Physics Question

[So]

Originally posted by: kd2777
fvck no it won't take off, there will be no lift.

Tire speed has nothing to do with a plane.

I can't believe "your group" was torn down the middle on this.

kd

How can one be so riteous and so wrong at the same time?

Assuming decent bearings in the wheels, It's 100% sure that it will take off. Read the thread.

 

[Armitage]

Originally posted by: kd2777
fvck no it won't take off, there will be no lift.

Your bold use of profanity is swaying me ... please explain why there won't be any lift?

 

[sygyzy]

Originally posted by: 911paramedic
OMFG, now there is a website posted saying it CAN take off...

http://www.straightdope.com/columns/060203.html

The "straight dope" is smoking dope.

OMFG, a 12 year old poster who's never heard of Straight Dope!

 

[The Godfather]

I got 99 problems but a b1tch ain't one.

 

[ta8689]

omfg are there any other pilots in here? honesly. And yes i have a private pilot's certificate.

 

[JujuFish]

Originally posted by: ta8689
omfg are there any other pilots in here? honesly. And yes i have a private pilot's certificate.

Why do you ask?

 

[cambre]

I'd like to take a stab at it.

First, please accept this as an addendum to the original question. Even though it was not worded as such, I believe it is within the spirit of the problem:

<addendum>
There is ample friction between the wheels and the treadmil such that traction is never lost via the plane's acceleration. Sure you could strap a saturn V on the back of the plane and incinerate the wheels as the plane heads up to escape velocity but I don't think that would follow the premise of the original problem. If traction is lost, then of course the plane will be able to move independent of the treadmill and the plane can take off. In other words, there shall be no skid marks on the treadmill.
</addendum>

It seems the schism here is between those who feel the wheels have absolutely no effect on the planes movement and those who do. Those who claim that wheel speed will simply double as the plane moves forward on the treadmill are of the "freewheel" camp. Please consider this scenario:

Plane is idle on the treadmill, engines off. Start the treadmill at 50mph. There are two possible outcomes:

1) Plane remains stationary, wheels are spinning at 50mph.
2) Wheels remain stationary, plane moves at -50mph.

I'm of the belief that the plane will be moving backwards and the wheels will be still. The freewheelers would think otherwise, the plane is stationary, the wheels are moving.

Scenario 2:

Plane is reversed on the treadmill such that they are both moving in the same direction. Takeoff speed for the plane is 200mph. Plane is idle, engines off. Start the treadmill and accelerate it to 200mph.

A) Plane remains stationary, wheels are spinning at -200mph.
B) Wheels remain stationary, plane moves at 200mph, lifts off from runway and promptly lands on runway as it quickly loses its forward speed where it is again acclerated up to 200mph, repeat. (At 200mph air resistance will likely be enough to overcome the static friction between the wheels and the treadmill causing the plane to start rolling backwards so you might have to speed up the treadmill a bit to get the plane to 200. Or, the terminal velocity of the plane may be less than 200mph and it will never take off with the wheels accepting the excess velocity but I think the logic still applies.)

Again, I see scenario B to be the more likely outcome. The freewheelers would think the plane just sits there. Going back to the first scenario and assume that the plane was moving backwards. Accelerate the plane such that it gains on the treadmill until you reach the origin. Now, back off on the accelerator so that you are no longer moving with respect to the ground. Here, I see the plane at zero speed relative to ground, treadmill at 50mph, wheels spinning at -50mph. Now, speed up the treadmill to 100mph. I would think the plane would again start moving backwards. If you can visualize this, then I think you can see that the plane can never advance if the treadmill is matching the plane's speed.

If the plane is independent of its wheels and therefore independent of the treadmill, then I can see how you think the plane can takeoff. This would only be the case if the wheels and bearings are frictionless which I don't think they are. Or if the air resistance of a stopped/low speed plane is greater than the friction between the wheels and the treadmill which I would also doubt. For example, in the first scenario if instead of air, the plane was trying to move through honey, there might be enough resistance there such that the treadmill would spin the wheels instead of move the plane. Also, I believe that whole issue of wheel driven versus thrust driven does not apply here. Those would be more relevent in the case of a dynamometer. A car on a dyno would not move. A plane on a dyno would.

As for the skateboard fan, it would not move as the fan is pushing itself backwards as much as it's pushing the board forwards. This would be the same thing as attaching a huge magnet a foot in front of your car and expecting it to move forward.

The idea behind pulling a rope is not a proper analogy. If you are on skates or whatever on a treadmill holding yourself by a rope, the rope is pulling you at the same speed that the treadmill is pushing you. If the treadmill increases speed, the rope will pull you harder. As long as traction is maintained between the wheels and the treadmill, the push/pull will be equal resulting in no net movement. If you now start pulling on the rope, you are adding a force outside of the system than can not be offset by the treadmill as any increase in the treadmill speed will still be taken care of by the rope. If the treadmill is pushing at -10mph, the rope is pulling you at 10mph. If you pull the rope at 1mph, treadmill increases to 11mph, rope pulls at 11mph, you're still pulling at 1mph, so now you're going 12mph. This cycle will continue until the rope shreds your hand or the rope breaks. The same occurs with the hand pushing the wagon from behind. If you think that the wheels spinning plays no part in the overall movement of the object, than you shouldn't need a rope or a hand to keep a wheeled object stationary on a moving treadmill.

P.S. I'm quite impressed at the discussion thus far. It has been rather civil and pretty open-minded. Hope it keeps up.

 

[ta8689]

Because if you are a pilot and have ever piloted an aircraft, then you should know that it will take off

EDIT: but then you can skid on the tredmil and still pull ytourself foreward, your landing gear can break off and at full thrust, you will still probably go foreward if the tredmill is moving

 

[edro]

I still think you are all on crack.

If the wheels are free to spin, and the engine generates forward thrust... it WILL take off. If the conveyor belt counteracts all forward motion of the plane, it will NOT take off.

It all depends on how the question is worded or translated.

 

[So]

Originally posted by: cambre
I'd like to take a stab at it.

First, please accept this as an addendum to the original question. Even though it was not worded as such, I believe it is within the spirit of the problem:

<addendum>
There is ample friction between the wheels and the treadmil such that traction is never lost via the plane's acceleration. Sure you could strap a saturn V on the back of the plane and incinerate the wheels as the plane heads up to escape velocity but I don't think that would follow the premise of the original problem. If traction is lost, then of course the plane will be able to move independent of the treadmill and the plane can take off. In other words, there shall be no skid marks on the treadmill.
</addendum>

It seems the schism here is between those who feel the wheels have absolutely no effect on the planes movement and those who do. Those who claim that wheel speed will simply double as the plane moves forward on the treadmill are of the "freewheel" camp. Please consider this scenario:

Plane is idle on the treadmill, engines off. Start the treadmill at 50mph. There are two possible outcomes:

1) Plane remains stationary, wheels are spinning at 50mph.
2) Wheels remain stationary, plane moves at -50mph.

I'm of the belief that the plane will be moving backwards and the wheels will be still. The freewheelers would think otherwise, the plane is stationary, the wheels are moving.

Scenario 2:

Plane is reversed on the treadmill such that they are both moving in the same direction. Takeoff speed for the plane is 200mph. Plane is idle, engines off. Start the treadmill and accelerate it to 200mph.

A) Plane remains stationary, wheels are spinning at -200mph.
B) Wheels remain stationary, plane moves at 200mph, lifts off from runway and promptly lands on runway as it quickly loses its forward speed where it is again acclerated up to 200mph, repeat. (At 200mph air resistance will likely be enough to overcome the static friction between the wheels and the treadmill causing the plane to start rolling backwards so you might have to speed up the treadmill a bit to get the plane to 200. Or, the terminal velocity of the plane may be less than 200mph and it will never take off with the wheels accepting the excess velocity but I think the logic still applies.)

Again, I see scenario B to be the more likely outcome. The freewheelers would think the plane just sits there. Going back to the first scenario and assume that the plane was moving backwards. Accelerate the plane such that it gains on the treadmill until you reach the origin. Now, back off on the accelerator so that you are no longer moving with respect to the ground. Here, I see the plane at zero speed relative to ground, treadmill at 50mph, wheels spinning at -50mph. Now, speed up the treadmill to 100mph. I would think the plane would again start moving backwards. If you can visualize this, then I think you can see that the plane can never advance if the treadmill is matching the plane's speed.

If the plane is independent of its wheels and therefore independent of the treadmill, then I can see how you think the plane can takeoff. This would only be the case if the wheels and bearings are frictionless which I don't think they are. Or if the air resistance of a stopped/low speed plane is greater than the friction between the wheels and the treadmill which I would also doubt. For example, in the first scenario if instead of air, the plane was trying to move through honey, there might be enough resistance there such that the treadmill would spin the wheels instead of move the plane. Also, I believe that whole issue of wheel driven versus thrust driven does not apply here. Those would be more relevent in the case of a dynamometer. A car on a dyno would not move. A plane on a dyno would.

As for the skateboard fan, it would not move as the fan is pushing itself backwards as much as it's pushing the board forwards. This would be the same thing as attaching a huge magnet a foot in front of your car and expecting it to move forward.

The idea behind pulling a rope is not a proper analogy. If you are on skates or whatever on a treadmill holding yourself by a rope, the rope is pulling you at the same speed that the treadmill is pushing you. If the treadmill increases speed, the rope will pull you harder. As long as traction is maintained between the wheels and the treadmill, the push/pull will be equal resulting in no net movement. If you now start pulling on the rope, you are adding a force outside of the system than can not be offset by the treadmill as any increase in the treadmill speed will still be taken care of by the rope. If the treadmill is pushing at -10mph, the rope is pulling you at 10mph. If you pull the rope at 1mph, treadmill increases to 11mph, rope pulls at 11mph, you're still pulling at 1mph, so now you're going 12mph. This cycle will continue until the rope shreds your hand or the rope breaks. The same occurs with the hand pushing the wagon from behind. If you think that the wheels spinning plays no part in the overall movement of the object, than you shouldn't need a rope or a hand to keep a wheeled object stationary on a moving treadmill.

P.S. I'm quite impressed at the discussion thus far. It has been rather civil and pretty open-minded. Hope it keeps up.

Your point about there being <some> friction is valid, but even at high speeds, it will be marginal compared to the thrust from the engines. Small enough that it can be neglected.

 

[Mrvile]

Alright, I only read through the first page. But here's my argument:

I know this sounds crazy but I gave it some thought.

Take Letdown's example on the first page. Picture a vehicle towing the plane on a belt. The vehicle is not part of the belt. The vehicle moves forward at 10mph, so the plane begins to move at 10mph, and the belt moves backwards at 10mph. So the plane's wheels are spinning at 20mph total. However, the belt only moves the plane's wheels, and doesn't affect the towing vehicle at all. So the towing vehicle will still be moving forward at 10mph, and so will the plane. The belt might be spinning at 1 billion mph, and the wheels 2 billion mph, but none of this affects the towing vehicle.

The engines of the plane act as the towing vehicle - they push air through the engines to propel the plane forward. The wheels aren't motorized, and exert no friction on the ground. So the conveyer belt would be doing nothing but spinning the plane's wheels - it won't actually be pulling the plane against itself.

So assuming the plane's wheels and the conveyer belt can move at infinite speeds, and there is no friction within the bearings of the wheel itself, the plane would take off.

I hope you guys understand this. This is a really cool topic.

 

[Armitage]

Originally posted by: edro
I still think you are all on crack.

If the wheels are free to spin, and the engine generates forward thrust... it WILL take off. If the conveyor belt counteracts all forward motion of the plane, it will NOT take off.

One more time ...
How does the belt countract the forward motion of the plane?

It all depends on how the question is worded or translated.

It seems very plain to me.

 

[Yossarian]

Originally posted by: ta8689
omfg are there any other pilots in here? honesly. And yes i have a private pilot's certificate.

yes, and drawing a free body diagram clearly shows that the plane will roll off whatever conveyor belt is supporting it. the conveyor can't stop the plane from moving.

 

[imported_Condor]

Originally posted by: 911paramedic
OK, a friend of mine frequently posts brainteasers for us on our site. (400+ members, all veterans or active duty) This is his latest question and we are deeply divided on the answer, just about down the middle. I would like to put this out there so I can get a very simple answer to explain it to him. (I am sure of my answer, but dont want to post it for fear of contamination) Here is his question:

"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 the opposite direction). Can the plane take off?"

Thanks in advance. (no, its not homework, you guys know me...duh)

No! It's all about airflow, not groundspeed.

 

[sxr7171]

Originally posted by: notfred
This thread's already about 3 million posts long, but I have to give my input. The plane WILL take off. The engines of an airplane move it relative to the air around it, not relative to the ground, like many people have said before.

Let's make an analogy. Pretend the plane is propelled forward by a giant winch. There's a rope attached to the front of the plane, and that rope is attached to a winch at the far end of the conveyor. Now, when the winch starts winding in and pulling the plane forward, do you think it matters how fast the conveyer under the plane moves? It doesn't the winch is going to keep pulling in rope at the same rate, and move the plane forward. The only thing that the conveyer will change is the rotation speed of the wheels, but the plane will keep moving forward at a constant rate because it's being pulled by a rope.

The engine on an airplane is like the rope, except the rope and the winch is air instead the engine pulls the plane through the air, independent of the movement of the ground.

Yeah you're right. So basically the wheel-conveyor belt interface is irrelevant. It may as well be a floating hovercraft with a conveyor belt under it.

 

[sxr7171]

Originally posted by: SophalotJack
So, you have to be kidding right?

There is a massive difference between moving a vehicle forward on a reverse-moving plane compared to that same vehicle taking off.


A jet powered car would do the same thing.... but we know for sure that it wouldn't take off because it's not an airplane.

Don't let the "wings" fool you all... you need "lift." Which can only be attained if wind is hitting the wings. Seeing as though the only wind generated is behind the airplane (from the jet engines), there is no lift.


The thing that the people (who think this airplane might take off) are severely not understanding is that just because a plane moves forward (countering the reverse covneyer belt) doesn't mean that the airplane's wings are getting any lift. The air in front of the plane is still perfectly still (except the air immediately around the jet intakes, and that air doesn't get to the wings at the intake speeds).


Maybe if there was a hurricane force wind blowing toward the airplane.... then it might take off.


Man, how to people not get this?

I suppose parachutes can open up on the ground based on the way most people think.

Buddy if you think this through the plane does move forward releative to the air in front of it and hence gets air over its wings and therefore flys.

 

[sxr7171]

Originally posted by: g8wayrebel
I read all the posts.
You are still wrong.
The plane would never have forward motion. If you are on a treadmill running 11 m/s (just under the world record) does the wind blow your hair back and your hat off? NO , it does not. There is no wind.
A plane , no matter the propulsion system, would not have any either.
It would initially move slightly forward to cause the conveyor to start according to the OP who says the conveyor "matches the speed " of the plane. From that point forward , the forward motion of the plane would be negated by the rearward motion of the conveyor.
The fact that the plane has wheels merely allows it to roll and accelerate on the conveyor faster.. Even if it were on it's belly , if it had enough thrust , it would begin to move and start the conveyor.
From that point forward , the result would be the same.It would be sationary relevent to the atmosphere and never develop lift.
THIS BIRD WON"T FLY!

This has been an extremely interesting question.. I am going to take this to another forum and see if I get any answers worth repeating.
BTW , I have a treadmill and am trying to convince a friend of mine to bring an RC plane over to prove the point. I will not stand in front of it , but don't expect any damage to plane or basement in the process. On second thought , guess I'll move the treadmill outside. There is more air out there. Yeah that's it , there's more air outside. LOL
If this takes place (doubtful) I will put it on video for all.

If you are on a treadmill, understand that it is your feet that is working against the treadmill and the treadmill against your feet. Should I push you while you are running on that treadmill, you will lunge forward.

 

[sxr7171]

Originally posted by: Chryso

Originally posted by: elkinm
Wow this is a long thread about almost nothing.

Anyway, if the conveyor matches the speed of the plane and the speed is directly proportional to the speed of the wheels and it had infinite friction between the wheels and the conveyor it cannot take of as from the point of view of an observer standing of the conveyor the plane is not moving.

But realistically, a plane is not a car and does not rely on the wheels for propulsion. The engines will still move it forward and if the wheels are free to rotate like they should be the plane WILL take off at whatever speed it needs and the conveyor will move in the opposite direction at the same speed and the wheels will spin backwards at double the speed. Somehting that was already said before.
And if the brakes are engaged then at some point the thrust will break the force of friction and simply slide on the conveyor until take off.

Have you ever tried a non powered treadmill. As you push to walk forward you push the tread back, or if you try to jump off a boat, what happens. The boat goes backwards and you land in the water (unless you jump really far.) But no imagine if you move not by pushing against the surface under you but from the air then you will move forward.

So a real plane in this scenario will take off.

How are the wings getting any lift if the plane is not moving?

Bad assumption.

 

[Mill]

Originally posted by: edro
I still think you are all on crack.

If the wheels are free to spin, and the engine generates forward thrust... it WILL take off. If the conveyor belt counteracts all forward motion of the plane, it will NOT take off.

It all depends on how the question is worded or translated.

I see what you are saying, but it seems to me the question implies that it is counteracting wheel speed. It shouldn't or can't prevent the thrust motion forward.

 

[sxr7171]

Originally posted by: edro
I still think you are all on crack.

If the wheels are free to spin, and the engine generates forward thrust... it WILL take off. If the conveyor belt counteracts all forward motion of the plane, it will NOT take off.

It all depends on how the question is worded or translated.

A conveyor belt cannot prevent the plane from moving forward no matter how fast it is travelling unless the wheels lock up.

I suppose you could argue that the wheels have a certain amount of friction and you could reach a speed where the wheels generate enough friction inside the wheels to actually couple the plane to the conveyor belt enough to stop the plane, but I can't imagine a real wheel not melting at that speed.

 

[sxr7171]

nm

 

[royalk4]

Ok this should settle the issue.

I asked Newton himself and he said "What is a plane?"

 

[letdown427]

The conveyer is not mimicing wheel speed. this leads to a paradox as has been said many, many times.

Second, stop discussing, oh the conveyer could get fast enough to stop the plane. Even if it could, at that speed, the plane would be flying. Please, read some of my earlier posts. The conveyers speed is relative to the speed of the plane. The speed of the plane is relative to the earth. I proved that in my earlier posts. Therefore for the conveyer to be moving at all, the plane must be moving.

For the plane to be going at say 200mph, the plane must be moving, relative to the earth, at 200mph. it would be flying at this point.

Please stop saying thrust doesn't equal lift. we all know that. Please stop argueing that airflow over the wings is needed. we all know that too. the airflow over the wings is created by the fact that the plane is moving.

hehe, this won't stop until we've converted every 135,000 odd anandtech members to the right way of thinking! hehe

 

[Rayden]

Here is a stupid example. Imagine that the conveyor belt is declined about 45 degrees. That is, it is significantly angled downward.

If it was off and you were to drop a ball on it, the ball would roll down.

If it was off and you held a plane on it, and then let go, the plane would roll off.

If it was on and you let the plane go... HOW IS IT SUPPOSED TO KEEP THE PLANE FROM ROLLING DOWNHILL?! The force on the plane from gravity will pull the plane down the incline despite the fact that the conveyor is moving.

The plane's engine provides force just like gravity. Therefore the plane will move.

 

[Kelemvor]

Well, I didn't read the whole thread but here's my take...

If the plane is thrusting with jets then it can take off. The conveyor cannot match the speed of the plane's wheels because the jets are pushing the plane forward through the air. The wheels are not what makes the plane go forward. So as the plane's jets fire, it will push the plane forward through the air. The wheels just move because they happen to be touching the ground. But the plane wil still move forward and if the convery starts to spin fast to try to stop the plane the converyor will eventually break beacause it's impossible.

Now if the plane was like a car and got it's movement from the tires driving, then No the plane could take off. It wouldn't matter how fast the plane is moving in relation to the converyor belt, it only matters how fast it's moving in relation to the air. The air (and air pressure) is what gives the plane lift. No movement = no fancy air pressure = no takeoff.