MythBusters tackles "plane on a conveyor belt problem"

By The_Decryptor
in Science News & Discussion

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Grand Master

jackwanders

Posted
Posted

Wouldn't a plane with a prop mounted on its nose wash air over the wings regardless of whether it had any foreward momentum?

Right. Well, half right. The propeller would push air backwards, thus pushing the plane forward (equal and opposite reaction and all that). The air pushed by the prop wouldn't be enough to lift the plane by itself, but it's enough to get the plane moving at a speed fast enough for lift-off.

I was thinking more about this on the way home, and I realized the inertia would be a big factor when trying to execute this for real.

Picture a stationary plane on a stationary treadmill. The engines are off. If you start the treadmill at a slow speed, inertia will cause the plane to want to stay at rest (relative to the treadmill), so it will move backwards (relative to the observer). However, if you start the treadmill at a very fast speed, the plane will remain stationary (relative to the observer), similar to quickly pulling the tablecloth off of a table, leaving all the plates and glasses on the table.

Hmm. Although, I guess once the plane's engines are turned on, it's going to overcome inertia either way.

Ugh, where are the Mythbusters on this one!!!

Grand Master

jackwanders

Posted
Posted

Why would the wheels spin twice as fast? Also if the plane is going to lift off, would it not imply the plane is moving over the treadmill?

Think about it this way.

Imagine the treadmill is moving 10mph and the plane is stationary over the treadmill (some giant hand is holding it in place). The wheels are moving 10mph in order to cancel out the speed of the treadmill.

Now take the same scenario and have the plane moving 10mph in relation to the treadmill. The wheels now have to travel 10mph faster than they were previously, meaning they'd be travelling 20mph, or twice the speed of the plane (assuming the plane and treadmill travel at the same speed).

Grand Master

MR_Candyman

Posted
Posted

LOL! What I find funny is the third year physics student thinking he's right! This is such an obvious answer I can not believe the stupidity I've read. It wouldn't matter how fast the treadmill goes, the plane will still take off because the wheels are in a free-rolling position (with minimal friction of course), the speed of the treadmill would not be enough to prevent the plane from taking off because it doesn't matter how fast the wheels spin, the backwards movement will be minimal. A car would be a different scenario because the tires are what actually is creating the movement of the car, so you could easily have a treadmill that counteracts it (heck, think of a dyno, that's essentially what it does)

Grand Master

_sphinx_

Posted
Posted

Who cares? :|

Not you? Ok, you can leave. Jeez I hate trollers.

Seriously, did Newton and Leibnitz just throw away their books on the ground and said "who cares?" when they faced the tangent and velocity problems?

Trying to understand something like that is self-fulfilling and challenging; it also gives you an insight into the field of aerodynamics. If you're a lazy slob who doesn't have anything to share, don't bother.

Veteran

mail

Posted
Posted

LOL! What I find funny is the third year physics student thinking he's right! This is such an obvious answer I can not believe the stupidity I've read. It wouldn't matter how fast the treadmill goes, the plane will still take off because the wheels are in a free-rolling position (with minimal friction of course), the speed of the treadmill would not be enough to prevent the plane from taking off because it doesn't matter how fast the wheels spin, the backwards movement will be minimal. A car would be a different scenario because the tires are what actually is creating the movement of the car, so you could easily have a treadmill that counteracts it (heck, think of a dyno, that's essentially what it does)

So the plane will MOVE is what your saying right so the treadmill will have to be the size of a runway ?

Collaborator

Notum

Posted
Posted

Hmm.. I think I remember this discussion being brought up before on this forum.

Oh well. The plane takes off. Just imagine that since the treadmill is always matching the speed of the tires, the landing gear then becomes nothing more than a set of very low friction sticks. This allows the force of the engines to push the plane forward which then gains speed and eventually takes off. Simple. It would be just like having skies or, as a previous poster said, pontoons instead of wheels.

Grand Master

_sphinx_

Posted
Posted

Exactly.

Hey Jack, I wanted to ask youj something since you seem to have a better grasp about this that others here....why didn't you just mention at first that the friction on the tires is in the direction of motion of the plane and therefor the overall force will will be in the forwards direction?

Mentor

Blues

Posted
Posted

allow me to simplify my ans for those still agruing thinking that it will take off

if its a perfect treadmilll (ie the ground ALWAYS matches the tires speed) then no lift will be created and the plane will not take off.

and for those of you arguing that eventually the thrust will overcome the treadmill, well no-duh, did you ever thinkn that the reason you come to this conclusion is becaus euyou want ot come to this conclision

Grand Master

Fred Derf Veteran

Posted
  • Veteran
Posted

allow me to simplify my ans for those still agruing thinking that it will take off

if its a perfect treadmilll (ie the ground ALWAYS matches the tires speed) then no lift will be created and the plane will not take off.

and for those of you arguing that eventually the thrust will overcome the treadmill, well no-duh, did you ever thinkn that the reason you come to this conclusion is becaus euyou want ot come to this conclision

You are assuming that the plane would go backwards (in the direction of the treadmill).

Why?

The engine would pull it forward regardless of the speed of the treadmill. The planes wheels are free spinning. It won't take much force to overcome inertia and get it moving forward.

Grand Master

jackwanders

Posted
Posted

allow me to simplify my ans for those still agruing thinking that it will take off

if its a perfect treadmilll (ie the ground ALWAYS matches the tires speed) then no lift will be created and the plane will not take off.

and for those of you arguing that eventually the thrust will overcome the treadmill, well no-duh, did you ever thinkn that the reason you come to this conclusion is becaus euyou want ot come to this conclision

that's cute. you're still thinking of a plane like it was a car.

the wheels are free spinning. the only force pushing the plane backwards is the friction of the wheel bearings and inertia. It would take litle thrust to overcome that and start the plane forward. Take a look at all the answers in here and all the external links. It's explained quite clearly.

Collaborator

Notum

Posted
Posted

allow me to simplify my ans for those still agruing thinking that it will take off

if its a perfect treadmilll (ie the ground ALWAYS matches the tires speed) then no lift will be created and the plane will not take off.

and for those of you arguing that eventually the thrust will overcome the treadmill, well no-duh, did you ever thinkn that the reason you come to this conclusion is becaus euyou want ot come to this conclision

Hmm... Imagine this:

We have the plane sitting with its engines off on the treadmill. Above there's a helicopter hovering. The helicopter hooks onto the plane using a long tether. Now, the helicopter accelerates along the the treadmill and starts pulling at the plane.

What happens? The plane will be pulled forward by the helicopter.

Now, we don't need a helicopter for this, as it only serves as an engine on a thether. The plane already has an engine that will pull it forward, regardless of what is beneath it.

Also, the whole riddle implies that the plane moves in the first place, because if the plane is not moving neither is the treadmill. Since the wheels do not provide the plane with thrust, they would not be moving if the plane was stationary. Thus the riddle answers itself.

Grand Master

Chicane-UK Veteran

Posted
  • Veteran
Posted

The most ridiculous question i've ever heard..

Of course the plane won't take off. It doesn't matter how fast the wheels are spinning.. what matters is airflow over and under the wings.. that is what helps generate lift and get it off the ground. No airflow (because plane is stationary and just the wheels are moving) then no take off.

Sheesh.. and I dropped out of university. Doesn't take a genius to work that one out!

Collaborator

Notum

Posted
Posted

The most ridiculous question i've ever heard..

Of course the plane won't take off. It doesn't matter how fast the wheels are spinning.. what matters is airflow over and under the wings.. that is what helps generate lift and get it off the ground. No airflow (because plane is stationary and just the wheels are moving) then no take off.

Sheesh.. and I dropped out of university. Doesn't take a genius to work that one out!

Not to be rude, but you are wrong. And it seems as though peoples education level has little or no bearing on if they get this riddle right or not. It has more to do with your ability to think outside of the box than your education level. Airplanes are not propelled by their wheels.

Grand Master

Laurë Veteran

Posted
  • Veteran
Posted

The most ridiculous question i've ever heard..

Of course the plane won't take off. It doesn't matter how fast the wheels are spinning.. what matters is airflow over and under the wings.. that is what helps generate lift and get it off the ground. No airflow (because plane is stationary and just the wheels are moving) then no take off.

Sheesh.. and I dropped out of university. Doesn't take a genius to work that one out!

You are assuming that the plane doesn't move relative to the air. It can because the engines will push it forward. The engines push against the air behind the plane, not the treadmill. So assuming minimal friction from the wheels, what they and the treadmill are doing makes sod all difference.

Now then, what if the treadmill was going the other way...

Grand Master

jackwanders

Posted
Posted

The most ridiculous question i've ever heard..

Of course the plane won't take off. It doesn't matter how fast the wheels are spinning.. what matters is airflow over and under the wings.. that is what helps generate lift and get it off the ground. No airflow (because plane is stationary and just the wheels are moving) then no take off.

Sheesh.. and I dropped out of university. Doesn't take a genius to work that one out!

Planes are not propelled by their wheels. Planes are not cars. As long as the engines produce enough thrust to overcome inertia and the small friction created by the wheel bearings as the wheels spin, then the plane will move forward (relative to the treadmill). If the plane moves, then air is moving over the wings, which creates lift.

Veteran

beh

Posted
Posted

(see attached image)

You people who think that the plane won't take off need to understand that the plane is pushed by propellers and/or thrusters and NOT by the wheels. The wheels cause the friction between the plane and the runway (or in this case treadmill) to be small enough to be overpowered by the force of the plane's propellers in the opposite direction, and the plane WILL be able to move forward.

Pretend the treadmill is made of ice, and the plane had skis on the bottom instead of wheels, it doesn't matter how fast the treadmill is going (unless it's going at some unrealistically high speed) the plane will be able to move forward.

post-43380-1165820008.gif

Grand Master

dreamz Veteran

Posted
  • Veteran
Posted

the question isn't particularly clear or well-defined, at least in its original form. for instance, it does not say that wheels do not provide propulsion or that the plane has an engine (it could very well be a glider) that provides all the propulsion. not to mention the fact that it argues over the question of wheels moving as fast as the treadmill, etc.

from what i understand, whoever thought up this question was simply wondering what would happen if an airplane on a treadmill were moving such that the treadmill would cancel out any forward movement.

let's say, in one case, that the wheels do provide propulsion. suppose a plane at rest. this situation is exactly the same as though the plane were moving at some speed matched by the treadmill. if the engine can pull the plane forward when the plane is at rest, it will surely do it when the plane's speed matches that of the treadmill, and there will be air moving over the wing.

but if the treadmill can match the speed of the plane, engines and all, such that it does not move, there will be no air flow over the wing.

Grand Master

snyper

Posted
Posted
but if the treadmill can match the speed of the plane, engines and all, such that it does not move, there will be no air flow over the wing.

Answer is there. ^^ (Y)

Speed ing wheels dont + jet engines dont make the plane fly.

Here is an example.

If you run on a tread mill hold a bad beside you open side facing in the direction you run...you will notice there is no wingd filling the bag.

Do the same not on the treadmill..wou will see the bag fill up.

You need this factor to lift the plane.

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