RE: "A plane is standing on a runway. . ." No, it's not. Here's why.

Cecil's Columns/Staff Reports
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Cecil,
I am new to this airplane on a treadmill conundrum (and straightdope.com), but I think your column on this issue from March 3, 2006 more scientifically addresses this thought experiment than any other I have read. However, that being said, I feel there is one key piece of information that many people (even you) have overlooked in the scenario below.

In your column you say in BR#2 if it were possible to accelerate a treadmill to a high enough speed to counteract the force of the plane’s thrust that the plane would stay in place. Now, I understand why you say that and you explain your reasoning quite eloquently, but here is why I believe the infinitely accelerating treadmill will still fail to keep the plane from achieving lift.

It is because air has mass and there is a lot of air resting on that treadmill. This can be proven by a simple experiment. Simply turn on a small treadmill at its highest rate of speed you can easily feel the slight breeze generated by holding your hand just above the surface of the treadmill.

If this mythical treadmill is now exponentially increased in both size and speed, it will also exponentially increase the rate of airflow generated by the treadmill. As the treadmill reaches several hundred miles per hour, I theorize that it will be generating enough wind speed on its own to give the plane lift effectually allowing the plane to still takeoff with minimal difficulty.

Does this theory hold any water or am I just thinking too hard on this? Anyway, thanks for fighting ignorance!! Someday I hope we all win that battle!!

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God help us.

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A list of previous threads on the subject:

Plane on a treadmill column

Comments:
Speaking of treadmills, read this.
Plane on a Treadmill, redux
A formulation of the airplane/conveyor belt problem that “works”?
Planes with “0” airspeed can fly?
Flight and the Conveyor Belt
Conveyor belt?
Plane on a Treadmill - SOLUTION
Airplane taxiing
The Airplane and the Treadmill
error in the “will the plane take off”
RE: A plane is standing on a runway. . ." No, it’s not. Here’s why.
Mythbusters – Plane on a treadmill
Mythbusters – Airplane on a treadmill!!
Mythbusters 1/30/08 Plane on a conveyor belt
Plane on a treadmill
Mythbuster PoaT episode (open spoilers in OP)
Plane on a Conveyor – Revisited
Plane on a treadmill
Plane on a Conveyor Belt
I’d rather not bump that thread. If this thread is allowed to live, would a passing mod add it to the ‘roundup’ thread?

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Actually, the OP raises an interesting question that I don’t recall having been discussed in the earlier threads. I hadn’t considered air being accelerated by the treadmill itself. Not that it matters of course, since the plane would take of anyway for reasons discussed in the other threads.

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OP does raise an interesting point but given the quite bizarre nature of the thought experiment for BR2 (the speed of the conveyor belt required to counteract the planes engines would very rapidly become so high that it would incinerate itself from friction with the air and its own bearings would seize) it’s a bit academic.

There are a dozen reasons why the experiment wouldn’t work in practice and this is just another of them.

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I’ve brought up the same point in the past, but I’m not about to go digging through all of the previous threads to find where it was.

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Me neither! :stuck_out_tongue:

.

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The Myth Busters busted this myth pretty well. Planes have no problems on treadmills no matter how fast the treadmill runs the other direction.

Hard data trumps theory no matter how elaborate the theory.

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And all the more so, when the hard data and the theory agree.

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Only when the hard data actually corresponds to the theory.

In the case of BR2 the theory proposed would be impossible to try because of the speed that the conveyor belt would need to achieve.

In any case, BR2 is effectively a tautology because all it’s saying is that if you apply a backwards force to an aircraft that exactly matches the forward force of the engine(s) the aircraft won’t move. It just happens to use a conveyor belt accelerating to impossible speeds to apply the backwards force.

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My bigger question has always been; how can a plane STAND on a treadmill? If I say something is sitting on something, I’m always told I should be using set.

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Who always tells you that?

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Result F in this post. The practical issue here (ignoring the multitude of “reasons discussed in the other threads” as you say) is that the treadmill is speeding far into the supersonic realm by the time enough boundary layer is developed to move the air at wing height up to liftoff speed.

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Mytbusters did it in a different way than we’re talking about here, though. Their plane was still accelerating forward relative to the ground. The concept we were talking with was that the treadmill would be motionless relative to the ground beneath the treadmill. In that (assuming a magical treadmill, yada yada), it wouldn’t fly because it’d never get enough lift (barring some freaky airplane which could get off with almost zero lift, yada yada).

However, I’ve seen at least three “versions” of this weird question, because the concepot is vague and variable enough to cover a lot of ground. Or not.

(Ba-dum KSSSH!)

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BigT said:

Some confusion here. If you are placing an object down somewhere, the verb is set. If you are placing yourself down somewhere, the verb is sit.

If an inanimate object is taking the active role (it is doing the verb), then treat it like yourself. The object is sitting itself down, not setting itself down.

Same way an inanimate object can stand.

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Except of course, that suns, moons, and airplanes can set. =)
Powers &8^]

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Isn’t that what one usually just calls a “runway”? Why have a treadmill at all if it’s going to be motionless relative to the ground?

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Nooooooooooooooooooooooooooooooooooooooooooooooo

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If you ask why we tread on a runway and run on a treadmill, I’m going to have to make you sit in a corner…

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it doesnt matter how fast the treadmill is moving under the aircraft. the aircraft can still take off. why?

the treadmill only effects the wheels. the thing that moves the aircraft is either a prop motor, or a jet engine.

if the wheels were the object that made the plane move to get its lift, then yes. a treadmill would keep the aircraft from taking off.

how ever this is not the case. this is something you could do at home. different objects are used but its basically the same thing.

get these items
treadmill
skateboard
rope

first the effects of a treadmill against propulsion that is made from the ground.

if you put the skateboard on the treadmill and get on it while the treadmill is running. you can skate along the moving floor and not go anywhere as long as your forward momentum is the same as the speed of the treadmill.

now tie your rope to the wall directly in front of you.
stand on the skateboard. the skateboard is now the body of your aircraft sitting on wheels that give it no forward thrust.
hold onto the rope with you arm. you arm is now your prop, or engine that gives the thrust you need to go forward. and the rope is the air the prop uses to create this motion.
when you pull on the rope you move forward. it doesn’t matter how fast the treadmill is moving under you. the pull on the rope will make you go forward.

its a very simple comparison. many wont understand it. so ill try to re-word it to try and make it simpler.

your aircraft is sitting on wheels that give no thrust. a treadmill can only spin the wheels.
a prop engine uses the air around to generate forward moment. when the engine is running fast enough the blades will pull the craft forward. it doesnt matter how fast the treadmill under the plane is moving, the craft will move forward because of the air around. all the treadmill can do is spin the wheels.

a jet engine works in the same way almost… only difference is instead of the engine pulling the craft through the air, the jet pushes. set a toy plane down on a treadmill. it doesn’t matter how fast the treadmill is running, if you push it forward it will go forward. just like a jet engine would do to a real plane.

in the end the treadmill would not generate the proper drag required to keep the plane from moving forward.

simple solutions for “complex” problems. you don’t always need a overly complex way of proving something to prove something.