Imagine a plane is sitting on a treadmill

See me after class Boy!

@ dolfan

So it takes off with zero airspeed? Wow.

Post #20

That doesnt solve anything, the basis of this questions, slowly for all of you, is that the plane and the belt move at the smae speed. The plane does not move. Its the a basic premise in the question. I dont give a rats ass about the wheels, the question says the PLANE and the belt, so if the plane speeds up the belt speeds up, forever until the engines explode or run out of fuel. So anyone who is arguing the plane will "move" faster than the belt, ie move forward, isnt incorporating a basic premise of the question and is thus guilty of a false conclusion. FACT!

Hahahahahahahah!

Fuck, I think I've lost the will to live.

Think of the plane on a huge mouse runner circle things. The weight of the plane will cause the circle to turn, the more force it uses the faster the wheel goes. The plane always stays at the bottom and never has enough air under its wings to lift off.

sGb your persistance is truly appreciated.

I'm off to bed but I'm sure this will still be going when I get up! Try not to lose sleep over it.

Since the question DW asked at the start of this thread has kept a discussion going for 308 odd posts (so far), I think I might try it out on one of my Advanced English classes tomorrow just so I don't have to talk for 2 hours.

OK, one last try, then I give up.

Look at the picture, there are two horizontal forces acting on the plane. That's it, it's these two forces that determine the horizontal acceleration of the plane. A forward thrust from the engine and a backwards force from the conveyor. Let's take the forward thrust as 200 kN, a typical thrust for a large passenger plane.

Let's now assume that the plane remains at rest. For this to happen, the backwards force from the conveyor needs to also be 200 kN. If you apply a 200 kN to the bottom of the wheels it will spin them up forwards - continuously, faster and faster, for ever. Assume 10 wheels, each of mass 100 kg and radius 1 m, using a bit of physics I make the angular acceleration of the wheels 400 rad/s/s. That is an obscene acceleration needed to prevent the plane going forwards. The conveyor would need to accelerate at 40g continuously to hold the plane still, that's 4000 mph after just 5 seconds!!! Given that we don't know of any infinitely powerful propulsion methods, the plane will be able to take off. q.e.d.

Thank you for changing the premise of the question. You have now confirmed what I have been saying. Thanks for playing.

Fine, I agree if in real life you put a plane on a threadmill it would go too fast for the treadmill to keep up and the plane could take off. But the premise of the question is that the plane and treadmill move AT THE SAME SPEED. You can drop a quarter on the ground, by the treadmill and the plane and the quarter will not change position in relation to each other. That is the premise of the question and in that situation, the plane absolutely cannot take off. If the plane isn't movving in relation the the quarter, then it isn't moving in relation to the air and if no air is getting under the wings, we have no takeoff.

Whats changed??

The critical point from the start which i and many others didn't think about is that the wheels are not driven.

Btw air under the wings is not how an aerofoil creates lift, try getting a piece of paper and holding it at one edge so it bends naturally. Blow over the paper and then under the paper and see what happens.

He is assuming that the plane is independent of the conveyor belt, which it is not until after takeoff is achieved and that the conveyor belt has limitations which it doesn't according to the premise of the question.

It's not independent, but the wheels only roll along the conveyor. There's no drive involved. The conveyor could go sideways and it'd make no difference provided the wheels could rotate that way.

You just said:


and DW originally said:


So let us stick a tail on the donkey. The plane starts at 0 and the conveyor at 0. Let us assume it's a regular crap plane and will NEVER EVER go above 500mph even when it is flying. So the conveyor is never ever allowed to go above 500mph. It is only allowed to MATCH the speed - it is not allowed to do a million miles per hour.

Now imagine a plane travelling at 500mph decides to "land" on the conveyor belt that is doing 500mph in the wrong direction. Will the conveyor belt stop the fucker dead? or will the wheels go round jolly quick? like 1,000 mph?

Hmm, you know I thought a bit more on my initial idea of the cheap treadmill that goes faster as you run/walk faster. Then it was pointed out that planes don't move forward because of the wheels moving (like a car) they are propelled forward from the jet engines. So I thought, ok, I'm on the same cheap treadmill, but attach some sort of powerful fan to my back (picture those ones they use in the everglades). At that point it wouldn't matter anymore how fast my legs were, the fan would blow me faster than the treadmill, because I would then be moving without the force of my legs. Now that I picture it that way, it makes perfect sense that the plane should still move forward, and take off. Of course, if the planes only source of movement were the wheels (like a car with wings) then no, I don't think it would move an iota.

edited to add that in the last bit (car with wings) it would be exactly like a car on a Dyno when you get the emissions tested.

You're just showing your ignorance here. It is perfectly valid to take a body and analyse it independantly of its surroundings if you include all forces acting on it. It's something I learnt in like A level mechanics and used throughout my degree.

The question clearly states the belt only goes as fast as the plane, I proved that the belts needs to accelerate at a constant rate of 40g to prevent the plane moving, which clearly can't happen if it's going the same speed as the plane.

Edit: Good analogy kitty-kat, for an even better one, imagine you were on a skateboard on the tread-mill...

Oh and this shit is going on everywhere:

http://answers.google.com/answers/threadview?id=428718

http://forum.physorg.com/index.php?showtop...155&#entry42611

There are some classic nuggets like:


I also like the notion of the plane being "pulled by a cable". Would it move? Course it would.

That point will only allow the plane to move forward, but not fly as I don't think under the premise of the question (all other things being normal) that the plane will be able to achieve the proper lift. The plane has to achieve a forward motion that produces a downward force on the air by the wings on a runway that is moving the same speed in the opposite direction. That includes all the surface area under the wings plus the frictional forces on the wheels. Is a commercial airplane capable of overcoming all that to fly, I don't think so.

Oh My God... So much to say... I want to say it, the same stuff I have been saying all day.

But its bed time... I hope you all work it out by the morning, it is a truly amasing thread.

Nice one DW

I will try to think up some nice examples to explain this with tonight...

NIGHT!

If it moves forward it can fly. This is how lift is generated. The air stays still apart from a small amount being drawn through the engines, the plane moves. Easy.

This one scared me for a moment:


Thinks...looks worried...hmmmmm.

But what will ACTUALLY happen in this scenario, is as soon as you take the brakes off, and apply a little
power the wheels will spin up to 200kph and completely knock out the effect of the conveyor. Plane is then free to fly away.

Yeah, after posting my original thoughts I decided to search up any possible answers and I found people claiming to be experts in this that or the other posting evidence for both sides. Seems everyone has an opinion and only those that say it will fly get biligerant about it. I think this may be one for myth busters.

I'm probably rehearsing a previous answer here but:

The wheels actually 'disconnect' the aircraft from the conveyer belt right?

Therefore the 'plane is propelled forward by engine thrust according to Newtons 3rd Law. The conveyer belt can turn as quickly as it likes but the wheels allow the 'plane to move independently of it. The major effect being that, as pointed out by earlier poster, the wheels spin twice as quickly as 'normal' because the 'ground' is moving backwards at the same velocity as the 'plane is moving forwards.

Consequently the plane is moving forwards, as normal, relative to the surrounding air and will generate lift - as normal.

The only thing that could prevent the above would be if the friction in the wheel bearings was dynamically variable such that it prvided a precise braking force to keep the 'plane stationary. This changes the premise of the question.

Oder?

Could it be because the people who understand the physics* get frustrated by people who refuse to believe that the instinctive answer is wrong?

* Very basic, Newton's third law.

Can it move forward fast enough to achieve lift on a normal runway though? Planes are at full throttle under normal conditions and it takes a hell of a lot of thrust and a long runway to achieve the correct forward speed at the best of times. Can you tell me for certain that the impact of a conveyor belt the length and width of a normal runway has zero impact or not enough to be significant? Planes don't go from zero to sixty in 3.2 seconds like some autos can and they weigh tons.

Well how does the conveyor slow the plane down? There will be rolling resistance & friction in the bearings. Both of these are pretty minimal or it would be too difficult for a plane to get off the ground under normal circumstances, never mind with the conveyor.

Showing your ignorance again... (commercial) planes never use full throttle to take off, the amount of throttle used is calculated based on plane loading, runway length, wind conditions etc, and it's not unusual to only use 50% power to take off.

Only if friction didn't exist.

Wheel bearings and tyres are designed to minimise friction & rolling resistance. A great deal of effort has gone into that technology because it has an enormous payoff in terms of fuel consumption in cars and other powered vehicles. Think about it - if you get a car moving on a flat surface it'll roll quite a long way.

You own a cat don't you?

Ditto for planes, think how far the plane would go if it landed and didn't brake Is this not the same situation as a conveyor trying to pull a plane backwards?

OK, the wheels "to all practical intents and purposes" disconnect the plane from the 'belt. Unless blah blah dynamically variable blah blah.

OK?

Without air brakes, reverse thrusters and brakes on the wheels the damn planes would keep going for miles.

Nope, and you're lucky badDoggie isn't here, you would have had a load of swear words and been accused of attending the university of makeshitup

Anyway, off to bed now chaps, i hope i don't dream about planes on conveyors!!

Imagine if you will the theoretical PERFECT FRICTIONFREE wheels and tyres. 100% zero friction. Conveyor I hope we all agree will have no effect (even at 100,000 mph). So plane will fly.

Then the less than perfect wheels with friction as fitted. Now we could argue that if the conveyor was allowed to keep speeding up to counteract all engine thrust then at 10,000mph or whatever it could counteract the thrust of the engines.

But that ain't allowed in this scenario. Conveyor is only allowed to go the speed of the plane.

Ok..

So I got a PM from DW and I can confirm that I have just finished my bottle of Baileys!

Imagine a conveyor belt moving in your direction at 10 mph. Now roll a ball onto it in the opposite direction at 10 mph. Is the ball going to appear to stay in one place while still rolling? Of course not, it's going to keep on rolling forwards until friction causes it to stop.

That's Perfect FrictionFree™ bearings from SKF?

Was that the clang of a penny dropping, perchance?

Whatever, I'm going to bed. 'Night!

Ah. Got it now, this makes it clear. Won't take off, absolutely right, what Wheel wrote is true.

I'm a, wait, I have to choose here: a moron, an embarrassment to myself, can't remember what else was suggested... I think I'll take the worst. Does any know what the worse was?

Oh I dunno. But if the treadmill is supposed to compensate for forward motion and, therefore, keeps the plane still then I cannot see how, without air movement on the wings, the plane can get any lift whatsoever... THat is the whole point of having a take off speed - so the air movement, combined with the positioning of the flaps, allows the plane to lift off the ground. Otherwise the plane would, theoretically, be able to take off as soon as it starts moving on a normal runway

Sigh. Couldn't keep away.

OK. A plane will take off if there is sufficient airspeed over the wings. Some short take off planes will take off at a standstill if they are facing into a 25mph wind. Clear?

The treadmill/conveyor has no effect whatsoever on the air- or ground- speed of the plane. The treadmill is not the ground.

Another expert says:

and another

Perhaps it would be easier to think of as if the wheels acted like the greased belly sliding along a greased conveyor belt- wouldn't matter how fast the conveyor belt was going against the plane, forces not connected to the conveyor belt are going to push it forward. Hence, the wheels make no difference in this case.

Yes baldrick eurovol, but what do you think? Thinking is so important.

The same country that is at the forefront of aeronautic/aerospace technology and has been so since the very definition of those words?

Just saying, but that's for another thread.

I really should be sleeping.

I have already stated, either add some distance (how much I couldn't say) to the runway or give this to myth busters. Just like with God, until you have experimental proof, all you got is a theory and everybody's got one of those.