Will it take off?

[Andy W]

I think folks are getting confused into thinking that the plane is somehow held stationary or the conveyor imparts a force on the plane the same way it would on a car. A car moves by transferring force to the tires. The planes tires do not impart or transmit any horizontal force on the plane so there is no force to counteract the thrust.

Exactly. If this were a car the car would appear to stand "still" to an obsever standing beside the conveyor.

Think of standing on a treadmill while wearing rollerskates. Once you got used to it, it would be pretty easy to maintain your position. Now, if someone hands you a rope that is attached to the wall beyond the treadmill and you pull on the rope, you'll move toward the wall independent of the speed of the treadmill. The thrust from the plane is equivalent to you pulling on the rope and the plane moves normally, independent of the speed of the conveyor.

Good analogy. That's the same thing I was trying to say with my dog analogy.

I just though of another way to explain this.

The original question:

A plane is standing on a movable runway (something like a conveyor). As the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction. The question is will the plane take off or not?

The new question:

A plane with levatation technology is hovering above a movable runway (something like a conveyor). As the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction. The question is will the plane take off or not?

This illustrates that the plane is not affected by the conveyor in any way. And the fact that a real plane has low friction wheel bearings is not far from being just like the new question posed here.

The force impated from the conveyor to the plane is very very small compared to the force of the engine on the plane.

OK, and for those who might think I'm reading too much into the question and assuming too much, consider this: the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction

If the plane was not moving (and therefore could not take off) the conveyor wouldn't be moving either.

[WS65711]

So . . . . .

If you were to suddenly shut off the airplane's engines, does the plane then remain motionless or does it move backward on the conveyor?

If the plane is not affected by the conveyor in any way, then it should remain motionless if the engines are shut down.

[Jay481985]

With the question now being revised for me to understand.... It should take off just fine as the engine thrust is enough air to move over the wings to take off.... Essentially its like a car being towed with its brakes on, the car still moves because the engine can overcome the conveyor belt and the minimal friction on the bearing.

[DrWho]

I think this thread is a classic example of the way people interact on forums...people just spew off opinions without regard, instead of directly commenting and adding to the comments of others. It's a most annoying scenario because everyone is making different assumption and drawing different conclusions. It's not so much a conversation as it is a wall of opinions...

Now if people were to actually sit down and read every post and make direct comments....[6]

Sorry for venting

[Coytee]

If we keep this up, "the most beautiful women in the world" thread will have some competition... (I know we have a ways to go)

.........................[sn]

[oldenough]

Dr Who, i don't think adding anything more to comments already made is going to improve others understanding of the situation. A number of contributors have explained it beautifully. The only thing that will resolve it for some is a demonstration. A picture is worth a thousand words.

[oldenough]

Oops, double post how'd that happen?

[DrWho]

aye, but people will continue to provide their own opinions without reading any of the elaborate answers...

[oldenough]

Very true Dear Dr.

[IndyKlipschFan]

A plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

the question is

will the plane take off or not?
==================================

I feel like I am sitting for my mensa test again. LOL

"A plane is standing on a movable runway." OK, stop here... It is stopped.

The conveyor will not move at all UNLESS the plane does.. (And we know again it is stopped.)

As for the question we have to assume a "plane" is a word used to describe something that flies. To fly, it has to originally take off. So the answer is Yes.. The plane described will fly, but not in this instance from this example.

From this example, because it is stopped, there is no movement at all from the plane or the conveyor. It will not take off in this example. But the plane itself should fly under different conditions..

my 2 cents

[oldenough]

IndyKlipschFan said,

I feel like I am sitting for my mensa test again. LOL....

Indy just for the record how did you do on your test????

[PhilMays]

I think this thread is a classic example of the way people interact on forums...people just spew off opinions without regard, instead of directly commenting and adding to the comments of others. It's a most annoying scenario because everyone is making different assumption and drawing different conclusions. It's not so much a conversation as it is a wall of opinions...

Now if people were to actually sit down and read every post and make direct comments....[6]

Sorry for venting

You proved your very point

[PhilMays]

I guess I keep getting stuck.

1. The conveyer belt will move as fast as the plane moves. To me that reads like it is cancelling out the forward motion.

2. A plane needs airflow over a wing to provide lift. Without any forward motion, where is the lift.

I obviously am not as smart as many (OK most if not all) on this forum, however it seems to be common sense that if the acceleration is cancelled out by an exact opposing acceleration (static momentum) there is no airflow over the wing to provide lift, unless it had a verticle take off system that would completely cancel out the need for the wheels.

[oldenough]

Phil, The ONLY result of sitting on the conveyor belt is that the wheels will turn faster, the jets/propellors will still thrust the airplane forward.

P.s Just to throw some more wood on the fire how about debating a landing under the same scenario. Sorry!

[sputnik]

I guess I keep getting stuck.

1. The conveyer belt will move as fast as the plane moves. To me that reads like it is cancelling out the forward motion.

2. A plane needs airflow over a wing to provide lift. Without any forward motion, where is the lift.

I obviously am not as smart as many (OK most if not all) on this forum, however it seems to be common sense that if the acceleration is cancelled out by an exact opposing acceleration (static momentum) there is no airflow over the wing to provide lift, unless it had a verticle take off system that would completely cancel out the need for the wheels.

No problem Phil. I like these discussions. The challenge is to cut through what doesn't matter and evaluate what does matter and then try to explain it clearly. To answer this problem, there is really no need to get into aerodynamics, or Bernoulli's equation or any of that stuff - we know it's a plane and it's capable of flight. We know that a plane needs airflow over it's wings to achieve lift. In still air, the plane must move forward to fly. So, the question boils down to whether or not the conveyor affects the forward motion of the plane.

The plane will accelerate when there is an unopposed force applied to it. It will move in reponse to the force according to Newtons second law of motion where Force = Mass times Acceleration. Force is a vector quantity, having both direction and magnitude, so the direction of the force will be the direction of the acceleration of the mass. For a plane, this force is applied by the action of the engine (propellor or jet thrust).

The crux of the question is: Can the conveyor impart a horizontal force on the plane to oppose the engine thrust? It really just boils down to that.

We know that the conveyor and plane's tires move in response to each other, but is that motion and associated force transffered to the rigid body of the plane? The mechanical connection between the tire and the plane is the wheel bearing. In a way it acts like a hinge. If you hang a door hinge to swing vertically and push on the lower end, it just swings up and the top part above the hinge doesn't move. If it were a solid bar (no hinge point), the force would be transferred to the top. The plane's landing gear wheel bearing sort of acts like a hinge and does not allow transverse forces to be transmitted to the plane from the tire. Think of the wheel on a rollerskate - you can turn the skate upside down and run your finger over the wheel and spin it without really applying any force to the skate itself. So, the conveyor doesn't really impart any horizontal force to the body of the plane because such a force cannot be transmitted to the plane through the wheel bearing. Common sense and experience tells us that there will be some resistance in the wheel bearing (friction) but this is a very small force compared to the engine thrust and it can be ignored in order to visualize the problem better. At any rate, there is no horizontal force transferred from the conveyor to the body of the plane that can significantly oppose the engine thrust, so the plane accelerates and takes off. The conveyor can be moving at any speed and still not really affect the plane's acceleration - that speed matching thing just gets people confused.

Someone asked if the plane would remain stationary if the conveyor were moving but the plane's engines were shut down - no force acting on the plane. Under the ideal frictionless scenario, yes, the plane would remain stationary while the conveyor just spun the plane's wheels. Did you ever see an orange or an onion that falls onto a moving supermarket conveyor just sort of roll in place. It's the same as that.

I'll stop for now. I hope that helps.

[Jay481985]

I guess I keep getting stuck.

1. The conveyer belt will move as fast as the plane moves. To me that reads like it is cancelling out the forward motion.

2. A plane needs airflow over a wing to provide lift. Without any forward motion, where is the lift.

not necessarily. The jet engine is on meaning air is pushing. The air will go under and over the wing proving lift. If you just had a jet engine on say a device like a lift it would still have a force forward because it is pushing the air in front of it to the back of it meaning that the air is physically being pushed.

I obviously am not as smart as many (OK most if not all) on this forum, however it seems to be common sense that if the acceleration is cancelled out by an exact opposing acceleration (static momentum) there is no airflow over the wing to provide lift, unless it had a verticle take off system that would completely cancel out the need for the wheels.

[bodcaw boy]

I saw this on another forum I frequent and found some of the commentary interesting. I don't know the answer although I've got an opinion. It WAS interesting seeing the answers/thoughts some people gave (2 pages worth).

I'm copying it exactly like he had it there

a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

the question is

will the plane take off or not?

(ps its been debated to death on other forums, its always fun to see how people present the theory behind there answer)

OKAY!! so i read it again. if the plane increases thrust then the conveyor goes faster right? and if it slows down the conveyor slows down, correct? it doesn't say the conveyor goes faster or the plane can thrust harder, it says in the exactly the opposite direction. so again, i say, no air over the wings, no lift!!

okay coyotee-o, what did i win? ribs and ice cream??[:^)]

boy!!

[bodcaw boy]

I think this thread is a classic example of the way people interact on forums...people just spew off opinions without regard, instead of directly commenting and adding to the comments of others. It's a most annoying scenario because everyone is making different assumption and drawing different conclusions. It's not so much a conversation as it is a wall of opinions...

Now if people were to actually sit down and read every post and make direct comments....[6]

Sorry for venting

okay so i was thinking, venting woofer cabinets to get more bass is a good idea.....

JUST KIDDING DOC!!

boy!!

[BS Button]

Is it an airplane?

BS

[Jay481985]

I saw this on another forum I frequent and found some of the commentary interesting. I don't know the answer although I've got an opinion. It WAS interesting seeing the answers/thoughts some people gave (2 pages worth).

I'm copying it exactly like he had it there

a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

the question is

will the plane take off or not?

(ps its been debated to death on other forums, its always fun to see how people present the theory behind there answer)

OKAY!! so i read it again. if the plane increases thrust then the conveyor goes faster right? and if it slows down the conveyor slows down, correct? it doesn't say the conveyor goes faster or the plane can thrust harder, it says in the exactly the opposite direction. so again, i say, no air over the wings, no lift!!

okay coyotee-o, what did i win? ribs and ice cream??[:^)]

boy!!

but Roy, doesn't the fact that an airplane gets thrust through moving air give thought that yes even though the wheel are pushing against it its the jets are moving enough air to make the air travel over the wings?