Mythbusters - airplane on a conveyor belt

Here are the best of the links that describe why the plane will take off so far:

http://youtube.com/watch?v=-EopVDgSPAk

http://www.straightdope.com/columns/060203.html

Did that Michigan engineering student ever come back?

Tre1nt wrote:

Jets thrust.

Wheels spin.

Relative to the ground the plane does not move forward.

Airspeed over wings does not increase.

Plane does not take off.

How does the force created by a treadmill on basically frictionless wheels = the amount of force created by engines (prop or jet) on the air?

Big Ten Engineering wrote:

Did that Michigan engineering student ever come back?

Yes, but he posted as Tre1nt.

Neb wrote:

How does the force created by a treadmill on basically frictionless wheels = the amount of force created by engines (prop or jet) on the air?

You've come around, Neb. Good boy.

wa runner wrote:

Big Ten Engineering wrote:

Did that Michigan engineering student ever come back?

Yes, but he posted as Tre1nt.

actually we disowned him, he's no longer one of us.

Ok, all this talk has me confused about what the question is.

So here is what I'm picturing in my head: There is a plane on a treadmill. As the planes engines begin to create thrust, the treadmill starts running and accelerates to match the thrust of the plane, so that the plane is at full throttle, but not moving forward as it is held essentially stationary by the treadmill.

Is this the question here? If a plane at full power but stationary in comparison to the ground will take off? Or did I misread something?

The confusion is coming because people are envisioning two separate scenarios.

In the first, the plane is not moving relative to anything around it. To someone watching from a block away, it would appear the plane is stationary because the treadmill matches the speed of the plane. This is analogous to a person running on a treadmill. You don't actually move forward. In this situation (not that I'm saying this is what would actually happen), the plane does not take off because there is no lift.

In the second, the plane does move forward along the treadmill. Although many before me have explained this better than I can, the thrust of the plane's engines "pulls" the plane forward. No matter how fast the treadmill goes, the plane will move forward because the engine pulls the air through. This is analogous to the rollerskater with the jetpack on. In this situation, the plane would take off because it is moving forward and creating lift.

Everyone clear now? It all depends on how you explain and envision the hypothetical question.

Ohio Rulz - we are not even close to the same understanding. Unless you've changed your view you think the wheels and the treadmill are going at the same speed. They are NOT.

Um wrote:

Is this the question here? If a plane at full power but stationary in comparison to the ground will take off? Or did I misread something?

The answer is that it is not possible for a plane to remain stationary on the treadmill. The engine's thrust will trump the treadmill's backward force, no matter what speed the treadmill is traveling at.

As for the Michigan student, he'll eventually learn two things:

1. Don't do that.

2. You wasted $150,000 on an education you coulda got for $1.50 in late fees at the public library.

The late fees don't come with a degree though.

Um wrote:

Ok, all this talk has me confused about what the question is.

So here is what I'm picturing in my head: There is a plane on a treadmill. As the planes engines begin to create thrust, the treadmill starts running and accelerates to match the thrust of the plane, so that the plane is at full throttle, but not moving forward as it is held essentially stationary by the treadmill.

Is this the question here? If a plane at full power but stationary in comparison to the ground will take off? Or did I misread something?

The picture in your head is wrong. The riddle says the treadmill matches the speed of the plane not the thrust of the engines. How can force created by the friction of wheels on a treadmill counteract the force created by the engines of a plane at "full throttle"?

The plane is not held in place - it moves forward. the only difference is that the wheels spin faster than they would on a normal runway. Keep in mind the treadmill is as long as a normal runway.

Big Ten Engineering wrote:

As for the Michigan student, he'll eventually learn two things:

1. Don't do that.

2. You wasted $150,000 on an education you coulda got for $1.50 in late fees at the public library.

...Were you gonna plagiarize the whole thing for us? Do you have any thoughts of your own on this matter? Or do you, is that your thing, you come into a bar, read some obscure passage and then pretend - you pawn it off as your own, as your own idea just to impress some girls, embarrass my friend?

The plane will take off because unlike a car the engines on a plane are independent of the wheels...such that the wheels can spin with out the engine. The wheels will rotate at twice the velocity of the plane but the plane will move forward as long as friction does not become a dominant force (which it will not). The engines will create the thrust that will push the plane until enough lift is generated to allow the plane to take off.

P.S. I an a mechanical engineer in my senior yeat at RIT

The best analogy given so far was riding on rollerblades being pulled by a rope tied to a truck. At 5 mph, it takes a certain amount of force for you to hold on. Now the truck's going 50 mph. Guess what? It still requires the same amount of force to hold on. The extra speed isn't being imparted to the wheels, they're just spinning freely. If you were to pull yourself forward along the rope to the truck, again, it would require the same amount of force no matter how fast the truck was moving.

Same thing with this situation. The speed of the ground underneath becomes irrelevant, but then as we increase our pulling force, we continue to move forward.

[[[[[[[[[[[Im an aerospace major.

Since I just took my final on aerodynamics yesterday, this is still fresh in my mind and Ill put in my 2 cents.

First, lets relate this plane to our own running bodies.

The thrust from say, jet engines, is comparable to our leg muscles.

Both will exert JUST enough force/energy to keep the person/aircraft in the same location on the treadmill.

The thrust from the jet engine will keep the plane moving "forward" on the mill, but really it is in the exact same place, just as our legs keep us in the same exact place in the room (on the mill)

The thrust produced pushes the body of the aircraft forward, but a jet engine doesnt make things fly or lift up.- wings do!

Airflow OVER the wings (as so many have stated already) generates lift (low/high pressure blah blah).

Knowing a little bit about treadmills, I know that when I run on one, the air around me in the room is STAGNANT, smelly, and humid. There is no airflow blowing against me from any angle.

That plane can be sitting (and keeping up) with a treadmill spinning at Mach 5, but its not going to lift into the air. Basically, there is absolutley NO air circulation, activity, or velocity around the wings whatsoever. PERIOD.

The fact that there are 9 pages of postings about this is sad.]]]]]]]]]]]

What is sad is that one day you may be designing aerospace technology that could possibly kill someone if not designed correctly, and yet you don't have a basic understanding of how a plane functions.

K.R.

it amazes me the people who come on here and say "i'm an aerospace engineer" or "my dad works as nasa and he says..." or "i teach physics..." and then proceed to give a long winded public display of their astonishing lack of intelligence.

A treadmill and a conveyer belt are not the same thing.

If you pose this as a "plane on a treadmill" that implies a scenario where the plane does not move relative to the ground or the air (and therefore it will never fly).

The "conveyer belt as long as a runway" is different matter. There must be air moving over (and under) the plane's wings to generate lift. So if the conveyer belt scenario allows for this this then yes, the plane will fly.

That's all there is to it.

Tre1nt - How is a conveyer belt different than a gigantic treadmill?

More importantly how does a conveyer belt allow for movement of the airplane - while a treadmill doesn't?

Conveyer belts move (or convey) things.

Treadmills keep the user (usually a runner) in place.

It's not just semantics. If the plane stays in place it does not fly. If it moves forward relative to the ground AND air it might fly.