A little riddle

[ANeonRider 3]

I am sure this is all over the internet by now, but, let's see what people know here

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).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

I know the answer, do you?

[vladd 26]

I am sure this is all over the internet by now, but, let's see what people know here

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).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

I know the answer, do you? 

<{POST_SNAPBACK}>

i'm guessing no, there is no wind passing over/under the wings which is how lift is created, not by the actual speed of the plane.

[Tirefryr 1,742]

I am sure this is all over the internet by now, but, let's see what people know here

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).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

I know the answer, do you? 

i'm guessing no, there is no wind passing over/under the wings which is how lift is created, not by the actual speed of the plane.

Agreed.

[JimJ 11]

Agreed again

No air movement over wings = no pressure differential = no lift.

[Supa_c 1]

Ever ride a bike on a tread mill, you dont go anywhere.

[dennisp 0]

Ever ride a bike on a tread mill, you dont go anywhere.

<{POST_SNAPBACK}>

A bike is propelled by its wheels, whereas the plane isn't. Big difference.

[Supa_c 1]

Well in that case it wouldnt make a difference if the runway moved foward or backward.

The plane is moving toward the air so it would definetly have lift when it reached the proper speed.

[vladd 26]

Well in that case it wouldnt make a difference if the runway moved foward or backward.

The plane is moving toward the air so it would definetly have lift when it reached the proper speed.

<{POST_SNAPBACK}>

but the plane isn't moving

[ANeonRider 3]

Supa & Dennis have the idea....

This is the answer someone else gave me on neons.org:

Vitor and neonbkw got it (except the part about ground speed), the plane will move forward regardless of what its wheels are doing and will be able to take off.

Ground speed is measured against a stationary point, and just because the airstip surface is moving won't have any effect on the amount of air pressure under the plane.  Ground effect is produced when the high-pressure air under the plane is forced into a cushion between the aircraft and the ground, it's relative to the speed of the plane through the air, not the speed of the ground under the plane.  This is of course not taking the affect of a rapidly moving surface on the surrounding air which would cause a breeze, but that would open up a whole new can of worms...

If the original poster's scenario was intended to mean: "If an airplane that was denied forward movement but nothing else, would it take off?" the answer would be no.  the plane would just sit there spinning its wheels with its afterburners glowing and making all kids of noise...and that's when you hit the stop button on the conveyor  :twisted:

Bingo!

A great vector diagram, complete with illustrations someone has made:

Any airplane (that is not extremely overloaded) can overcome its own force of weight & friction, mute point. They do it everyday, I watch them.

If the plane is not moving forward... then is the conveyor moving backwards to counteract the forward motion?

This riddle is worded like crap, which is why it is a riddle. Speed means that something is moving, without speed, the conveyor does not move, and if the conveyor moves, then that means the plane is moving.

If the wheels were actually driving the plane, then it would not move, but that is not the case. The engines use Newton's Second Law to provide thrust, and the wheels are freewheeling (not frictionless, or weightless, but still freewheeling) over the conveyor at twice the "air speed" of the airplane. Even a prop plane will be able to take off, because it too uses the air to propel itself to the point where lift takes over.

Sooo, if the plane is moving at 150mph down a conveyor runway, that means that the wheels are turning at 300mph, because the conveyor is turning at 150mph in the opposite direction, and the plane lifts off into the blue yonder.

Here is something that is on an aviation news forum:

Conveyer-Belt Runway

What I learned from Old Hack was that an updated version of a question aimed at confusing folks over relative measurements of airplane motion and the medium in which it operates had shown up on the Internet, and it was causing the fracas in the Lounge.

The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is shown: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

My comment: Notice that the question does not state that the conveyor's movement keeps the airplane over the starting position relative to the ground, just that it moves in the direction opposite to any movement of the airplane.

Initially, about a third of the folks here said that the airplane could not ever takeoff, because the conveyor would overcome the speed of the airplane and it could never get any airspeed. The rest said the airplane would fly.

The "It won't fly, Rocky" group said that the conveyor would hold back the airplane. They asked us to imagine a person running on a treadmill. As he or she sped up, the treadmill would be programmed to speed up, just as the conveyor in the problem, and the person would remain over the same locus on the earth, while running as fast as possible.

The argument was that if the airplane started to move forward, the conveyor program was set up to move the conveyor at exactly that speed, in the opposite direction, thus, the airplane would never move relative to the ground, and, because the air was calm, it could never get any wind over its wings. One of the analogies presented was the person rowing at three mph upstream in a river on a calm day. However, the current was flowing downstream at three mph, so the resultant speed with reference to the stream bank and air was zero, and thus there was no wind on the rowboat.

I watched and listened to the disagreement for a while and was fascinated to see that the argument seemed to split between those who had some engineering or math background, all of whom said the airplane would takeoff and fly without any problem; and those with some other background, who visualized the airplane as having to push against the conveyor in order to gain speed. Because the conveyor equaled the airplane's push against the conveyor, the airplane stayed in one place over the ground and in the calm air could not get any airspeed and fly.

It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff. One guy even got one of those rubber band powered wood and plastic airplane that sell for about a buck, put it on the treadmill someone foolishly donated to the Lounge years ago, thinking that pilots might actually exercise. He wound up the rubber band, set the treadmill to be level, and at its highest speed. Then he simultaneously set the airplane on the treadmill and let the prop start to turn. It took off without moving the slightest bit backwards.

http://www.avweb.com/news/columns/191034-1.html

If you still disagree, don't fret, a guy I know with a Physics Degree seems to not be able to grasp this either.

Imagine putting a rocket booster on your back, with rollerskates, on a conveyor trying to match your speed. If the rollers on the skates don't disinegrate first, the conveyor won't do a thing to slow you down. The friction of the rollers on the conveyor, nor the friction of the bearings, or the force of your weight downwards will have any effect that will be noticeable to that rocket on your back.

Edited November 29, 2005 by ANeonRider

[ANeonRider 3]

Well in that case it wouldnt make a difference if the runway moved foward or backward.

The plane is moving toward the air so it would definetly have lift when it reached the proper speed.

<{POST_SNAPBACK}>

If the conveyor moved forward, the wheels would spin at 1/2 of the speed that the plane is travelling. So, using my example, if the plane was travelling 150mph, the wheels would be travelling at 75mph, and the plane would still get enough speed to provide itself with lift.

[Chevy350TPI 12]

But if you think about it ... maybe it would take off. Don't planes accelerate relative to the air? It's not like a car ...

Ah well, waaaayyyyy too much to think about right now.

[ANeonRider 3]

Steve, your right

[vladd 26]

conveyer and wheel speed shouldn't matter for crap, its like taking off in an ultralight with a heavy headwind, you can jump in the air real quick (not steadily or safely mind you) but at a much lower speed because the speed that the wings see is much higher

[ANeonRider 3]

It doesn't matter, just putting it in perspective...

The conveyor really does not matter at all, the friction on the wheel bearings will increase slightly because of the speed at which the wheels are turning, but, will not stop the plane from taking off.

Edited November 29, 2005 by ANeonRider

[dennisp 0]

Here's a silly visual which probably has little relevance but it's hilarious.

http://videos.streetfire.net/player.aspx?f...8D-D6BA1A43A06B

[Beat_Dominator 2]

I answered the riddle before I even read it

[vladd 26]

maybe i'm misunderstanding, but lets say you're from a 3rd person perspective watching this plane, directly perpindicular to it, would it be "moving" forwards or backwards? i thought it was stationary, the plane's moving forward on the belt thats moving backwards just as fast.

if it can take off, how? i mean lets assume we're measuring its "air speed", shouldn't that be 0?

i'm not arguing, i mean if a reputable source says its right, i know i'm not smarter than 99% of non-inbred people, just trying to understand

[Beat_Dominator 2]

Yer brain is just messing with you. It is used to things being moved by their wheels. THe plane will move.... say right to left. The tires on the plane will be spinning 2x faster than normal. That's all the conveyor does.

[ANeonRider 3]

The wheels are just a medium for the plane to get to a destination on the ground, they provide no acceleration, only weight bearing in a take-off scenario, read the hovercraft example in the link, same idea, just more friction (which everyday, thousands of planes overcome, I watch them at work).

The force that acts on the plane to accelerate it is the thrust provided by the propultion of the air from the prop or jet engine. The tires are just along for the ride until the airspeed has reached the level to provide lift. If there is a conveyor underneath the tires, it will do nothing more (besides a slight increase in the friction on the wheels because of friction, which is pretty much neglible) than turn the wheels faster, the weight of the plane does not increase, therefore, the plane is not slowed down any. The plane continues to accelerate, even if its ground speed is increasing, since the tires have no ability to slow it down, unless the brakes are applied.

[vladd 26]

i understand the tires don't accelerate, but i'm saying is the plane actually moving in relation to say...the flight tower. If they were to watch the plane on the radar would it be stationary or moving? because stationary, no matter what else is happening, it can't take off unless there is air moving under the wing, i mean if this wasn't true, don't you think aircraft carriers would use this instead of the huge counterweight slingshot they use now?

[dennisp 0]

i understand the tires don't accelerate, but i'm saying is the plane actually moving in relation to say...the flight tower.  If they were to watch the plane on the radar would it be stationary or moving? because stationary, no matter what else is happening, it can't take off unless there is air moving under the wing, i mean if this wasn't true, don't you think aircraft carriers would use this instead of the huge counterweight slingshot they use now?

<{POST_SNAPBACK}>

They plane is stationary compared to a spectator/radar/flight tower/etc

[ANeonRider 3]

i understand the tires don't accelerate, but i'm saying is the plane actually moving in relation to say...the flight tower.  If they were to watch the plane on the radar would it be stationary or moving? because stationary, no matter what else is happening, it can't take off unless there is air moving under the wing, i mean if this wasn't true, don't you think aircraft carriers would use this instead of the huge counterweight slingshot they use now?

<{POST_SNAPBACK}>

They plane is stationary compared to a spectator/radar/flight tower/etc

<{POST_SNAPBACK}>

Nope, the plane is moving, and everyone would see it doing so, at the same rate as it would without the conveyor. Read the link Dennis

The conveyor would not increase or decrease the length of runway needed.

Edited November 29, 2005 by ANeonRider

[Nikuk 1]

rofl... yes, this IS all over the net. I posted it up on a few other forums as well

Yes, the plane flies.

[UndercoverPunk 0]

I hate planes...

[Nikuk 1]

^ LMAO