Airplane and Conveyor Belt Debate

[neutrino]

This has been posted on other forums and has cause massive arguments

Why do you think this forum is any different?

There are a many threads on this question, here, open and locked. Use the search feature.

 

[Danger]

Welcome to PF, Mtaylor. This subject has been beaten absolutely to death. Do a quick forums search and you'll find more than you'll ever want to read about it. The simple, straight answer is that it will take off. There's no point in debating it again.

 

[joshd]

It can take off just fine. The plane moves forward as usual (the wheel bearings are the magicaly frictionless ones, OK?) and takes off, since the plane's engines are pushing against the air, not the groud, the moving ground has no effect apart from to make the wheels spin twice as fast as it takes off.

 

[leright]

It can take off just fine. The plane moves forward as usual (the wheel bearings are the magicaly frictionless ones, OK?) and takes off, since the plane's engines are pushing against the air, not the groud, the moving ground has no effect apart from to make the wheels spin twice as fast as it takes off.

good call. I wasn't thinking properly when I posted that.

 

[rcgldr]

Unless the tires explode from rotating at twice the speed, as posted the jet takes off because the engines exert a force against the air not the ground , taking only a tiny bit longer because more work is used up adding kinetic energy to the wheels and tires.

 

[Doc Al]

As Danger said, this topic has been done to death. Check the archives!

 

[Xori]

Plane on a treadmill

I read this puzzle recently,

There is a plane on an endless treadmill. The thing about this treadmill is that however fast the plane is moving forward, the treadmill goes the same speed backwards.

Would a plane be able to take off of this treadmill?


The only answer I can come up with is that the wheels would eventually blow up and the airplane crashes.

But this is probably not the right answer.

Any thoughts?

 

[D H]

Not again! I predict this thread will soon be locked or deleted. Look at this thread instead.

 

[Doc Al]

This one's been beaten to death several times over in various threads.

 

[russ_watters]

I know Doc already locked this, but I'm going to throw in my $.02: This is a stupid question. What matters for an airplane is how fast the air is flowing over the wings. Since the question badly worded, there is no way to know how fast the air is flowing over the wings without arguing over the wording of the problem and various assumptions and complicating factors. Thus: stupid question.

 

[solidon]

Read this about the airplane on treadmill problem. Is it true?

A plane is on a conveyor belt or treadmill, a very very very long one. It is slowly accelerated to a speed of 700MPH backwards. The treadmill is designed with a moving structure placed behind the air plane stopping drag/air-resistance from affecting the airplane. That is the plane is moving back without its wheels spinning, just as if it were at rest.

The plane begins accelerating foward, and reaches a top speed of 600 MPH.(the moving structure designed to cancel drag air-resistance follows behind it.). Does the plane move foward with relation to the Earth or backwards?

The person claims it moves backwards, but some people claim it moves foward with respect to the earth. Who's right?

 

[chroot]

This has been answered so many times on this forum that it's no longer welcome (or interesting).

- Warren

 

[Cyrus]

Plane on a conveyor belt

http://video.google.com/videoplay?docid=-7083501107445407444&q=plane+on+treadmill&total=24&start=10&num=10&so=0&type=search&plindex=0

Wow, what a shocker. Now, if you search this on the internet, PF is one of the hits on the first page of google, so I KNOW they had to have seen the answer posted here.

 

[D H]

Unfortunately, I doubt that that video will stop the psychoceramics. I predict we will be plagued with (and locking) plane-on-a-conveyor belt threads for years to come: "The MythBusters didn't set things up right." "That wasn't a real conveyor belt." "That wasn't a real airplane." "That video doesn't prove anything."

 

[mgb_phys]

Did they have birds flying off perches inside it ?

psychoceramics

Brilliant - I hadn't come across that term before.

 

[Jimmy Snyder]

I'm new to this myth. The plane was going 40 mph forward and the belt 40 mph backward and so the plane should have been stationary relative to the ground. But it wasn't, you could see it moving past the cones. Therefore, I conclude that the plane's speedometer measures speed through the air, not relative to the ground. If the plane is going at 40 mph through the air, then I suppose that is sufficient to rise. The motion of the belt seems irrelevant.

Now I see the point. The airplane moves by pushing the air back with the propeller, not by pushing forward with the wheels. That's a pretty elaborate experiment to confirm this simple fact.

 

[mgb_phys]

The motion of the belt seems irrelevant

It is, but the only way to get that fact into some people heads appears to be with a hammer and chisel!

 

[leright]

I'm new to this myth. The plane was going 40 mph forward and the belt 40 mph backward and so the plane should have been stationary relative to the ground. But it wasn't, you could see it moving past the cones. Therefore, I conclude that the plane's speedometer measures speed through the air, not relative to the ground. If the plane is going at 40 mph through the air, then I suppose that is sufficient to rise. The motion of the belt seems irrelevant.

Now I see the point. The airplane moves by pushing the air back with the propeller, not by pushing forward with the wheels. That's a pretty elaborate experiment to confirm this simple fact.

yeah, that's a lot of work to prove this obvious fact.

 

[tribdog]

wow, so a plane can take off no matter how fast it's wheels are turning. who woulda thunk it.

 

[D H]

I'm new to this myth.

The silly question has hit this forum many times since your join date. Those discussions must have either flown right over your head, or since the mods got sick of the silliness quickly, the threads must have been locked/deleted before they took off.

I predict that this thread, for instance, will soon be locked because some pyschoceramic will chime in with their bits of wisdom. At the forums at the MythBusters website the post-show discussion is now over 40 pages long (), mostly by people who claim that they didn't bust the myth. The plane took off right before their eyes, the MythBusters even did one of their "Warning: Science Content" bits to explain why the plane does take off, and they still claim the plane won't take off.

 

[Jimmy Snyder]

The silly question has hit this forum many times since your join date. Those discussions must have either flown right over your head, or since the mods got sick of the silliness quickly, the threads must have been locked/deleted before they took off.

I don't pay attention to all the threads. I probably only enter about 30% after reading the subject line. Perhaps someone could do a psychological profile on me based on what threads I enter. It may be that two threads started some time apart have the same subject line and yet I entered one and not the other. I not only didn't hear of this myth from the forum, but not from any other source either. That's not surprising, most of the myths I hear of, I hear of from this forum.

 

[Mk]

The plane took off right before their eyes, the MythBusters even did one of their "Warning: Science Content" bits to explain why the plane does take off, and they still claim the plane won't take off.

Wait a second, I thought I understood this myth.

I understood it to be, a plane uses some sort of force, like a gas jet, to attempt to propel itself forward. However, the conveyor belt keeps the plane's x-coordinate constant. Is that right?

 

[D H]

However, the conveyor belt keeps the plane's x-coordinate constant. Is that right?

No real conveyor belt, acting on the plane solely through its freely-rotating wheels, can do that. The plane has wheels for the very reason that wheels reduce friction. If you insist on endowing the conveyor belt with some magical force, I in turn insist on endowing the plane with magical, frictionless wheels. With this, the silly "plane-on-a-conveyor" question reverts to the equally silly but age-old problem of an irresistible force versus an unmovable object.

The basic problem with the myth is that it has a flawed premise. Such a conveyor belt contraption can indeed keep a car stationary because a car has driven wheels. The flaw in the premise is that a plane does not have driven wheels. End of story, end of myth.

One of the things the MythBusters did was to put a model car on a treadmill. While the car was in gear, the treadmill was easily able to keep the car stationary. Then they took the car out of gear. Because wheels only reduce rather than eliminate friction, the can began to move backwards. However, it only took a very small force (exerted by Adam's hand) to keep the car stationary.

The same applies to the plane on a conveyor belt. The conveyor belt exerts a tiny force backwards on the plane. A plane moves forward by forcing air to move backwards relative to the plane. The forward-directed force resulting from the plane's propellers forcing air to move rearward easily overcomes the tiny rearward-directed force resulting from the plane's wheels moving forward with respect to the conveyor.

The only way to keep the plane stationary is to strap it down to the conveyor. A plane securely strapped down to the conveyor (or to the tarmac) will not take off. However, that is not how the question was posed.

 

[FredGarvin]

A pox on the person that put this question out to the world. Fie! Fie!

 

[RonL]

The only other time i made the "mistake of watching" myth busters, they were building a steam powered cannon, boy what i could have done with the money they used to make that program

 

[Lord Ping]

Just remember not to switch the treadmill on before switching the plane on... otherwise it really will go backwards.

 

[chemisttree]

The plane didn't really fly... the Earth just moved away beneath it. Simple optical illusion.

 

[russ_watters]

Wait a second, I thought I understood this myth.

I understood it to be, a plane uses some sort of force, like a gas jet, to attempt to propel itself forward. However, the conveyor belt keeps the plane's x-coordinate constant. Is that right?

Now you see where the argument comes from. The question is ill-formed, so the entire argument is over that point and the minutae of how it could come to be true. It's just dumb.

 

[Dark Prism]

Airplane on a conveyor belt (sorta)

Before I get the, "This has already been talked about in another thread," let me just state my case.

The question here is not about taking off, but of landing.

I was discussing the Mythbuster's experiment of the airplane on a conveyor belt with some friends of mine, who all agreed that the experiment was flawed in it's wording and/or approach. It seemed obvious to us that the plane would take off, being that the propeller was pulling the plane forward through the air, and the wheels had little to do with it. I assume this is the general consensus on a physics forum as well. The point of this, however, is that my friend posed this question-

If a plane were to land on a conveyor belt moving in the opposite direction to it, wouldn't it be able to land with almost no distance?

What followed was a very lengthy discussion, the outcome of which was that everyone thought I was nuts because I argued that while it might be possible to stop the plane in a very short distance, it would cause countless injuries because it would stop so abruptly. what they argued was that the plane would slow down like normal because of the conveyor belt.

I would very much appreciate some intellectual input on this. Please let me know if I haven't made the problem clear.

 

[mgb_phys]

You need the plane to stop in the same time as before, to keep the same acceleration and so the same force on the passengers.
But the wheel speed on landing will be higher because you have the extra conveyor speed so you would have to apply more wheel braking - assuming you are using wheel brakes to slow you down. If you used purely engine braking (thrust reverses) it would make no difference.

 

[Dark Prism]

For arguments sake we'll say that braking is done the way a 747 usually brakes, that being reverse thrust and wheel brakes.

Your answer seems like it doesn't follow the question, although that is probably my fault. Let me restate.

A plane is landing. Instead of a long runway, you have a short conveyor belt. The plane is travailing 100mph north. The conveyor is not going anywhere, but it's speed is 100mph, south. The plane lands on the conveyor belt and the conveyor is slowed from 100mph to a stop. If the plane lands on the conveyor belt, and its forward momentum is transferred into the conveyor belt, will it stop without the need for a long runway with no adverse effects on the plan or passengers? Will the conveyor belt negate it's forward inertia?


My argument is that when the plane touches down, if it does stop, it will probably destroy the plane and/or kill the passengers because it's inertia is cut abruptly, being that I don't believe that the conveyor belt will "catch" the plane's inertia. Alternatively, is it even possible to stop the plane on the conveyor belt, as the plane's 100mph is air speed, and the conveyor belt's 100mph is ground speed?

 

[Danger]

I agree with Mgb. Under normal circumstances, with a light passenger aeroplane, you don't use the toe brakes until it's time to turn from active to a taxiway. Runways are long enough that the bird slows down just fine by itself; differential braking is used for steering. If you did apply the brakes on a conveyor belt, something would come unglued.

 

[mgb_phys]

The plane lands on the conveyor belt and the conveyor is slowed from 100mph to a stop.

If it takes the same time for the conveyor to stop the plane - there is no difference, if it takes less time there will be more force on the plane and passengers.
If the plane lands on the conveyor doing 100mph in the opposite direction then you just have to (wheel) brake more gently.
It's exactly equivalent to landing with a 100mph tailwind - you would have the same indicated air speed but would probably blow all the tires !

 

[FredGarvin]

The thing that people fail to realize is that, no matter what the wheels are doing, the forward flight of the aircraft is the wing relative to the surrounding air. When you're coming in at, say, 100 MPH IAS, that means the wing has a relative velocity to the wind of 10 MPH. The ONLY way this would work is if you had a wind generating device that matched the required wind speed at the second the wheels hit the ground.

 

[D H]

OMG. Kill one plane-on-a-treadmill myth and another just as ludicrous in from its ashes.

The MythBusters already addressed this new incarnation as well. They drove a car up a ramp trailing behind a moving truck. The car's wheels changed their rotation rate nearly instantly as the car transitioned from rolling at speed with respect to the road to nearly at rest with respect to the ramp. The bulk of the car's momentum is in the car, not the wheels. The same applies to the plane landing on a moving treadmill. The bulk of the plane's momentum is in the plane itself, not the wheels.

In this case, problems would arise should the pilot stupidly lock the brakes the instant the plane hit the conveyor belt. So what? Problems would arise should the pilot stupidly lock the brakes the instant a plane hits a run-of-the-mill tarmac.