MythBusters tackles "plane on a conveyor belt problem"

By The_Decryptor
in Science News & Discussion

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Grand Master

Rohdekill

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The laws of physics should be universal right? I say yes, plane will take off. I've a previous experiment to verify my hypothesis. So lets say my test case is a human running on a treadmill, call him the unexpecting classmate. Now, if unexpecting classmate can go flying-off a treadmill - and humans are not meant to fly - does this not mean that same principal applied to an airplane, which is meant to fly- would work? I think so.

*This is wrong on so many levels, including experimenting on humans which I do not condone.

Answer me this. While running on treadmill, do you feel a breeze you create from running? No!! No breeze, no lift.

Grand Master

Astra.Xtreme

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If lift is dependent on velocity, then once that certain velocity is reached, it will create lift.

The plane should be reliant on the ground to get that necessary velocity needed for life.

Once it's in the air, it shouldn't go below that velocity, so you can't compare that.

Grand Master

+Xinok Subscriber²

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I'll try to explain this as simply as possible:

The plane is moving forward at 20mph.

This means the conveyor belt is moving in the opposite direction at 20mph.

This means the wheels are moving at 40mph.

Think of it this way:

You're on a conveyor belt, and you're on a skateboard. (or wearing a pair of skates, whatever makes you happy).

You're pulling on a rope to pull you forward.

Is the conveyor belt gonna prevent you from moving forward? No.

The same concept applies to this myth. Instead of pulling on a rope, the jet engines essentially "pull" on the air to move the plane forward.

Grand Master

HawkMan

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it WONT be stationary. the conveyor will be moving back at the same speed as the plane's forward speed. but the trick is that the plane's speed is NOT dependent on the GROUND (otherwise how the HECK do you think it flies after leaving the ground?!). the plane moves by pushing the air back and not the ground back.

so what happens is that the conveyor's speed is totally irrelevant to the plane. it will still move forward, because it doesn't depend on the ground to move forward (only to keep it from "falling to the center of the earth" while it doesn't have enough windspeed to sustain itself).

the plae still has weight pushing down. now say the conveyor belt is moving backward at practically unlimited speed. the engines would need to give the same force to keep the plane stationary(yeah the conveyor belt moves backward at the same speed as the plane "would" have moved forward, but t's the same thing) In addition to this to get th eplane moving forware it would need to at least doule that force again.

afterall the myth says the converyor belt moves as fast as the plane can ever counter. so the converor would essentially be pushign the plane backwards at it's own weight.

but then agai, I voted for the plane taking off :) but seeing as the pilot guesed wrong, I'm trying to figure out how it could not take off...

Grand Master

Rohdekill

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i contacted the leader of a RC plane club a while ago over this and he said it wouldn't take off in theory, i reckon the same but lets see where mythbusters take this...

like some dweeb from an RC plane club is an expert. lol

Grand Master

kaffra

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Probably on a smaller aircraft where the propeller provides some airflow across the wings.

the key is the airflow across the wings. no lift no take off.

i dont see this possible with the bigger jets, the engines are used to provide trust. and as the plane is moving forward, the airflows across the wings to provide the necessary lift.

if the plane is standing still(because of the conveyor belt) and the exhaust air of the engine doesnt flow across the wings, then i dont see it taking off.

Grand Master

Astra.Xtreme

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I think you guys are thinking too much into reality.

They are testing it as if there were a possible way to keep the plane stationary by using a conveyor belt. Or am I wrong?

If they are testing if the plane will outrun the conveyor, then that's obvious that it will.

Also, wouldn't just setting the brakes and blasting the engines have the same effect as to having it roll on a belt?

Mentor

Hooya

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Posted
I'll try to explain this as simply as possible:

The plane is moving forward at 20mph.

This means the conveyor belt is moving in the opposite direction at 20mph.

This means the wheels are moving at 40mph.

Think of it this way:

You're on a conveyor belt, and you're on a skateboard. (or wearing a pair of skates, whatever makes you happy).

You're pulling on a rope to pull you forward.

Is the conveyor belt gonna prevent you from moving forward? No.

The same concept applies to this myth. Instead of pulling on a rope, the jet engines essentially "pull" on the air to move the plane forward.

At least somebody else in this thread understands physics! Thank you for restoring my faith in the human brain.

They are testing it as if there were a possible way to keep the plane stationary by using a conveyor belt. Or am I wrong?

That is indeed what they are essentially testing. There is NO WAY the conveyor has ANYTHING to do with the thrust of the engines, and therefore the speed the plane will move while on the ground. All that will happen is the wheels will spin faster than they would if they weren't on a conveyor. That's all. The plane will act as if the conveyor wasn't there in every other way (unless the steering is controlled by the wheels, in which case they're screwed!!!)

Grand Master

Doli

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Do MythBusters ever have experiments go wrong and still show it on TV?

I just dont see how that can pull this off getting the speed right with a large conveyor belt and the weather condition (wind).

Enthusiast

Fred 69

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I think the plane will take off.

My reasoning behind it-

If the plane wasn't moving and the conveyor belt was:

Wheels wouldn't move and plane would be moved backwards.

If the plane was moving and the conveyor belt wasn't:

Wheels would move as the plane moves through the "air" to allow it roll over the ground.

If both the plane and the conveyor belt is moving:

Initially the plane will be stationary but as it speeds up the plane will reach nearer to the speed the engines are pulling air at while the conveyor belt will be moving at the same speed the engines are pulling air at, BUT the wheels will be spinning the sum of these.

So while the plane's engines will be running at say 500Kph and the conveyor belt at 500Kph, the wheels at (perhaps not exactly?) 1000Kph, the velocity of the plane will be increasing steadily towards 500Kph but never quite reach it until the plane lifts off.

At least that's how I understand it.

Mentor

Lewism

Posted
Posted
Do MythBusters ever have experiments go wrong and still show it on TV?

I just dont see how that can pull this off getting the speed right with a large conveyor belt and the weather condition (wind).

Watch Wednesday's show and see it

Grand Master

shakey_snake

Posted
Posted (edited)
I'll try to explain this as simply as possible:

The plane is moving forward at 20mph.

This means the conveyor belt is moving in the opposite direction at 20mph.

This means the wheels are moving at 40mph.

Think of it this way:

You're on a conveyor belt, and you're on a skateboard. (or wearing a pair of skates, whatever makes you happy).

You're pulling on a rope to pull you forward.

Is the conveyor belt gonna prevent you from moving forward? No.

But it will take twice as much force on the rope for you to overcome your inertia and begin moving forward...

The conveyor belt functions exactly like an incline (minus the angle of attack issues with the airfoils)

The question is, does the plane have enough engine power? Will the engine be able to hit it's "power band"?

So, I say, this depends on the plane. The smaller the plane, the more likely, since the engine power/weight ratio changes as the plane scales.

Edited by shakey_snake
Contributor

golemn

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The jet plane will fly, the wheels do not affect the speed of the aircraft at all. Everyone that thinks it wont is thinking that the wheels are what create thrust. The wheels spin freely has nothing to do with the plane. Just imagine it and it will become clear.

Grand Master

gigapixels Veteran

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  • Veteran
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The jet plane will fly, the wheels do not affect the speed of the aircraft at all. Everyone that thinks it wont is thinking that the wheels are what create thrust. The wheels spin freely has nothing to do with the plane. Just imagine it and it will become clear.

No, nobody's thinking that the wheels create the thrust, we're saying that the motion of the plane through the air will create lift... Since it's not moving through the air, it's not creating lift.

Grand Master

HawkMan

Posted
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The jet plane will fly, the wheels do not affect the speed of the aircraft at all. Everyone that thinks it wont is thinking that the wheels are what create thrust. The wheels spin freely has nothing to do with the plane. Just imagine it and it will become clear.

Must be why all those planes keep rolling off the carriers when they start moving.

noones saying the wheels is propelling the plane or that the wheels aren't spinning freely.

but let's scale this thing up from the microlight they are testing with. put a passenger jet weighting over a hundred tons or so on a conveyor and start moving the conveyor backward, no matter how freely the wheels spin, the Jet engines will need to provide twice the thrust to keep the plane still to keep it moving forward as fast as the converyor moves back. and that's not counting that the conveyor is moving backward at the same speed as the plane "would" be moing forward.

on the other hand, the conveyor doesn't start moving untill the wheels do, wich means by the time the converyor is countermoving to the wheels, the plane has allready achieved momentum, wich makes it unlikely the conveyor woudl ever be able to counter this momentum. But this is only a propelm iin reality, not in the thought experiment where there would be no such lag.

Contributor

golemn

Posted
Posted (edited)
No, nobody's thinking that the wheels create the thrust, we're saying that the motion of the plane through the air will create lift... Since it's not moving through the air, it's not creating lift.

hmm, the plane will move through the air.

Lets say i am on a trendmill on a skateboard thats attached to me. I am holding a rope attached to a car. The car accelerates, i am going to fly off that trendmill on the skateboard the bearings spin freely. Now if i had wings, and reached 140 mph ( varies on wing span and weight ) I would literally fly.

The thrust coming from the jet engines is like the acceleration from the car.

I dont think I painted a good enough picture, but of this i am certain. The plane will create lift and fly.

Must be why all those planes keep rolling off the carriers when they start moving.

noones saying the wheels is propelling the plane or that the wheels aren't spinning freely.

You know they have the wheel locks... I have seen one roll off a carrier...

Where in Norway do you live Hawkman, I have been there multiple times?

Edited by golemn
Grand Master

The_Decryptor Veteran

Posted
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  • Veteran
Posted
The important part to remember is:

If the pilot guessed wrong, it must be the opposite of what everyone assumes.

I was talking to a pilot (at least he claimed he was one), and he swore that planes couldn't take off using engine power alone. Don't assume they know the right answer (or even how a plane works).

Answer me this. While running on treadmill, do you feel a breeze you create from running? No!! No breeze, no lift.

Answer me this. Are you an airplane?

The most common problem with this question, is people start thinking the plane behaves with a car (probably because the type of ground it's on is specifically mentioned), airplanes don't move by spinning the wheels, they move by using their engines (which pulls/pushes the entire plane forward)

Grand Master

sundayx Veteran

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  • Veteran
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It pushes the plane forward so enough air can move through the wings. The engines don't give the plane lift, the wings does. Being on the treadmill, there won't be enough air through the wings, only through the engines, which is working it's ass off trying to draw air in.

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