Will it take off?

[sputnik]

It's no wonder that all of our high tech jobs are going to India and China.

[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!!

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?

good point but remember that the air is being sucked in the engine and thrust out the back, not over the wing and if the plane is not moving, the air could be going "around in circles"; kinda like a woofer trying to play 30 Hz just laying on the floor...

boy!!

[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.

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.

The problem I see here is that the jet wash is not what I feel provides lift to the wing. Case in point. MD-80's have the jet engines on the tail, as do Fokkers. Their is no jet wash over or under a wing in that configuration. Perhaps I'm getting caught up in a minor meaningless detail, but I do not believe an engine can provide the the air for the lift.

My company used to have (thank God we sold it) a twin engine Beachcraft Baron. I used to fly up front with the pilot. Before take off he would rev the engines to darn near 100% to get the trim, etc going and to test the engines. The brake was applied, and we never experienced verticle takeoff. Only when we got to say 100MPH (depending on head wind) would the plane leave the ground.

[sputnik]

Was this too much to read through?

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.

[PhilMays]

It's no wonder that all of our high tech jobs are going to India and China.

It just takes all types my friend. I really do not claim to know. Hell I'm a hotel guy. I would not expect high tech people to know squat about my field in how to finance a multi-million dollar property and what CAP rate to accept much less the knowledge on accepted accounting practices or general operational proceedures.

This is a thought provoking thread. It only bothers me when people make attacks at others just because they feel they "know it all" or that others may not get it. Doesn't make anyone dumb, just not their field.

This is no place for attacks. We debate and move on.

[PhilMays]

Was this too much to read through?

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.

No.

I read it and it does make sense. I can see your point. However in my mind something is not right. I am a very visual person and would almost need to see it. Perhaps I am getting too caught up in something else.

Some of the posts here have actually made me go from "no it can't fly" to "why hell, it just might fly".

[sputnik]

It's no wonder that all of our high tech jobs are going to India and China.

It just takes all types my friend. I really do not claim to know. Hell I'm a hotel guy. I would not expect high tech people to know squat about my field in how to finance a multi-million dollar property and what CAP rate to accept much less the knowledge on accepted accounting practices or general operational proceedures.

This is a thought provoking thread. It only bothers me when people make attacks at others just because they feel they "know it all" or that others may not get it. Doesn't make anyone dumb, just not their field.

This is no place for attacks. We debate and move on.

Phil,

That comment was merely an attempt to inject a bit of humor. It was not directed at you or anyone in particular. In fact, I admire your tenacity in sticking with the problem.

[DrWho]

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

JUST KIDDING DOC!!

Just kidding?!? So that means ported horns = bad? [][][][<)][6]

[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?

good point but remember that the air is being sucked in the engine and thrust out the back, not over the wing and if the plane is not moving, the air could be going "around in circles"; kinda like a woofer trying to play 30 Hz just laying on the floor...

boy!!

Well put in this case, most engines are below the wing but the actual exhaust is behind the wing meaning that the bottom air is being sucked and pushes the air behind the wing creating a vaccuum like above the wing which means the air passes over too.... which means the air on top has to go faster than the air on bottom hence low pressure on the bottom and lift.

[oldenough]

My Sister is an operations manager for an air-cargo company out of Vancouver (YVR) .She has posed the question to two of her co-workers both pilots, one answer back so far and that one is a yes, it will fly, the other pilot is probably in a downtown bar getting tanked up before flying tommorow.

[WMcD]

It is silly.

But. The question says "the plane moves". Why or how it moves is not stated. Why it would not keep moving at increasing velocity is not stated either.

If the riddle is the meaning of unclear words, then there is no clear answer.

Gil

[PhilMays]

My Sister is an operations manager for an air-cargo company out of Vancouver (YVR) .She has posed the question to two of her co-workers both pilots, one answer back so far and that one is a yes, it will fly, the other pilot is probably in a downtown bar getting tanked up before flying tommorow.

Hey, I've flown with him before!

[sheltie dave]

Decided to call kid brother, and he talked to his squadron commander. Thought he would go into the various charts to use regarding headwind speed, what bell they had to answer(ahead flank,) and how much power the pilots used - all they got.

Instead, the admiral created a scenario that satisfied the question, and the answer is evident. A free standing conveyor freewheels at the same negative vector as a plane provides in positive from the point of origin. The plane will never move from the origin.

Bolt a chain to the wall and hook it to the tail of the plane, apply an exact opposite force, and watch it try to take off. As long as a conventional winged plane dooes not move from the point of origin, there will be zero venturi flow of air to allow pressure displacement.

Equal and opposite forces always cancel out in physics. Only unequal forces allow a plane to take off and fly.

[klipschaholik]

Goes to show you, Admirals don't know everything!

[sputnik]

Decided to call kid brother, and he talked to his squadron commander. Thought he would go into the various charts to use regarding headwind speed, what bell they had to answer(ahead flank,) and how much power the pilots used - all they got.

Instead, the admiral created a scenario that satisfied the question, and the answer is evident. A free standing conveyor freewheels at the same negative vector as a plane provides in positive from the point of origin. The plane will never move from the origin.

Bolt a chain to the wall and hook it to the tail of the plane, apply an exact opposite force, and watch it try to take off. As long as a conventional winged plane dooes not move from the point of origin, there will be zero venturi flow of air to allow pressure displacement.

Equal and opposite forces always cancel out in physics. Only unequal forces allow a plane to take off and fly.

It is pretty alarming that these guys, up to the level of an admiral, would go through irrelevant analysis and reach the wrong conclusion. The conveyor does not act like a chain attached to the tail.

Someone explain how any horizontal force from the conveyor is transferred to the plane to oppose thrust. It just doesn't happen.

[sputnik]

Here is a cutaway view of a roller bearing - similar, in function, to what would be found on the plane's landing gear. This is the mechanical connection between the tire and the plane. It sort of looks like a round conveyor. So, when the plane is placed on the conveyor, you essentially have two conveyors in contact that cancel each other's motion (one will spin clockwise and the other will spin counter-clockwise). The inner ring of the bearing is not affected by the turning of the outer ring. The bearing isolates the plane from the movement of the conveyor. In simple terms, the horizontal force from the moving conveyor cannot get past this. If horizontal thrust is applied to whatever the inner ring is attached to, it moves in the direction of the thrust independently of the spinning of the outer ring.

The plane takes off.

[Andy W]

[Joe Pesci]OK, OK, OK, OK[/Joe Pesci]

Lets imagine that the conveyor is really really really big, and the conveyor is moving 1,000 miles per hour in the direction opposite that the plane is pointing, and the plane is moving with the conveyor - backward to an obsever standing on a motionless obect hovering above this huge conveyor. The conveyor is so big it has its own gravity and pulls the air along with it. The pilot starts the engine and attempts to take off. Can the plane take off?

The earth itself is that conveyor, folks!

If you think that the plane in the original question can't take off, then planes would only be able to take off when they are facing EAST! The ground at the equator of the earth is moving ~1,000 mph to the east.

In fact, most planes in the northern hemisphere take off facing WEST (prevailing winds).

[Ray Garrison]

I can't believe that (a) there was ever any question about this, and ( this thread is still going on. Makes me sad about our public education system.

Of *COURSE* it will take off. What a stupid question. The velocity, or lack thereof, of the runway on which the plane is sitting has absolutely *NOTHING* to do with the motion of the plane, except for the (presumeably negligable) friction of the wheel bearings.

Look at it this way. The plane is flying, wheels up, over the conveyor belt. Obviously, it doesn't matter one whit to the plane what the conveyor belt is doing - moving forward, backward, sideways, doesn't matter - the plane is flying. Now, the plane lowers its wheels onto the belt. If the belt were moving in the same direction as the plane at the same speed, the wheel would not rotate, but the plane would keep flying. Suppose the conveyor belt is stationary. The plane's wheels, upon contact, would begin rotating at, uh, whatever measure of rotational stuff is appropriate in this scenario (angular velocity?), at the same rate as the plane is moving forward above the belt. The plane continues to fly. Now, have the belt move in a direction opposite to the plane's direction at a speed equal to the plane's speed. When the wheels contact the belt, they will begin to rotate really fast, twice as fast as they would if the belt were stationary, but the plane continues to fly. The *ONLY* possible effects here are (a) the wheels have lousey bearings, and the friction is so high that it imparts sufficient drag on the airframe to slow its velocity to some degree, or ( the tires explode due to the high forces involved, setting the aircraft on fire, which then crashes and burns like the Concord did.

And if anyone feels that the fact that the plane is flying prior to lowering its gear in my scenarios is somehow different than taking off and raising its wheels, then I dispair of our educational system and retreat into my cave, working on my version of A Book For Everyone and for Nobody.

[Champagne taste beer budget]

Hey Ray, welcome to my cave! I've been in here a while now just watching out the door.

Care for some fresh roasted bat wing? Goes great with Merlot.

[Jay481985]

[Joe Pesci]OK, OK, OK, OK[/Joe Pesci]

Lets imagine that the conveyor is really really really big, and the conveyor is moving 1,000 miles per hour in the direction opposite that the plane is pointing, and the plane is moving with the conveyor - backward to an obsever standing on a motionless obect hovering above this huge conveyor. The conveyor is so big it has its own gravity and pulls the air along with it. The pilot starts the engine and attempts to take off. Can the plane take off?

The earth itself is that conveyor, folks!

If you think that the plane in the original question can't take off, then planes would only be able to take off when they are facing EAST! The ground at the equator of the earth is moving ~1,000 mph to the east.

In fact, most planes in the northern hemisphere take off facing WEST (prevailing winds).

I am not saying you are wrong but isn't the plane and earth rotating at the same time so the relative speed = 0?