Imagine a plane is sitting on a treadmill

To produce lift, an aerofoil - ie a wing, has to move through the air. It can only produce lift when it moves through the air. As the aircraft doesn't move in relation to it's surroundings, it cannot produce lift and thus cannot take off.

The theory of flight explains it all really.

lol mate.. go read the thread

What, all of it?

are they queuing up outside the door, or something?

Single file, one at a time.

Well, I look at it this way:

1. There are no variables given, so therefore there are none. Zero, zilch, nada. There is no head-wind. The air-density is taken as normal.

2. The jet/s is/are at take-off thrust. The plane moves forwards. The conveyor moves backwards.

The rub comes on the interpretation of opposing speed. The intake/output of the jet/s do not generate lift. It/they merely suck air through a port, or ports, compress said air and thrust it backwards. If the plane was not in contact with the conveyor belt then obviously, it is already flying.

STB summed it up pretty well with the glider scenario. This is where the devil in the detail was exposed. The plane moves forwards at (eg) 300mph, and the conveyor belt moves backwards at the same speed. The passing speed is 600mph, and because the plane has forward momentum it gathers lift and up it goes. It is the scenario when the plane moves forwards at 0mph because the conveyor is matching it's speed that causes all the problems. This can be proved by watching a plane firing it's jet/s without wishing to move. The air moves behind the aircraft, but it does not move forwards because of the brakes/chocs. In one interpretation, no matter how fast the wheels spin, the conveyor belt acts as the virtual brake, and no forward momentum is gained, no airflow, no lift, no take-off.

And this is the beauty of the riddle. It never fully satisfies which scenario we are working to, so the correct answer is, Not enough information.

However, the marketing boys have already sold 27 conveyor belt launch systems to Saudi Arabia, and taken a 50% deposit, so we'd better get the thing to work PDQ.

Are those laser guided conveyor belt launch systems?

Where did I say that?

I bet a very clever person came up with the riddle, and is now chuckling away at how many millions of posts on the internet have been "wasted" debating what the original author really intended Either that, or a relatively normal person just forgot to write "air-speed" or "wheel-speed" in the question.

@righter

Why do you think the plane is not moving through the surrounding air?

Hang on, where's the plane - I never spotted any plane

Come on guys. I've got 55 minutes until I break up for Xmas. Surely we can fill that small void with this thread until then?

The propulsion used matters not in this scenario. I find it funny that everybody keeps coming back to jets versus cars.

I believe the original question used "wheel-speed".

Look again.

The plane isn't moving because the speed of the conveyor belt in relation to the thrust the engines are producing and the speed the wheels are turning cancel each other out. It wouldn't even get it's nose of the ground. If it would work, aircraft carriers would use it instead of catapults.

@eurovol
No it doesn't.

My point was only that the conveyor belt speed is irrelevant to begin with, which obviously some people haven't yet gotten. the plane is going to go as fast as it would normally have gone because of the thrust of the engines, irregardless of the conveyor belt matching its' speed, speeding up, or stopping completey.

@ righter

Except that the treadmill, if it is producing any thrust acts on the wheels, which don't slow the plane because they are free to rotate as fast as they like.

Meanwhile, totally independently the engines are acting on the atmosphere and moving the plane forward according to Newton's third law.

OK, last word from me then, I think this covers it.

Interpretation 1: Conveyor speed = planes air speed (or what the GPS in the plane would read). Result: Plane takes off as usual but with wheels spinning twice as fast

Interpretation 2: Conveyor speed = planes wheel speed (ie working how a car speedo does), this means the conveyor does whatever is necessary to keep the plane stationary relative to the ground. Result: A conveyor with ever increasing speed and a stationary plane. As a further discussion, the conveyor could generate enough wind to lift the plane eventually (as the conveyor speed is ever increasing). But before that happens the wheels will probably break off and the plane will be catapulted backwards by the conveyor, so not really worth discussing.

Seriously, try and read at least some of the thread before ignorantly posting nonsense that was answered back on page 4. If the plane isn't moving then nor is the conveyor belt.

Pay attention and look at what I quoted. Fachidioten.

HOW?! HOW is it going to move forward. Unless the thrust:weight ration is greater than 1 AND the engines are blowing at an angle below horizontal, it ain't going to happen. The air never gets a chance to flow over the wings because the plane can't roll forwards since the belt moves in conjunction with the wheels.

woof.

@ eurovol

The original question does not say wheel-speed. End of.

Get a toy car, eg matchbox and a sheet of paper.

1. Put car on sheet of paper.
2. Holding car with left hand slowly push it towards the left as you...
3. ... hold the paper with your right hand and jerk it backwards and forwards under the car.

Is the car still moving steadily to the left even though the paper is moving all over the place? What effect did the movement of the paper have on the forward motion of the car?

@ BD

Aren't you a pilot?

that was the sales guys. your marketing guys branded it conyeoplane, and did a massive launch party on a rented aircraft carrier in the bahamas (making sure of course not to invite the GM's as they will, turn up in badly fitting suits, eat all the canapes and drink so much that al they can do is chat up the "air hostesses" and scare the clients away)

Try to comprehend and read what I quoted. DW's posting isn't the "original".

That's the scary thing.

OK, let's try another way.

According to the first law of thermodynamics, (conservation of energy) energy cannot just disappear.

We are all in agreement that the engines of a large plane can produce an enormous amount of energy.

According to the plane-will-fly people, that energy is used in accelerating the plane forward.

According to you non-fly people, exactly where is all the energy going?

How does it get there?

This nonsense is still going on - 'triffic.

Energy cannot be created nor destroyed.
me thinks.

add "from the fuel" on the end then...

BTW the energy argument is a good one, well done.

I love how each contingent thinks the others are complete and utter morons. It's pretty funny.

A smart engineer uses it to power the conveyor belt so they can go the same speed.

Well the fuel contains potential energy (in the form of chemical bonds), which when mixed with the air and ignited, is released.

The energy from the engines is clearly being used to power the conveyor belt.

edit ...too slow...

edit 2- but does that not make some sense? the conveyor clearly uses/creates energy/heat/whatever... ah, forget it, I am way over my head.

And he harnesses that energy how, exactly?

Edit: same question to you Tim - q. says everything else is normal.

I think that was just a joke so he could avoid answering the question - something all the no-fly people will no doubt do too.

With a windmill of course.

Come on, try and answer the question. Or does your theory not allow for this large energy offset?

@ eurovol

Is that you officially capitulating? Because the q. does not mention windmills, it says everything else is normal.

But it wont fly! Lift cannot be generated if the aircraft isn't moving through the air. To all intents and purposes, it is stationary. I could walk around it while it huffs and puffs and gets nowhere.

To those that say it will fly, at which moment will the nose lift? When it is travelling at 0mph? Or ten minutes later, when it is still travelling at 0mph?

Yeah, but I find the "yes" crowd to be the worse of the 2 at the moment. There is a lot more name calling and "read the thread" from them as if it had all been proved further back. The fact that people are still disagreeing over it means that it wasn't resolved and no amount of re-reading will change that.

Variation on sgb27's thread summary...:

[newPerson] it won't take off because its not moving so no thrust
[flyPeople] Why is it not moving?
[newPerson] IT DOES NOT GET ANY LIFT BECAUSE ITS NOT MOVING!!!
[flyPeople] ARRGGHH!!! *bang head*
<big argument>
[newPerson] ok i see now

Take the matchbox test. Get your fingers behind a matchbox car sitting on a sheet of paper and push it in one direction at a steady pace. Now whip out the paper in the opposite direction... Did the car still keep moving forwards, even though the paper was travelling in the opposite direction..even at a much faster speed?

The second law of thermodynamics prevents all that energy from being transferred directly to the windmill and on to the conveyor belt. As Homer Simpson says: "In this house we obey the Laws of Thermodynamics".

@ SiM

The argument has been won. Slam dunk.

Laws were made to be broken.