MythBusters tackles "plane on a conveyor belt problem"


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serious

I bet 100$ that the plane (at least theoretically) takes off. Like so many of you are saying, the wheels will spinn faster than the plane is moving forward. The plane WILL move forward, no matter how fast the conveyor is moving backwards. The wheels don't exercise any kind of force, no movement or anything.

Just think about it. Imagine you keep the plane still with some kind of rope/wire and fires of the engine. The airplane will exercise a forward force, but the ropes will hold it still - it won't take off (obviously). Now, place the rig on a conveyor and start the band. The wheels will move in one direction but the plane won't go anywhere (because of the ropes).

Now, start the engines and remove the ropes. The airplane will go forward, no matter the speed of the conveyor. The wheels have nothing to do with the force forward or backward (in fact this is kind of false as there's some friction involved).

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Sethos

Why does no one stop to ask, what would happen if you put the conveyor belt on the plane?

Think about that

:ninja:

( :p )

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Slugsie
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.

Planes get ALL of their forward thrust by pushing the air backwards. Be it in the air or on the ground, and yes they do do that entirely by the force of the engines. If they don't entirely rely on the engines, then please do tell exactly what else is pushing an aircraft forward. The ground provides exactly ZERO force with regards to getting an aircraft moving. It contributes exactly ZERO of the air-speed required for the Bernoulli effect to come into play.

while on the ground they arent moving fast enough to generate a lift

Erm, if they can't generate enough lift whilst on the ground, how does any aircraft ever take off? Clue... at standstill, the engines push air backwards, this causes the aircraft to slowly move forward. They push more air backwards, the aircraft gets faster. This keeps happening until the aircrafts forward speed is sufficient for the lift generated by the wings to lift the aircraft from the ground. The only function of the wheels is to keep the aircraft off the ground whilst the wings aren't generating enough lift (which is just as relevant at standstill as it is whilst it's half way down the runway).

Edited by Slugsie
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monkey13

Ok the first problem is that the current theory of flight is just that a theory. I think there are also three competing theories. I can only find links for 2.

Anyone quoting the bernoulli effect (what I and most of you were taught at school). You're wrong see NASA

Also 2 other incorrect theories (NASA again) here and here

Two of the current theories (can't find the third)

Lift From Pressure Area

Lift From Flow Turning

Both of which require movement through the air.

So if the plane is stationary relative to ground and air whilst on the belt I'm going with it doesn't fly.

The straight dope answer continually quoted in this topic assumes that the plane is moving forward on the conveyor. If the plane is moving forward then what is the point in the conveyor? It does nothing except make the wheels spin faster.

Also the treadmill argument is a joke. Imagine running on a treadmill you are running as fast as you can and the treadmill is set to this speed. How do you move forward then? You can't.

Edited by monkey13
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Kushan
Planes get ALL of their forward thrust by pushing the air backwards. Be it in the air or on the ground, and yes they do do that entirely by the force of the engines. If they don't entirely rely on the engines, then please do tell exactly what else is pushing an aircraft forward. The ground provides exactly ZERO force with regards to getting an aircraft moving. It contributes exactly ZERO of the air-speed required for the Bernoulli effect to come into play.

Think of it this way:

When the aircraft moves forward (normal aircraft, normal situation), it pushes itself into the air in front of it. This air goes over the ENTIRE aircraft, including the entire wingspan, generating the lift once it's going fast enough.

But if the plane can't move forward, then the air around the plane isn't moving over the plane, it's just surrounding it. The only air being moved is that from the engines, which aren't enough to generate the air flow needed to create the lift (plus they'd only push air over certain parts of the craft anyway).

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Slugsie
The straight dope answer continually quoted in this topic assumes that the plane is moving forward on the conveyor. If the plane is moving forward then what is the point in the conveyor? It does nothing except make the wheels spin faster.

And that is exactly the point. The conveyor does nothing. As I stated before, if (as you and other postulate) the aircraft isn't moving, then by your very own argument, neither is the conveyor (the conveyor moves at the same speed as the aircraft remember). So if the conveyor isn't moving, then how is it any different to a normal piece of runway tarmac? It isn't.

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Kushan
And that is exactly the point. The conveyor does nothing. As I stated before, if (as you and other postulate) the aircraft isn't moving, then by your very own argument, neither is the conveyor (the conveyor moves at the same speed as the aircraft remember). So if the conveyor isn't moving, then how is it any different to a normal piece of runway tarmac? It isn't.

Aren't you now arguing the wording of the scenario more than what will actually happen to the plane?

I was under the impression that the conveyor was just a way of saying "the plane cannot move forward, it's wheels can spin but it wont move forward, will it still be able to take off?".

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Slugsie
Think of it this way:

When the aircraft moves forward (normal aircraft, normal situation), it pushes itself into the air in front of it. This air goes over the ENTIRE aircraft, including the entire wingspan, generating the lift once it's going fast enough.

But if the plane can't move forward, then the air around the plane isn't moving over the plane, it's just surrounding it. The only air being moved is that from the engines, which aren't enough to generate the air flow needed to create the lift (plus they'd only push air over certain parts of the craft anyway).

OK, please tell me exactly how a conveyor belt, acting through free running wheels (i.e. no brakes or anything like that) is able to physically arrest the movement of the aircraft. IT CAN'T, and IT DOESN'T. The fact that the conveyor is running backwards simply means that the wheel are spinning at twice the speed of the aircraft. Nothing more, nothing less.

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Kushan

But it's not on free running wheels, it was stated that it matches the exact speed of the wheels of the aircraft....

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Slugsie
Aren't you now arguing the wording of the scenario more than what will actually happen to the plane?

I was under the impression that the conveyor was just a way of saying "the plane cannot move forward, it's wheels can spin but it wont move forward, will it still be able to take off?".

Nope, if the point of the scenario was to prevent forward motion of the plane, then why bother with a conveyor. Why not just tie it to a huge big spike in the ground?

The point of the conveyor is to introduce a scenario where the ground is moving, and moving contrary to the attempted movement of the aircraft.

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Kushan

The ground is moving, but it's being kept in the same position, so it has no air speed.

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monkey13

If the aircraft is moving down the conveyor then what is the point in this experiment?

It doesn't affect the aircraft except to make the wheels go round faster. It would be like asking if it would take off with wider wheels or a different tread. If the aircraft is still able to progress along a "runway" (conveyor or otherwise) and get ground + air speed up of course it's going to take off.

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Slugsie
But it's not on free running wheels,

No, it is on free running wheels, as in the wheels are free to spin at any speed, and in any direction (forwards or backwards) they want. If the wheels weren't free running then they would either be braked or powered.

it was stated that it matches the exact speed of the wheels of the aircraft....

It's stated that the conveyor matches the speed of the aircraft, not the speed of the wheels.

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monkey13
The point of the conveyor is to introduce a scenario where the ground is moving, and moving contrary to the attempted movement of the aircraft.

Well then by that argument it has to stop it otherwise it isn't "moving contrary to the attempted movement of the aircraft".

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Slugsie
The ground is moving, but it's being kept in the same position, so it has no air speed.

HOW is the aircraft being kept in the same position? The only way the ground can prevent the movement of the aircraft is if it is able to exert a force on the aircraft. Given that the wheels aren't braked or powered, then there is no way a force can be exerted.

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Slugsie
Well then by that argument it has to stop it otherwise it isn't "moving contrary to the attempted movement of the aircraft".

Nope. If the aircraft is moving forward at 100mph, then the point of the scenario is that the conveyor is moving backwards at 100mph. This will result in the wheels spinning at 200mph and thus the forward speed of 100mph of the aircraft is maintained. Thus it is moving through the air, and generates lift, and thus can take off.

Like I have already stated. If the conveyor is able to prevent the aircraft moving in any way shape or form, then by your very arguments, the conveyor is also not moving (it maintains the same speed as the aircraft remember).

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ring0

this is turning into one of the ol' faithfuls of the internet. any time this issue is brought up, it automatically turns into the dumbest f'ing argument you'll ever see.

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Kushan
Nope. If the aircraft is moving forward at 100mph, then the point of the scenario is that the conveyor is moving backwards at 100mph. This will result in the wheels spinning at 200mph and thus the forward speed of 100mph of the aircraft is maintained. Thus it is moving through the air, and generates lift, and thus can take off.

Like I have already stated. If the conveyor is able to prevent the aircraft moving in any way shape or form, then by your very arguments, the conveyor is also not moving (it maintains the same speed as the aircraft remember).

But the wheels of the plane are NOT free to move whatever way they want. They will be exerting a force onto the ground up until the point the plane takes off. Essentially, until those wheels lift off the ground, they act like any other wheeled vehicle out there (which is where people get confused when you start comparing it to cars on conveyor belts). That's where the confusion really lies with this, are we testing if a plane can take off from a standing position, or are we testing if a conveyor belt will prevent it from moving forward? I think you're arguing towards the latter and I'm arguing about the former.

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Guest jgrodri

Ok,

perhaps i need to explain myself better. On an airplane, the engines are doing two things to make the plane take off:

- "sucking up" air and expelling it towards the back. This creates thrust for the airplane.

- by this expelling, the plane is moved forwards. this high speed movement creates lift for the airplane as wind moves around the wings.

the combination of thrust and lift makes an airplane take off and continue flying.

An airplane on a convener belt will generate all the thrust it could possibly need but because it will not move forward it will encounter no additional air resistance.

No resistance means no additional air flow on the wings. This translates into zero lift. Thrust on its own will not lift and airplane.

i dont know how good my analogy may be but it would be like trying to make a bird fly in a vacuum chamber (assuming it wouldnt die): it can generate power to move but it wont be able to leave the ground.

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sundayx

I use to think that, I was wrong. The conveyor belt matches the plane speed, but does nothing to control it's velocity. The plane is an independent entity that accelerates and takes off.

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monkey13
I use to think that, I was wrong. The conveyor belt matches the plane speed, but does nothing to control it's velocity. The plane is an independent entity that accelerates and takes off.

As I said before then. What is the point in the conveyor? If the plane is able to accelerate and move along it it has no effect on the plane apart from making the wheels move round faster.

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sundayx

To generate this argument and confusion.

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Slugsie
But the wheels of the plane are NOT free to move whatever way they want. They will be exerting a force onto the ground up until the point the plane takes off. Essentially, until those wheels lift off the ground, they act like any other wheeled vehicle out there (which is where people get confused when you start comparing it to cars on conveyor belts). That's where the confusion really lies with this, are we testing if a plane can take off from a standing position, or are we testing if a conveyor belt will prevent it from moving forward? I think you're arguing towards the latter and I'm arguing about the former.

The only force the wheels will be exerting is the one required for keeping the aircraft from falling to the centre of the planet. They do not act like any other wheeled vehicle, because other wheeled vehicles (i.e. cars, bikes etc) have powered wheels.

However, your argument that an aircraft cannot take off from a standing position is correct. It can't. I'm also arguing that a conveyor is not capable of arresting the forward motion of an aircraft, thus it it not trying to take off from a standing start.

One last try at an analogy (not specifically aimed at you Kushan).

You are standing on a skateboard (i.e. device whose wheels are unpowered and unbraked). There is a rope that you can pull on to move yourself forward. You are capable of pulling that rope with sufficient force to move forward at 5mph. This is a reasonable analogy to an aircraft in normal takeoff. You're pulling instead of the aircrafts pushing, but the point is the motive force is not as a result of a force applied to the ground. Now, put a tread-mill underneath the skateboard, set to run at 5mph. Does anyone seriously believe that if you apply the same pulling force to the rope that you will not continue to move forward at 5mph? The wheels on the skateboard will indeed be spinning as if you were moving along at 10mph, but that has no relevance. If your take-off speed was 5mph, you'd now be airborne.

As I said before then. What is the point in the conveyor? If the plane is able to accelerate and move along it it has no effect on the plane apart from making the wheels move round faster.

And again, there is no point to the conveyor. It is little more than a distraction. It has no, and cannot have an, affect on the forward motion of the aircraft, and thus has no effect on the aircrafts ability to achieve lift-off speed.

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OPaul

Without going through and reading all the post here. Isn't a plane taking off more about wind speed then forward momentum. For instance, a strong gust towards a stationary plane would cause the plane to lift because the air is flowing over the wings. So what's really needed is the air to move, or the object to move through the air. Seeing as a plane on a conveyor doesn't move, and the air isn't moving at any considerable speed... the plane will get no lift. Or is that all wrong?

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SkyLost1984

I agree with slugsie, the analogy of the skateboard is something you could try if you were feeling brave. If you put a skateboard on a treadmill, and stood on the skateboard, the turned the treadmill on, you would not move, the wheels would spin as fast as the treadmill was moving, but the overall object will not move relative to the treadmill. Therefore pulling on the rope would make you go forward.

If a plane was stationary on a conveyor, and the conveyor started to rotate, then again, the wheels of the plane would spin but the plane itself would not move position. When the engines on the plane were powered, the plane would exert a force on the air and begin to move forward, with the wheels spinning at the combined speed of the forward speed of the plane plus the rotational speed of the conveyor.

So you can see that the conveyors effect on the plane or skateboard is negligable.

Sorry if this has been explained before, but i havent read the whole thread.

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