Plane on a Treadmill

[htownclone]

We've been over the differences between us running on a treadmill and a plane on a treadmill multiple, multiple times.

Try this if you have a treadmill at home. Put on some rollerblades an jump on the treadmill, now have your wife or someone give you a push from behind. Do you move forward?

And I agree, a car moving forward has nothing to do with this, that's kind of the point. A plane moves in much different ways than a car. Can I request for my son to never have you for science?

If the plane is moving as fast as the belt, there is absolutely no wind. Yes I could put on rollerblades and have someone push me forward and I'd hit the front of the treadmill. This relates in no way to a plane either. You must have a decrease in pressure below the wings for lift.

I'm pretty sure by your reasoning that the belt and level platform on the treadmill would need to be like 1/4 mile long, so that the plane could be moving forward on the belt, rather than at the same speed.

 

[CyPlainsDrifter]

I've seen the light as well. The treadmill does nothing (at least any real world treadmill) to prevent the plane from moving forward. The plane will move forward easily will only a small amount of extra resistance. When the plane takes off, it will take off at normal take-off speeds even the the wheels touching the treadmill will be going 2 times as fast as normal.

The wheels decouple the plane from the surface. What the surface is doing and what the plane is doing are independent of each other. The movement of the plane is something that occurs between the plane and the air. It has nothing to do with the ground.

Welcome brother.... I took the wrong side on first glance too. It's OK to say you were wrong ..... I did. Join the club.

 

[jbhtexas]

I've read through much of it, and it seems it's all the same crap. The examples of cars moving forward does not relate in any way to how a plane takes off. Where is the wind, which is needed for take off, magically coming from. The plane is stationary on the treadmill and there is no air moving over the wings, thus no lift. Have you ever run on a treadmill before? You do not feel a breeze in your face because you are stationary relative to the earth.

When you are running on the treadmill, you don't move forward or backward relative to the treadmill because your brain contains a very sophisticated feedback algorithm that controls the motion of your legs and feet such that they match the speed at which the treadmill's conveyor is moving, and you essentially "stand still" relative to the surrounding air. If your brain tells your legs and feet to stop moving, you fall off the back of the treadmill.

The wheels and landing gear of the airplane would be analogous to your feet and legs. Except that in the case of an airplane, the landing gear has nothing to do with generating movement of the airplane. The landing gear is passive, and it is just there to support the airplane, because there is much less rolling resistance when the plane is rolling on wheels as opposed to sliding down the runway on its belly.

The airplane moves because the engines are generating thrust F=(mdot)(delta V) by changing the velocity of the air from inlet to outlet of the engine) and pushing the plane forward.

net force = (m)(a). The engines generate force (thrust) in the forward direction and which tends to push the plane forward. The rolling resistance between the tires and treadmill, the friction losses in the wheel bearing assembly, and wind resistance oppose the motion of the plane. If the thrust of the engines exceeds the total force of the resistances (which is a positve net force, and will occur in every "practical" case), the plane will accelerate forward and eventually reach lift off velocity if the treadmill is long enough. The wheels just spin at whatever rotational speed is required to account for the linear velocity of the plane relative to the treadmill belt.

If you want to use the analogy of person "running" on a treadmill, you would need to strap on rollerblades and a jetpack, where the jetpack engine outlet nozzel is parallel to the horizontal plane. What do you think would happen in that case when the jetpack engine is fired up?

 

[CyinCo]

I've read through much of it, and it seems it's all the same crap. The examples of cars moving forward does not relate in any way to how a plane takes off. Where is the wind, which is needed for take off, magically coming from. The plane is stationary on the treadmill and there is no air moving over the wings, thus no lift. Have you ever run on a treadmill before? You do not feel a breeze in your face because you are stationary relative to the earth.

What you are thinking is the initial reaction of most people. I was the same way. You can't think of anything else on the treadmill except an object that is propelled by thrust. Any object that propells itself from contact with the ground will experience what you are indicating. But objects that use thrust, such as a plane, apply that thrust to the air around them. The air around them isn't changing simply because a treadmill is present. The thrust will still be applied in the same manner. The wheel speed (even though we don't care about it) will be greater than that of a normal take-off, but the plane will still move forward and take-off.

 

[CyPlainsDrifter]

Yes I could put on rollerblades and have someone push me forward and I'd hit the front of the treadmill. This relates in no way to a plane either.

It relates quite well. The free-wheeling of the roller blades is like that on the plane wheels. The push you get is like the thrust of the engines. The forward motion you feel would be what gains the speed to provide the lift as the air passes over/under the wings of the plane.

As a science teacher, you really need to stop and think about this for a minute...... because you are wrong.

I ask again, what force is keeping the plane from moving in a forward direction? The treadmill acting against the wheels that are free-wheeling? What is countering the thrust of the engines working on the surrounding air????

 

[CyinCo]

Welcome brother.... I took the wrong side on first glance too. It's OK to say you were wrong ..... I did. Join the club.

My name is Dean and I was wrong. It doesn't happen very often. I love this kind of stuff that gets you thinking, though. At first, I would have bet money it wouldn't fly. But, when you really think about it, the treadmill isn't even relevant to the question. It is funny how it seems important but really isn't at all.

I would now bet anything that the plane takes off.

 

[htownclone]

I ask again, what force is keeping the plane from moving in a forward direction?

Are you trying to say we're using an infinitely long treadmill?

 

[cmoneyr]

If the plane is moving as fast as the belt, there is absolutely no wind. Yes I could put on rollerblades and have someone push me forward and I'd hit the front of the treadmill. This relates in no way to a plane either. You must have a decrease in pressure below the wings for lift.

You are so wrong it hurts. That is a perfect comparison. The rollerblades spin freely and are analogous to the planes wheels. The push forward from your wife = the planes thrust. The treadmill = the treadmill. All we are saying is the plane will move forward with outside thrust, i.e. the engines thrust, after it starts moving it's simple airflow over the wings that generates the lift.

 

[jbhtexas]

Are you trying to say we're using an infinitely long treadmill?

An infinitely long treadmill is not required.

 

[htownclone]

If you want to use the analogy of person "running" on a treadmill...

I never wanted to use that analogy, that was someone else.

 

[htownclone]

An infinitely long treadmill is not required.

But in order for you guys to be right, you would need a very long treadmill in order for it to have room to move forward from the thrust, which is not at all included in this scenario. If that were the case, no crap it would fly, but that, I believe is something you've added to the problem on your own.

 

[cmoneyr]

Are you trying to say we're using an infinitely long treadmill?

Obviously you haven't read the thread because that is answered in post #1.

 

[CyPlainsDrifter]

Are you trying to say we're using an infinitely long treadmill?

The original post in the thread reads: - the question is, if a plane is on an infinitely long treadmill that is capable of accelerating to match the speed of the plane, will the plane take off?

It really doesn't matter though..... the treadmill's presence is nothing more than a distraction to complicate the issue in people's minds.

If it were long enough, the plane could reach lift-off speed before leaving the treadmill. If it were too short, the plane would just run off the end of the treadmill and continue to rated speed, although the wheels would make a step change DOWN in speed as the plane left the end of the treadmill.

 

[htownclone]

But in order for you guys to be right, you would need a very long treadmill in order for it to have room to move forward from the thrust, which is not at all included in this scenario. If that were the case, no crap it would fly, but that, I believe is something you've added to the problem on your own.

I will admit I was wrong about the infinitely long treadmill, definitely didn't remember reading that. If the plane could accelerate at a rate faster enough on that treadmill to generate enough lift on the wings from the decrease in pressure below the wings, then it could take off.

 

[cmoneyr]

My name is Dean and I was wrong. It doesn't happen very often. I love this kind of stuff that gets you thinking, though. At first, I would have bet money it wouldn't fly. But, when you really think about it, the treadmill isn't even relevant to the question. It is funny how it seems important but really isn't at all.

I would now bet anything that the plane takes off.

Welcome to paradise, feels good doesn't it, air seems fresher, sun's brighter, it's a beautiful place. :smile:

 

[CyPlainsDrifter]

But in order for you guys to be right, you would need a very long treadmill in order for it to have room to move forward from the thrust, which is not at all included in this scenario. If that were the case, no crap it would fly, but that, I believe is something you've added to the problem on your own.

Huh, how is the thrust induced movement not included in the scenario. What in the world makes a plane move forward OTHER than the thrust from its engines?

And for the record..... the infinitely long treadmill IS part of the original question, although again, it DOESN"T matter. Unless you assume that at the end of the treadmill there is some structure that the plane would crash into. In that case, the treadmill would need to be long enough to avoid the contact.

 

[cmoneyr]

I will admit I was wrong about the infinitely long treadmill, definitely didn't remember reading that. If the plane could accelerate at a rate faster enough on that treadmill to generate enough lift on the wings from the decrease in pressure below the wings, then it could take off.

You're getting there, but basically the point is that it doesn't even matter that it's on the treadmill to begin with. A pilot wouldn't even notice if his plane were sitting on a treadmill, it would feel exactly the same as solid ground.

 

[cmoneyr]

Now we just need to show herbicide the way and we can be on with our lives.

 

[CyPlainsDrifter]

I will admit I was wrong about the infinitely long treadmill, definitely didn't remember reading that. If the plane could accelerate at a rate faster enough on that treadmill to generate enough lift on the wings from the decrease in pressure below the wings, then it could take off.

It would easily reach speed since the rolling treadmill does literally NOTHING other than doubling the speed of the plane's wheels. Oh.... and FWIW, the plane lifts off due to the air pressure being less ABOVE the wing, not below it.

 

[htownclone]

You're getting there, but basically the point is that it doesn't even matter that it's on the treadmill to begin with. A pilot wouldn't even notice if his plane were sitting on a treadmill, it would feel exactly the same as solid ground.

If you look at this scenario on different websites around the internet, the question is always that the belt has to be moving the same speed as the plane, so the plane is never moving forward on the belt of the treadmill, it is still stationary to earth. This isn't the first time I've heard this question and when I read it on CF, I didn't notice it didn't include that. If you used every other example from the internet, that the belt MATCHES the speed of the plane, it would not take flight.