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Geek Culture / Plane on conveyor belt problem

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Libervurto
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Posted: 14th Dec 2007 14:05 Edited at: 14th Dec 2007 14:10
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No, you need the air moving under the wings to take off otherwise 747s would just levitate into the air
The fact that the wheels are turning makes no difference, they are just keeping it stationary.

Quote: "The plane will take off. As the driving force for a plane has nothing to do with the runway itself, the plane will be traveling at 10m/s relative to the air and 20m/s relative to the runway."

It isn't travelling at 10m/s relative to the air because it isn't moving through the air
It isn't travelling at 20m/s relative to the runway because it isn't moving across the runway
the only thing the aeroplane has is potential energy, if released (conveyor belt stops) it would catapult out at a much greater speed than from stationary. Maybe they should launch planes like that

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Peter H
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Posted: 14th Dec 2007 14:06 Edited at: 14th Dec 2007 14:11
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I hope this makes it painfully obvious that the plane will take off.



[edit] made it slightly more painfully obvious.

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BiggAdd
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Posted: 14th Dec 2007 14:08 Edited at: 14th Dec 2007 14:16
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Quote: "No, you need the air moving under the wings to take off otherwise 747s would just levitate into the air
The fact that the wheels are turning makes no difference, they are just keeping it stationary."


Are you saying the plane won't take off? Air is moving under the wings, the plane is moving at 10m/s, even though the conveyor belt data is given in the question, it has nothing to do with the problem.

Its like me saying:
How far would I have to stand away from PeterH if I threw a stone at him in order to hit is head.
The initial velocity of the stone is 3m/s
Acceleration due to gravity is -9.81m/s
The angle the stone is thrown is 30 degrees
PeterH's head is 1.7m off the ground
The drag coefficient of a jar of jam is 0.34

The drag coefficient of a Jar of Jam in this question is completely useless. Just like the runways speed is useless in the original question.

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Van B
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Posted: 14th Dec 2007 14:17
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Nope, that's not painfully obvious, just painful.

Am I wrong in saying that the point of the question is that a plane being held in the same position due to a treadmill is not actually moving?

If we can agree that then we can agree there's absolutely no way a plane can take off without forward movement or a huge amount of wind being thrown at it.

The way I see it is that if the plane moves forward 1 inch then the treadmill pushes it back 1 inch, keeping it in the same position, with no thrust despite any number of badly drawn rockets .

We need Dark Coder to come along and elaborate on the details .


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Libervurto
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Posted: 14th Dec 2007 14:17 Edited at: 14th Dec 2007 14:21
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@Peter H
You've proved that the plane wouldn't take off
Vt = Vp

@BigAdd
Air is moving under the wings all the time even when the engines are off, but it can't take off because there isn't enough force from the air.
I think myth busters need to try this out

I am convinced it wouldn't take off, all the plane is doing is burning fuel.

[edit]
@Van B
Exactly, I was thinking that wind idea too might actually work lol Although you'd need a wind powerful enough to hold back a plane which might destroy the thing haha

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BiggAdd
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Posted: 14th Dec 2007 14:23 Edited at: 14th Dec 2007 14:37
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Speed is relative!! - Most of you seem to forget.

If I were to measure the speed of the plane relative to the conveyor belt, it would be 20m/s

As the speed is being measured relative to the earth then the plane, as stated, is moving at 10m/s relative to the air.

If a car was driving on the very same runway at 10m/s and the belt was moving in the opposite direction at 10m/s, the car will still be moving at 10m/s, if the car was moving at 10m/s relative to the runway and the runway was going at 10m/s relative to the ground in the opposite direction, then the car will be stationary relative to the ground.

Darkcoder's question is somewhat flawed as he has not stated what the speeds where measured relative to.

Quote: "Air is moving under the wings all the time even when the engines are off, but it can't take off because there isn't enough force from the air.
I think myth busters need to try this out

I am convinced it wouldn't take off, all the plane is doing is burning fuel."


These speeds are purely hypothetical, of course we all know that 10m/s gust of wind isn't going to lift a plane. But if I were to get a mega big fan and blow air at it say at 300m/s, the plane will lift.

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IanM
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Posted: 14th Dec 2007 14:32
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Here are better explanations of why the plane does take off than I could come up with:
http://www.straightdope.com/columns/060203.html
http://www.straightdope.com/columns/060303.html

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Peter H
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Posted: 14th Dec 2007 14:34 Edited at: 14th Dec 2007 14:42
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Quote: "Am I wrong in saying that the point of the question is that a plane being held in the same position due to a treadmill is not actually moving?"

yes. because the treadmill can't hold it there.

Quote: "@Peter H
You've proved that the plane wouldn't take off
Vt = Vp"

I've proved you know nothing about physics. you are right that to an outside observer the two velocities are equal and opposite... so it follows that the plane is moving with twice of it's own speed with respect to the treadmill.

[edit] IanM's links appear to be correct. The negligible friction in my diagram isn't even necessary...

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Jeff032
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Posted: 14th Dec 2007 14:44 Edited at: 14th Dec 2007 14:46
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Quote: "@Peter H
You've proved that the plane wouldn't take off
Vt = Vp"

Nope, it takes off, because it moves, because you forgot about the force of the rocket. There is nothing to counteract it, so the plane moves forward.

Quote: "Am I wrong in saying that the point of the question is that a plane being held in the same position due to a treadmill is not actually moving?"

Yes. I agree that if the plane was held stationary by the treadmill, then it couldn't take off.

but, the plane is not held stationary by the treadmill. It will move down the conveyor belt almost exactly as if it was stationary, and take off.

The wheels serve as an almost frictionless barrier between the plane and the runway/conveyor. Therefore, let us replace the wheels with a frictionless block of ice, as they are not used for driving the plane. Now which way will this go:



And where's Raven....

[EDIT] IanM get a post in before me, oh well, that basically sums it up.

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BiggAdd
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Posted: 14th Dec 2007 14:44 Edited at: 14th Dec 2007 14:48
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IanM's links basically say what I was trying to say, but in a less rubbish format.

It's given me a headache how some of you know little about basic physics.

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Libervurto
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Posted: 14th Dec 2007 14:44 Edited at: 14th Dec 2007 14:47
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Oh I am an idiot
I didn't think about the plane's propellers!!
I still don't understand how the air moves over the wings

[edit]
are you saying that as far as the plane is concerned it IS moving along the conveyor belt?

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Jeff032
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Posted: 14th Dec 2007 14:49
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Quote: "are you saying that as far as the plane is concerned it IS moving along the conveyor belt?"


As far as everything is concerned, the plane is moving. The plane's global coordinates change.

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BiggAdd
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Posted: 14th Dec 2007 14:49
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It is moving relative to the conveyor belt at 20m/s (which is not important when lift is concerned)
and 10m/s relative to the air which is important when lift is concerned.

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Peter H
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Posted: 14th Dec 2007 14:52 Edited at: 14th Dec 2007 14:59
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actually, i just realized this problem, as stated, is ambiguous

Quote: "So here's the problem; you have a runway sized conveyor belt and a plane at one end(a 747 for argument's sake). The conveyor belt will move at the exact inverse of the plane's speed, so if the plane is moving across the runway/belt at 10M/s the conveyor belt's surface moves at 10M/s in the opposite direction(like a normal treadmill would). The question is, can the plane take off?"


case 1) as several of us have been arguing the plane has engines and is moving with 20 m/s with respect to the treadmill

case 2) the plane is moving with 10 m/s with respect to the treadmill (I.E. no thrust... just sitting there with wheels spinning)

really the question can be read either way because it never tells us what the velocities are relative to or if the plane is using it's engines or if the breaks are on or if it is taxiing (sp?)... etc

but yes... if you take an airplane and drop it on a treadmill runway (treadmilling at a constant speed), turn on the engines, take off the brakes, it will take off.

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TDK
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Posted: 14th Dec 2007 14:57
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As I eventually realised, the forward movement of the aircraft comes from the thrust of the engines against the surrounding air - not the wheels.

As such, the wheels and conveyor belt are unconnected and totally irrelevant. I originally said it wouldn't take off and was wrong because I mistakenly thought of the plane's propulsion working like a car - which clearly it does not...

The plane will move forward at the same rate as it normally would - the only difference being that with the conveyor belt, the wheels would spin much faster than without it.

TDK_Man

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Van B
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Posted: 14th Dec 2007 15:02
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Ahhh, see I couldn't see past how the plane is actually moving.

What a horrible, horrible question to ask on a Friday .


less is more, but if less is more how you keeping score?
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Libervurto
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Posted: 14th Dec 2007 15:06
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yes but even with the propellers pulling it forward, because it isn't moving no air current is being produced, or is it?
I don't understand aeroplanes

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Peter H
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Posted: 14th Dec 2007 15:08
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it is moving if the propellers are moving it forward.

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BiggAdd
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Posted: 14th Dec 2007 15:09 Edited at: 14th Dec 2007 15:11
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Quote: "actually, i just realized this problem, as stated, is ambiguous"


In a usual scenario, if not stated, you take the speed to be measured relative to the same point in space. In this case, the stationary air.
Quote: "
What a horrible, horrible question to ask on a Friday"


I concur! This is meant to be my relaxing day away from university work!

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dark donkey
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Posted: 14th Dec 2007 15:10
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The plane would move along the conveyor belt no problem. But it would then gather speed just as if it was like on a run way. It would then create lift from the air being pushed under the wings and take off.
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Mr Tank
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Posted: 14th Dec 2007 16:01
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So you're saying the treadmill is designed to go at the same speed as the plane is going relative to the moving surface of the treadmill? Therefore the plane is necessarily stationary in the world.

Unless the treadmill is very wide and pulls the air with it, the friction from the undercariage is usually insignificant, and unless the treadmill is going hellaciously fast, the plane should take off as normal. However, since you say the treadmill is capable of going fast enough to keep the plane in place, you have a situation where the treadmill is going like a million miles an hour so the rolling resistance equals the engine thrust.

If what you're saying is that the treadmill surface moves relative to the world/air at opposite velocity to how the plane moves relative to the world/air, then it shouldn't make much difference, and the plane takes off as normal.

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dark donkey
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Posted: 14th Dec 2007 16:14
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Quote: "Therefore the plane is necessarily stationary in the world."


Not quite. If it was powered by moving wheels this would be the case but it is not. The wheels are free moving so the wheels have no effect. The plane has power so it can push itself through the air, correct?. So the plane is moving for would because the wheels are moving twice as fast because of the tredmill. i hope you get it because im not very good at explaining.
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Mr Tank
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Posted: 14th Dec 2007 16:30 Edited at: 14th Dec 2007 16:32
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dark donkey

i understand what you are saying. It all depends on what the question is. It isn't posed clearly. If it is as you interpret it- that, if the plane is moving at 10m/s forward (relative to the world), then the treadmill is moving at 10m/s backward (relative to the world), etc, then you are right- it makes next to no difference.

If the treadmill moves backwards at whatever speed is required to stop the plane moving (relative to the world), then the plane doesn't move. Of course then the treadmill will then be moving extremely fast, and the plane's tyres will explode.

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dark coder
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Posted: 14th Dec 2007 16:44
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To clarify this. I put a pole in the ground, which does not move at all. My runway conveyor belt is static too. If the plane moved at 10M/s down the runway relative to this static pole, i.e. our air speed is 10M/s. The conveyor belt's surface will move at 10M/s relative to the pole in the exact opposite direction. There is no wind at all, and if people still have issues understanding the scenario then the conveyor belt could move at a minimum speed of 10M/s in the opposite direction to where the plane will go as well as move at the inverse of the plane's air speed. The actual speed of the conveyor belt isn't so important.

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Mr Tank
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Posted: 14th Dec 2007 17:03
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When you put it like that, the plane should takes off fine, although the tyres might not be designed to rotate at twice normal takeoff speed.

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code master
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Posted: 14th Dec 2007 23:53 Edited at: 14th Dec 2007 23:54
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A simple way to visualize this is to fill a bowl with water, pour a little food coloring or pepper in one distinct spot. Then spin the bowl.

More or less, the water will remain in the same GLOBAL place, due to the near absence of friction with the bowl.

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Matt Rock
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Posted: 16th Dec 2007 05:48 Edited at: 16th Dec 2007 05:59
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I seriously can't believe you people are arguing about this! Apparently not enough of you play M$ Flight Simulator, so let me draw it out for you...

[img]
For a wing to work, air must be moving over and under the wing. You can strap a turboprop to a car, but it's not going to start flying around (not unless it (A) has wings, or (B) is moving extremely fast without enough downforce to keep it on the ground). Being on a treadmill will not produce wind under and above the wings. While the landing gear's wheels might be spinning, the plane is not moving, and therefore the plane isn't going anywhere.

So you can sit an F-18 on a treadmill and you can jam the throttle forward and kick up your afterburners and burn through a full tank of gas... but all you'll achieve is setting your treadmill on fire . How, might you ask, is that?

While the engines are producing a crap load of thrust, THEY ARE NOT PRODUCING LIFT. And if the gear is on a treadmill (and it would have to be one hell of a fast treadmill), the plane isn't moving forward. If the plane isn't moving, air isn't moving over and under the wings, so lift isn't being created. Put that same plane on a runway, and the thrust will push the plane down that runway until it builds up enough momentum for the wings to gain lift, and voila, you're flying. The only sort of vehicle that has an engine producing both thrust AND lift is found in a helicopter.

There, debate over . And I'm guessing with all of my MS Flight Simulator experience I think I'm the closest thing to a certified pilot we have on TGC. I've flown around the world twice in MSFS, in real time (no cheating), using a Beechcraft King Air 350 the first time and a Leer the second. All in all I have well over 7,000 flight hours spread out over four different MSFS games and with nine different aircraft. AND, to make me even MORE of an authority than anyone else here, I have a Saitek X52 Flight Control System, the best joystick ever made! Until someone comes in and trumps all of that, consider me and the "it can't fly" crowd as 100% correct

Edit: No, I think someone here might be able to top me... David I think? The mod David I mean, I always mix the two of them up lol.


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Benjamin
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Posted: 16th Dec 2007 06:10 Edited at: 16th Dec 2007 06:12
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We all understand how lift works... and the plane would get lots as it would be moving forwards. Please read the entire thread.

Quote: "And I'm guessing with all of my MS Flight Simulator experience I think I'm the closest thing to a certified pilot we have on TGC"

But unfortunately you fail at physics so it all means nothing.

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hyrichter
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Posted: 16th Dec 2007 06:13
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Erm, Matt, I hate to tell you this, but....


The treadmill has almost NOTHING to do with keeping the plane still. The fact is, the plane IS MOVING FORWARD!!! regardless of the treadmill spinning the wheels. Umm, didn't we just have two pages discussing this?

Put on a pair of rollerblades, grab onto the handle of a treadmill, and pull yourself forward. No matter how fast your treadmill is spinning (well, realistically, that is), you'll be able to easily pull yourself forward, as the wheels are NOT what's powering you forward like in a car.

Oh, and the mod you're thinking about is David T.

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Matt Rock
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Posted: 16th Dec 2007 06:15
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How is wind moving over the wings if the plane is static on the treadmill? The plane might be moving on the treadmill, but where exactly is the wind coming from? When you go running on a treadmill, you don't get wind in your face . I guess you could with a fan attached to it, otherwise... no air movement, no flight.


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Benjamin
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Posted: 16th Dec 2007 06:19
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Quote: "How is wind moving over the wings if the plane is static on the treadmill?"

Your mistake is assuming it's static on the treadmill.

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Matt Rock
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Posted: 16th Dec 2007 06:23
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I must be worse at physics than I realized. That and this is a pretty odd question, but... if I go running on a treadmill, I'm moving in place. if the plane;s landing gear is matching the speed of the treadmill, it's doing the same thing, so even if the treadmill were turning at the plane's minimum takeoff speed, it isn't moving forward any faster than the treadmill is spinning.


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hyrichter
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Posted: 16th Dec 2007 06:29
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Matt,
The planes landing gear doesn't propel the plane forward. As Ben has said, it is there to reduce(or add) friction. Try the rollerblade idea.

Actually, why not try this:
Find a nice long conveyer belt at a factory or something. Take a toy car and tie a string to it. Now, turn on the conveyer belt, and pull the car along the opposite way the conveyer belt is spinning. You should have absolutely no problem at all as the wheels on the toy car aren't doing anything but reducing friction between the car and the surface it is on. They don't provide the power. All that will happen is the wheels will turn faster than normal.

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Benjamin
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Posted: 16th Dec 2007 06:30 Edited at: 16th Dec 2007 06:32
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Realise that a plane doesn't turn its wheels to go forwards, they are just there for the purpose of eliminating friction between the ground and the plane (you wouldn't go anywhere if the bare underside of the plane was touching the ground ). The plane propels air behind it to move, the wheels turn freely.

[edit] Too late!

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bitJericho
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Posted: 16th Dec 2007 06:32
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But a plane without wheels makes for good explosions. Or awesome land on truck scenarios... That of course is if it's coming in for a landing.

Only newbs try to take off in planes with no wheels.


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Matt Rock
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Posted: 16th Dec 2007 06:41
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Oh... so the plane can move forward... yeah... you're right, I'm stupid lol. I was thinking the plane was static, picturing it like a runway. It's sort of a trick question, there's two ways to see it in your head, like one of those pictures where you see an old lady, blink a few times, and it's a young girl.

Quote: "Only newbs try to take off in planes with no wheels."

lol I tried once, belly landing in MSFS, didn't work


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Wiggett
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Posted: 16th Dec 2007 07:06
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so we have figured it out now? wasn't this thread posted a while back or do I just visit too many forums.

Basically the treadmill IS irrelevant, planes don't go "ok travel x meters down the tarmac and you will take off", and the wind doesn't go "oh look there is a treadmill let's speed up!".

The forces that lift the plane come from the plane moving a distance at a speed, creating enough pressure under the wings (which reading this thread is brought up many times!), people seem to think that a plane can just keep still and once the engine gets up to a high enough speed the plane suddenly lifts off. Well maybe with harriers and other jet fighters I don't know, but I do know that the world is 3d (4d if you're a playstation fan) and people need to start thinking in 3d to fully understand how a plane flies.

which brings me to my next question, how is it that we say we are standing still when we are actually moving at millions of miles an hour through space?

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Libervurto
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Posted: 16th Dec 2007 09:46 Edited at: 16th Dec 2007 09:46
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The plane is only moving in the 2D world of the treadmill, it isn't moving in the 3D world.
The only way the plane can take off is if the propellers can pull it off the treadmill.
No airflow, no take-off
I was a bit confused with how the propellers would work but I am certain now that this is impossible.

[edit]
If you added a giant fan blowing air at the plane to this puzzle would it take off?

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Chris K
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Posted: 16th Dec 2007 14:49
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Ha love it.

Can't wait for the next person to come on here and explain that the plane has to be moving forwards to create lift, therefore it won't take off... then realise they are wrong about three posts later.

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IanM
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Well here's the second post Chris. Let's hope OBese87 gets it soon

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Posted: 16th Dec 2007 16:38
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The plane has to be moving forwards to create lift, therefore it won't take off.


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hyrichter
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Posted: 16th Dec 2007 16:54
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Haha, I can't believe some of you guys! Can't you see that it's basically a trick question? The wheels don't drive the plane at all. How many times do we have to say it? The plane IS MOVING FORWARD! It's GLOBAL position is changing as its engines provide thrust. I'm tired of repeating myself here.

This is kind of like the question of the water rising 1 foot an hour; how long until it will reach the deck of a ship that's 20 feet above the water?

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Posted: 16th Dec 2007 16:56 Edited at: 16th Dec 2007 16:59
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the friction against the plain moving forerds would still be less than the forwerd thrust so the plain would move, but the wheels would rotate faster than normal. the plain would take longer to reach take off speed but it is lickly to take off

Quote: "This is kind of like the question of the water rising 1 foot an hour; how long until it will reach the deck of a ship that's 20 feet above the water?"


eather a few seconds while gravaty pulls the ship into the water or an infanate amount of time if the ship is always 20 feet above the water

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Jeff032
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Posted: 16th Dec 2007 19:06
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Quote: "Quote: "This is kind of like the question of the water rising 1 foot an hour; how long until it will reach the deck of a ship that's 20 feet above the water?"

eather a few seconds while gravaty pulls the ship into the water or an infanate amount of time if the ship is always 20 feet above the water"

Never, the ship floats on the water.


The airplane moves down the runway, and therefore takes off
The airplane moves down the runway, and therefore takes off
The airplane moves down the runway, and therefore takes off
The airplane moves down the runway, and therefore takes off
The airplane moves down the runway, and therefore takes off
The airplane moves down the runway, and therefore takes off

Clear yet?

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Insert Name Here
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Posted: 16th Dec 2007 19:27 Edited at: 16th Dec 2007 19:28
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The plane has to be moving forwards to create lift, therefore it won't take off.[b][/b]


Sudoku arts, the rabi and Nancy DrewG
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Benjamin
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Posted: 16th Dec 2007 19:35
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Quote: "The plane has to be moving forwards to create lift"

Yes, this is why the plane does take off.

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Insert Name Here
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Posted: 16th Dec 2007 19:41
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You do realise that I'm being sarcastic, right?


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Peter H
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Posted: 16th Dec 2007 20:18
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yeah, your exact quotation of chrisk almost directly after he posted it gave it away to me...

was funny to see everybody get mad at you lol.

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Posted: 16th Dec 2007 20:29
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Yup. It is actually a word for word copy and paste quotation.
Guess you gotta read closer next time, hey guys?


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Peter H
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Posted: 16th Dec 2007 20:45
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except you capatilized the T in the...

tone of speech is hard/impossible to know over the internet so i don't blame them much.

One man, one lawnmower, plenty of angry groundhogs.
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