Plane on a Treadmill

**tim_redd** · 01-26-2008, 09:36 AM

At first I thought it wouldn't take off, but now I think it would. Think of a person on roller skates, er roller blades on a treadmill. Think of two cases, the treadmill not running, and when the person on the treadmill is stationary, ie wheels and treadmill at the same speed. Now think of an outside force such as another person pushing the guy on the treadmill. This would be the thrust of the jets. It would take the same force to push the person forward the same relative amount whether or not the treadmill is on or off, assuming frictionless bearings.

I think the key part of the whole question is the speed of the wheels. If I understood this question, the speed of the wheels means absolutely nothing in the whole scheme of things.

**ISUKyro** · 01-26-2008, 09:36 AM

If it helps some to think of it. Imagine a building just to the side of the belt. As the planes wheels are going 300mph against the belt, if the plane is standing still relative to the building then it is not going to take off. Even though the engines would be producing a great deal of thrust, there would be no lift. No lift, no flight.

**jumbopackage** · 01-26-2008, 09:38 AM

If the forward motion was being generated by the wheels of the plane, the plane obviously isn't going to go anywhere. The difference here, being that the forward thrust of the aircraft is generated by either a prop or a jet (or potentially a rocket), so it doesn't matter how fast the wheels are traveling, merely how fast air is traveling over the wing. That's the difference between indicated airspeed, and true groundspeed.

The problem we have here though, is that to move the plane forward through the air, the wheels have to spin relative to the air moving over the wing while the plane is on the ground, not just relative to the treadmill. The whole earth isn't moving any faster, just the treadmill, so the air won't be moving any faster as the wheels accelerate. Of course the whole system depends greatly on the treadmill being able to determine the amount of thrust, and hence how fast to turn. That's going to be the key part of the whole thing.

My vote is that the plane will not take off, but the treadmill will make the difference. The treadmill is going to have to go VERY VERY fast in order to keep up, and the wheels will probably seize and snap off at some point, since the entire thrust of the aircraft will be just trying to overcome the friction caused by the spinning wheels, which for most aircraft is pretty small, as opposed to pulling the whole plane through the air.

The more thrust you have, and the faster it's applied, the harder it will be for the treadmill to keep up. If you slap a rocket plane on there, I don't' think it's probably feasible for the treadmill to keep up, though theoretically possible (probably). Put a Piper cub on there and slowly throttle the engine up, and I bet it stays on the ground given a fast enough treadmill. The treadmill is going to have to spin WAY faster than the plane is capable of normally going though.

**JRCampy** · 01-26-2008, 09:38 AM

Originally Posted by IsUaClone2

This summarizes best without using engineering language. Lift is created as the difference in speed relative to the air and the wing. If the air doesn't move, the plane doesn't move. The difficulty in this excercise is keeping the treadmill pace equal to the plane's attempt at acceleration.

Another way to look at it is that it is easier to take-off against the wind rather than with the wind. Accordingly, if the turbines are not allowed to propel the edges of the wing because the turbine force is negated by the rotation of the wheels against the treadmill, it is a no go. Again the trick will be to match the treadmill and plane acceleration.

Guess that's why I only made it through two years of Aerospace Engineering, couldn't talk the talk...

**jumbopackage** · 01-26-2008, 09:45 AM

Originally Posted by cmoneyr

If a plane was landing on a treadmill that was matching it's speed exactly would it just come to a hault as soon as it hit the ground? No.

The difference being that the aircraft has already acheived a positive forward velocity, enough to keep it airborne. Upon landing, if the treadmill was spinning fast enough that the resistive force generated by the wheels spinning is enough to match the forward energy stored in the aircraft, yes the plane would come to an immediate stop.

The treadmill will not be matching the relative velocity of the world around the aircraft, but the forward force applied to the aircraft by it's engines and it's relation to the mass of the aircraft.

**tim_redd** · 01-26-2008, 09:45 AM

Originally Posted by jumbopackage

That's the difference between indicated airspeed, and true groundspeed.

Of course the whole system depends greatly on the treadmill being able to determine the amount of thrust, and hence how fast to turn. That's going to be the key part of the whole thing.

I think this is the key to the question, and we understood it differently. I thought the treadmill just matched the speed of the wheels, regardless of the thrust. In which case I think it would takeoff. But if the treadmill matches the speed of the thrust, and the groundspeed is zero, the plane obviously won't takeoff.

**cyadict** · 01-26-2008, 09:46 AM

I'm guessing that the plane won't be lifting off.

**cyfan964** · 01-26-2008, 09:49 AM

Originally Posted by tim_redd

I think this is the key to the question, and we understood it differently. I thought the treadmill just matched the speed of the wheels, regardless of the thrust. In which case I think it would takeoff. But if the treadmill matches the speed of the thrust, and the groundspeed is zero, the plane obviously won't takeoff.

Exactly... it's more a question of wording. Everyone is aware of the fact that if the plane is not moving in relation to the ground, or more importantly the air, that it won't take off. I thought the question was based on wheel speed... which has no bearing at all on whether or not the plane flies, other than a small amount of easy to overcome friction.

**cytech** · 01-26-2008, 09:50 AM

My prediction is about 50% that the plane will take off due to failure of the treadmill. 40% the treadmill works and the plane doesn't take off. 5% the treadmill works and the plane takes off. And 5% something terribly bad happens that hasn't been predicted.

**ericlambi** · 01-26-2008, 09:51 AM

If this is posted, I am going all-in on WON'T take-off.

**jumbopackage** · 01-26-2008, 09:52 AM

Originally Posted by tim_redd

I think this is the key to the question, and we understood it differently. I thought the treadmill just matched the speed of the wheels, regardless of the thrust. In which case I think it would takeoff. But if the treadmill matches the speed of the thrust, and the groundspeed is zero, the plane obviously won't takeoff.

Yes, but the speed of the wheels, at least while the plane is on the ground, is directly related to the amount of thrust being provided. The plane cannot move forward while the wheels are not moving in relation to the earth. If they are spinning, but only in relation to a treadmill, the forward velocity relative to the earth is still 0, and there will still be 0 lift as air (which is generally stationary in relation to the earth) will still be going 0 MPH over the wings. It doesn't matter where the forward thrust is coming from.

What is weird about this one is that how fast the wheels are turning during the test doesn't really have any relation to how fast they might turn under normal conditions. The wheels can go infinitely fast, with frictionless bearings, and the plane still won't go anywhere. All the engine(s) are trying to overcome, in this case, is the friction of the wheels turning.

**Wesley** · 01-26-2008, 09:55 AM

Option Three:It may only fly off backwards if the mooring cables break.

**cyfan964** · 01-26-2008, 09:55 AM

Does it make a difference if this is the plane?????

Just for the record this really exists.....

[ame=http://www.youtube.com/watch?v=y9633v6U0wo&NR]YouTube - Fairey Rotodyne[/ame]

**jumbopackage** · 01-26-2008, 10:07 AM

I'd say the odds are about 90% the treadmill fails, 9% the treadmill works and the plane stays put, and 17% everything works but Adam makes fun of Jamie's moustache.

**twittkop** · 01-26-2008, 10:10 AM

Originally Posted by redrocker

So MythBusters got a Big Treadmill, huh?

I agree, it will not take off, but what do I know.

From the previews, it looks like they got a treadmill that they are going to use as a runway for a model air plane, which will be able to take off. For their full scale approach, they got a really long tarp that they are going to pull with a truck.

This does sound like a great bookie event. There are enough people that would be on both sides of this issue.

There seems to be 2 different ways to test this myth:
1. Have the treadmill going the same speed the wheels are going.
2. Have the treadmill match the forward velocity of the plane relative to the ground.

In scenario 1, they will not be able to have the treadmill match the speed of the wheels because the velocity of the wheels is going to be the forward velocity of the plane relative to the ground + the speed they are running/pulling the treadmill. Assuming the plane is going to be able to move forward on the treadmill because of the thrust produced by the prop, this scenario is going to be busted because a + b never equals b if both a and b are greater than 0.

If scenario 1 is busted, scenario 2 is automatically busted because in scenario 1 the faster they run the treadmill, the faster the wheels are going to turn, which will be much less than the forward velocity of the plane.

I believe this myth is going to be busted for the same reason that people have already stated, the wheels on a plane are not the cause of the forward motion of the plane.