Mythbusters 1/30/2008 - Airplane on Conveyor Belt!

Jonathan_S · 01-31-2008, 09:46 AM

Well, they put an (ultralight) airplane on a (simulated) conveyer belt and it took of just fine. Myth busted.

(I kind of wish they'd shown whether it took slightly more distance than normal to take off. But then that wasn't the myth, so I don't blame them for not bothering)

wmcbrine · 01-31-2008, 10:15 AM

It wouldn't take any more distance. What they showed was basically that what the speedometer of an airplane (unlike that of a car?) measures is actual forward speed -- so as long as the speeds on the speedometer are the same, takeoff times will be the same.

If you're saying that the airplane could be dragged back a bit if the conveyor belt started rolling before the plane did, that would technically make the takeoff distance shorter rather than greater. Really, it would just mean that the plane was dragged back beyond its starting point, and initially given a negative speed to overcome before it could hit its takeoff speed.

So, it might take more fuel -- more power to reach the same speed. But it would be a subtle difference.

DanB · 01-31-2008, 12:00 PM

I really didnt get this one. I guess that's why they say it's a big "web" discussion.

If the conveyor and plane are going at the same ground speed but in opposite directions, I would think that there shouldnt be any relative ground movement of the plane.

I was looking at this more in the terms of when enough lift would take place that the plane would "get lighter" -- removing some rolling resistance -- which would allow the plane to move forward.

I wonder what would happen if the conveyor and plane were accurately controlled to keep the two in exact speed synchronization. Then I'd think it would be entirely a question of when lift occurs (kinda like a wind tunnel).

Zen98031 · 01-31-2008, 12:19 PM

It doesn't matter how fast the belt is going, it could be going at twice the speed of the plane and the plane will still take off. The only difference is the wheels will be spinning a lot faster than they normally would be but that is it. The propeller is not pushing against the ground it is pushing against the air, does not matter if the ground is moving.

I think they could have done one final demonstration with that RC car and put a propeller on it and showed that it would move forward regardless of how fast the belt was moving. The tires just spin faster but the air is still at the same speed as it would be if the belt were not moving at all, and it is the air that the propeller is pushing against.

Mitch

5thcrewman · 01-31-2008, 12:27 PM

Wouldn't a 'gel' type shaving 'cream' expand more?

I think anyone who fills a car with expanding foam deserves to drink a pint of same said foam!

brettatk · 01-31-2008, 12:31 PM

Yeah, I dont see what all the discussion was about in the other threads. As soon as I read the first thread I selected "Yes" it would take off. Seemed pretty easy to me at the time but then again I didnt sit there and think about it for hours like others did. Looks like it only confused them.

wmcbrine · 01-31-2008, 12:52 PM

This episode would make for a good back-to-back airing with the Knight Rider myth (driving into a moving truck).

flatcurve · 01-31-2008, 12:55 PM

some people just had a hard time wrapping their heads around it. It's not too complicated if you think about it though... the plane moves by moving air with it's propeller. The resistance of the air is what allows the plane to be pulled forward. The movement of the ground has nothing to do with it. It's only there to hold the plane up while the lift generated by the wings builds up enough to overtake the force of gravity.

Jonathan_S · 01-31-2008, 01:00 PM

Quote:

Originally Posted by wmcbrine

So, it might take more fuel -- more power to reach the same speed. But it would be a subtle difference.

I'm assuming that the plane normally takes off at full throttle, so it can't just use more power to get up to speed, it's already at full power.

So if the tarp managed to impart any noticable force to the plane it would result in a longer takeoff length and/or time. (Time is probably easier to measure).

Clearly the engine is more than capable of overpowering minor force the tarp might have excerted, I'm just kind of wondering if that force was noticable. (Partly, because to me it did look like it might have taken just slightly longer to take off from the tarp than from the bare runway)

flatcurve · 01-31-2008, 01:18 PM

friction generated by airplane wheels is almost negligible. if they produced significant resistance, they wouldn't be used because of the effect it would have on take off.

andyw715 · 01-31-2008, 02:14 PM

So if a plane was tethered to a wall by the tail (tail to wall rope) and the throttle is gunned...sooner or later the airplane would "take off" (not really go anywhere but at least "levitate" ?

DevdogAZ · 01-31-2008, 02:14 PM

I think the best explanation proffered in the HH threads about this was from the Straight Dope website, which used this analogy. Pretend you're wearing rollerblades and you're standing on a treadmill. You are holding onto a rope which is attached to the forward wall. If you turn on the treadmill and hold onto the rope, you will remain stationary, but if you try to pull yourself forward (hand over hand) using the rope, you'll be able to move forward without any additional effort. The wheels may have to spin faster for you to move forward, but because the source of propulsion (pulling on the rope) is independent of the wheels, you won't have any trouble moving forward.

DevdogAZ · 01-31-2008, 02:17 PM

Quote:

Originally Posted by andyw715

So if a plane was tethered to a wall by the tail (tail to wall rope) and the throttle is gunned...sooner or later the airplane would "take off" (not really go anywhere but at least "levitate" ?

NO! The whole point of the myth is that the forward movement of the plane is unaffected by the wheels or the ground, because the thrust is caused by the propeller/jets pushing the surrounding air. Therefore, the plane moves and air moves over the wings despite the treadmill. However, if the plane itself is tethered, that negates the thrust of the engines and the plane wouldn't move, thus there wouldn't be any movement of air over the wings to provide lift.

DevdogAZ · 01-31-2008, 02:18 PM

Why did they choose fruit flies for the radiation experiment? Don't they only live 24-48 hours under normal circumstances?

Jonathan_S · 01-31-2008, 02:22 PM

Quote:

Originally Posted by andyw715

So if a plane was tethered to a wall by the tail (tail to wall rope) and the throttle is gunned...sooner or later the airplane would "take off" (not really go anywhere but at least "levitate" ?

Not unless the rope broke, or the anchor pulled out of the wall.

But that's because the engine wouldn't be able to overpower the force excerted by the tether.

andyw715 · 01-31-2008, 02:31 PM

ok....the roller skate analogy is good.

5thcrewman · 01-31-2008, 03:14 PM

When did Adam put spinnaz on his Segway?

SeanC · 01-31-2008, 03:20 PM

Quote:

Originally Posted by 5thcrewman

When did Adam put spinnaz on his Segway?

You saw that too?

That gets a

from me.

I really hate spinnaz. Hey people can spend money however they want, but man, they just look so stupid.

scottykempf · 01-31-2008, 03:37 PM

Great, now this thread is going to be like the Discovery forum about plane on a treadmill and people will just blather on and on for hundreds of pages.

JMikeD · 01-31-2008, 06:21 PM

Quote:

Originally Posted by scottykempf

Great, now this thread is going to be like the Discovery forum about plane on a treadmill and people will just blather on and on for hundreds of pages.

I think the Discovery Forum people learned that from us.

stalemate · 01-31-2008, 07:00 PM

Quote:

Originally Posted by scottykempf

Great, now this thread is going to be like the Discovery forum about plane on a treadmill and people will just blather on and on for hundreds of pages.

Whatever you do, stay out of Happy Hour then.

YCantAngieRead · 01-31-2008, 07:33 PM

I think I misunderstood the concept to begin with, for some reason. I didn't think about propellers at all. I was just thinking about lift. I guess I was assuming that it meant a jet.

The cockroach myth was gross. Ick.

doom1701 · 01-31-2008, 07:52 PM

The first time I gave this any thought was the first thread in HH--but the OP posted it by asking what would happen if the conveyor belt was moving at the same speed as the plane's wheels. That's a no brainer--no, the plane could never take off. If the wheels and the conveyor are moving at the same speed, that means that the plane is exerting just enough force to stay stationary on the belt.

I noticed that the show did that same "misdirection" a few times as well. It's like a good magician--keep pointing the audience toward something unrelated, and you can make them think anything is possible.

When you realize the role the wheels play, and understand that they really aren't part of the equation at all, it surprises me that there's so much discussion about the topic.

Mars Rocket · 01-31-2008, 10:27 PM

Quote:

Originally Posted by YCantAngieRead

I think I misunderstood the concept to begin with, for some reason. I didn't think about propellers at all. I was just thinking about lift. I guess I was assuming that it meant a jet.

The cockroach myth was gross. Ick.

Jet or propeller makes no difference - they both push against air to move the plane. The treadmill could be stationary, moving at the same speed as the wheels (whatever that means) or moving backwards at twice the speed of sound - all the treadmill does is spin the wheels. The plane will still move forward if the engine (jet or propeller) has enough thrust to move it.

MrGreg · 01-31-2008, 10:53 PM

Quote:

Originally Posted by doom1701

... what would happen if the conveyor belt was moving at the same speed as the plane's wheels.

That's also how I've always heard it. If the belt is moving fast enough that the plane is not moving forward, what happens? The plane has no lift and won't take off. Similar to the "tied to the wall" scenario mentioned earlier in this thread.

Granted, because the resistance is soooooo small through the wheels, you would need a very very fast treadmill to overcome the force from the propeller.

vertigo235 · 01-31-2008, 11:02 PM

The conveyer belt can not move fast enought to prevent the plane from moving forward...

That is the point of the myth...

However, if in fact ANYTHING prevents the plane from moving, then no, it can not fly.

Worf · 01-31-2008, 11:28 PM

Planes fly because their wings generate lift when a chunk of air moves over them. (It's the movement of air relative to the wings that generates lift).

The ground can move, or it can be stationary. The landing gear of a plane freewheels on its axle - no power from the engine goes to the wheels directly.

If you have a strong enough wind, any plane can take off and land like a helicopter (i.e., vertically) - all that has to be done is that air is moving over the wings at a sufficient rate to generate enough lift to keep the plane airborne. If the wind is moving faster than the plane needs to generate lift, it'll rise - if the plane slows down, it'll.... fly backwards (yes, it has happened).

That's why planes always take off and land into the wind - the wind reduces the ground speed thus lowering the amount of runway needed to takeoff and land. If you land with the wind or takeoff with the wind, all else being equal, the landing or takeoff will take significantly more runway.

If you put a plane on a conveyor belt running at takeoff speed, and held it still without using the engine, it won't take off (e.g., you slope the conveyor downwards enough to counteract any friction in the landing gear). As far as the plane is concerned, the ground is irrelevant to flying - it's strictly an air machine. How fast air moves over the wings determines how it flies, not the ground. Put a plane in front of a fast enough fan, and it'll start to fly.

MrGreg · 02-01-2008, 01:37 AM

Quote:

Originally Posted by vertigo235

The conveyer belt can not move fast enough to prevent the plane from moving forward...

This would be true only if the wheels were frictionless. There is a coefficient of friction, however small, so the conveyer belt DOES apply some force to the plane. Now in order to overcome the thrust from a prop (or jet) the conveyer belt may have to be moving at ludicrous speed, but it is still possible. And in that case there will be no air movement over the wings, so no lift, and no flight.

Of course landing gears aren't built to handle forces in that direction, so they would probably sheer off first, or the tires would overheat and explode.

Church AV Guy · 02-01-2008, 12:57 PM

Quote:

Originally Posted by Worf

Planes fly because their wings generate lift when a chunk of air moves over them. (It's the movement of air relative to the wings that generates lift).
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That's why planes always take off and land into the wind - the wind reduces the ground speed thus lowering the amount of runway needed to takeoff and land. If you land with the wind or takeoff with the wind, all else being equal, the landing or takeoff will take significantly more runway.

That is what I have thought all along. the conveyer belt moving would be the same as a plane trying to take off with the wind rather then into it. The plane would still take off, but would use the equivalent of more runway to achive lift.

If I remember correctly, and it's pretty hazy so I might not, but I thought the origin, or basis of the roach radiation myth was a study done in the Arazona desert where a large area (a square mile or so?) was given a steady dose of a low-level of radiation analogous to the physical conditions after the initial hard radiation of a blast had worn off. This had the effect of lowering the life-expectancy of almost everything the researchers studied, except the roaches, who seemed unaffected by the background radiation increase. It has been some time, and I don't remember where I heard about this. My brain might have even made it up.

markb · 02-01-2008, 01:33 PM

Quote:

Originally Posted by MrGreg

This would be true only if the wheels were frictionless. There is a coefficient of friction, however small, so the conveyer belt DOES apply some force to the plane. Now in order to overcome the thrust from a prop (or jet) the conveyer belt may have to be moving at ludicrous speed, but it is still possible. And in that case there will be no air movement over the wings, so no lift, and no flight.

The wheels are going to contribute some drag, which is going to work against the plane taking off, but the conveyor speed should make no difference in the amount of drag. The is going to be a slight difference in rolling friction between the runway and the conveyor, but only because the material is different, and thus the coefficient of rolling friction is different. But speed does not come into play when calculating rolling friction.