A jet plane on a large treadmill

[Cornholio]

Cornholio, what is your frame of reference? Where does the treadmill's speed come from? The speed of the wheel compared to the treadmill or the speed of the aircraft as a whole from a stationary point (say, the control tower).

Let's say there's a string connected to the tail of the plane. The instant (remember, I said no lag in the control system) the plane starts moving forward, it tugs on the string. As soon as the treadmill detects the string tug, it starts moving forward until there's no tugging. (this all happens instantaneously, or there's a good predictive algorithm that rapidly adjusts for dynamic fluctuations in string tension.)

[cj001f]

As you can see, it only matches the speed, not the force.

If the forces aren't matched the plane will accelerate and not maintain position on the treadmill.

[Big Blue]

I think I'm also assuming the control system is perfect-- that is, there's no lag (or some great prediction) between the sensing of the speed and the treadmill movement. That's fair enough for this hypothetical.

Then plane moving has nothing to do with imperfections in the control system. The plane moves forward with respect to a fixed point (say a tree on the edge of the runway). The runway moves backward at an equal speed. So, as the plane is doing 50 mph with respect to the tree, the runway is doing negative 50 mph with respect to the tree. So the wheels are spinning at 100mph. The fact that the wheels have to spin a little faster doesn't mean that the plane can't take off.

[Cornholio]

If the forces aren't matched the plane will accelerate and not maintain position on the treadmill.

This is true.

In all reality, as I read and re-re-read the problem, it's stated too poorly to decide anything.

But, as I read it, the treadmill functions to make the plane's velocity, relative to the control tower, zero.

[Big Blue]

I feel like I'm taking crazy pills. This is so mind-bogglingly retarded. I've completely lost respect for some individuals here.

[Beaver]

What this thread needs is a big snapper.

[cj001f]

I feel like I'm taking crazy pills. This is so mind-bogglingly retarded. I've completely lost respect for some individuals here.

So have we

The plane moves forward with respect to a fixed point (say a tree on the edge of the runway). The runway moves backward at an equal speed. So, as the plane is doing 50 mph with respect to the tree, the runway is doing negative 50 mph with respect to the tree. So the wheels are spinning at 100mph.

[EPSkis]

There are too many unstated vaiables, ie: weight of the plane, power generated by the engines, frictional resistance between the wheels of the plane and the conveyor belt, whether or not there is an instantaneous reaction by the belt, etc.

[PulverSchwein]

Beaver's got it all correct, absolutely. Everyone who says, oh try jumping on a treadmill and see what happens when speeds change is missing the fact it is the friction force of your feet to the treadmill that will send you flying backwards.

The only contact the jet has to the treadmill is wheels that spins essentially freely and without friction (relative to the massive thrust force the jet can generate). Apply X million pounds of force to something that can roll freely and it will move. The only change that occurs by being on a treadmill that rolls backwards at same speed the jet moves forwards is that it's wheels will spin at double speed.

[MeatPuppet]

Ummm, Beaver is right. The only thing slowing the 747's velocity RELATIVE TO AIR is the rolling friction from the treadmill.

Simple example: A person is on a treadmill wearing roller skates and a rocket pack. The tredmill starts moving, and the person moves backward slowly due to rolling friction. The person flips on the rocket pack and blasts forward off the front. Why? The propulsion does not come from the wheels, it comes from the rocket pack.

Complex example: If the minimum takeoff speed for the 747 required 200 mph airspeed (speed of air moving over wing), the treadmill will be moving at 200 mph, the wheels will be spinning at 400 mph. The plane will be slowed by friction so the engines will need to overcome this. In theory the plane will take off.

Now to actually answer this in a more real situation I need to know what the strength and rolling resistance of the tires are, and the maximum thrust of the 747's engines. There are two ways the plane would not take off: 1. The tires blow out because they are rotating too fast to achieve the required airspeed, or 2. The rolling resistance is too great for the engines to overcome.

Got to go, Ogre is knocking on my door.

I've only read up to DJ's post and I don't have any time to read any further right now. But...

My physics prof would have ripped everyone, who posted up to this point, a new one for even attempting to answer a question without having enough information to answer it. Too many assumptions, too many unknowns. DJ has provided examples that do have enough information to come up with an answer.

That is all.

[13]

Oh my god, some of you are really fucking stupid.

[Big Blue]

But, as I read it, the treadmill functions to make the plane's velocity, relative to the control tower, zero.

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).

It says nothing about making the speed zero. the plane has to have some velocity for the treadmill to have a velocity. If the plane stays still, the treadmill doesn't move.

[EPSkis]

C'mon, Beav - this thread beat any of the Ogre threads to hell and now I can't participate...

[Dr. Send]

I feel like I'm taking crazy pills. This is so mind-bogglingly retarded. I've completely lost respect for some individuals here.

Seconded. I've only made one post and I give up already.

[cj001f]

My physics prof would have ripped everyone, who posted up to this point, a new one for even attempting to answer a question without having enough information to answer it. Too many assumptions, too many unknown

I dunno, it took the Science Teachers forum 133 pages

Forward Force doesn't produce lift; airflow over the wing does. You need to be moving relative to the air to acheive lift. As long as the plane is never moving relative to the air (and if it's on a treadmill, it's position is stationary) it has no lift - it stays on the ground. It's speed relative to the treadmill will approach the speed of light/failure speed of the treadmill because there is no friction, BUT IT WON'T FUCKING TAKE OFF!

[Cornholio]

My physics prof would have ripped everyone, who posted up to this point, a new one for even attempting to answer a question without having enough information to answer it.

Heh.

I think you'll find that you never have all the information you need to solve a problem. Assumptions are the only way we find the mathematical approximations that we do.

[mcsquared]

Beaver is right. I had to think about it for a bit. Imagine a 3 wheeler with the front (middle, non-drive) wheel on a treadmill and two rear wheels (drive wheels) on the regular ground.

[cj001f]

Beaver is right. I had to think about it for a bit. Imagine a 3 wheeler with the front (middle, non-drive) wheel on a treadmill and two rear wheels (drive wheels) on the regular ground.

You need to be moving relative to the air to acheive lift. As long as the plane is never moving relative to the air (and if it's on a treadmill, it's position is stationary) it has no movement relative to air. Therefore no lift - it stays on the ground. It's speed relative to the treadmill will approach the speed of light/failure speed of the treadmill because there is no friction, BUT IT WON'T FUCKING TAKE OFF!

[Cornholio]

You need to be moving relative to the air to acheive lift. As long as the plane is never moving relative to the air (and if it's on a treadmill, it's position is stationary) it has no movement relative to air. Therefore no lift - it stays on the ground. It's speed relative to the treadmill will approach the speed of light/failure speed of the treadmill because there is no friction, BUT IT WON'T FUCKING TAKE OFF!

There's no need to yell.

It's just a different reading of the problem.

I think it's all relative to the stationary object, like the control tower.

They think differently. (although I still don't see what they're measuring speed against.)

[Sensisnow]

The more I think about it, the more feasible Beaver's assessment of this problem seems to be. Imagine if the treadmill were rotating in the opposite direction, it wouldn't make it any easier to take off (assuming 0 friction in the wheel bearings). The plane would just sit motionless on the treadmill until it throttled up at which time the jets would push against the air causing it to move forward until eventually reaching takeoff speed.

[belgian]

Lets think about this for a second... If you have EVER studied the premise on which modern airplanes manage to stay in the air you know that it is due to the shape of the wing. The straight bottom of the wing creates high air pressure whilst the rounded top of the wing creates low pressure because the air takes longer to get over it. Without airflow, that bitch is going no where. Unless of course you strap an ariane 5 to it

Since there is no air flow over the wing (which, btw is what an airplane requires to takeoff, not just thrust. That's also why gliders fly!!!) You are technically doing the same thing as parking a 747 at the start of a run way and flicking the thrust onto max, is the plane gonna take off right then and there? nope. It needs to get up to a FORWARD MOVING speed so that there is enough airflow over the wing to create lift. unless we are talking rocket or space-shuttle type thrust on the 747 (which it simply does not have, sorry) it still needs to roll. And if you COULD theoretically get the conveyor belt to work, don't you think that your local international airport would have implemented that already (or at least Japan)?


Here's your sign.

[Cornholio]

Lets think about this for a second... If you have EVER studied the premise on which modern airplanes manage to stay in the air you know that it is due to the shape of the wing. The straight bottom of the wing creates high air pressure whilst the rounded top of the wing creates low pressure because the air takes longer to get over it. Without airflow, that bitch is going no where. Unless of course you strap an ariane 5 to it

Since there is no air flow over the wing (which, btw is what an airplane requires to takeoff, not just thrust. That's also why gliders fly!!!) You are technically doing the same thing as parking a 747 at the start of a run way and flicking the thrust onto max, is the plane gonna take off right then and there? nope. It needs to get up to a FORWARD MOVING speed so that there is enough airflow over the wing to create lift. unless we are talking rocket or space-shuttle type thrust on the 747 (which it simply does not have, sorry) it still needs to roll. And if you COULD theoretically get the conveyor belt to work, don't you think that your local international airport would have implemented that already (or at least Japan).

OK, this doesn't make much sense.

We all agree that if there's no airflow (forward velocity) then there's no takeoff.

The question is if there's forward velocity.

I say no, because that's how I read the problem.

[dubu]

The more I think about it, the more feasible Beaver's assessment of this problem seems to be. Imagine if the treadmill were rotating in the opposite direction, it wouldn't make it any easier to take off (assuming 0 friction in the wheel bearings). The plane would just sit motionless on the treadmill until it throttled up at which time the jets would push against the air causing it to move forward until eventually reaching takeoff speed.

clap clap clap clap clap clap,

and for the people that think that the plane will not take off, what are the JET engines pushing against?

that is the treadmill pushing against?, no its not the plane.

[13]

The correct answer.

["The Big One"]

this question has caused so much chaos all over the internet, im glad it is doing the same here.