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Question posed on Facebook

But in the case of roller blades the wearer would be static like a runner on a treadmill as forward motion is gained by a V motion of the skates against the belt.

Give the wearer a Jet pack though and we have the same scenario as the plane. :)
 
Guys we might just be over thinking this. I don't think the challenge was supposed to be that technical. It's a coffee break conundrum. Let's say the belt speed is variable and starts at zero and can be speeded up. It can't start at full speed, you wouldn't be able to get the luggage and lemon scented napkins on board.

The answer is yes it would take off and the wheels, until the plane rotates and leaves the runway, would always be rotating at twice the speed of the belt. In relation to the plane, Newton's third law is in effect and whatever thrust goes out the engines equates to forward movement through the 'world' as it were.

The plane moves in relation to the earth, not the belt. Just remember that if the plane brakes were off and you started the belt very quickly, the plane would momentarily stand still with the wheels moving before it gradually built up some momentum caused by what little friction there was in the wheel axles. Just like the tablecloth and dishes trick. And then the plane would be carried backwards. If they stopped the belt suddenly, of course the plane would then continue to roll back until it stopped. But all of this is because at this point the belt is doing all of the driving.

The pilot could then raise the throttles to counter that backwards movement and hold the plane completely stationary. All he'd be doing would be countering very slight frictional 'drive' as it were. Then when opening the throttle fully the plane would move forwards.

See I said we're over thinking it.

OK so we have a boat with a decent engine and we point it into a very strong current like the Menai Straits. With no revs the boat goes backwards with the current. With half throttle, the boat 'hovers' and with full throttle, it makes headway out to sea. Just like the damn plane. Obviously with a boat, you reach a point where it's almost impossible to defeat the current - like a tsunami. Same with a plane if the head wind is more than the max thrust of the engines, like in a hurricane; it simply blows the plane off the runway.
 
The post did not say the engines were running so the plane would fall off the end of the belt.

If the engines were running it would move forward over the belt and take off . Who changed my font?
 
No it wouldn't. Because the question says the belt is designed to match the speed of the wheels. So if the wheels aren't moving, then neither is the belt. Ahhh haaa!!! So the plane has to move in order to start the belt you see.

Also the engines can be running, but with zero thrust. Don't you just love this.

Next we'll get onto the debate about the plane that flies faster than a rifle bullet, which then fires its guns whilst flying. No please don't.
 
I’m with Chris on this, so without any thrust the wheels would be stationary and thus so would the conveyor belt.

As thrust is applied, the wheels would turn and so would the conveyor, but neither would affect the thrust which is only relative to the air around the plane, including the wings, so lift would be achieved, and a successful take-off.

As for firing bullets, leave that to the RAF, this is a 747 FFS :lol:
 
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A bullet fired out of the back of a plane going at the same speed as mussle velocity would fall vertically to the ground.

The only stationery part of a travelling vehicle is the bottom of the wheel which touches the ground. If that were moving there would be constant tyre smoke. That's assuming the ground is stationery.
 
The thing is its a dead weight until it has wind speed assisted lift so the plane would fail to reach even 1mph .
 
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Plane flies. It has absolutely nothing at all to do with the ground. End of story.

What about landing. If the belt is travelling in the opposite direction when the plan comes to land, are we saying as soon as it touches the belt it stops dead? I doooon't think so.

Frank, not seen a faster than bullet plane with rear facing guns. The point of the conundrum is that they are forward facing and the plane is as fast as the projectile.
 
Chris said:
Plane flies. It has absolutely nothing at all to do with the ground. End of story.
Click to expand...

I'm genuinely not certain of the answer and trying to understand but I'm struggling with some of the arguments - perhaps I'm being extra dense today.

....for a plane to take off from the ground, it has to establish enough lift (usually via the wings) to overcome its own weight/gravity. I think we are all agreed on that ? To achieve this it uses the engine's thrust to move itself forward at speed, creating "wind" over the wings, which creates lift and ultimately the plane takes off. (Let's assume there is no headwind for simplicity).

If the airplane is on a conveyor belt which matches it forward speed, then it is stationary and cannot generate any wind over its wings and therefore cannot generate any lift. This is regardless of the fact that wheels are not driven like a car's or how fast they might be rotating on the conveyor belt. The plane's engine only provides thrust - not lift. It doesn't matter how much thrust the engines provide, as that is countered by the belt - resulting in the plane being stationary. Which in turn results in no air passing over the wings and no lift.

Chris said:
What about landing. If the belt is travelling in the opposite direction when the plan comes to land, are we saying as soon as it touches the belt it stops dead? I doooon't think so.
Click to expand...

Essentially yes. Imagine that a plane is landing at 100mph on to a conveyor belt that is running at 100mph in the opposite direction (let's assume that the belt will match the plane's deceleration). When the plane "lands" - to an observer inside or outside the plane it will appear stationary or as if it had stopped dead.

Another scenario - imagine a 100m race with Usain Bolt. From 50m onwards the track is replaced with a treadmill/conveyor belt, that will match his speed at that point. So to observers Bolt will start the race and run the first 50m as usual and then hit the treadmill and effectively "stop dead" - though he is still running at full pelt.

The analogy to the plane in the above example is that Bolt will be slicing through the air in the first 50m, creating turbulence and perhaps lift if he had big enough wings etc. whereas he will be stationary in relation the to air around him when he hits that treadmill and at that point will be creating no more turbulence or lift, than a bystander at side of the track.

Have I missed something in my theory?
 
Yes. The wheels of the plane are not being driven. Unlike Bolt's legs. You have to discount the wheels completely. It's utter misdirection. It doesn't matter how fast the wheels spin on the belt, they're simply there to allow the plane to move on the ground. They're castors. The belt does NOT MATCH THE FORWARD SPEED OF THE AIRCRAFT. It matches the rotational speed of the wheels.

When the plane lands on the moving belt, you cannot dump the inertia of 200 tonnes of plane. All that will happen is that the wheels will spin much faster than if it had landed on a static runway, but the plane will still have forward motion on the earth. You can't stop a jumbo dead in its tracks can you.
 
Thrust creates forward speed creates lift and plane takes of irrespective of ground conditions. They can take off from water or be dropped from another plane without their engine running and fly for a bit due to air speed. As in the x planes.

A box kite can fly with zero ground speed.
 
Chris said:
Frank, not seen a faster than bullet plane with rear facing guns. The point of the conundrum is that they are forward facing and the plane is as fast as the projectile.
Click to expand...
Ah but you see, the projectile is already travelling at the same speed as the plane so when its fire it should still move faster then the plain, same thing as standing in a plane and throwing a ball forward. I think Mr Newton has a created a law about this somewhere.
 
Float plane is a good example Frank. Headed up river on a fast flow the plane would still take off even if the river got faster and faster with the plane's speed.
 
Correct Hamba. The projectile is at the same speed as the plane and when fired, the round MUST travel down the barrel. It has gas behind it. It can't 'not' move. Yes there is air pressure being forced down the barrel as the plane flies but not sufficient to overcome rapidly expanding hot gas from the propellant.

But, the speed it is travelling at in relation to the plane and the ground are different. Once it leaves the muzzle it can no longer be accelerating and very quickly its trajectory will begin to decay and the bullet fall under gravity in a parabola. But just as a normal bullet would so someone on the ground would see a normal firing. The plane moves ahead but only because it has an engine and can maintain a constant speed. It could of course slow a bit and fly alongside the bullet if the pilot was skilled enough and had good eyesight!

Or of course the plan might just shoot itself down.
 
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