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MythBusters tackles "plane on a conveyor belt problem"

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serious    1

Hold on a second, can I change my bet?

After been thinking some more about this problem it's quite obvious the plane's going to take of. The intuition says no, but when you think twice the speed of the wheels do nothing at all. Imagine the wheels to roll friction less against the rest of the plane. Even if the conveyor belt was moving backwards at 40000km/h the plane would still move forward when the engines were turned on, the only thing that would happend is that the wheels would spinn in 40000km/h+the speed of the airplane, the speed of the airplane remains the same.

In the experiment the wheels will spinn twice the speed of the airplane.

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Colin-uk    135
I'm confused now why they would even test this.

If the plane has enough power to outrun the belt, then it will.

If the belt has more power, the plane will go backwards.

So what's the myth?

The myth is, what if both are going at the same speed ?

ive always thought that it wouldnt take off, but the conveyer/rollerskate/rope analogy made it a bit clearer that it will..

just found this on youtube:

:p

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Kushan    27

But hold on here, I think everyone is focusing far too much on the wheels here, when we can pretty much agree that they've got no real affect on the plane.

Basically, the plane is standing in the one spot, the treadmill is to prevent it moving forward (which is why the wheels just spin). Think of it like a human treadmill, you're running but you're not moving forward because the treadmill keeps you in the same spot.

Now, I am certainly no expert, but I was under the impression that lift (in the case of airplanes) is created because air flows faster over the wing that it does under the wing.

wing.gif

But where's the air flow with this? Doesn't that depend entirely on the placement of the engine on the plane and thus render all of these arguments completely null and void?

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gigapixels    99

Kushan's post is right on the mark.

The point of the treadmill is to keep the plane at a standstill. If it moves as fast as the wheels turn, it will keep them in place and thus there is no friction for the plane to use to move forward, despite what thrust is available.

If that's not the point of the treadmill, then there is no point to the treadmill at all and you've got a regular old runway.

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David Scaife    0
The point of the treadmill is to keep the plane at a standstill.

But it doesn't. You don't move when you run on a treadmill because your forward movement is created by your legs, which act directly on the treadmill, which is moving backwards, so (as stated) you don't move.

With an aircraft, however, the wheels are merely there to allow the craft to move forwards on the ground. The thrust is created by the engines, which act on the air, and thus the aircraft will still move forwards regardless of how fast the treadmill is moving backwards. The wheels will just be spinning a lot faster.

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Slugsie    1,123

OK, it's very simple, the plane will take off. No if's, no but's, it will take off.

Everyone who is arguing that it won't has failed to understand the physics of how an aircraft flies.

The best analogy stated so far is standing on a skateboard on a treadmill, and pulling on a rope. If the bearings are well oiled (i.e. low friction) then the effort required to pull yourself forward will be virtually the same as it would be if your skateboard was sat on solid ground.

Just think about this... Imagine if the wheels/bearings where completely frictionless (not physically possible, but treat this as a pure thought experiment). If you stood on such a skateboard on a treadmill, and the treadmill started running, you would not move. The only reason you do move in reality is because the friction in the wheels/bearings imparts a (small) force on you. What this means is that whilst you can argue that the conveyor will impart some force on the aircraft, it'll be insignificant, and far from enough to halt the forward movement caused by the thrust of the engines (jet or propeller doesn't matter).

Edited by Slugsie

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Sjokkel    8

The belt is just adding a groundfactor. Groundspeed doesn't matter here. The plane will be moving air to gain thrust. The wheels are just there to keep it away from the ground. The plane will be creating airspeed no matter what is happening underneeth it's wheels. After he get's enough airspeed Bernoilli will do the rest.

If the plane would have no thrust then the story would be different. While doing a hangglider course the course director demonstrated to me that with enough airspeed there would be no need for groundspeed, pointing the glider with it's nose in to a 25 km/h headwind he just lifted up in the air without even putting a step forward. If the airspeed would not be enough and he would need to start running to add some more forward speed to get the air flowing beneath the gliders wings at 25 kms/h, then his legs are putting in the power to the ground. In this case running on a conveyor belt would not make him able to lift off. Not in case with a plane with either a prop or jet engine.

I don't even understand why they would need to do a test for this? The plane takes off without a problem.

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Kushan    27

But sjokkel, aren't you essentially saying that the whole test depends on the engine used (be it a jet engine, propeller or good ol' human legs)? If that's the case, then why are people arguing at all, since we don't know what kind of plane/engine they plan on using?

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se7en.hu    0
Curious why velocity != 0 if it will be stationary...

^^ By the way, the military wouldn't shorten the runways, or else how the hell would the planes land?? haha didn't think of that one did ya?

Land onto a conveyer belt moving at the speed of the plane at touchdown :p

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Slugsie    1,123
But sjokkel, aren't you essentially saying that the whole test depends on the engine used (be it a jet engine, propeller or good ol' human legs)? If that's the case, then why are people arguing at all, since we don't know what kind of plane/engine they plan on using?

Nope, because all powered aircraft work on the same principal. Something - either a jet engine or a propeller - grabs air from in front of it, and forces it behind it at a speed greater than the forward motion of the craft. Any system that relies on pushing against the ground (i.e. wheels or legs) is doomed to fail the moment the aircraft takes off.

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curme    31
I don't even understand why they would need to do a test for this? The plane takes off without a problem.

Don't understand why they need to do a test?! :omg:

After all of this discussion, aren't you at least curious as to the outcome?

Will the question be :busted: ?

I actually look forward to the discussion of how they did the experiment, the pros and the cons. I just hope that there is a YouTube link or something!

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kaffra    16
But hold on here, I think everyone is focusing far too much on the wheels here, when we can pretty much agree that they've got no real affect on the plane.

Basically, the plane is standing in the one spot, the treadmill is to prevent it moving forward (which is why the wheels just spin). Think of it like a human treadmill, you're running but you're not moving forward because the treadmill keeps you in the same spot.

Now, I am certainly no expert, but I was under the impression that lift (in the case of airplanes) is created because air flows faster over the wing that it does under the wing.

wing.gif

But where's the air flow with this? Doesn't that depend entirely on the placement of the engine on the plane and thus render all of these arguments completely null and void?

This is what i mentioned earlier as well. all correct if the plane is still. what the other side of the argument is that the conveyor belt is not able to keep the plane still. the focus on the wheels is that it doesnt have any power in it, like a car in neutral gear.

if so then the lift will be there, just the wheel is spinning much faster and perhaps some sliding.

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inStereo    0

Maybe if it was a windy day!

A plane takes off because of the air speed around the wings, not how fast it is moving along a runway.

If the forward friction of the engine is counteracted by the conveyor belt then that renderds the jet engines useless, doesn't it?

I'm sure it will make for interesting veiwing.

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Kushan    27

To be honest, I don't think half of the people here are going to change their opinion, no matter what happens on the show.

This is what i mentioned earlier as well. all correct if the plane is still. what the other side of the argument is that the conveyor belt is not able to keep the plane still. the focus on the wheels is that it doesnt have any power in it, like a car in neutral gear.

if so then the lift will be there, just the wheel is spinning much faster and perhaps some sliding.

But where is the air flow coming from? Normally, that comes from the engine "pulling" the plane along the runway, but because it's stationary then there would be no air flow except for that from the engines, which wouldn't be enough to give lift.

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James7    659
To be honest, I don't think half of the people here are going to change their opinion, no matter what happens on the show.

LOL You're probably right there!

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Slugsie    1,123
But where is the air flow coming from? Normally, that comes from the engine "pulling" the plane along the runway, but because it's stationary then there would be no air flow except for that from the engines, which wouldn't be enough to give lift.

Where in the problem does it state that the aircraft will be stationary? It states that the conveyor will be moving at the same speed but opposite direction as the aircraft. That is not the same as the aircraft being stationary. If the aircraft was propelled on the same principles are an car then yes it would be stationary. But it isn't. So it won't be.

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Kushan    27

But that's the point of the conveyor belt, to stop the aircraft moving forward...why else would it be there? (to be fair, the scenario is pretty poorly worded).

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Guest jgrodri   

I don't think it will take off and here's why:

because of the free-spinning wheels and the conveyor belt vs. the engines, this simulates a stand still. everyone agree up to here?

This stand still would then be the equivalent of having the plane suspended by a rope (to get rid of the friction).

would a plane suspended by a rope take off? I doubt it. planes accelerate on the ground to get enough air moving in the opposite direction. because of the shape of the wings this creates a bernoulli effect. The lift of the bernulli effect makes the airplane fly so long as the airplane is moving fast enough to create an effect that can lift a whole airplane (which is why you don't see airplanes standing still in the air waiting to descend).

my guess is that windspeed at ground level will not create a significant bernoulli effect to lift even a light glider.

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Slugsie    1,123
But that's the point of the conveyor belt, to stop the aircraft moving forward...why else would it be there? (to be fair, the scenario is pretty poorly worded).

But how can it stop the aircraft moving forward? All it can do is run at the same speed in opposite direction to the aircraft. Given that the wheels of an aircraft are free spinning, how exactly does the conveyor belt assert any force on the aircraft and thus prevent it from gaining any ground speed? The conveyor belt will assert a force on the wheels sure. But because the wheels are free spinning, virtually none of that force will ever hit the aircraft. Thus the aircraft does gain airspeed (which is all that's relevant), and takes off.

This may boil your noodle a little... If the conveyor really was able to arrest the speed of the aircraft, then the conveyor would in fact never move. The problem states that the conveyor moves at the same speed as the aircraft. So if the conveyor stops the aircraft, then it follows that the conveyor is also stopped. You cannot have the situation that the aircraft isn't moving (as so many of you seem to believe) but the conveyor is. So, if the conveyor isn't moving (because the aircraft isn't moving) then its effect on the aircraft is in fact no different from a standard tarmac runway.

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Kushan    27
But how can it stop the aircraft moving forward? All it can do is run at the same speed in opposite direction to the aircraft.

That's exactly how it stops the aircraft moving forward, though. Think of a teradmill that you run on. Your legs are running, but you're not moving forward because you're running at the same speed as the treadmill. The engines will speed up and get faster and faster, but will they be able to produce enough thrust to make the plane take off?

If that were the case, then the wings wouldn't be needed at all, but we all know the wings ARE needed(obviously).

The only thing that the engine could possibly do is generate enough airflow to go OVER the wings (Normally this would be generated by the plane moving forward, but it's essentially in the same spot due to the conveyor belt) and this is possible, but depends heavily on the types and size of the engines used.

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Slugsie    1,123
I don't think it will take off and here's why:

because of the free-spinning wheels and the conveyor belt vs. the engines, this simulates a stand still. everyone agree up to here?

No, that's the whole point, as that is precisely what doesn't happen. If the aircraft worked on the same principles as a car, then yes you would have a standstill. It doesn't, so you don't.

This stand still would then be the equivalent of having the plane suspended by a rope (to get rid of the friction).

would a plane suspended by a rope take off?

In a limited fashion (due to the angular forces applied to it by the rope) yes it would. It would fly in circles around the fulcrum of the rope.

I doubt it. planes accelerate on the ground to get enough air moving in the opposite direction.

Just think for a second the exact mechanism the aircraft uses to accelerate. Does it do it by pushing the ground backwards? Nope, it doesn't. Does it do it by pushing the air backwards. Yes! So, because it is not exacting a force on the ground (other than its weight which is not relevant to the equation) the direction of motion of the ground is not relevant.

because of the shape of the wings this creates a bernoulli effect. The lift of the bernulli effect makes the airplane fly so long as the airplane is moving fast enough

THROUGH THE AIR (that is what is important)

to create an effect that can lift a whole airplane (which is why you don't see airplanes standing still in the air waiting to descend).

my guess is that windspeed at ground level will not create a significant bernoulli effect to lift even a light glider.

Wrong. Normal winds are quite capable of lifting light gliders etc. They often need air-speeds of less than 40mph, which isn't exactly uncommon. Put it this way, if you faced a Boeing 747 head-on into a 200mph wind (that was stead and not gusty) it would take off quite happily without having any apparent ground-speed.

Edited by Slugsie

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golemn    1

OK i got it, this is best way to explain it...

Lets say I drop a car at 35000 ft (With throttle to the max). Will the go forward or fall straight down?

Lets say a drop a jet at 35000 ft (With throttle to the max). Will it fall straight down?

The Jet will thrust forward... wheels dont matter it will move forward thus creating lift.

Does not matter what surface the planes on it will move.

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Guest jgrodri   

slugsie, i think you are misunderstanding the third point i made:

airplanes indeed work by pushing air backwards and not the ground but they need an initial help.. there is a reason why airplanes accelerate on the ground otherwise planes would look like rockets on a 45 degree launch platform. that reason is that while on the ground they arent moving fast enough to generate a lift. When on the air they are already moving at a great speed and thus the lift is generated so long as they keep that speed (there come the turbines) but to reach that (sort of) self sustaining state they need to have created one of those two conditions first. They do this by forcing air to flow faster backwards (ground acceleration) through the power of the engines.

The bottomline of my argument is that for a plane to take off there needs to be enough air moving in the opposite direction, this is achieved by the friction of the air when you accelerate and although engines help a lot in this, they wont cut it by themselves.

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furby    0

If the plane takes off, how would it happen. Would it slowly fly off the conveyor? Or would it fire off really quickly once it left the ground?

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The_Decryptor    1,105
But that's the point of the conveyor belt, to stop the aircraft moving forward...why else would it be there? (to be fair, the scenario is pretty poorly worded).

To make the question seem harder than it actually is.

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