DescentBB

Ok, this is a big argument. Given this:

An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

Also, it's said that the conveyor is designed to \"match the speed of the plane\".

Fundimentally, this question is flawed, but here we go.

Will it fly or not?

(I e-mailed NASA on this)

I would think not because of little or no air flow for lift.

why no air flow?

If the moving conveyor belt is going the opposite direction and is designed to match the speed of the plane that nulifies one another as far as movement goes - so no airflow for lift.

http://www.allstar.fiu.edu/AERO/airflylvl3.htm


Newton’s first law states a body at rest will remain at rest, or a body in motion will continue in straight-line motion unless subjected to an external applied force. That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, there is a force acting on it. Newton’s third law states that for every action there is an equal and opposite reaction. As an example, an object sitting on a table exerts a force on the table (its weight) and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction.

put it this way

take out plane and replace with rocket

thust vs motion not speed vs distance

boxed thinking will kill you

would the conveyor belt affect the rockets thrust? so why would you think it would affect a planes thrust... just because it has wheels and the rocket doesnt??

Call in NASA!!

Yes, the plane will fly as long as the belt doesn't prevent the plane from getting in to launch position (facing down the runway). If the belt is going in either direction it won't null out the thrust of the engine that will be pushing the plane along the runway. The wheels might be going very fast by the time they reach rotation speed.

RC

Of course it will fly. Take for example, a Cessna with a fixed pitch propeller. The plane will move forward because the propeller, via its pitch, is screwing through the air pulling the plane along.

Since the wheels of the plane are not tied down and are free to move, the plane will take off whether the runway matches its speed or not.

The moving runway is irrelevant.

Bettina

Cuda's first response is good, as is the link in his second response. Ignore all the Newtonian jabber in his second post and ignore Bub's post entirely unless you want to get more confused. Think of it as a crude relativity problem.

For lift, the plane's wings need to be in motion relative to the air, not the ground. To achieve this, planes usually have to travel down runways real fast (on days when the air is nearly motionless WRT the ground, i.e. little to no wind). However, I've seen small planes lift off at nearly zero mph off the runway...on really windy days. So what matters is the plane's velocity relative to the surrounding air, not relative to the ground. Your plane on a conveyor belt isn't moving at all WRT the air. It's like putting an F1 car on a dyno without it's front and rear spoilers/wings/foils and expecting it to lose traction. Now, put both the plane and conveyor belt in a wind tunnel and it'll take off.

EDIT: RC and Bett posted while I was typing. I must say I can't understand a word of RC's post; I think he may have misread the original question. Bettina's post indicates she believes planes fly due to their thrust, but that's only partially true. The thrust is only a means to move the wing WRT the air.

Duper's scenario is akin to putting the plane's brakes on then maxing out thrust. In both the conveyor belt and brakes-on scenarios, the plane is thrusting, is not moving WRT the ground, and is not tied to the ground, meaning it is free to lift off. Now watch this video posted here a couple of months ago and see what happens:

http://video.google.com/videoplay?docid ... 4752382399

As an aerospace engineer, I can tell you that lift is proportional to the square of the relative wind speed. If the air is not flowing over the wing, the relative speed is 0, 0 squared is still 0, and anything proportional to that is also 0.

Now, if the airplane is powerful enough (like some jet fighters), if it were somehow able to rotate upwards so that its thrust was counteracting gravity, it could take off. Rockets, for example, don't use lift to fly. But as long as the thrust is pointing backwards, it won't fly.

Yeah, I can say with certainty that it won't go anywhere if its not mocing forward. To to create lift, you need airflow over the wings, which then pull the plane up. Look up that effect on wikipedia if you want...
Unless the jets point down (a la the Harrier) it can't take off, no matter how much thrust those engine put out. The engines just make it go forward, not up.

You need to take friction between belt, wheels and bearings into account. As I think more about it, a break-even point will however be easily reached, where friction becomes neglectable, and hence the influence of the belt is null. Consider: The plane does not use a force applied to the belt to speed up, but to the surrounding air.

It would be different with a car, where propulsion happens due to friction with the ground.

The plane is not tied down and is free to move. The conveyer belt and plane are both stopped. Nothing is moving.

The planes engine is then started, and with a fixed pitch prop, the engine throttle is advanced.

The plane begins to move forward because the propeller is \"biting\" into the air, or screwing threw it.

The conveyer begins to match the rotation speed of the planes wheels making its wheels go twice as fast. Totally irrelevent.

Because the plane is moving forward, there is airflow over its wings producing lift, and as it picks up speed it will lift off.

This is simple physics my cuties.... It will fly. I stand by what I said.

Bettina

Bet51987 wrote:The conveyer begins to match the rotation speed of the planes wheels making its wheels go twice as fast. Totally irrelevent.

Totally irrelevant? Not so. You forget friction. Read my post.

Of course, Bettina is right.

The plane is propelled by it's engines, not by its wheels. As long as the plane is moving fast enough with respect to the ground and the air (not the conveyor belt) it will lift off. Whether the conveyor belt is moving has no bearing on it.

I think this is why Duper said at the outset that it is a flawed question anyways.

If there was enough friction, the plane would not fly. But there won't be.

Diedel wrote:

Bet51987 wrote:The conveyer begins to match the rotation speed of the planes wheels making its wheels go twice as fast. Totally irrelevent.

Totally irrelevant? Not so. You forget friction. Read my post.

There is no friction. As the wheels move one way, the conveyer moves in the other.

The wheels are only there to support the plane. The planes engine is what makes it move thru the air.

We had this question a year ago in class.

Bettina

this has been going around forums for a while now

http://www.elisetalk.com/forums/showthread.php?t=19011

@Diedel
I don't think that the conveyor belt/wheels would create enough friction to keep the plane from moving forward. In fact, the purpose of wheels in planes is to reduce the friction to the ground as much as possible.

If there was enough friction, the plane would not fly. But there won't be.

Compare it to someone in a very strong current of water, pulling himself along against the flow on a rope someone has handed to him. Unless the current is too strong (friction of water and body!), he will eventually reach a point further up the river, even if the speed of the water would match his speed moving against it. What the rope is for the swimmer, the air is for the plane. Actio - reactio propelling the plane do not depend on the system 'plane - belt', but 'plane - air'.

Pandora,

Generally spoken, wheels must never reduce friction too much, or they would start to slip. Rolling friction > sliding friction, that's what you learn at school here. But you're right insofar as compared to the forces exerted by jet turbines, wheel and bearing friction is neglectable. For take off, the plane would even be better of with a friction free wheel system (unless moving slowly enough to be affected e.g. by wind). Not so for landing however.

It will take longer to fly--but it will fly.

Now, if the case relied on a car whose only method of getting airborne was to get moving very very fast by wheel speed alone, it wouldn't fly--however, since there is no theoretical limit on the speed the wheels can turn on an airplane, and the airplane's METHOD OF POWERING ITSELF is not based on the moving ground, but on stationary air, the plane will be able to pull itself forward. Realistically, it won't go as fast, and might even not be able to get up enough speed to take off because of the friction of the wheels in their bearings--but that's a moot point for this theoretical problem.

Bet51987 wrote:There is no friction. As the wheels move one way, the conveyer moves in the other.

Rubbish. There is a small amount of friction between wheels and belt, and in the wheel bearings. But it doesn't affect the entire system. But if you would increase friction insanely - like by putting some extremely sticky stuff on the belt and in the bearings - the plane might have a hard time working against the belt.

Diedel wrote:

Bet51987 wrote:There is no friction. As the wheels move one way, the conveyer moves in the other.

Rubbish. There is a small amount of friction between wheels and belt, and in the wheel bearings. But it doesn't affect the entire system. But if you would increase friction insanely - like by putting some extremely sticky stuff on the belt and in the bearings - the plane might have a hard time working against the belt.

Of course there is, but my answer was in ref to the opening post scenario that friction is less than if the plane was standing on an actual runway It was not important.

The scenario isn't flawed BTW. If you built the conveyor belt and controls as described, the plane will take off as normal. This was a great question because it takes you down the wrong path initially.

B

Diedel, it doesn't matter if there is no friction or 100% friction between any parts of the plane's wheels and each other or the conveyor belt. Since the velocity of the conveyor belt is defined to be of the same magnitude and opposite direction as that of the plane, neither the conveyor belt nor the plane will move. Ever.

Bettina has it competely correct. What generates uplift is the jet engine/propellor/other device, NOT the motion of the plane along the ground.

Lemme boil it down to make it simple. Airplanes operate completely on airspeed, and ground speed doesn't matter a whit. Also, airplanes take off by creating airspeed, not by creating ground speed. So, the conveyor can be moving in whatever direction it wants, at whatever speed it wants, and the plane will still take off normally. That is provided that the beginning of the plane's run is relatively close to 0 air speed, and that you don't burn up your tires or bearings. Deidel's point is that if you do burn up your tires, or freeze your bearings, you may not be able to take off because friction will suddenly become a huge source of drag.

DCrazy,

#1 It does matter.

#2 Bet is right, but the plane will move and lift off.

Wheels or no wheels. I don't think that there is enough resistance to hold the plane in place.

Like I said from the get-go. This \"equation\" is FLAWED. There are fundimental problems with it. \"match the speed of the plane is too vague. What, exactly is that supposed to mean?

For arguments sake, there is no wind and standard mechancial tolerances do not apply. (i.e. velocity toleraces of bearings, tires etc.)

Also, you really need to break this down into 2 questions.

1, if the conveyor moves at the velocity of the plane as though it were taking off from a standard runway.

2, if the conveyor tries to move fast enough to stop the planes forward progress.


Celic Adams of the Chicago Reader put it this way that it is like saying A=A+5.

Why is everyone concentrating on wheels, conveyor belts, engines, none of that means anything.

As has been stated by an engineer and several others already...

It's airspeed, not ground speed that matters

Planes wings develop the lift, the engines provide thrust. In some cases (fighter jets as an example) the thrust is sufficient to lift the plane against gravity.

If we are speaking about any commercial aircraft (not jets), then the wings develop the lift and therefore the only thing that matters is the air speed, meaning the speed of the air over the wings.

If the air over the wings is sufficient for the specific plane, then it will take off irregardless of groundspeed.

In other words, a car won't move, but a plane will. A car pushes off the ground to accelerate; a plane pushes air to accelerate. All the conveyor will do is make the wheels on the plane spin twice as fast as it takes off (and given the nature of the question, we are going to assume that the tires won't explode in the process).

The assumption here is that the conveyor moves in the opposite direction of the vehicle, matching the speed of its wheel rotation so that the vehicle is stationary. If the vehicle is a car, it all makes sense. No matter how fast the wheels spin, the car is stationary and the belt moves as fast as the wheels.

But if the car were actually to move forward (through an outside force), the \"belt is moving backward fast enough to keep it stationary\" rule no longer applies. if you are winching the car forward with a cable, no matter how fast the belt goes backwards it can't keep the car stationary anymore. If there is a computer controlling the conveyor that is trying to compensate for the forward speed of the vehicle, it will increase the belt speed to infinity. Its just a question of what fails first, the belt, wheels, whatever.

An alternative rule for the belt would be, \"move fast enough to match wheel speed\". So whatever distance would be covered by the wheel if it was traveling over the ground, that's how much the belt should move over a given period of time. The problem here is that the belt and the wheel are connected at the contact point of wheel and belt. If there is no slippage, it is another impossible task. As the car is winched forward, the wheels must rotate an additional amount according to the distance the car moves. There is no way for the belt to match up this distance; any additional belt speed is already added in to the wheel rotation. So the conveyer computer again takes the belt speed to infinity.

So what I think will happen is the plane will move forward, and the conveyor belt will be failing to move fast enough to achieve the impossible.

SuperSheep wrote:Why is everyone concentrating on wheels, conveyor belts, engines, none of that means anything.

As has been stated by an engineer and several others already...

It's airspeed, not ground speed that matters

Because the question never mentioned air speed. It did mention belts, engines, and wheels, and without the engines there would be no airspeed. (In the question that was asked).

When this came up in class a year ago, the teacher also said that if the brakes on these large jumbo jets were set, or applied, the total power of all the engines would not be enough to overcome the brakes and it wouldn't move. Wow.

Bet

Oh, boy. As was stated in the beginning, this question is flawed, because the answer completely depends on the assumptions you make about things like friction, wind, orientation of the plane, and the effect of the conveyer belt.

That said, let's RE-clarify a couple of things:

First (and foremost), for normal jet or propeller-driven aircraft, the upward \"lift\" force which allows it to get off the ground is generated by a pressure differential between the top and bottom of the wing. This pressure differential is caused by air flowing over the wing. So, directional airflow relative to the wing (from the front of the wing to the back) = lift.

Second, for normal jet or propeller-driven aircraft, the forward \"thrust\" force from the engine or propeller is designed to move the aircraft through the air, causing the . So, forward thrust does not directly cause lift; it indirectly affects lift, but only if the thrust causes airflow over the wings (i.e. the plane moves through the air).

(Of course, all this discounts the effect of wind. For example, a plane can be moving forward at 100MPH, but if the wind is coming from behind it at 100MPH, it drops like a rock, because there is no airflow over the wings. This is why those high-wind \"microbursts\" are so dangerous to aircraft.)

Here are a couple of examples of assumptions which result in different conclusions:

1. Assumption: The plane does not move, because the engine is running, but friction and the conveyer speed exactly counteracts any motion relative to the ground, (so except for the wheels, the plane does not move), and the thrust of the engine is only horizontal. In this case, the plane does not fly.

2. Assumption: The plane moves, because the engine is running, but the motion of the conveyor belt only speeds up the wheels, and friction is not enough to keep the plane from moving forward. In this case, the plane does fly once it gets enough airspeed.

Foil,

just make real world assumptions, and the plane will take off. Air flow around the wings due to the plane's increasing speed will cause lift. Btw, because the wings are more convex on their top than on their bottom, there is lower air pressure above than below the wings, and the pressure from below causes the lift.

Bet,

if this is true, that would be because of the friction between tires and runway. I could however imagine that a Jumbo could break it's landing gear when grounded, full brakes and full thrust.

Diedel wrote:Foil,

just make real world assumptions...

My point was just that. You see the problem one way (your concept of how it would work in the "real world", where there's no way a conveyer could keep the plane stationary), and others see it as a hypothetical situation (where anything can happen, for the sake of academic exercise).

Since the question was vague, the "right" answer to the question depends on how you interpret it.

A better wording of the question (that I heard on another forum) is:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction). Will it take off?

And heres the simplest answer I got from that forum:

The wheels can be turning backwards with the belt moving them while the plane is still moving forward at whatever speed it needs to go to fly. The wheels do not move the plane. The propellers/engines do.

There will still be forward movement as long as the wheels are not locked. The belts effect on the plane will be lost through the neutral characteristics of the wheels. The wheels will be spinning backwards at the same rate of the belt, but this will have no effect on the forward momentum of the plane since the engines will be pulling it.

And heres proof that it will fly!

The plane would roll right off the belt and take off...if it survived the crash through the hangar bays doors.

A conveyer belt relies on gravity pulling a vehicle toward it for there to be any traction between belt and tire, which would stop the vehicle's forward movement if the belt matched it's ground speed.

Planes, however, do not rely on wheels for motion; they rely on the thrust from the engines being greater than force of gravity acting on the plane. Since gravity no longer has an effect on the plane, it is no longer being pulled toward the belt, and it loses traction in the wheels, leaving the forward thrust from the engines as the greatest forces acting on it. The plane would move forward, roll off the belt, and take off like normal.

This effect of forward motion counteracting gravity is also present in bicycles; it won't stay upright while at rest, but it will stay upright as long as it's in motion. The only difference is that bikes go use tire traction for movement.

So the short answer: Yes, a plane would fly if placed on a conveyer belt designed to match its ground speed...it just won't stay on the belt.

LOL, man this made a long discussion! It got me at first as well, good one.

The conveyer belt shouldn't affect the plane at all (besides making the wheels spin faster) sence the plane's engines push against air and not the ground.

OK, The other point of view here is that the conveyor belt has no relavance and the engine plays some role in lift. Lift comes from air flow across the wings. The propeller/jet engine alone will not create enough air flow for lift without air speed being in the equation and the conveyor belt apears to nulify that part. The only way a jet engine will over come this is to point the air craft nose up 15 or 20 degree's (Kinda like the harrier jump jets used in the Balkins), maybe more to push against gravity like a rockett, but I am making an asumption that this a level take off attempt. So with no forward phyiscal motion where is the air flow comeing from because air flow around the wings is key to achieve lift in a level take off. Thats why ships, which are also moving, point into the wind for planes to take off.
The A7 is what I worked on the most and it was reffered to as a flying brick. When air speed went under 100 knots it stalled and for every one foot of forward motion it went down two feet. A cessna (I think) needs 15 or 20 knots of air speed to achieve lift or it goes down.

DOH - I took to long to write this out and you Beat me on posting with the answer to my question, and I think you are right.

The conveyer belt shouldn't affect the plane at all (besides making the wheels spin faster) sence the plane's engines push against air and not the ground.

Cuda68 wrote:OK, The other point of view here is that the conveyor belt has no relavance and the engine plays some role in lift. Lift comes from air flow across the wings. The propeller/jet engine alone will not create enough air flow for lift without air speed being in the equation and the conveyor belt apears to nulify that part. The only way a jet engine will over come this is to point the air craft nose up 15 or 20 degree's (Kinda like the harrier jump jets used in the Balkins), maybe more to push against gravity like a rockett, but I am making an asumption that this a level take off attempt. So with no forward phyiscal motion where is the air flow comeing from because air flow around the wings is key to achieve lift in a level take off. Thats why ships, which are also moving, point into the wind for planes to take off.

Cuda, the engine alone is what provides forward motion, thus providing airflow over the wings, thus providing lift. There is negligable resistance or friction from the moving belt...Thats about it...

Suppose it was the X15 rocket plane and you lit that off.... it would fly just like the plane..

Bettina

the plane will end up taking off as if everything was normal.

That belt can be going as fast as possible, but it won't matter.

Because the engine thrust will overcome the rolling friction of the wheels.


You can see this in action by doing the following: make a paper airplane, get in your car, find a street clear of traffic (shouldn't be hard to do) and see if the plane lifts away when you stick it out the window and let go.

Ferno wrote:...the engine thrust will overcome the rolling friction of the wheels.

Clearly true in the "real world"! I guess being a "head-in-the-clouds" student of Mathematics, I'm more interested in the hypothetical, where the conveyor keeps the plane motionless (which is theoretically possible, if the force of the thrust is not enough to overcome the friction).

Ferno wrote:You can see this in action by doing the following: make a paper airplane, get in your car, find a street clear of traffic (shouldn't be hard to do) and see if the plane lifts away when you stick it out the window and let go.

Only if it's a well-constructed paper airplane, with the type of wings that generate lift.