Originally Posted by Steve B
.when the wheels leave the ground, you're going from 0mph linearly to the speed required to lift the plane, linearly. I think you'd fall out the skyno, the plane is travelling at 180mph (or whatever) at the moment the wheels lift off, at which point it starts flying at 180mph.
The plane is moving, it just happens to be moving along a thereotical treadmill instead of a runway. It still requires the same length to reach its take-off speed, the only difference is that the (irrelevant) wheels are spinning faster. That is the only effect of the movement of the treadmill. The plane is not stationary!If a plane could take off from a treadmill then why do they take a 'run up' to take off. They do need to be moving as said in previous replies
people stop this madness!
Vodka
Yes, it's moving, but as said in the previous posts it needs to be pushing against the air to get the uplift. It's not just it's wheels spinning that makes it fly else then it could take off without a run-up.
at the moment the wheels lift off, the plane will no longer be restricted from moving linearly, therefore will need to accelerate to 180 mph, if this is the speed required for acceleration. It will inevitably lose thrust.
It would take off, as long as the plane is moving from one end of the treadmill/runway to the other end of the treadmill at normal take off speed AND providing the wheel bearings could handle the extra speed/friction.
All that would happen is the wheels would be moving at {normal speed}+{the speed of the treadmill} (with and adjustment for friction taken into account). If the wheels can cope with it it would take off. The plane would still move.
Comparing it to a car on a treadmill is completely wrong, its not the wheels that generate the forward momentum, it's the trust of the jets. The wheel are just there to keep the belly off the ground. Thats partly why planes fly - you can put wings on a car but that does not mean it can fly! Remember, the wheels have nothing to do with moving the plane, they are not used during flight.
In theory, it would be a normal take off - it's just that the wheels would be spinning much faster as it travels down the moving runway.
If you still cant get it, try this - imagine you are a massive giant who could push the plane. Imagine pushing the plane down the treadmill, you can still do it no problem - but the wheels will be screaming. Does that help? Does for me anyway.
Last edited by autopilot; 15-12-2006 at 03:23 AM.
the plane is not moving though, its stationary
Not read all the thread so apologies of someone else has posted the same/similar answer.
The plane will take off. It will roll of the treadmill and take off, regardless of how fast the treadmill is turning.
The point everyone is where the thrust for the plane's movement comes from.
Now, if this were the wheels, then yes, they'd just make the treadmill go around and the plane would just sit there, but the thrust is from the dirty great jet engine, which has nothing to do with the wheels.
So, the jet engine is pushing against the AIR and NOT against the floor (treadmill), so regardless of how the plane rests on the floor, (water skis, snow skiis, wheels or whatever), it will start to move as the thrust isn't against the floor but against the air...
Some roller skates on a tread mill doing 50 mph. If you held onto the rails you'd stay on with the treadmill going like the clappers and you stationary to the surroundings.
Now, if I came along with a broom handle and pushed you towards the front of the treadmill with the broom, you'd move forward, regardless of how fast the treadmill was going... because the force being exerted on you is external to the treadmill/roller skate system that is in place... I am standing still next to the treadmill exerting a force on you, so you will move forwards... The same thing applies with the plane and the air...
So the plane will just roll off the treadmill, trundle along the runway and then get airborne, even if the runway is one enormous treadmill....
I thought the point was that the treadmill is matching *whatever-the-hell* speed the plane is doing... if the plane starts to move down the treadmill a bit, the speed of the treadmill increases to match this, depending how quick it reacts the plane wont move at all.So the plane will just roll off the treadmill, trundle along the runway and then get airborne, even if the runway is one enormous treadmill....
The thrust of the engines is creating (well attempting to create) forward motion down the treadmill, this would make the wheels turn faster, BUT the treadmill then matches this wheelspeed... The wheels could be going at 200mph, but the treadmill would be turning the opposite way @ 200mph, therefore the plane stays stationary.
There's no air moving over the wings to create lift. Why do they have fans on running treadmills in gyms? so you get air blown in your face! if you turn the fans off, you're just effectively running on the spot. no air moving at you like there would be when running outside.
But the thing is the treadmill can't affect the speed of the plane as the wheels are free spinning. It can be going at the speed of sound and as long as you assume zero friction bearings then the plane is unaffected.
If the plane had zero friction bearings in the wheels and just sat there engines off and you turned the treadmill it would stay still.
Yes it is moving, the "dirty great jet engines" are pushing it forward *through* the air. The wheels could be doing the tango for all we care. The plane still takes off.
Ok, simple way to explain this.
Take a floatplane... a water-borne plane that lands and takes off on water.
Now, there's NO thrust or force of any kind from the floats and the water (except for drag and water suction etc) but the point is, there's no FORWARD force from the floats, ok?
The plane gets airborne because the propeller or jets thrust air behind the plane.
The relationship between the floats/water is the same as the relationship between the treadmill/wheels.
Even if the entire runway was a giant treadmill running at the speed of sound, the wheels (supposing there was no friction and the brakes weren't on) would just spin really fast and the plane would stay still.
BUT... when the pilot opens up the throttles, a second force is introduced, a pushing force from the ENGINES against the AIR. This has nothing to do with the interaction between the treadmill/wheels so the plane will move forward.
So, seeing as the wheels are free spinning and not the means to drive the plane forwards, the wheels will spin faster than the treadmill as the plane is now pulling the wheels forward too.
Even if the speed of the treadmill is increased, the plane will carry on moving as the treadmill cannot affect the engines vs air thrust.
Opening up the throttles introduces a new set of forces on the plane which are independent from the wheels/treadmill.
The wheels wouldn't spin unless there was forward motion, think about it. Thus in this example the treadmill is a (forwards) moving treadmill. The plane will take off.
To err is human. To really foul things up ... you need a computer.
I thought this was a massive treadmill for planes...was disappointed that it was just speculation.
It won't leave the ground because to do that air has to be moved past the wings which isn't happening because the plane is on a treadmill and therefore not physically moving in a direction. Would a be a bit worrying if you just turned the engines and it took off.
Stealth Geek - And Proud!
but that is what happens every minute of every day!! How else would a plane take off ??(apart from steam catapults on aircraft carriers)
Nick's last post is 100% correct. end of story.
this is not how a real plane works
Vodka
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