed up negating the effect of thrust.

Thrust alone will pull SOME air across the wings, it would take an enormous amount of thrust, spread out over an enormous airsurface for this to occur.

It will take ever so slightly (a fractional amount) more thrust than what would be needed for a normal take off from a normal runway. The resistance from the treadmill would be miniscule at best. In other words:

The treadmill doesn't matter.
The treadmill doesn't matter.
The treadmill doesn't matter.
The treadmill doesn't matter.
The treadmill doesn't matter.


HA!!!! Threw ya'll off that time!

 

[87GNSteve]

So according to the it will fly folks, please contact the US Navy so they can install conveyors onto the back of Cruisers and Destroyers so that they have the abilty to carry and launch F-22 Raptors.
I mean whats the sense of having a large air craft carrier/(floating air strip) that is such a great torpedo target?
The rocket/rollerskate analogy is completely off base because that is a completely different form of propulsion.
Simple-- for a fixed wing aircraft to take off it is required to propel foward to reach a physical speed to generate lift(take off speed, and it varies from design to design). If the conveyor is EXACTLY MATCHING in the opposite direction there will be no foward progress thus no take off speed achieved.

 

[Pitbozz]

So according to the it will fly folks, please contact the US Navy so they can install conveyors onto the back of Cruisers and Destroyers so that they have the abilty to carry and launch F-22 Raptors.
I mean whats the sense of having a large air craft carrier/(floating air strip) that is such a great torpedo target?
The rocket/rollerskate analogy is completely off base because that is a completely different form of propulsion.
Simple-- for a fixed wing aircraft to take off it is required to propel foward to reach a physical speed to generate lift(take off speed, and it varies from design to design). If the conveyor is EXACTLY MATCHING in the opposite direction there will be no foward progress thus no take off speed achieved.

Wow. Just...wow.

Please tell us why the plane will remain stationary and NOT accelerate down the runway/treadmill like it normally would?

 

[Geoff87]

And tell them (the Navy) that they no longer need the catapults, nor do they need to turn into the wind anymore either...

 

[87GNSteve]

Wow. Just...wow.

Please tell us why the plane will remain stationary and NOT accelerate down the runway/treadmill like it normally would?

Im glad your impressed!

This experiment would be very hard to carry out.
But to explain:
Lets say the conveyor control is computer controlled and hooked into the plane's TPS w/ 3 second initial delay (to get the wheels rolling) and as the throttles are opened then the plane will atempt to move further, but is counter acted by the conveyor. So if it cannot make forward progress it cannot generate air flow over and under the wings.
If it is done precisley the plane will be screaming at max power while staying in the same spot.

Still dont believe me think about calling the Navy.:wink:

 

[Pitbozz]

Im glad your impressed!

This experiment would be very hard to carry out.
But to explain:
Lets say the conveyor control is computer controlled and hooked into the plane's TPS w/ 3 second initial delay (to get the wheels rolling) and as the throttles are opened then the plane will atempt to move further, but is counter acted by the conveyor. So if it cannot make forward progress it cannot generate air flow over and under the wings.
If it is done precisley the plane will be screaming at max power while staying in the same spot.

Still dont believe me think about calling the Navy.:wink:

Dude...there is so much flawed logic in your post, I truly am impressed now. Why won't the plane roll out and take off like it would on a normal runway? There is ABSOLUTELY NOTHING holding the plane in place. The only thing that would occur is the wheels would be spinning twice as fast before lift off. You do understand that a plane is not wheel driven, right? And the rollerblade scenario is EXACTLY the same principle.

 

[jpratt]

Im glad your impressed!

This experiment would be very hard to carry out.
But to explain:
Lets say the conveyor control is computer controlled and hooked into the plane's TPS w/ 3 second initial delay (to get the wheels rolling) and as the throttles are opened then the plane will atempt to move further, but is counter acted by the conveyor. So if it cannot make forward progress it cannot generate air flow over and under the wings.
If it is done precisley the plane will be screaming at max power while staying in the same spot.

Still dont believe me think about calling the Navy.:wink:

The treadmill counteracts the planes movement but it does not stop it. The thrust push the plane forward the treadmill does not hold it back it merely spins the planes wheels. The plane still moves forward and will take off.

 

[Pitbozz]

And tell them (the Navy) that they no longer need the catapults, nor do they need to turn into the wind anymore either...

Okay. Right back at ya'. If the treadmill did in fact prevent the planes from moving or taking off, then running them in the SAME direction would shorten the take off time by a remarkable distance, right? Why aren't there treadmills on a carrier instead of catapaults and turning into the wind?

 

[Pitbozz]

Im glad your impressed!

This experiment would be very hard to carry out.
But to explain:
Lets say the conveyor control is computer controlled and hooked into the plane's TPS w/ 3 second initial delay (to get the wheels rolling) and as the throttles are opened then the plane will atempt to move further, but is counter acted by the conveyor. So if it cannot make forward progress it cannot generate air flow over and under the wings.
If it is done precisley the plane will be screaming at max power while staying in the same spot.

Still dont believe me think about calling the Navy.:wink:

And the experiment is not very hard at all to carry out.

 

[jpratt]

Dude...there is so much flawed logic in your post, I truly am impressed now. Why won't the plane roll out and take off like it would on a normal runway? There is ABSOLUTELY NOTHING holding the plane in place. The only thing that would occur is the wheels would be spinning twice as fast before lift off. You do understand that a plane is not wheel driven, right? And the rollerblade scenario is EXACTLY the same principle.

This is kinda like an illusion. Some people see it as magic and then others SEE what really happened.

It will fly, oh by the way, IS THE TREADMILL RELEVANT?

 

[Pitbozz]

This is kinda like an illusion. Some people see it as magic and then others SEE what really happened.

It will fly, oh by the way, IS THE TREADMILL RELEVANT?

The treadmill is irrelevant.
The treadmill is irrelevant.
The treadmill is irrelevant.
The treadmill is irrelevant.
The treadmill is irrelevant.


LOLOLOLOLOLOL!:biggrin:

 

[87GNSteve]

Ok guys whatever you say,
BTW the Pentagon opens at 0730 EST let me know what they say when you tell them of this new development.

 

[Pitbozz]

Ok guys whatever you say,
BTW the Pentagon opens at 0730 EST let me know what they say when you tell them of this new development.

I don't know how else to tell ya' man. The plane will take off just like it normally would. Re-read the posts here about why it will move and take off just like it would on a normal runway. Maybe it will click. If it does click, come back and save a little face.

 

[87GNSteve]

What others have to say.....

The question is slightly unclear: if the conveyor "tracks the plane's speed" and keeps it exactly zero, then obviously, no, the plane would not take off as it would be stationary with respect to the air. I don't see what could possibly be difficult to understand about that.

The way the question is worded though, it could mean to say that the plane moves forward at 150kts, while the conveyor belt moves backwards at 150kts, for a rotational speed of 300kts for the wheels. In that case, as long as the wheels don't burn off, the plane would take off.

Perhaps the wording is all that is causing the problem there...



Looking only at friction, and assuming the aircraft could take off if unhindered the cases are as follows:

Case 1: If the treadmill is frictionless and the wheel bearings have any amount of friction, the treadmill will move (in the direction of flight), the wheels will not turn, and the plane will lift off.

Case 2: If the treadmill has any friction and the wheel bearings do not, the treadmill will not turn, the wheels will turn, and the plane will lift off as if on a conventional runway.

Case 3: If the treadmill and wheel bearings have infinite friction, the aircraft will sit there under maximum power and not move, unless maximum power overcomes the friction between the wheel and the treadmill, then all hell breaks loose.

Case 4: If the treadmill and wheel bearings both have some friction, but not enough to stop the plane from reaching takeoff velocity, either the treadmill or wheels will turn, or both the treadmill and wheels will turn, and the plane will take off (albeit with a longer takeoff roll than in Cases 1 & 2)

Case 4 is what actually happens and has a few subcases to it:

Case 4a: There is a threshold of treadmill friction where, if the wheel bearings have a much smaller friction, the wheels will turn and the treadmill will not.

Case 4b: Conversely, it's possible for the wheel bearings to have a relatively high friction compared to the treadmill where the treadmill turns and the wheels do not.

Case 4c: It is also possible for combinations of treadmill/wheel bearing friction to have both the treadmill turning AND the wheels turning during the takeoff roll.

 

[Geoff87]

Why aren't there treadmills on a carrier instead of catapaults and turning into the wind?

Ummm.. I don't know. Maybe catapults are more EFFICIENT than a treadmill would ever be, and by turning into the wind, MAXIMUM AIRFLOW can be achieved....