How to start an argument with physics

[Tyson]

Ah Jaysus lads.

 

The wheels rolling along a conveyor or a runway has nothing to do with the plane getting into the air. The lift is generated by the movement of air over the wings. The movement of air is provided by the engines thrusting the plane forward.

 

Technically the conveyer could be running in the same direction that the plane needs to travel to generate lift, faster than the planes wings are passing through the air, meaning the wheels could actually be rolling in reverse and the plane would still take off as the movement of the air over the wings has no relationship to the conveyor speed or the direction in which the wheels are turning. Regardless of which direction or how fast the conveyor is moving the air above it is static and it is the engine's affect on the air that produces forward movement of the plane, which generates the lift.

 

But the plane isn't moving through the air as it's stationary, the engines don't pull air over or under the wings they are there to push the plane through the air the flaps and ailerons divert air to create the lift needed not the engines. Obviously it needs engines to get the plane travelling fast enough for their to be enough lift created by the air to take off. Another thing to think about is the force of the engine thrust on the runway as the nose is lifted this effectively pushes the aircraft up off the runway as the engines are angled at the runway.

 

E.g if you run on a treadmill is air rushing past you ?

[Digsy]

Once the thrust that you mention counters the wheel friction then the wheels are going faster than the conveyor. The Question says this imaginary conveyor belt will always match the speed of the wheels so the plane will always be in the 1 spot on this conveyor so no lift for the wings...

 

It will match the speed of the wheels, not the speed of the plane.

Assuming the conveyor is moving backwards, it still cannot physically move the plane backwards, or even decelerate it, by acting through its tyres unless the brakes are on or the bearing friction is extremely high.

 

I say assuming because that's what everyone (including myself) has been doing. However I'm not even sure that's correct anymore.

 

Looking at the attachment:

 

In step 1, the plane is at rest on the runway.

In step 2, the engines are on and the plane moves forwards by a minute amount. The tyre contact patch is actually moving backwards with respect to the plane. The marker on the tyre stays more or less above the marker on the conveyor (assuming the plane only moves a little way).

In step 3, if you interpret the question literally, the conveyor has to move forwards, as it is matching the speed of the wheel but in the opposite direction. The conveyor rotates the wheel back to its start position where the two markers line up. However the wheel centre has still moved, therefore the plans has moved, therefore there is airflow over the wings, so it will take off.

 

Looking at it this way, even the wheel bearing friction ceases to be a potential issue as the wheel never really rotates at all.

 

If they conveyor moved backwards, it would be moving in the same direction as the wheel, doubling it speed of rotation but still not actually moving the plane itself backwards.

 

Within the context of the question steps 2 and 3 would actually happen simultaneously.

[Digsy]

E.g if you run on a treadmill is air rushing past you ?

 

If you run on a treadmill you are propelling yourself forwards with respect to the treadmill by pushing against the surface of the "road" which itself is moving backwards at the same rate.

A jet aeroplane moves itself forwards by pushing the jet exhaust against the air behind the engines* (which itself remains stationary). Your analogy would only hold true if the plane was being driven down the runway by its wheels.

 

*edit* more correctly, pushing against the air and expelling exhaust mass backwards to create an equal and opposite reaction forwards through conservation of momentum, as stated by pedro below.

[pedrosixfour]

But the plane isn't moving through the air as it's stationary, the engines don't pull air over or under the wings they are there to push the plane through the air the flaps and ailerons divert air to create the lift needed.

 

E.g if you run on a treadmill is air rushing past you ?

 

But the engines, pulling air into the compressors, adding fuel and directing the expanding gases rearward, supply the forward momentum. The wheels provide nothing in terms of movement except reduced friction compared to sliding the plane on the ground. So the speed at which they are turning is irrelevant. The air entering the engine's is unaffected by the conveyor.

[Tyson]

But the engines, pulling air into the compressors, adding fuel and directing the expanding gases rearward, supply the forward momentum. The wheels provide nothing in terms of movement except reduced friction compared to sliding the plane on the ground. So the speed at which they are turning is irrelevant. The air entering the engine's is unaffected by the conveyor.

 

Agreed but as you said the air is being pulled through the engine not around the wings so there will be no lift generated.

[pedrosixfour]

Agreed but as you said the air is being pulled through the engine not around the wings so there will be no lift generated.

 

But once the engines are doing their job, which you must understand is totally unaffected by the wheel or conveyor speed due to the surrounding air being static in relation to the conveyor, the forward momentum is there and once enough momentum is generated by the engines the plane must move forward and there will be sufficient airflow over the wings to generate lift.

 

The wheels, friction drag aside, are free wheeling, whether on a runway or on a conveyor causing them to rotate at a faster rate. They are not powered by the engines so no power is used to match the speed of the conveyor.

 

That bit about the conveyor matching wheel speed is impossible to attain anyway. Once the wheels start to turn, even slightly, the conveyor will start to move backwards, causing the wheel speed to increase, which the conveyor will try to match, causing the wheels to turn even faster still. If rolling resistance was taken out of the equation the plane could sit there with the engines powered down but the wheels and conveyor would increase their speeds until they simply disintegrate.

[Ric]

Yes, it takes off, i've seen mythbusters.

[Attero]

I haven't seen mythbusters...

 

Yes it will take off.

 

All it needs is thrust. The wheels have no purpose or meaning other than to safely transport a plane on ground. They are free moving and so the plane will happily move forward as per normal.

[JackyBoi]

Of course it will take off. Like others have said the wheels are free rolling - doesn't matter if they're spinning at 200mph or 2000mph, the engines will generate enough thrust to overcome this

[Pete]

Oh no, not this again... can't find the last thread.

[Dnk]

I wasn't going to reply but oh well

 

I didn't take physics so here's my simple logical opinion.

 

If the plane isn't moving forward because its on a conveyor belt rotating underneath it then no air is passing over the wings

thus no lift created so it wont go anywhere, all the belt is doing is rotating the wheels

[Al Massey]

I'll just like to reiterate, that a conveyor belt, or a runway act in the exact same way.

 

And a plane takes off on a runway

[Dnk]

Only if the brakes are on the wheels or they'd just rotate and the plane stays put

[Alex_GT]

Why does it have to be an aircraft with turbofan engines? Surely it would have made much more sense to use an aircraft with no propulsion system of its own like a glider?

[Dnk]

 

http://nslog.com/2008/01/31/stupid_people_refuting_mythbusters_over_planeconveyor_belt

[JackyBoi]

I wasn't going to reply but oh well

 

I didn't take physics so here's my simple logical opinion.

 

If the plane isn't moving forward because its on a conveyor belt rotating underneath it then no air is passing over the wings

thus no lift created so it wont go anywhere, all the belt is doing is rotating the wheels

 

But the plane uses air to move forward, and the air isn't moving with the conveyer. Imagine the conveyer is travelling at 200mph backwards, and the plane is turned off, the plane will go backwards. But when the engines come on (which for example on a 747 is 1/4 of a million pounds thrust each) that is PLENTY of energy to overcome the friction being made by the wheels on the conveyer, even if the bearings were worn. The wheels would need to be creating an equal or greater amount of energy that the engines are producing to prevent it taking off, which is near impossible. I hate this myth!

[Digsy]

OK to save this eating up all the server space like it did last time...

 

The question isn't really about physic at all. It is deliberately badly worded to be ambiguous and to demonstrate how people interpret things in different ways, and once they choose an interpretation, how difficult it is to convince them otherwise.

 

The question isn't clear enough about what it means when it says the conveyor belt moves "at the same speed as the wheels". Which part of the wheels? The outer radius of the tyre (i.e. the tangential velocity of the wheel), or the forward speed of the axle? Many people assume it is the forward speed of the axle, and then assume it follows that this means the conveyor belt stops the plane moving forwards.

 

However the question does tell us that the wheels move. If the wheels didn't move, then there would be no speed for the conveyor belt to match. If the wheels move, then the plane is moving. In reality there is no physical mechanism that can act through the wheels to decelerate the plane once it starts moving. At best the conveyor will change the reative speed between the "ground" and the plane, and therefore speed that the wheels rotate, but it cannot affect the forward speed of the plane. If the plane is moving, the wings will generate lift and the plane will take off. This is the mechanically correct answer to the question.

 

However, if you accept the boundary conditions of the question as "the backwards motion of the conveyor cancels out the forward motion of the plane" (despite there being no practical way that this could ever happen), then no, the plane would not take off.

[bignum]

Wheels are only fitted to a plane to stop it rubbing across the ground when taking off or landing, what happens underneath the wheels has no bearing on the plane taking off.it just means the wheels will spin faster before the plane takes off.

[Big Mark]

A plane needs to move forward so that air passes around the wings to generate lift, as the plane would effectively be stationary then it won't generate lift and therefor not take off. Simples..................

[Digsy]

^^^^ Anyone else loving the irony of Mark's post, and the attachment?

[Kendo11]

The plane does take off.

 

It's a 747 using old Harrier thrusters, therefore it takes off vertically.

[bignum]

^^^^ Anyone else loving the irony of Mark's post, and the attachment?

 

Lol, come on Mark, and i thought you were an engineer?

[Big Mark]

???

 

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[Style]

Mythbusters did this episode.

 

The plane took off.

 

The wheels are free rolling, so technically the runway already acts as a conveyor belt as the ground and the wheels move at the same speed.

 

Mythbusters didn't re-create the problem properly though. They just ripped a giant carpet out from under a plane effectively. The runway needs to match the speed of the wheels in the opposite direction exactly at all times. AKA as soon as the wheels even think about moving forward, the belt immediately counters that movement and keeps the plane firmly in place. Mythbusters couldn't recreate that so it was kinda debunked

[Style]

The plane needs to move in the forward direction relative to the ground in order to generate the lift to take off. But if the ground is matching that exact momentum produced in the opposite direction then the plane will remain in equilibrium and never go anywhere/take off.

 

I guess you could argue that the smallest break in friction at the contact surface will set the whole thing off and it will eventually leave the ground. But I'm taking this as classroom physics conditions.