Plane on a Conveyor Belt

[tristanhilton85]

I did vote yes, but I really had to think hard about... not an easy question and one that I'll have fun asking to others later.

[Missouri Swede]

A flight surgeon got this right when the engineers got it wrong?  Scary thought.

[Ernest Jones]

Originally Posted by Golfingdad

Ok ... I get it. I'm picturing it "running in place" but in actuality it would still be moving the same speed regardless of whether or not the runway is moving since the thrust comes from the jets, is that it?

(I still vote no ... The friction on the wheels caused by them moving too fast will cause them to burn up and the plane will have to abort its takeoff ;))

Maybe I'm slow but I still picture the plane "running in place" in this scenario so I don't understand where the "lift" is coming from? Can someone slow this down so I can get it? Under my understanding the answer is definitely no, but I think I'm misunderstanding the scenario??

[tristanhilton85]

Originally Posted by Ernest Jones

Maybe I'm slow but I still picture the plane "running in place" in this scenario so I don't understand where the "lift" is coming from? Can someone slow this down so I can get it? Under my understanding the answer is definitely no, but I think I'm misunderstanding the scenario??

The plane is not "running in place." It is moving forward... the wheels on a plane do not propel the plane forward as they do on a car, the jets do that.

If you couldn't see the conveyor belt, everything would appear to be normal. It's not like the plane appears to be standing still and then all of a sudden is flying. The only thing the conveyor belt actually does is make the wheels spin faster (twice as fast to be exact).

[Ernest Jones]

If an object is moving forward 100 mph on a surface that is moving in the opposite direction at a speed of 100 mph wouldn't the object in question be stationary? And if said object was a plane, wouldn't it be incapable of liftoff? Not trying to argue just trying to understand why I don't get it....

[tristanhilton85]

Originally Posted by Ernest Jones

If an object is moving forward 100 mph on a surface that is moving in the opposite direction at a speed of 100 mph wouldn't the object in question be stationary? And if said object was a plane, wouldn't it be incapable of liftoff? Not trying to argue just trying to understand why I don't get it....

No.... because the wheels are essentially just there to hold the plane up when it's on the ground... they just spin but they do nothing to move the plane forward, the thrust from the jets or propeller do that. The only thing that the conveyor belt does is makes the wheels spin faster, so the plane is going 100 mph forward, the conveyor belt 100 mph backwards, meaning the wheels are spinning at 200 mph.

[Ernest Jones]

The airport I frequent (ironically) has a moving sidewalk, it is possible to walk against the "flow" in a way that makes you stationary, if you were to do this and then leap into the air you would not be "leaping" forward, you would just be hoping up and down no? What difference does it make whether the "thrust" is created by the friction drive of the wheels or the propulsion drive of the jets or propellers? Again, not trying to be an ass and this is obviously and old riddle but I can't quite grasp the logic behind it and I keep thinking that there is a flaw somewhere....

[Ernest Jones]

Ahhhh, did a little research on this question via the interweb and the pretense is that, if the conveyor belt were moving "backwards" at 100 mph (or any other arbitrary speed) the plane would be stationary as the wheels would just spin, I am not sure I buy that. I think that if you were to put a model plane on the surface of a record player (for example) the plane would spin around with the record and not sit stationary with the wheel spinning. If anyone has a record player and a model plane handy I would like very much to see a video of the result.

I suppose if you were to introduce the thrust of the jets or propellers the plane would be able to remain stationary BUT in order to take off the thrust would necessarily be greater than ( and not equal to) the opposing force of the moving runway in order to generate the aerodynamics required for liftoff...

[Ernest Jones]

Originally Posted by Ernest Jones

Ahhhh, did a little research on this question via the interweb and the pretense is that, if the conveyor belt were moving "backwards" at 100 mph (or any other arbitrary speed) the plane would be stationary as the wheels would just spin, I am not sure I buy that. I think that if you were to put a model plane on the surface of a record player (for example) the plane would spin around with the record and not sit stationary with the wheel spinning. If anyone has a record player and a model plane handy I would like very much to see a video of the result.

I suppose if you were to introduce the thrust of the jets or propellers the plane would be able to remain stationary BUT in order to take off the thrust would necessarily be greater than ( and not equal to) the opposing force of the moving runway in order to generate the aerodynamics required for liftoff...

Of course, I am not a scientist by any stretch of the imagination so maybe I just don't get it.

[billchao]

The airport I frequent (ironically) has a moving sidewalk, it is possible to walk against the "flow" in a way that makes you stationary, if you were to do this and then leap into the air you would not be "leaping" forward, you would just be hoping up and down no? What difference does it make whether the "thrust" is created by the friction drive of the wheels or the propulsion drive of the jets or propellers? Again, not trying to be an ass and this is obviously and old riddle but I can't quite grasp the logic behind it and I keep thinking that there is a flaw somewhere....

Imagine you are wearing roller skates on that moving sidewalk, going against the direction of the belt. Now imagine there is a long rope tied on to a pillar beyond the length of the sidewalk. Can you pull yourself through the conveyor belt with the rope? That's how thrust independent of the ground works. Also if you are stationary on a conveyor and jump straight up, you would land behind your original position. That might be the piece you are missing. While the plane may appear stationary (relative to our perspective) it is actually moving forward (per its perspective). Remember that engines on planes will move air regardless of whether they are on wheels or affixed to a specific position.

[Ernest Jones]

This is gonna take some serious "Bong Time" lol.

[Ernest Jones]

Pulling on the Long Rope attached to the pillar would mean I'm travelling faster than counter-motion of the belt no? I don't disagree that if the planes forward thrust is greater that the belt's "backwards" thrust then the plane can take off, but if they are equal...????

Would the planes ability to take off not require some degree of differential??????

[billchao]

Originally Posted by Ernest Jones

Pulling on the Long Rope attached to the pillar would mean I'm travelling faster than counter-motion of the belt no? I don't disagree that if the planes forward thrust is greater that the belt's "backwards" thrust then the plane can take off, but if they are equal...????

Would the planes ability to take off not require some degree of differential??????

I guess I didn't explain it as well as I thought about it in my head...

The plane takes off via air pressure differential on its wings. The engines propel air, causing air around the plane to move faster than the surrounding atmosphere. As long as the air movement is fast enough to create lift, the plane will take off. Therefore, as long as the plane's engines are creating enough thrust upon the plane, it will fly.

Maybe try thinking about a mattress strapped on top of someone's car moving down the highway. The mattress is stationary in relation to the car (much like the plane is to the conveyor) but the air movement caused by the propulsion of the car lifts up the mattress. The relation of the mattress to the earth is irrelevant.

[billchao]

Originally Posted by Ernest Jones

Ahhhh, did a little research on this question via the interweb and the pretense is that, if the conveyor belt were moving "backwards" at 100 mph (or any other arbitrary speed) the plane would be stationary as the wheels would just spin, I am not sure I buy that. I think that if you were to put a model plane on the surface of a record player (for example) the plane would spin around with the record and not sit stationary with the wheel spinning. If anyone has a record player and a model plane handy I would like very much to see a video of the result.

I suppose if you were to introduce the thrust of the jets or propellers the plane would be able to remain stationary BUT in order to take off the thrust would necessarily be greater than ( and not equal to) the opposing force of the moving runway in order to generate the aerodynamics required for liftoff...

That's because the model airplane has very little mass and therefore little inertia, which means it is easily moved by the record player. A plane, having more mass, is more likely to remain stationary, causing the wheels to spin.

My wife tells me that my mattress example isn't very good, either, so I apologize if you're still confused. I guess the important thing here is that a plane's movement is through the air and independent of the ground.

[iacas]

Good discussion guys. Thanks. ;)

Ernest, the airplane's wheels just spin. If the plane was stationary (i.e. speed of 0), then the conveyor belt that "matched" that speed would also be at 0. When the plane is going 100, the conveyor belt is going 100 in the opposite direction. If that negated the plane's forward progress, then the plane would be going 0, and the conveyor belt would be going 0 again. It's an impossibility.

Anyway...

The wheels just spin. They provide nothing except to reduce the friction between the ground and the plane to virtually nothing.

This reduction in friction allows the actual forces - which are applied BY THE PLANE TO THE AIR (not to the ground, like a car or a runner) - to pull the plane against the air (the rope in the example someone provided above).

The plane pushes or pulls the air. It moves the plane through the air column (that's sitting just above ground level prior to impact). The wheels just let the plane roll more easily when this force wants to push/pull the plane.

[Chipless]

I have to disagree with the consensus of this thread in three areas...

If the conveyor belt is going 170 mph in a backwards direction, and the plane engines are providing enough thrust for the the plane to go forward at 170 mph, then the plane would actually be sitting still and would have no forward movement with regards to other stationary objects.  There would also be no air movement over the wings, unless the wind was blowing.

[quote] The only thing that the conveyor belt does is makes the wheels spin faster, so the plane is going 100 mph forward, the conveyor belt 100 mph backwards, meaning the wheels are spinning at 200 mph. [/quote]

In the example I gave above, the wheels of the plane would be moving at 170 mph, not 340 mph.  There is no force that would double the speed of the wheels.  The conveyor belt is going backwards at 170 mph, and the plane wheels are going forward at the same speed.  170 - 170 = 0, which is the speed of the aircraft.

[quote] I'm a pilot, and the answer is yes. Forward movement of an aircraft is caused by the thrust of its propeller or jet, not by it's wheels like that of a car. Therefore, the movement of the conveyor in no way affects the aircraft's ability to move forward relative to its position through the air which is what generates lift and allows for flight.[/quote]

No offense, but si nce the plane is sitting still relative to the air surrounding the wings, there would be no lift, and this plane has no chance of taking off.  The rotation speed required for takeoff must be in relation to the air moving across the wings.  I think this is called "indicated airspeed".  So with a 20 mph headwind, you'd only need 150 mph of actual land speed.  But with no aircraft speed, you might need ~ 170 mph headwind to take off.

This would be a completely different story if we were talking about a plane specifically designed for vertical takeoff...

[iacas]

Originally Posted by Chipless

1) If the conveyor belt is going 170 mph in a backwards direction, and the plane engines are providing enough thrust for the the plane to go forward at 170 mph, then the plane would actually be sitting still and will have no forward movement with regards to other stationary objects.

No it won't. The conveyor belt can not keep the plane still.

The plane does not exert its force against the ground. It exerts its force against the air. The plane is pulling itself through the air.

The wheels will simply be spinning twice as fast.

Originally Posted by Chipless

2) In the example I just gave, the wheels of the plane would be moving at 170 mph, not 340 mph.  There is no force that would double the speed of the wheels.  The conveyor belt is going backwards at 170 mph, and the plane wheels are going forward at the same speed.  170 - 170 = 0, which is the speed of the aircraft.

If the speed of the conveyor belt matches the speed of the plane, and the plane is still, then the belt will be still. You can't have a conveyor belt that matches the speed of the plane going 170 MPH when the plane is not moving at all.

When the plane is moving 170 MPH forward, the belt is moving 170 MPH backwards, and the wheels are thus spinning the equivalent of 340 MPH.

Originally Posted by Chipless

3) Since the plane is sitting still relative to the air surrounding the wings, there would be no lift, and this plane has no chance of taking off.  The rotation speed required for takeoff must be in relation to the air moving across the wings.  If I remember correctly, I think this is called "indicated airspeed".  So with a 20 mph headwind, you'd only need 150 mph of actual land speed.  But with no aircraft speed, you might need ~ 170 mph headwind to take off.

It's not sitting still.

[Chipless]

OK.  Looks like I misread the initial post, and now have my tail tucked between my legs.