Airplane and Conveyor Belt Debate

[russ_watters]

So perhaps we could see the initial question...? The wording of the initial question makes all the difference here.

 

[Wilbur Wright]

So perhaps we could see the initial question...? The wording of the initial question makes all the difference 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?

The answer is that the plane will take off with little difference from the manner it would on solid ground.

We know the vessel must move, or there can be no lift.

There are only two forces at work here, the force of the engines pushing the craft, and the force of friction on the wheels.

Since the conveyor is only moving at the same speed as the vessel, rather than with equal and opposite force, the force of friction on the wheels will never come even remotely close to equal or overcome the force of the engines.

The conveyor could even be moving considerably faster than the fuselage, and flight will still occur because the force of friction is so much lesser than the force from the engines.

One needs to remember that unlike a car, the wheels on an airplane do nothing but spin freely. It's no different that pushing a matchbox car up a treadmill, where the wheels on the matchbox are the landing gear, the car itself is the aircraft, and your hand is the engine of the plane. The speed of the treadmill makes next to no difference in the force exerted by you to move the car against the rotation of the belt.

 

[G01]

Sorry, this is not right. The aeroplane's wheels are not powered, - the forward motion of an aeroplane on takeoff is produced by thrust from the engines. For these purposes, it makes no odds what speed the runway is moving relative to the plane, since the undercarriage wheels are free to spin at whatever speed they're being driven at.

Yes, I see my mistake, you're right, my bad.

 

[DaveC426913]

BTW, as far as the conveyor goes, consider this:

If the conveyor is able to supply ANY non-trivial amount of drag upon the plane's forward movement then - clearly, the wheels are NOT doing their job! They're not supposed to cause drag! If they are, it means they're ALSO going to cause drag when the plane is going down a regular runway to take off!

 

[rcgldr]

Edit: what's being ignored on my part is that the conveyor belt just speeds up how fast the tires spin. Otherwise the plane just takes off as normal. If the plane was tied down so it couldn't move horizontally, then then propwash would have to draw enough airflow to create lift.

 

[MythBusted]

http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B"
This may help you get a visual on the problem

 

[Moe88]

plane and a conveyor belt

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?

 

[rcgldr]

There is already another thread here on this.

Edit: The following would only apply if the plane was tied down so it couldn't move, not the same situation.

The answer is only if the planes engines can produce enough airflow to cause the plane to fly even though it's not moving relative to the ground.

No full scale plane could do this, but there are some high powered radio control models that can do this. The most extreme examples are "27 cell" (old ni-cad terminology), F5B limited motor run gliders. The motors produce over 2 1/2 horsepower, geared to drive a 16 or 17 inch prop (folding) on a model weighing about 5 pounds.

 

[Cyrus]

lol mythbusted, I heard what you said in the end of that video...ahem...

 

[rcgldr]

Wow, brain fade on my part, wasn't paying attention, assuming that the friction from the tires rotating isn't excessive, the plane takes off in normal fashion. The free wheeling tires just spin faster while the plane takes off.

 

[rcgldr]

Wow, brain fade on my part, wasn't paying attention, assuming that the friction from the tires rotating isn't excessive, the plane takes off in normal fashion. The free wheeling tires just spin faster while the plane takes off.

All the conveyor belt does is cause the tires to spin faster. Other than some energy consumed by rotational kinetic energy and friction from the tires rotating faster, the plane is going to take off pretty much as it normally would.

If the plane was tied down down via a tow hook so it couldn't move forwards, then my extreme power situation where the propwash draws enough air across the wing to lift the plane would apply.

 

[russ_watters]

It's all about the wording of the question, the phrase "The plane moves in one direction" implies that the plane is moving with respect to the air, so the plane does take off. See the other thread...

 

[pervect]

If I'm understanding the operation of the conveyer property, it would not even stop a car. A car whose spedometer read 60 mph would be going 30 mph in one direction (say east), while the road went 30 mph in the other direction (say west)

 

[pervect]

If I'm understanding the operation of the conveyer properly, as described it would not even stop a car. A car whose spedometer read 60 mph would be going 30 mph in one direction (say east), while the road went 30 mph in the other direction (say west).

It's unclear if this was the intent of the question.

If the conveyer was intended to stop a car, the conveyer will reach its maximum possible speed in order to attempt to stop the plane. This could be achieved by deriving the feedback signal to the conveyer not from a measurement of the speed of the plane relative to the ground, but by deriving a feedback signal from the rate at which the wheels of the plane were turning. (The original question didn't really clarify exactly where the feedback signal was coming from, hence the ambiguity of the question).

It would probably be good to include a physical upper limit to the speed at which the conveyer can operate in the problem statement if the problem is modified so that the conveyer actually does attempt to stop a car.

 

[Noodler]

Maybe you guys can help settle a little argument here.

My friends and I have been having a physics based argument for some time and we have reached a point where we need a resolution. I believe for someone trained in physics it is a very easy question and I was wondering someone could shed a little light on this.

Thanks a lot!

The situation:

A plane is standing on 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 the plane move forward or stay stationary?

 

[brewnog]

https://www.physicsforums.com/showthread.php?t=101259

This has been done to death.

 

[KingNothing]

Good question, one that makes you think twice. Not because of the conveyor, but because of how planes work. Yes, the conveyor could stop a car, because a car uses the ground to move.

Think about it like running on a treadmill. If you just run and the treadmill keeps up with you, you won't go anywhere. Now think about if you had legs that could move incredibly fast (like the wheels on the plane), and simply pushed off the wall. You would still move.

 

[webtim-1]

Airplane On Conveyor Belt?

See what you guys think
Taken from another forum, this question was put forward and argued with incredible conviction from 5 angles or more, to reach either of two possible answers.
It's a pretty simple question but had some brilliant rows about all sorts of things going on to justify the answers.




Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

 

[moose]

https://www.physicsforums.com/showthread.php?t=101259

Yes it will take off

 

[pallidin]

A plane does not require wheels to provide for forward motion.
Perhaps a way to look at this is similar to your description:
Imagine a plane, without wheels, yet it's belly is on a sheet of slick ice. Will the plane move forward? Yes.
Perhaps some of the confusion is with respect that a car will NOT go forward in the same scenario you described. In a car's case, the wheels ARE providing the forward motion by virtue of being DIRECTLY COUPLED to the engine output. A plane DOES NOT have that set of circumstance.

 

[russ_watters]

Duplicate thread...locking.

 

[AJHank]

Air

The plane would rise if the air on its lifting surfaces was moving across them faster than the plane's stall speed. Aircraft carriers head into the wind so that the speed over ground is lessened, relative to the speed of the (moving) ship. If the plane must go 140 kts relative to the air it is flying through, and there is no wind, it will only need to go 120 kts relative to the deck of a carrier going 20 kts in the same direction as the takeoff. Into a headwind of 20 kts, in that scenario, the plane will have sufficient life to take off at what would appear to be 100kts to someone standing on that same carrier. Speed through the air is what counts. A plane could beon a conveyor going 1000 kts and keeping that pace, but if the air over the control and lift surfaces (wings) does not move, the plane will not go up.

 

[curvedlogic]

If you go to pprune/forums/jet-blast you will see what pilots and engineers think of this problem. you may never want to fly again.

 

[Dan Shenk]

Aside from the question of whether or not the conveyer belt can actually prevent forward movement of the plane, and all other esoteric mumbo jumbo, planes fly because of airspeed, not becaues of ground speed. If air does not flow across the wing, the plane will not fly.

Care to rephrase your question, Smoke?

 

[mewmew]

I have riddle type of question

So on another forum we are in a huge argument and so I come to you guys for help with the following:

Suppose you have an airplane on a runway that is a huge conveyor belt. The conveyor belt moves with the opposite velocity of the plane at all times, what happens when it tries to take off?

Now, I think we have two cases at hand, one where the runway moves with the opposite speed of the plane itself, and one in which the runway moves with the opposite speed tangental to the outside of the tire.

Can anyone shed any light onto how this would actually work, assuming just Newtonian mechanics and what not.

I think it will take off in both cases. but am looking for a solid argument for each. Thanks

 

[simon009988]

i not sure what u mean but i think the plane wouldn't take off because a plane only files because there is a difference in air pressure on the wings and if only the runway is moving and not the air molecules the plane would stay still (if only the tires were moving exactly in opposite to the runway)

 

[mathman]

The ability of a plane to take off depends on its speed relative to the air, so what is happening on the ground is relevant only as it affects air speed. An example of this principle is taking off from an aircraft carrier. The carrier is pointed against the wind and the plane takes off against the wind. Similarly at airports the preferred direction for takoff is against the wind. An analogous logic is used for landing, also against the wind, to get minimum ground speed for given airspeed.

 

[mewmew]

I understand that, I know how a plane flys(well, the basics atleast) and know that this problem comes down to if the plane can move or not. I, as well as others I have talked to, say that the conveyor belt will only make the wheels spin faster but have little effect on the plane. The wheels aren't locked like on a car, so the plane can generate forward movement relative to the Earth independent of how fast the wheels move. Say the plane is going 100 m/s relative to the earth, the coveyerbelt is going -100 m/s relative to the earth, so the wheels, or a point on the wheel, should spin at 200 m/s, if this was a car it would be easier, but because its a plane the situation is harder to imagine.

 

[berkeman]

Who cares what the wheels are doing. If the plane is not moving relative to the Earth and the associated air, the wings cannot generate lift. This seems like a pretty straightforward puzzle. Why are you guys debating it somewhere? Care to make a little wager... :-)

 

[mewmew]

It is not as easy as you make it sound, just because the belt moves backwards and the plane is moving forward(with equal and opposite speed) doesn't mean the plane is going to feel the backwards motion, because it has wheels independent of its engine that can spin however fast they want. It isn't really a straight forward question.

 

[berkeman]

Wanna bet? You got a PayPal account? :-)

 

[mewmew]

No where in the question does it say the plane is stationary in respect to the earth.

Here is what I am thinking:

The moving belt should only cause a small change in the planes speed, but it will cause the wheel of the plane to spin. If you have a treadmill and something on wheels you can try it, as it is easier to imagine or even actually do. If you turn on the treadmill you can put something on it that has wheels like a tonka truck and then with little force move it in the opposite direction the treadmill is moving, can you not? If this is the case than certainly the plane could do it because its engine is just like your hand and then it could of course take off.

I am more interested in what the forces all are that cause this to happen and what is going on with those forces, as I don't really understand all of that part of the question.

 

[Danger]

How bloody many threads do we have on this thing, anyhow?
The plane will take off unless the belt is moving so fast that the wheel bearings seize up.

 

[berkeman]

No where in the question does it say the plane is stationary in respect to the earth.

Well, I guess it was where you said this:

Suppose you have an airplane on a runway that is a huge conveyor belt. The conveyor belt moves with the opposite velocity of the plane at all times, what happens when it tries to take off?

"Opposite velocity at all time" is the part where I guess you threw me. You need to define your reference frame better (the earth?), and define what the velocity of the treadmill is with respect to it, and what the velocity of the plane is with respect to the reference frame.

 

[RandallB]

So on another forum we are in a huge argument

I hope you’re talking about some other forum web site, not double threading on this one.

Now, I think we have ...
one in which the runway moves with the opposite speed tangental to the outside of the tire.

No this would be the same a stationary runway, the tangential speed of the tire in contact with the runway is always ZERO. The top of the tire would be moving forward at double the speed of the axle. Tires don’t leave skid marks on takeoff.

This is a logic problem that requires assumptions: assuming zero wind and the airplane speed unlike a car is measured against the wind and (because no wind) the stationary markers on the sides of the treadmill runway.
And since the runway only moves as a complement to the movement of the airplane it’s only important the runway move backwards at takeoff speed. As defined in the problem it only gets there if the plane also does so moving forward & therefor takes-off.
Note also that the bottom of the tire is still moving at Zero with relation to the treadmill which is moving backwards. The axle moves with the plane so the top of the tire is moving at 4 times takeoff speed.

Pretty simple, what’s to argue about?
I assume since you can afford one expensive runway you can afford wheels that don’t seize up when over spun.