Imagine a plane is sitting on a treadmill

What JE meant about the car needing wheels is that a car needs wheels to provide the friction for acceleration.

A plane does not, it just needs wheels to provide a low rolling-resistance platform to scoot along on.

A plane will work with skis, you put 4 skis on your car where the wheels are and try to go anywhere (flat).

OK.

1. Are there any Communists on the plane?

2. Is there any Marmite in the hold?

3. What are the bust measurements of the female cabin staff (this has a great effect on the equation)?

You have changed the force equation. The engines are now acting against gravity rather than perpendicular to it. The treadmill no longer exerats a reactive force to gravity and therefore none on the rocket.

Can all the people saying the wheels have no effect explain how the planes stops crashing through the conveyer belt before it generates lift?

The conveyor belt may be moving at a grizillion miles an hour, but that doesn't mean the plane is moving backwards at this speed. Just exert enough force to counteract friction and the plane will stay where it is as the wheels spin at a grizillion mph minus a little bit. Exert more force and the plane will move forwards and take off.

Edit: forget the minus a little bit, of course they will spin at a 1 gph

Check the question. The speed of the belt and the plane stay the same. Nothing to provide lift.

The wheels/belt have nothing to do with the speed of the plane with respect to the AIR. The plane thrusts against the air, not the conveyor belt

What is slowing the plane down? Nothing. It'll move forward as normal.

Wheels, on a plane, are only there as they provide a low friction means of letting the plane move forward - the actual force, that provides thrust, has fuck all to do with wheels - nothing whatsoever so it's irrelevant if they're on a treadmill or not UNLESS you fit a locking diff to the wheels of the plane, but that would be stupid.

Tom17 and sGb27 have been right from the start. The answer is absolutely obvious. The conveyer belt moves at the same speed as the plane. The plane moves at 10klm/h, the conveyer spins at 10klm/h the other way, and the wheels spin at 20klm/h. Regardless, the plane is still moving forward. Eventually it will reach take off speed and lift off. At this point, the wheels will be spinning at twice their usual speed, but this makes no difference, as the wheels are free to spin at any rate they want.

The force vector from the conveyer belt.

Hutcho - Its air spped that counts.

Yes, and as they are moving in opposite directions point A on the plane will move away from point B on the conveyor built at a rate roughly equal to that of their combined speeds.

Even a locking diff would be ok-ish as its going straight for the most part.

Would get a bit hairy though if he had to make directional adjustments during the take-off

inverted momentum. as long as the wheels are on the ground and wieght is being applied the plane will have to go faster than the belt. it doesnt matter if its proped or jet powered (pushed or pulled) new picture to explain. the thrust takes place outside the frame of referance of planes speed forward and belt backwards.

This is true but that does not slow the plane down because its engines move it relative to the air (& therefore the earth) not the conveyor belt. The wheels are undriven, and assuming the brakes aren't on they can spin as fast or slow as they like, in whatever direction they like, makes no difference.

The plane moves forward at the normal speed - therefore, lift.

You might get the plane off the ground with full thrust, the lightest load possible and the plane mounted vertically on the conveyor belt. But, the planes jets would burn holes in the conveyor belt, and it would almost certainly fall off the end of the conveyor belt before it got enough energy to lift off.

Still, it would be fuckin' awesome watchin' the damage, and I'm fully behind the geeks at NASA funding such a project... so long as I can watch the DVD afterwards with a couple of beers laughing my socks off.

And that force will be approximately equal to the force in the wheel bearings and tyre carcass... i.e. not much at all. Insignificant in the decision of "will it take off or not"

The only force from the conveyor on the plane is due to the friction in the wheel bearings. Seriously. Try it. Put a model car on a sheet of paper and pull out the sheet of paper. The car stays pretty much in the same place relative to the Earth. Now try "very gently" pushing on the car as you pull the paper out with great force in the other direction. What happens?

Again, if the plane is not moving downwards then the conveyer must be appling a force in the opposite direction.

but remain static to point c outside of the converyor belt.

@ Exile

Not opposite the propulsion though, opposite gravity.

I got my word back and agree with sGb27. The only difference between the case of a moving conveyor and fixed one here is that in the former case, the wheels spin twice faster than that for the latter. Take off happens in both cases.

Wheel, Aren't you pulling your hair out yet?
You are absolutely correct and it has been explained throuout the thread, yet some people just can't get it.
Somehow you still have the patience to explain further.

Hang on! Hang on!

Of course the plane can take off. It depends on the direction and speed of the conveyor belt.

If the conveyor is moving forwards at enough speed and the pilot applies the brakes ••• forward momentum, and up we jolly well go.

OK, what would happen if you had a helicopter with a tow rope attatched to the plane and pulled the plane along the treadmill with that? Would the plane move along or stay still?

HEY! I have been trying too

Yes - 9.81N the same as the normal runway would - but that upward force is only required until the planes forward motion wrt the air is as great enough to induce 9.81N of lift.

This is a great post.

Incase there is any confusion I support the plane taking off scenario.

No but I have banged my head on the desk a couple of times.

i have lots of things inthe office but no model cars!!!

And what if one of those musclebound cretins pulled the plane along by a rope clenched between his teeth (obviously not from on the conveyor belt... Oh! I dunno. That would be worth filming).

Use a pencil then... pull the paper perpendicular to the pencil. Similar deal.

Its also a similar deal to the old "pull the sheet from the dinner table" trick.

So Gideon - you have joined the dark side then? Cos I am looking at that diagram and those fucking bras are gonna fall off the end of that conveyor belt if some big lump pushes them.

Yes, you have.
Giving credit where credit is due.

Gideon, in your little diagram (very good btw!) the guy is only pushing with the force needed to counteract the friction under the box.

The plane, however, is at full take-off thrust... Far more force than the friction under the plane from its wheels... So it moves (with respect to the air it is pushing against).

If your guy pushes harder, the box will move forward (with respect to the guy its pushing against).

I'm ignoring vertical forces, it's pretty obvious that until the plane takes off the vertical forces cancel out.

We need to analyse horizontal forces acting on the plane to see if it starts to move or not. Basically, you have one big f-off forward force from the engines, and a tiny little backwards force due to the friction in the wheel bearings (and some drag once the plane starts moving, but that's normal during any take off). The conveyor CANNOT apply a force to the plane because the wheels are free to rotate. The key point is to try and get people to visualise this as clearly they don't/can't believe the physics.

Here's another attempt then. Get a normal treadmill and set it to maximum speed. Gently place a model car in the middle and hold it stationary (relative to the earth) on the conveyor. How hard is it for you to hold there? Can you push a bit harder to make it move forwards (relative to the Earth)? What, you can? Now I wonder what would happen if the car had a jet engine on the roof?

it cannot take off because it does not have the required air coefficience which allows the plane to remain at a constant speed in the air, for the plane to take off it requires down facing engines which are positioned vertically otherwise in my view... its impossible.

the air goes through the plane under the wings to get it to lift, with speed, it can achieve lift off. without the air, it simply cannot.

I know its airspeed that counts. Plane has an airspeed of 10klm/h. Conveyer belt is pushing back at 10klm/h, and therefore the wheels are spinning at 20klm/h. This doesn't matter though, cause the plane is still moving at 10klm/h relative to the ground outside the conveyer belt.

Exactly.

Let's try it the other way around - what happens if the conveyor moves the other way? Would it speed the plane up assuming the brakes were off?

Edit: typo

how where is the inertia involved. thats my point. we start from plane 0 belt 0 dont we? the plane has to increase speed. each foce is moving relative to itself. there is no shove effect.

sorry i need someone to film this happening.

the wheels are a red herring.

But the air is there! Fast moving air at that! The plane uses its jet engines to get off the ground, not power through the wheels. Just before it takes off, it is going to have normal take off speed (lets say 400klm/h) relative to the stationary ground, the conveyer is going to be spinning at 400klm/h the other way, and the wheels will be going crazy at 800klm/h. The plane is still going to lift. How anyone could read this whole thread and still disagree is beyond me.

This diagram might help...

Answer: Yes, but only a bit, due to the friction in the tyres and wheel bearings

Or would the wheels just freewheel backwards?

Edit: bum, loads of posts in between.

Imagine if you will that the plane is a large stiff cock. Then imagine it is being forced against the opening of Paris Hilton's well lubricated flangepiece.

Not helping?

I thought not, but it seemed a good excuse to spice up the thread.

OK I now admit I was wrong about the forces.

Slowly, people are seeing the truth

I have only scanned this thread, but I saw at one point that Johnny English had bet a testicle on something. Just wanted to know how that turned out.

No, because they freewheel. If the plane was standing on bricks instead of wheels you'd be right.

o crap. i am now seriously worried. i just did the paper and pencil thing. pencil stayed in same position relative to my desk. if this pencil had wings. it would not have had air flowing fast enough to create lift. i no longer wonder how airbus could fuck and planes crash. they are designed by engineers.

The speed of the wheel at the point it touches the conveyor belt is proportional to the radius of the wheel itself: V = wr, where V is the speed, w is the angular speed and r is the radius. At the center of the wheel r = 0, and so V = 0 with respect to the aircraft.

So, if we assume a point of reference NOT on the conveyor belt, the wheels could be moving at 2000mph at the point at which they touch the conveyor. It doesn't mean the aircraft is moving w.r.t. the air itself.

No airspeed, no lift.