Okay Krupski or other knowing people answer this please

[1 Patriot-of-many]

I've been arguing with my cousin about this, I say it wouldn't fly, he says it will.

Imagine a 747 sitting on a conveyor(treadmill) as long and wide as a runway. The conveyor is designed to move in the opposite direction at the exact speed of the wheels on the jet. Can the jet take off?


I say no because the jet will not move relative to the ground if the treadmill moves exactly the opposite direction at the exact speed of the wheels, it will never develop airflow under and over the wings to create lift. It will stay in one place. Anyone?

[Viking350]

I've been arguing with my cousin about this, I say it wouldn't fly, he says it will.

Imagine a 747 sitting on a conveyor(treadmill) as long and wide as a runway. The conveyor is designed to move in the opposite direction at the exact speed of the wheels on the jet. Can the jet take off?


I say no because the jet will not move relative to the ground if the treadmill moves exactly the opposite direction at the exact speed of the wheels, it will never develop airflow under and over the wings to create lift. It will stay in one place. Anyone?

You are correct. If it were on a treadmill of sorts, there would be no airflow over the wings so no lift would be generated.

[alismith]

You are correct. If it were on a treadmill of sorts, there would be no airflow over the wings so no lift would be generated.

Yup. The airfoil design of the wings uses air pressure to create lift. Ground speed, on a stationary plane, has nothing to do with making the plane fly. The only reason runways work is that the plane is able to travel fast enough for the air to create lift on the wings.

All a treadmill will do is wear the tires out.....not create lift.

[imanaknut]

Very interesting...

[davepool]

yep, air speed not ground speed makes you fly. in my hangliding days there were times when i had zero ground speed, but was flying at 20-25 mph air speed, it;s called static soaring

[N/A]

If the jet is not moving forward, then its wheels are not moving/turning. If the wheels are not turning, then the conveyor is not moving either.
If the wheels can not turn because the conveyor counteracts their forward movement, thus the conveyor can not turn because it neutralized the force that caused it to turn.

The solution is simple, set the brakes on the wheels, apply full power and slide down the conveyor without the wheels turning.

[Krupski]

You are correct. If it were on a treadmill of sorts, there would be no airflow over the wings so no lift would be generated.

Yup. The airfoil design of the wings uses air pressure to create lift. Ground speed, on a stationary plane, has nothing to do with making the plane fly. The only reason runways work is that the plane is able to travel fast enough for the air to create lift on the wings.

All a treadmill will do is wear the tires out.....not create lift.

You miss the point that the only thing that makes the wheels spin is the forward motion of the jet.

Now, imagine a car with wings that has to drive fast enough to lift off. Driving on the treadmill would make the wheels spin, but the car would have zero motion relative to the air and would not fly. Of course, the car on a road would lose thrust from the wheels as soon as it lifted off and it would come back down to the road in a repeating liftoff - land - liftoff - land cycle.

[Viking350]

I may be overthinking it, but a conveyer belt has its own source of motion. If I walk or run on a treadmill, I feel no air movement on my sweaty face. If there is no relative air movement then no lift could be generated.

[Krupski]

I may be overthinking it, but a conveyer belt has its own source of motion. If I walk or run on a treadmill, I feel no air movement on my sweaty face. If there is no relative air movement then no lift could be generated.

OK, if the airplane were on a treadmill or conveyor and the airplane brakes were on and the treadmill moved the airplane forward above stall speed, the airplane would lift off the treadmill (and promptly land back down due to no longer having the "thrust" of the treadmill). As in post #14, the airplane would fly - land - fly - land......

[alismith]

You miss the point that the only thing that makes the wheels spin is the forward motion of the jet.

Now, imagine a car with wings that has to drive fast enough to lift off. Driving on the treadmill would make the wheels spin, but the car would have zero motion relative to the air and would not fly. Of course, the car on a road would lose thrust from the wheels as soon as it lifted off and it would come back down to the road in a repeating liftoff - land - liftoff - land cycle.

What do spinning wheels on a stationary jet have to do with making it fly? If the jet remains stationary to the air around it, there is no lift created, no matter how fast the wheels are spinning. The jet must move, relative to the air around the wings in order for lift to be created.

In the "thought game" of the OP, the jet remains stationary relative to the air; only the wheels and conveyor belt are moving opposite to each other. In this case the wheels have absolutely nothing to do with causing the air to move around the wings.

No air movement means no pressure differences above and below the wings, therefore no lift. No lift means no flying. The jet will just stay on the treadmill with its wheels spinning.

Without wings, a car can't fly. If it could, there would be a lot of airborne crashes at racetracks.

[N/A]

Then the question comes up, "How do you get the 747 on the conveyor belt to start with?"
If the taxi ways are normal, then the 747 pulls around to the end of the runway, lines up, and then proceeds to pull up on the conveyor belt. s soon as the nose wheels hit the conveyor belt it starts to turn. In this thought experiment, the minute the nose wheels are on the conveyor belt, the belt turns and the wheels can go no farther; but the main landing gear is still sitting on solid runway and can still move forward.
Thus, you would seem to have a part of the plane that couldn't move forward and part that can still move forward. As Krupski said, it's the engine thrust that propells the plane forward, not the mechanical turning of the wheels like on a car. The planes tires just freewheel, like bearings.
So, even tho the nose wheels are on the belt, they just freewheel as the 747 continues to taxi onto the conveyor belt. When it comes time to apply thrust for the take off roll, the wheels will just freewheel over the conveyor belt as thrust moves the plane forward. All the conveyor belt will do is match the "speed" of the wheels, but in the opposite direction.

The only way this could affect the plane is if, like a car, the plane had to get it's forward momentum from mechanically turning the wheels. Otherwise, the wheels are just freewheeling rubber bearings that have no impact on the thrust pushing the plane forward.

[Krupski]

Without wings, a car can't fly. If it could, there would be a lot of airborne crashes at racetracks.

I was talking about flying cars with wings.

1964 homebuilt flying car

[alismith]

I've been arguing with my cousin about this, I say it wouldn't fly, he says it will.

Imagine a 747 sitting on a conveyor(treadmill) as long and wide as a runway. The conveyor is designed to move in the opposite direction at the exact speed of the wheels on the jet. Can the jet take off?


I say no because the jet will not move relative to the ground if the treadmill moves exactly the opposite direction at the exact speed of the wheels, it will never develop airflow under and over the wings to create lift. It will stay in one place. Anyone?

Did your cousin get this idea from Season 1; Episode 6 of The Republic of Doyle?

The reason I'm asking is that I just saw this same debate in the first part of that episode and was curious about where this came from....

If so, tell him the bartender was right....there is 0% chance of the jet taking off. Physics won't allow the plane to take off because the conveyor belt doesn't generate lift.

[1 Patriot-of-many]

Did your cousin get this idea from Season 1; Episode 6 of The Republic of Doyle?

The reason I'm asking is that I just saw this same debate in the first part of that episode and was curious about where this came from....

If so, tell him the bartender was right....there is 0% chance of the jet taking off. Physics won't allow the plane to take off because the conveyor belt doesn't generate lift.

My cousin used this example https://www.youtube.com/watch?v=YORCk1BN7QY One. the plane isn't on a conveyor belt, you can clearly see the wheels are in contact with the ground. 2. This doesn't show the speed of their tarp in the reverse direction being the exact opposite of the wheels on the plane.

[Krupski]

I've been arguing with my cousin about this, I say it wouldn't fly, he says it will.

Imagine a 747 sitting on a conveyor(treadmill) as long and wide as a runway. The conveyor is designed to move in the opposite direction at the exact speed of the wheels on the jet. Can the jet take off?


I say no because the jet will not move relative to the ground if the treadmill moves exactly the opposite direction at the exact speed of the wheels, it will never develop airflow under and over the wings to create lift. It will stay in one place. Anyone?

The jet moves forward due to the thrust of the engines. The forward motion also causes the landing gear wheels to rotate. If the "treadmill" moved as fast as the jet, the effect would be that the wheels would be stationary, but the airplane would still take off and fly...

[1 Patriot-of-many]

The jet moves forward due to the thrust of the engines. The forward motion also causes the landing gear wheels to rotate. If the "treadmill" moved as fast as the jet, the effect would be that the wheels would be stationary, but the airplane would still take off and fly...

? I'm confused by your answer, the conveyor is moving at the exact speed in the opposite direction, that would counteract any forward movement no matter how much thrust was applied as in the OP the treadmill always keeps the same speed in reverse as the rolling wheels.

[1 Patriot-of-many]

The jet moves forward due to the thrust of the engines. The forward motion also causes the landing gear wheels to rotate. If the "treadmill" moved as fast as the jet, the effect would be that the wheels would be stationary, but the airplane would still take off and fly...

How does a plane fly? It flies when the air movement is sufficient to provide lift. If the airplane is in the same spot relative to the ground it cannot provide lift. The treadmill is always canceling out any thrust/wheels turning except keeping up with the treadmill.

[Krupski]

How does a plane fly? It flies when the air movement is sufficient to provide lift. If the airplane is in the same spot relative to the ground it cannot provide lift. The treadmill is always canceling out any thrust/wheels turning except keeping up with the treadmill.

If you look at the shape of a typical aerofoil:





You see that the top is curved more than the bottom. The air flows over the top and bottom of the wing. Since the TOP flow has further to go, it has to move faster to "keep up". Bernoulli's law says that if the flow is faster, it's pressure must be lower.

To imagine this, think of a shoulder-to-shoulder crowd of people walking past an aerofoil shape (wing) standing on end. The people on the "top" side of the "wing" have further to walk, so they must speed up in order to keep up with the people on the "bottom" side of the "wing". Since they walk faster, they temporarily move further apart from each other (their density decreases).

Going back to airflow, decreased density means lower pressure. Therefore, there is a LOW PRESSURE area at the top of the wing. The wings are not pushed up by the airflow, they are SUCKED UP by the low pressure above the wing!

Ever take an air hose and blow the air straight up and suspend a ping-pong ball on the airstream? This works because the air flowing over the curve of the ball has to speed up as it flows around the ball, creating a low pressure area that pulls on the ball.

If the ball, say, tries to fall off to the left, the right side is exposed to more airflow, the pressure on the right side gets lower and the ball is "sucked" back into the center. The ball does "dance around" in the stream, but that's just due to turbulence in the airflow. But the ball is always kept inside the "column" of moving air due to Bernoulli's principle.

[1 Patriot-of-many]

If you look at the shape of a typical aerofoil:





You see that the top is curved more than the bottom. The air flows over the top and bottom of the wing. Since the TOP flow has further to go, it has to move faster to "keep up". Bernoulli's law says that if the flow is faster, it's pressure must be lower.

To imagine this, think of a shoulder-to-shoulder crowd of people walking past an aerofoil shape (wing) standing on end. The people on the "top" side of the "wing" have further to walk, so they must speed up in order to keep up with the people on the "bottom" side of the "wing". Since they walk faster, they temporarily move further apart from each other (their density decreases).

Going back to airflow, decreased density means lower pressure. Therefore, there is a LOW PRESSURE area at the top of the wing. The wings are not pushed up by the airflow, they are SUCKED UP by the low pressure above the wing!

Ever take an air hose and blow the air straight up and suspend a ping-pong ball on the airstream? This works because the air flowing over the curve of the ball has to speed up as it flows around the ball, creating a low pressure area that pulls on the ball.

If the ball, say, tries to fall off to the left, the right side is exposed to more airflow, the pressure on the right side gets lower and the ball is "sucked" back into the center. The ball does "dance around" in the stream, but that's just due to turbulence in the airflow. But the ball is always kept inside the "column" of moving air due to Bernoulli's principle.

There would no no airstream to provide lift, whatever speed the wheels are moving forward, the treadmill is running backward at the exact speed. The plane would never move, and there would be no airflow in the question I pondered. How does it fly when it essentially never moves from one spot? I'm aware of how lift works, flew a plane as a student pilot, with a student pilot certificate. I'm thinking a couple people including you missed the part that the treadmill is always running the opposite direction as the plane's wheels.

[N/A]

[QUOTE=1 Patriot-of-many;486954]There would no no airstream to provide lift, whatever speed the wheels are moving forward, the treadmill is running backward at the exact speed. The plane would never move, and there would be no airflow in the question I pondered. How does it fly when it essentially never moves from one spot? I'm aware of how lift works, flew a plane as a student pilot, with a student pilot certificate. I'm thinking a couple people including you missed the part that the treadmill is always running the opposite direction as the plane's wheels.[/QU

It really doesn't matter about the treadmill and the wheels. As I pointed out the wheels are free wheeling, like bearings in a machine. They are not mechanically linked to the frame to impart any forward motion. Even sitting on a solid surface, those wheels will not move until thrust is applied. Then all the wheels do is act like bearings between the plane and the runway.
The same with the conveyor belt, it doesn't matter how fast the conveyor turns in the opposite direction, the wheels are still free wheeling bearings that have nothing to do with imparting movement to the plane. Thrust from the engines propels the plane forward completely independent of what the conveyor does.