[Dakar²]

Originally Posted by Railroader 

The only thing funnier than this thread is AirPlane the movie.

Roger Murdock: Flight 2-0-9'er, you are cleared for take-off.
Captain Oveur: Roger!
Roger Murdock: Huh?
Tower voice: L.A. departure frequency, 123 point 9'er.
Captain Oveur: Roger!
Roger Murdock: Huh?
Victor Basta: Request vector, over.
Captain Oveur: What?
Tower voice: Flight 2-0-9'er cleared for vector 324.
Roger Murdock: We have clearance, Clarence.
Captain Oveur: Roger, Roger. What's our vector, Victor?
Tower voice: Tower's radio clearance, over!
Captain Oveur: That's Clarence Oveur. Over.
Tower voice: Over.
Captain Oveur: Roger.
Roger Murdock: Huh?
Tower voice: Roger, over!
Roger Murdock: What?
Captain Oveur: Huh?
Victor Basta: Who?

Reading that still makes me laugh, let alone watching it.

[badidea]

Originally Posted by Gossamer 

...but the rotational inertia of the tires accelerating does. But it's miniscule compared to the thrust of a jet engine.

Except that the tires would run at infinite speed the moment the plane starts moving...which would result in infinite friction...which would not let the plane move at all!


Why?
Well, 1mph plane speed would only mean 1mph tire speed on a normal runway!
But this stupid runway moves 1mph in the opposite direction...wich would result in 2mph tire speed (if the plane should still be going 1mph)!
But 2mph tire speed would mean that the runway would move 2mph in the opposite direction...which would result in 4mph tire speed (if the plane should STILL be going only 1mph)!
...8
...16
...32
...
compute the rest!

So the plane only takes off if there is no friction involved (or if the runway does not really immediately adjust to the same tire speed)!


edit: wait? Is there something like infinite friction?? I'm not really sure right now!

[Gossamer]

Originally Posted by badidea 

Except that the tires would run at infinite speed the moment the plane starts moving...which would result in infinite friction...which would not let the plane move at all!

Why?
Well, 1mph plane speed would only mean 1mph tire speed on a normal runway!
But this stupid runway moves 1mph in the opposite direction...wich would result in 2mph tire speed (if the plane should still be going 1mph)!
But 2mph tire speed would mean that the runway would move 2mph in the opposite direction...which would result in 4mph tire speed (if the plane should STILL be going only 1mph)!
...8
...16
...32
...
compute the rest!

So the plane only takes off if there is no friction involved (or if the runway does not really immediately adjust to the same tire speed)!

Well a Boeing 737 has to hit about 150mph to take off. Are you saying that the tire speed is double what it would be otherwise, or exponentially greater? I doubt the wheels would have too many problems at 300mph. But it's not a problem of the plane not being able to move, it's a problem of the bearings overheating or tires exploding.

[TETENAL]

Originally Posted by badidea 

Except that the tires would run at infinite speed the moment the plane starts moving...

That is because the conditions of the treadmill are impossible if you take "The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction." as meaning that the treadmill matches the tangential speed of the wheel. The speed of the wheel is the sum of the speed due to the moving airplane plus the speed of the moving treadmill (both a pushing the wheel) and at the same time the speed of the weel is exactly the speed of the moving treadmill (as per definition). The only time both conditions can be met is if the airplane is standing still. As soon as the airplane moves both conditions can not be met any more and such a system would be impossible.

Therefore you can only take "The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction." as meaning the treadmill moving at the speed of the plane in the opposite direction. The wheels will just spin twice as fast as usual and the plane will take off.

[d4nth3m4n]

if it were worded so that the treadmill was moving backwards at the speed needed for the plane to take off and stayed at that speed (someone in this thread already said 150MPH) the entire time (including when the plane had 0 groundspeed), would people have the same issue?

say the plane started on a stable piece of tarmac and rolled onto the belt if you need something like that.

[KrazyKoot]

I'm starting from scratch using common sense logic:

Imagine an airplane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the airplane take off?

Assumption 1: The conveyer can move as fast as the speed the plane needs to take off.

Assumption 2: The speed referred to in the problem is the linear speed of the wheels.

Assumption 3: The plane and conveyor can reach take-off speed instantaneously - all you techies out there forgive me!

We start with the plane and conveyor standing still.

The plane starts up, it is moving foward and reaches almost to the end of the runway moving at take off speed.

The conveyer starts up. The wheels are rotating clockwise. The conveyer moves in the opposite direction anti-clockwise i.e. from front to back of the runway matching the speed of the wheels.

The plane is therefore stationary with respect to the conveyor. BUT the conveyor is moving the plane BACKWARDS with respect to the ground.

NO WAY IT CAN TAKE OFF unless I have got some facts wrong!!!!

[analogika]

You have got some facts wrong.

In fact, every single thing about your post is wrong, starting with the timing.

Somebody, please lock these five pages of hell and idiocy.

[jokell82]

Originally Posted by KrazyKoot 

NO WAY IT CAN TAKE OFF unless I have got some facts wrong!!!!

You have your facts wrong. Even with the conveyor moving at the same speed opposite the tires the plane moves forward.

[analogika]

Yes. Because THE TIRES ARE NOT THE MEANS OF PROPULSION.

[Sherman Homan]

Can we merge this thread with the Airbus thread?
Then the answer is:
If the plane is a Boeing then it takes off and soars through the sky, effortlessly.
If the plane is an Airbus then it can't take off and the engines explode...

[Gossamer]

Originally Posted by Sherman Homan 

Can we merge this thread with the Airbus thread?
Then the answer is:
If the plane is a Boeing then it takes off and soars through the sky, effortlessly.
If the plane is an Airbus then it can't take off and the engines explode...

Proabably the only worthwhile post in this thread.

[Railroader]

I disagree Gossamer. This is:

Originally Posted by analogika 

Somebody, please lock these five pages of hell and idiocy.

[iLikebeer]

Originally Posted by Dakar² 

Oddly enough, that's not far off one of the thought experiments I had when I was trying to figure this one out.

Exactly, on top of that, if it makes it easier to understand, get the car wheels up to speed first without moving. Now put it in neutral and light off the rocket and watch it fly. There is no transmission linking the powerplant to the ground in a plane. You put enough thrust on an airplane and it will take off with the wheels locked at 0 rpm which would make the conveyer act like a regular runway.

Or get a twin turboprop plane with a LOT of power. As much as 50% of the lift can be generated by the propwash at low speeds and high power settings. Ground speed could be 0 in that scenario and possibly have enough airflow over the wing to generate enough lift to rotate into ground effect and build up enough airspeed through propwash lift and forward thrust to get into the air, even if you use some people's flawed idea of how much friction the wheels would cause. That's why you never chop the power at low speeds and high angles of attack in twin props, your nose will drop and ruin your day.

[Dakar²]

Yeah, it's been a month, I don't care anymore.

[dlefebvre]

At that point, I took the train and I'm already relaxing at home...

[centerchannel68]

I flew a plane once.

[Tiresias]

So what was the fruitful outcome of this 5 page debate—does the plane take off or not?

[Gossamer]

Originally Posted by Tiresias 

So what was the fruitful outcome of this 5 page debate—does the plane take off or not?

Yes.

[Y3a]

Harrier Jump Jets get off the ground without forward motion...

The plane WILL fly.. The thrust will move the whole plane gear/wheels included. The speed of the wheels and the conveyer belt don't make a difference. The engines will overcome the belts influence. The wheels roll freely so they won't cause braking . The engines provide LOTS of thrust so you'll move the entire airframe forward. A string is not the same thing as rolling the runway backwards.

[Sherman Homan]

So what was the fruitful outcome of this 5 page debate—does the plane take off or not?

No.

[analogika]

Originally Posted by Sherman Homan 

No.

[Dakar²]

Originally Posted by Tiresias 

So what was the fruitful outcome of this 5 page debate—does the plane take off or not?

Maybe.

[phantomdragonz]

these threads are great... really shows the dumb ones.....

(the ones who say no)

Yes the plane will take off... the wheels are just there to reduce friction, the propellers and moving air move it so it can take off...

Zach

[SVass]

Imagine an airplane that has a 100 knot rotation speed (takeoff) sitting still on a runway with a 100 knot tailwind. It will take off using zero runway length.

Imagine the same plane with a 100 knot landing speed at zero ground speed flying into a 100 knot head wind. It will land using zero runway length.

All the rest is misuse of the English language. sam

[design219]

As the question was asked, no the plane will not take off. As explained may times... in different ways... the conveyor is not a factor to take off. Lift comes from air moving over the wings, not the ground moving under the wheels.

[Dakar²]

Originally Posted by design219 

Life comes from air moving over the wings, not the ground moving under the wheels.

Sounds like an inspirational poster in Sky Captain's office.

[JohnM15141]

Imagine an airplane that has a 100 knot rotation speed (takeoff) sitting still on a runway with a 100 knot tailwind. It will take off using zero runway length.

That statement is as wrong as wrong can be...that airplane is going to have to achieve a ground speed of 200 knots before it can take off. First it will have to accelerate to 100 knots to over come the 100 knot tailwind and then another 100 knots to reach rotation speed and that will require a lot of runway.

Second statement is correct...

[design219]

Yeah, that's what I was thinking! That's brilliant, if I say so myself.

[Dakar²]

**** KrazyKoot for brining this thread back to life.

[design219]

Why is this so hard to understand?

[Dakar²]

Because the question was poorly phrased.

[JohnM15141]

Why is this so hard to understand?

Because the phrase "the speed of the wheels" from the original poster is not clearly defined and everyone has a different opinion of what it means...

[kikkoman]

Originally Posted by JohnM15141 

Why is this so hard to understand?

Because the phrase "the speed of the wheels" from the original poster is not clearly defined and everyone has a different opinion of what it means...

That's the trick part of the question. It implies the wheels have some effect on the plane taking off but the fact is they do not. It's purposely phrased to throw people off and obviously worked.

[centerchannel68]

Originally Posted by kikkoman 

That's the trick part of the question. It implies the wheels have some effect on the plane taking off but the fact is they do not. It's purposely phrased to throw people off and obviously worked.

Depends on how you look at it. The reason it's confusing is that it's a very poorly worded question.

[design219]

Dude, that's what he said.

[analogika]

Originally Posted by SVass 

Imagine an airplane that has a 100 knot rotation speed (takeoff) sitting still on a runway with a 100 knot tailwind. It will take off using zero runway length.

Headwind, not tailwind.

[Tiresias]

Originally Posted by Y3a 

Harrier Jump Jets get off the ground without forward motion...

The plane WILL fly.. The thrust will move the whole plane gear/wheels included. The speed of the wheels and the conveyer belt don't make a difference. The engines will overcome the belts influence. The wheels roll freely so they won't cause braking . The engines provide LOTS of thrust so you'll move the entire airframe forward. A string is not the same thing as rolling the runway backwards.

Just one question: Don't the jet thingys on Harrier Jets point down? Isn't that what enables them to make a near-vertical take off? A passenger jet's engines are fixed.

[Tiresias]

analogika, you seem to be the most outspoken advocate of the "take off" school.

I'm in the "too dumb to expound an opinion school", but intuitively feel that it can't take off. Therefore, I find your conviction intriguing, and have two questions I'd like you to answer:

FIRST QUESTION: The plane is on a conveyor belt that is moving in the opposite direction to the plane's forward thrust at the same speed as that forward thrust. The upshot of this is that to a stationary observor standing next to the plane, but not on the conveyor belt, the plane appears stationary. There is no wind mentioned, so well asume there is no wind.
Is this, in your opinion, tantamount to the plane simply being stationary? If we eliminate the conveyor-belt runway and instead give the plane metal landing feet like a moon-lander so that it cannot move forward, will it take off?

SECOND QUESTION: Remembering that there is no wind, doesn't the plane need to move to create lift? You cannot fly a kite on a windless day. But you can fly a paper airplane or a model glider—so long as you propel it into the air.

Thanks in advance for clearing this up.

[analogika]

I've cleared both of those up numerous times over the past five pages, as have other people.

Thrust is not generated by the wheels; it is generated by the JETS. The answer to your first question is: If you nail it to the ground so that it cannot move, then HELL NO, IT WON'T FLY. Let me rephrase your first question: If we eliminate the conveyor-belt runway and instead give the plane metal landing skids like a polar carrier so that it cannot move forward WITHOUT THRUST, will it take off?

What exactly is it that you are not getting?

A plane will fly if air moves over the wings at a certain speed. Whether this air moves across the wing because the aircraft is moved forward by wheel, jets, a catapult, or a huge ****ing hand pulling on a string (which is in essence how gliders fly), or because the wind is blowing so strongly from the front, or a combination of the two, is COMPLETELY IRRELEVANT to whether it will fly or not.

SECOND QUESTION: Has it occurred to you that planes actually have engines for a reason? The engines serve to PROPEL THEM INTO THE AIR.

I'm seriously having difficulty figuring out how people can be so dense, or where exactly the fundamental brain-hang lies.

[design219]

Imagine an airplane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the airplane take off?

You know, I think I was thrown off by the "sitting on a..." part. Rethinking about this, it doesn't say that the airplane remains still as the conveyor moves under it, but just "can the plane take off."

It wouldn't remain still, as the airplane engines are going to move the plane forward through the air (generating lift on the wings), as the conveyor moves even faster underneth.

Analogika, you are correct. And I stand corrected.

[itistoday]

Wow. What a beautiful thread.

My first instinct was to say that the plane would not take off, but then I saw that there were 5 pages to this thread and that analogika (whom I respect) was saying it would. Then I realized that I had misunderstood the nature of the original question, and that it was in fact a trick question.

The key to understanding why the plane will take off, is to first understand the question correctly. The question says that the conveyor belt will move in the opposite direction of the wheels and match their speed exactly. The point of saying this is to trick you into thinking that the plane will remain still relative to the ground, just like a jogger on a treadmill, which is not true. Yes, obviously if you think that the plane will remain still then you would be correct in saying it won't take off. Everyone in this thread understands that.

The thing is though, is that it will not stay still. The easiest way to understand this I think, is to think in terms of forces. It is forces that cause objects to move and therefore to see why the plane will take off we simply need to think about what forces are acting on the plane. There are two forces acting on the plane (other than gravity):

1) The force that the question is designed to trick you into thinking is relevant, and that is the force exerted on the plane through the wheels by the conveyor belt.

2) The force of the jets on the plane.

So, from this we clearly see that the force of the conveyor belt on the wheels is only used in turning the wheels, not to push the plane back. The wheels will spin really really fast, the conveyor belt will match that speed, but only a minuscule amount of force (from friction) will go into pushing back on the plane. Meanwhile the plane will be pushed faster and faster by the jets, causing it to go forward, and that will generate lift under the wings.

Presto! It takes off.

[iLikebeer]

The confusing part for most probably has to do with how the plane gets ground speed. If the conveyer belt turned at the same speed as the wheels with no slippage, then the plane WOULD be in the same place and not be able to take offf. The wheels would have to slip or skid to accelerate the plane. There would be no other way for the plane to ever achieve forward acceleration.

[itistoday]

Originally Posted by iLikebeer 

The confusing part for most probably has to do with how the plane gets ground speed. If the conveyer belt turned at the same speed as the wheels with no slippage, then the plane WOULD be in the same place and not be able to take offf. The wheels would have to slip or skid to accelerate the plane. There would be no other way for the plane to ever achieve forward acceleration.

No. It would accelerate just fine. Did you read what I said?

F = ma = jets - (small amount of frictional force from ball-bearings)
F = ma = 100000 - 10

Therefore 'a' (acceleration) is a large, positive number.

[f1000]

Originally Posted by iLikebeer 

The confusing part for most probably has to do with how the plane gets ground speed. If the conveyer belt turned at the same speed as the wheels with no slippage, then the plane WOULD be in the same place and not be able to take offf. The wheels would have to slip or skid to accelerate the plane. There would be no other way for the plane to ever achieve forward acceleration.

"Butthead."

[iLikebeer]

Originally Posted by itistoday 

No. It would accelerate just fine. Did you read what I said?

F = ma = jets - (small amount of frictional force from ball-bearings)
F = ma = 100000 - 10

Therefore 'a' (acceleration) is a large, positive number.

Yeah, no duh, i understand that's why it will takeoff. I'm commenting on why so many people get confused, on both sides.

People that say no see that it wouldn't be possible for the aircraft to accelerate if the wheels and conveyor belt maintain a 1:1 speed ratio. People that say yes say "twice the speed as normal" without wondering how it magically gets there. It's not magic, it's wheel slipping, skidding or whatever you want to call it. It happens, because, like you've shown, the jet engine's thrust is a brute force compared to the friction of the wheels. The jet takes off just as surely as I would get scooted across the ground if Warren Sapp was pushing me, no matter how deep i could stick my feet into the ground.

That is why it's confusing to some. It's written in a way that makes people focus on one part of it that can be very convincing, even if wrong.

[itistoday]

Originally Posted by iLikebeer 

Yeah, no duh, i understand that's why it will takeoff. I'm commenting on why so many people get confused, on both sides.

People that say no see that it wouldn't be possible for the aircraft to accelerate if the wheels and conveyor belt maintain a 1:1 speed ratio. People that say yes say "twice the speed as normal" without wondering how it magically gets there. It's not magic, it's wheel slipping, skidding or whatever you want to call it.

Argh... no, that is not what we're saying. The wheels do not need to slip or skid or slide. If the wheels and conveyor belt maintain a 1:1 ratio as you say, it will still take off.

Edit: Try to think of the wheels as being separate from the plane.

[iLikebeer]

Originally Posted by itistoday 

Argh... no, that is not what we're saying. The wheels do not need to slip or skid or slide. If the wheels and conveyor belt maintain a 1:1 ratio as you say, it will still take off.

Edit: Try to think of the wheels as being separate from the plane.

heh, no. It will take off, but not with a 1:1 ratio. The rollerskate on a string analogy misses one key point. The conveyor isn't accelerating with the wheels. If it does, the skate still moves forward, but the wheels will skid a bit or the skate will jump around on the belt.

The only way for the plane (or skate) to accelerate and the wheels to maintain a 1:1 ratio with the conveyor is for skidding/slipping of the wheels. The awesome power of the jet engines compared to that skidding is enough to do that no problem.

[itistoday]

Originally Posted by iLikebeer 

heh, no. It will take off, but not with a 1:1 ratio. The rollerskate on a string analogy misses one key point. The conveyor isn't accelerating with the wheels. If it does, the skate still moves forward, but the wheels will skid a bit or the skate will jump around on the belt.

The only way for the plane (or skate) to accelerate and the wheels to maintain a 1:1 ratio with the conveyor is for skidding/slipping of the wheels. The awesome power of the jet engines compared to that skidding is enough to do that no problem.

I didn't read this entire thread so I don't know what you're talking about with the rollerskate analogy. Did you read my post? If so, what part of it do you disagree with?

[iLikebeer]

Originally Posted by itistoday 

I didn't read this entire thread so I don't know what you're talking about with the rollerskate analogy. Did you read my post? If so, what part of it do you disagree with?

Yes, I read it, and I agree with the entire post. Again, I'm only pointing out why it's so confusing to some. The wheels will skid, otherwise there could be no forward movement in relation to the ground. That post you pointed to had nothing in it about that though.

You can't look at the wheels as disconnected from the airplane in the context of the problem. They don't exist in different dimensions. Either the wheels spin 1:1 with the conveyor and friction is somehow so great as to keep it from taking off. NO. Or the wheels spin at a 1:1 with the conveyor with a lot of skidding and black marks as the plane accelerates to take off.

That's my entire point. There is a disconnect with the people that say no because the airplane shouldn't be able to accelerate if the wheel and conveyor speed is the same. The skidding because of the overwhelming power of the jets allows the plane to take off within the parameters of the initial riddle.

[jokell82]

Originally Posted by iLikebeer 

heh, no. It will take off, but not with a 1:1 ratio. The rollerskate on a string analogy misses one key point. The conveyor isn't accelerating with the wheels. If it does, the skate still moves forward, but the wheels will skid a bit or the skate will jump around on the belt.

The only way for the plane (or skate) to accelerate and the wheels to maintain a 1:1 ratio with the conveyor is for skidding/slipping of the wheels. The awesome power of the jet engines compared to that skidding is enough to do that no problem.

You are completely wrong. Go try it sometime instead of just thinking about it.