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MythBusters tackles "plane on a conveyor belt problem"

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Sethos    270

Can someone call Mythbusters :laugh:

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.fury    0

Red Herring :sleep:

Please demonstrate why it will not fly...

Seeing as I can't be bothered to type out a full explanation myself... (don't know if any of this stuff has been quoted in the thread already)...

#1

"The weight pulls down on the plane opposing the lift created by air flowing over the wing."

#2

"Lift is a force on a wing (or any other solid object) immersed in a moving fluid, and it acts perpendicular to the flow of the fluid. (Drag is the same thing, but acts parallel to the direction of the fluid flow). The net force is created by pressure differences brought about by variations in speed of the air at all points around the wing."

#3

Good old Newton:

"Air flowing over the wing is ultimately deflected downward by the angle of the wing, and Newton said there has to be an equal and opposite reaction, so the wing is forced upward."

What it comes down to is that the air movement past the wings of the plane cause lift to be created, as there is a pressure difference between air moving over the wing and air moving under it. If the plane is sitting there doing nothing WITH RESPECT TO THE AIR AROUND IT then no lift will be created!

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Toxicfume    7

Okay I didn't get it straght away and changed my mind a few times over, and my conclusion is Yes, the plane will fly.

I chose yes in the poll in the beginning, and i thought damn it! It wont, and thenthinknig it over again, I made the right choice.

Fred Derf speaks the truth my friends!

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Fred Derf    212

Well, I originally thought "no" because I was thinking of a theoretically perfect treadmill that could move fast enough to keep the plane still.

Since the wheels are free-spinning (i.e. not connected to an engine like a car in gear) then they would turn very easily and thus it would take a ridiculous amount of force to make an effect with the relatively small resistance of the wheels.

If we assume that the treadmill can only match the speed of the plane, then the wheel resistance will easily be overcome by the plane and it will move forward on the treadmill fast enough to get lift. In fact, if the treadmill is moving backwards at the same speed that the plane is going forward then it ultimately will have a minimal impact on the takeoff. The free-spinning wheels will be going like crazy (and potentially generating a fair bit of heat) but so long as the tires don't explode the plane will have no trouble lifting off.

Mythbusters could never create the theoretically perfect treadmill that could keep the plane from moving forward and thus taking off. There just wouldn't be much wheel resistance to work with. The theoretically perfect treadmill would need to rotate several orders of magnitude faster than the plane's takeoff speed to make enough of a dent. In fact, it would also need to predict the acceleration of the plane to keep it stationary (from a 3rd party perspective). Essentially, that is not going to happen. The energy requirements would be enormous.

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.fury    0

Yeah, on second consideration of the implications of the question, the plane would take off. It is quite ambiguous, the way I first read it, the question asked if the plane could just suddenly tilt the airfoils and lift off the ground, which it obviously couldn't. If the plane is allowed to create thrust of its own via the engine, which is how i now read it given some other posts, then it will, as the wheels are independent.

Apologies.

It would have to be a bloody long treadmill though!

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jackwanders    0

Fred Derf speaks the truth my friends!

No respect, I tells ya. No respect at all. The guy with the tag gets all the credit.

/5000

//milestone threads are for suckers, past, present and future company excluded of course

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Jason S.    1,276

no, it wont fly. if the plane isnt moving forward, there's no lift created under the wings. it's like saying if youre running on a treadmill, are you just going to fly forward into the control console and hurt yourself? as physics says, the net velocity is 0. the plane goes nowhere b/c the treadmill is going the same speed in the opposite direction. w/o forward velocity, there's no lift created.

if anything, you could try turning the plane around and using the opposite direction of the belt to provide thrust like an aircraft carrier.

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choppa    14

I don't see how it's going to be able to create lift if the treadmill and wheels are spinning at the same time. A one prop airplane won't be strong enough to create lift.

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Laura    145

Yeah, on second consideration of the implications of the question, the plane would take off. It is quite ambiguous, the way I first read it, the question asked if the plane could just suddenly tilt the airfoils and lift off the ground, which it obviously couldn't. If the plane is allowed to create thrust of its own via the engine, which is how i now read it given some other posts, then it will, as the wheels are independent.

Apologies.

It would have to be a bloody long treadmill though!

You could have tried reading more of the thread before posting.

no, it wont fly. if the plane isnt moving forward, there's no lift created under the wings. it's like saying if youre running on a treadmill, are you just going to fly forward into the control console and hurt yourself? as physics says, the net velocity is 0. the plane goes nowhere b/c the treadmill is going the same speed in the opposite direction. w/o forward velocity, there's no lift created.

if anything, you could try turning the plane around and using the opposite direction of the belt to provide thrust like an aircraft carrier.

You didn't read the thread did you? This has been explained enough times already.

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jackwanders    0

no, it wont fly. if the plane isnt moving forward, there's no lift created under the wings. it's like saying if youre running on a treadmill, are you just going to fly forward into the control console and hurt yourself? as physics says, the net velocity is 0. the plane goes nowhere b/c the treadmill is going the same speed in the opposite direction. w/o forward velocity, there's no lift created.

if anything, you could try turning the plane around and using the opposite direction of the belt to provide thrust like an aircraft carrier.

you probably would've been advised to read the thread first.

Just because the plane and treadmill move at the same speed doesn't mean the plane is stationary. The plane's speed is not canceled out by the treadmill because the wheels are not used to propel the plane.

This is not a "if the plane is stationary, will it take off" question, it's a "if the treadmill matches the plane's speed, will the plane take off". Thinking the plane is stationary is a faulty assumption.

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quigley0    0

oh man, this thread again.

I swear it was last december that we went over this.

The ground has nothing to do with a plane taking off. The engines Pull (or push if rocket) the plan. So the plane PULLS or PUSHES itself against the AIR. The wheels being free-rolling, will just spin faster and faster as the plane continues to move forward with no regard to the "treadmill" at all.

In fact, I believe that the wheels end up spinning twice as fast as the plane is moving. So, if the plane is moving forward at 100mpg, and the treadmill is at -100mph, then the wheels end up spinning at 200mpg.

But the plane moves forward as it should, and it takes off, and the airport struggles to determine why the hell they spent a billion dollars on a treadmill runway.

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Jason S.    1,276

i'll believe it when i see it. like everyone else says, call in the Mythbusters.

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Jason S.    1,276

realizing that the wheels actually dont do anything, i still dont believe the plane/engines would create enough lift w/o the plane physically moving forward.

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Fred Derf    212

realizing that the wheels actually dont do anything, i still dont believe the plane/engines would create enough lift w/o the plane physically moving forward.

Again, the problem is that you assume the plane would not move forward.

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quigley0    0

Again, the problem is that you assume the plane would not move forward.

Exactly. The plane moves forward as it should, generates lift as it should, and takes off as it should.

A plane (jet engine or propeller based) is sitting on a treadmill, the treadmill will always speed up to match the rotation of the wheel's, in reverse, no matter how fast the wheel's turn, the treadmill will match*.

Actually, i just noticed that the phrasing of this makes this a logically impossible puzzle. I believe that the correct wording should be the treadmill matches the speed of the plane, not the wheels.

See, as the plane moves forward the wheels will spin twice as fast as the treadmill. So, the treadmill will match the speed of the wheels, thus making the wheels spin even faster. So, if the wheels spin 10miles per hour, and the tread mill will now go 10mpg. The wheels will spin to 20mpg, and the treadmill can never match the speed of the wheels.

A=A+10

not possible.

Here is something for all of the "non-believers" to think about: WHere are your test-taking skills? Why on earth would someone create a question like this, and have the easy obvious answer to the masses be the correct one?

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linsook    149

at first i thought no... but i changed my mind. yea it will fly.

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christracy    0

I am under the impression that the speed of the wheels will be matched dynamically. Those saying that thrust by the engine is what causing the plane to take off then answer this, why don't all planes just sit at the end of a runway with the brakes on and just crank up the engines? The plane has to move and if that movement is negated by treadmill counteracting any force being sent to the wheels then there is no movement.

The trust of the engine is transferred to the wheels causing a plane to move through the air creating lift. If that trust is negated by the fact that the treadmill is moving in opposite direction at the same speed then the plane is not moving through the air. Without moving through the air creating lift the plane will not take off.

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dreamz    4
I am under the impression that the speed of the wheels will be matched dynamically. Those saying that thrust by the engine is what causing the plane to take off then answer this, why don't all planes just sit at the end of a runway with the brakes on and just crank up the engines? The plane has to move and if that movement is negated by treadmill counteracting any force being sent to the wheels then there is no movement.

The trust of the engine is transferred to the wheels causing a plane to move through the air creating lift. If that trust is negated by the fact that the treadmill is moving in opposite direction at the same speed then the plane is not moving through the air. Without moving through the air creating lift the plane will not take off.

the thrust isn't negated by the treadmill because thrust is a function of air, not the ground. as i explained before, hang an airplane from a piece of string and start the propellor. does it move forward? of course it does, but that's because the fan is acting on the air around it. it has nothing to do with the wheels.

as for the case where the plane is on the ground, all the engine has to do is overcome all the forces that keep the plane in place on the ground, e.g., friction. that isn't too hard.

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Michael_C    0

Think about what happens if you switch the engines off and sit the plane on the treadmill, it is going to fly off the back. The engines will need to counteract this force and so the plane may not be able to take off depending on the speed the treadmill is moving at.

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MR_Candyman    114
I am under the impression that the speed of the wheels will be matched dynamically. Those saying that thrust by the engine is what causing the plane to take off then answer this, why don't all planes just sit at the end of a runway with the brakes on and just crank up the engines? The plane has to move and if that movement is negated by treadmill counteracting any force being sent to the wheels then there is no movement.

The trust of the engine is transferred to the wheels causing a plane to move through the air creating lift. If that trust is negated by the fact that the treadmill is moving in opposite direction at the same speed then the plane is not moving through the air. Without moving through the air creating lift the plane will not take off.

Thrust is NOT transferred to the wheels!

Michael_C, yes, that is what would happen, but on bearings there's so little resistance that there's no poaaible way a treamill would be able to counteract even a tiny little plane, since the only possible factor is the friction in the bearings of the wheels connected to the plane that could possibly make the plane not take off. Now if you had some really tight bushings that provided a ton of resistance, then it would be do-able, but that's not the point of the question at all. I said it before on page 1 or 2, I can't believe people have a hard time with this question.

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Laura    145

They can't be bothered to read what others have written.

They can't be bothered to think about what others have written and how/why it might be right.

This is because they are typically arrogant and convinced "they must be right". People on the internet for you.

Either that or they are just stupid.

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shine_victim    2

i am confused as to how many ppl have put yes. Surely the plane wont take off if no air is passing under the wings and all the plane is not actually moving that cannot happen, so my answer is no!

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sharpy2k4    10

the plane will take off shine_victim, as it overcomes the friction, it would just behave as if it was on a normaly tarmac airstrip, just the wheels would be moving alot faster. so their would be air going over the wings as the engine is pulling it forward, and this would create the lift required for it to take off.

Thats how i see it i think after reading the thread,

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[f]    0

I havent read the whole thread, but I have read thru some of the longer replys. What most people who are answering YES are leaving out is the fact that the plane will leave the treadmill...the treadmill isn't going to make the plane not move just by matching the speed of the wheels. Similar to how if u tied a rope to the front of the plane and started winding it on a winch, it would leave the treadmill, whether or not the treadmill matched the speed of the wheels (it sure is hard to imagine, but has to be the result).

The jet acts like the winch, or even better, like 500,000 men pushing at the rear of the plane. The result has to be that the plane leaves the treadmill (and therefore, given enough distance, could take off), not (like it sort of might appear form some of the replys) that the plane will take off FROM the treadmill (not possible given the scenario)

Anyway, I think I'm right (lol, I ALWAYS do).

i am confused as to how many ppl have put yes. Surely the plane wont take off if no air is passing under the wings and all the plane is not actually moving that cannot happen, so my answer is no!

Exactly, flight is a function of lift (air molecules traveling over and under the wing), not wheels acting on tarmac or overcoming friction.

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