A jet plane on a large treadmill

[stuckathuntermtn]

Man this is harder than the whole gravity stretches time thing.

[MagnificentUnicorn]

um.....................................
um.....................................
I still can't picture how the plane moves forward relative to the air if the wheels and the treadmill negate each other.
Yeah, I know it's being powered by the jets. So what?
If it were on floats and pointed up river and the river speed matched the speed the floatplane would be going at full power, it wouldn't move relative to the river bank (or the air).

We're doomed.

[Ted Striker]

WHO'S TAKING OFF NOW BITCHES???!!

[NorthernSpotted...]

If the treadmill matches the speed of the plane, the plane takes off. If the treadmill matches the speed of the wheels, the wheel bearings seize before the plane takes off.

It's really a matter of interpretation of the question. The plane has to move forward to generate lift. If the treadmill matches the wheel speed then no lift is generated. If (as in the mythbusters scenario), the treadmill matches the speed of the plane then the plane still moves forward and lift is generated.

Everyone is right, and everyone is wrong.

If we're talking speed relative to the to the stationary ground and - lets just assume - air next the treadmill and and the plane, then the treadmill will only only match wheel speed if the plane is stationary. The second jet thrust is applied and the plane moves forward relative to the ground and air - as it most certainly will - the wheels move faster than the treadmill and lift will be generated.

[Jethro]

I still don't get it. It's wheels are the only part touching the ground. You're basically saying, yet it would take off because it would overpower the treadmill.
I thought we were saying the treadmill matches the wheels speed and negates it. I don't see why it matters where the thrust comes from, as long as it's forward(backward?) and not up(down?).

Think of it like walking up the down elevator. if the airplane was driven by its wheels, you would stay in the same place, just like walking up the down elevator. The confusion comes from the fact that the plane is not driven by it's wheels in contact with the conveyor belt, it gets it propulsion from its jet engines pushing air.
Now if you had a rocket pack on at the bottom of the stairs, it would shoot you up the stairs at a fast rate, regardless of how fast the escalator went down, or how fast your legs ran.

[doughboyshredder]

If we're talking speed relative to the to the stationary ground and - lets just assume - air next the treadmill and and the plane, then the treadmill will only only match wheel speed if the plane is stationary. The second jet thrust is applied and the plane moves forward relative to the ground and air - as it most certainly will - the wheels move faster than the treadmill and lift will be generated.

Yes, but, if we had a magical treadmill with a motor controlled by a pid that is linked to the wheels of the jet, the treadmill would increase in speed sufficiently to keep the jet from moving forward. The jet wheels and treadmill would reach terminal quickly and bearings would seize, shit would go kaboom. LOL.

If the jet is moving forward relative to the ground it will achieve lift at some point.

In the original hypothetical question (not the op's, but the original argument starter posted online) the real question is can the imaginary treadmill be set up in such a way to keep the jet from moving forward by matching the wheel speed caused by the jet's thrust. In my imaginary world the wheel bearings seize, and then depending on the amount of thrust the plane provides either the nose hits the treadmill and the plane disintegrates, or the plane overcomes the drag of the melting tires and takes off.

[doughboyshredder]

Think of it like walking up the down elevator. if the airplane was driven by its wheels, you would stay in the same place, just like walking up the down elevator. The confusion comes from the fact that the plane is not driven by it's wheels in contact with the conveyor belt, it gets it propulsion from its jet engines pushing air.
Now if you had a rocket pack on at the bottom of the stairs, it would shoot you up the stairs at a fast rate, regardless of how fast the escalator went down, or how fast your legs ran.

In your example, the planes wheels would HAVE to spin at a higher speed than the conveyor belt is moving.
The confusion comes from the concept that the conveyor belt could be somehow linked to match the speed of the wheels, therefore never allowing the wheels to spin at a higher speed, causing the bearings to seize.

It doesn't matter where the propulsion is coming from. If the wheels can never move faster than the conveyor belt the plane will not be moving forward relative to the ground. In order to move forward relative to the ground the wheels MUST move faster than the conveyor belt.

This is why the answer to the question depends on whether the conveyor belt/treadmill is moving at the same speed as the plane, or at the same speed as the wheels, and why stuck is so confusered.

[mtngirl79]

I think it would be helpful to get two toy cars.

One regular hot wheels type car, or maybe a toy airplane. That would be better, a toy airplane with free rolling wheels.

Then get one of those toy cars that you roll backwards and then they go forward on their own...

Then take them to treadmill.

The toy airplane, you will be its power, sort of like a real airplanes jet engines. You can put it on the treadmill, and push it forward, against the direction the treadmill is going and the wheels just spin, right? So the toy airplane's speed is decided by how fast you push it (like a real airplane's speed is dependent on how fast its jet engines push it). The wheels are just along for the ride.

Now take your toy car, and roll it backwards to charge it up. It will be powered by its wheels. If you set it on the tread mill, against the flow, it will probably go backwards, or forward pretty slow because its speed is dictated by the speed of its wheels...

See the difference????

[doughboyshredder]

I think it would be helpful to get two toy cars.

One regular hot wheels type car, or maybe a toy airplane. That would be better, a toy airplane with free rolling wheels.

Then get one of those toy cars that you roll backwards and then they go forward on their own...

Then take them to treadmill.

The toy airplane, you will be its power, sort of like a real airplanes jet engines. You can put it on the treadmill, and push it forward, against the direction the treadmill is going and the wheels just spin, right? So the toy airplane's speed is decided by how fast you push it (like a real airplane's speed is dependent on how fast its jet engines push it). The wheels are just along for the ride.

Now take your toy car, and roll it backwards to charge it up. It will be powered by its wheels. If you set it on the tread mill, against the flow, it will probably go backwards, or forward pretty slow because its speed is dictated by the speed of its wheels...

See the difference????

LOL, you're still not getting it.
In your example the wheels on the plane that you are pushing are moving FASTER than the treadmill. In order to create lift, the wheels will ALWAYS have to move faster than the treadmill. If the wheels CANNOT move faster than the treadmill, then the plane CANNOT generate lift.

The wheels aren't "just there". The plane is sitting on them. They are the connection of the plane to the ground. Remove the wheels and replace them with saucers, and the plane takes off. In a scenario where the treadmill is matching the speed of the wheels, the wheels prevent the plane from getting any ground speed which is necessary for lift.

It's pretty basic really.

[doughboyshredder]

[mtngirl79]

LOL, you're still not getting it.
In your example the wheels on the plane that you are pushing are moving FASTER than the treadmill. In order to create lift, the wheels will ALWAYS have to move faster than the treadmill. If the wheels CANNOT move faster than the treadmill, then the plane CANNOT generate lift.

The wheels aren't "just there". The plane is sitting on them. They are the connection of the plane to the ground. Remove the wheels and replace them with saucers, and the plane takes off. In a scenario where the treadmill is matching the speed of the wheels, the wheels prevent the plane from getting any ground speed which is necessary for lift.

It's pretty basic really.

Actually, you don't get it...

The original question...

A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The question is:

Will the plane (747 passenger jet) take off or not?

The conveyor is tracking the PLANE'S speed, NOT the wheel speed, and the wheels are free rolling, so their speed is irrelevant, and YES they CAN move faster than the treadmill, and faster than the plane, because they are free rolling! They will go however fast they need to go to keep up with the plane unless the friction becomes too much, and they break...

[GeezerSteve]

Remove the wheels and replace them with saucers, and the plane takes off.

Uh, you lost me there

[doughboyshredder]

Actually, you don't get it...

The original question...



The conveyor is tracking the PLANE'S speed, NOT the wheel speed, and the wheels are free rolling, so their speed is irrelevant, and YES they CAN move faster than the treadmill, and faster than the plane, because they are free rolling! They will go however fast they need to go to keep up with the plane unless the friction becomes too much, and they break...

That's the OP's question, but as I posted not the original question floating around the nets for so long.

Of course if the conveyor is tracking the planes speed the plane takes off.

Stuck and others have posted about the other hypothetical where the conveyor matches the wheels speed. In that scenario the bearings seize and the plane falls over (unless it has so much power it just drags the melted carcass of the tires across the treadmill and takes off.

This is one of those questions designed with an ambiguity for the purpose of creating pointless arguments.
Which can be fun, I suppose.

[Ted Striker]

Uh, you lost me there

They're flying saucers.

[stuckathuntermtn]

WHO'S TAKING OFF NOW BITCHES???!!

priceless.


And a helicopter on a turn table would take off. The thrust is downward. The spin is torque.

[MagnificentUnicorn]

That's the OP's question, but as I posted not the original question floating around the nets for so long.

Of course if the conveyor is tracking the planes speed the plane takes off.

Stuck and others have posted about the other hypothetical where the conveyor matches the wheels speed. In that scenario the bearings seize and the plane falls over (unless it has so much power it just drags the melted carcass of the tires across the treadmill and takes off.

This is one of those questions designed with an ambiguity for the purpose of creating pointless arguments.
Which can be fun, I suppose.

Even if the conveyor matches the wheel speed, I'm sure that the wheel bearings could tolerate the 350 kts(twice a 747s takeoff speed) and still take off. Like I said, we're doomed.

[DJSapp]

Even if the conveyor matches the wheel speed,

The conveyor cannot match wheel speed if the airplane is moving. Per the question, the conveyor will not move until the plane moves.

Here's an example using time as a variable:

Wheelspeed(t+0.001) = airspeed + conveyor speed

Conveyor speed = wheelspeed(t)

Therefore

Wheelspeed(t+0.001) = airspeed + wheelspeed(t)

Once the plane moves, the wheelspeed/conveyor speed would accelerate until the wheelbearings created enough friction to stop the airplane (i.e. seize the bearing). It's a circular reference.

[Flyoverland Captive]

I weep for the future of this country. Jesus fucking christ, is this really so hard? Really?

Plane takes off. If you don't understand why the treadmill has no fucking affect on the jet engine's ability to move the plane forward with respect to the atmosphere, then you are an idiot. The wheels can be on ice. In quicksand. On a treadmill going 9000mph in reverse, or 9000mph forward. It makes no difference. If you really don't get it, then you should burn your GED paperwork and go back to middle school. Fuck I'm glad I don't have kids.

[Jethro]

Here is another question, from high school physics 25 years ago. Two identical cars, each traveling 50mph crash into one another in a perfect, straight on head on crash. they come together at a speed of 100 mph. Do the occupants experience a wreck equal to 100mph into a solid concrete wall, or 50 mph into a wall?

[doughboyshredder]

I weep for the future of this country. Jesus fucking christ, is this really so hard? Really?

Plane takes off. If you don't understand why the treadmill has no fucking affect on the jet engine's ability to move the plane forward with respect to the atmosphere, then you are an idiot. The wheels can be on ice. In quicksand. On a treadmill going 9000mph in reverse, or 9000mph forward. It makes no difference. If you really don't get it, then you should burn your GED paperwork and go back to middle school. Fuck I'm glad I don't have kids.

Effect, not affect. Who's the idiot?

Really.

If the treadmill is moving at 9000mph backwards then the jet's wheels must rotate at 9200mph. I doubt the wheel bearings and rubber could handle that. So, you are wrong. The speed of the treadmill does have an EFFECT on the jet's ability to move forward.

[doughboyshredder]

Even if the conveyor matches the wheel speed, I'm sure that the wheel bearings could tolerate the 350 kts(twice a 747s takeoff speed) and still take off. Like I said, we're doomed.

If the conveyor matches the wheel speed, then the plane never generates any speed relative to the ground, and therefore does not generate lift, and does not take off.

It's really not difficult at all to understand. The plane must generate about 200mph of ground speed to take off, so the wheels must spin at 200mph faster than the conveyor. If the conveyor is matching the wheel speed this never happens. If the conveyor is matching the ground speed of the airplane, then the wheels are spinning faster than the conveyor. Once the plane is traveling at 200mph the conveyor will be moving at 200mph and the wheels will be spinning at 400mph. Voile, the plane takes off.

[Chainsaw_Willie]

Here is another question, from high school physics 25 years ago. Two identical cars, each traveling 50mph crash into one another in a perfect, straight on head on crash. they come together at a speed of 100 mph. Do the occupants experience a wreck equal to 100mph into a solid concrete wall, or 50 mph into a wall?

Because the cars are identical then the force experienced by the occupants of the cars is the same as hitting the concrete wall.

If one car is heavier than the other then the occupants of the smaller car experience more force while the occupants of the bigger car experience less. Not being a physics geek I don't know if the difference in forces is proportional to the difference in weights, but I suspect it is.


Flyoverland Captive - I'm with you because that's the practical answer to this question. What DJsapp and Doughboy are talking about is a different take on the question that focuses on the wheel speed vs. conveyer speed, and they are right as long as we are talking about the rotational speed of the wheel. The only way that the wheel speed can be the same as the conveyer is if the plane is not moving, and in the real world the only way that would happen is if the plane was anchored to a stationary point off the conveyer and it's engines didn't come into play OR the engines are tuned to provide just the perfect amount of thrust to perfectly counteract the rolling resistance and bearing friction in the wheels and thus hold the plane in place.

I have only ever seen this question asked the way it was asked in the OP of this thread - that the conveyer matches the plane's speed. In that scenario the wheels are pretty much irrelevant.

[Cross]

If the conveyor matches the wheel speed, then the plane never generates any speed relative to the ground, and therefore does not generate lift, and does not take off.

How can the conveyor prevent the wheels from going faster then it goes? I do not see how the conveyor can determine the speed of the jet through the wheels thus preventing it from taking off. The jet's engines force is on the air surrounding the plane. It pushes/pulls against the air to move the jet forward. For the force of the jet's engines pushing against the air to be negated(keeping the jet stationary) the treadmill would need to apply an equal or greater force in the opposite direction to the plane through the free spinning wheels to the landing gear. Simply matching the speed of the plane would not apply a force great enough to hold the jet in a stationary position.

[Chainsaw_Willie]

That's why the wheel speed = conveyer speed idea is not a very realistic idea and is tougher to wrap your head around than the conveyer = plane speed idea.

Let's look at this in the real world and pretend that someone did indeed build a giant conveyer belt upon which rests a conventional airplane. When the conveyer is turned on, the airplane will move backwards at the same speed as the conveyer belt. This is because the rotation of the wheels isn't frictionless in the real world. There is rolling resistance of the tire and friction in the wheel bearings. In order to make the wheels turn at the same speed as the conveyer, something needs to keep the plane motionless in relation to the ground. As I mentioned above, this could be accomplished by tying the plane to anchors off the conveyer, or by using the plane's engines to provide just the right amount of thrust to overcome the rolling resistance of the wheels & bearing friction. This satisfies the requirement that the conveyer speed always matches the wheel speed and in order for this to be true the plane cannot be moving with relation to the ground. The only way the plane can take off in this scenario is if it's somehow provided with a head-wind strong enough to generate the required lift. So, put a fan in front of the conveyer that can generate a 200mph wind and the answer becomes yes, the plane will take off. Otherwise in this scenario it can't because it's constrained by the artificial requirement that the conveyer speed matches wheel speed.

[Skirotica]

I don't know if someone already explained this to you retards or not but I can't leave this untouched.

The plane would never take off. What causes a plane to fly is not thrust directly. Thrust is a means of producing the movement necessarily to achieve lift. Lift is a change in air pressure between the air below and above the wing. That's why wings are flat on the bottom android d on top. The round tops create a larger surface area leading to air passing over more slowly than the air passing under. The delta in that air speed creates lift.

So if the plane is motionless the. There is no air passing over the wings thus no lift and no takeoff. No matter how high the thrust of he engines.