It's Friday..

[Sooty Stoo]

That's not a real plane or conveyor.

If you look carefully you can see where they filmed the moon landings.

[David G]

Originally Posted by dave archer

More evidence the plane moved forward, similar to a normal takeoff.

Well that is because it did/does

The points you need to accept to see that it will fly are these:

1. The engines push air; they do not drive the wheels.
2. The wheels freely rotate.
3. When the plane moves through the AIR at x mph, the conveyor belt moves backwards at x mph, so the free-wheeling wheels rotate at 2x mph.
4. The conveor belt has NO ABILITY to arrest the planes movement due to the free-wheeling wheels.

Perhaps I will have more success convincing you of the following?:

This is Blue
This is Green
This is Red


David.

[SoggyFox]

Originally Posted by Scu-bamboo

On the bus to college, we always seem to have really serious talks about what we would do if we had Bernard's watch! What would you do if you had Bernard's watch?

probably a long time in prison.

[big_si]

Originally Posted by David G

Well that is because it did/does

The points you need to accept to see that it will fly are these:

1. The engines push air; they do not drive the wheels.
2. The wheels freely rotate.
3. When the plane moves through the AIR at x mph, the conveyor belt moves backwards at x mph, so the free-wheeling wheels rotate at 2x mph.
4. The conveor belt has NO ABILITY to arrest the planes movement due to the free-wheeling wheels.

This was the bit that made it click for me... After a bit of thinking

Compare it to a sea plane with an imaginary current whizzing against the direction of the plane. Although this theoretical current would be matching the speed of the plane the plane will still take off.

As the drive is happening by the propellor / jet acting on the air and the sea/ski (or in the other case wheels / runway / conveyor) is just acting as a "frictionless" surface for it to move on then it will still move forward and take off.

Clear as mud still? It's only the last 2 minutes of thinking about it has made me think this... Until then I was thinking "what's this idiot talking about! Of course it won't take off!"

[NeilB]

Originally Posted by pwood2457

say for argument sake call it a train that would give us the answer that a fly colliding with a train would stop it.

Not enough mass in a fly to stop a train.....

Newton's 1st law states 'An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an outside force.'

So the fly will affect the trains speed by so little it would not be measurable.

I think......

[uwila]

Originally Posted by David G

Well that is because it did/does

The points you need to accept to see that it will fly are these:

1. The engines push air; they do not drive the wheels.
2. The wheels freely rotate.
3. When the plane moves through the AIR at x mph, the conveyor belt moves backwards at x mph, so the free-wheeling wheels rotate at 2x mph.
4. The conveor belt has NO ABILITY to arrest the planes movement due to the free-wheeling wheels.

Listen to David G for he speaks wisely. Think about the forces involved.

The aircraft engine develops thrust which accelerates the plane forwards because the force is acting directly on the airframe. The conveyor can only exert a force on the tyre. This force causes the wheel to rotate and is not acting directly on the airframe. So the wheel merely rotates and does not exert much backward force on the plane. The only backward force on the plane is caused by the friction in the wheel bearings. Since this is (hopefully) small, the aircraft moves forwards at almost exactly the same speed as it does on a normal runway and will take off at the usual airspeed, but it takes a little bit longer to get to that speed.

[dave archer]

Originally Posted by uwila

Listen to David G for he speaks wisely. Think about the forces involved.

The aircraft engine develops thrust which accelerates the plane forwards because the force is acting directly on the airframe. The conveyor can only exert a force on the tyre. This force causes the wheel to rotate and is not acting directly on the airframe. So the wheel merely rotates and does not exert much backward force on the plane. The only backward force on the plane is caused by the friction in the wheel bearings. Since this is (hopefully) small, the aircraft moves forwards at almost exactly the same speed as it does on a normal runway and will take off at the usual airspeed, but it takes a little bit longer to get to that speed.

ka-ching! That's the sound of a penny dropping. I see it now, forces are of ccourse the way to think of this. It's not true to say the conveyor has NO ABILITY to arrest the planes movement, but it is true the ability is very limited as the wheels are free wheeling, hence only friction in the bearings.

Too long since I did any physics

The darn plane is not taking off without going forward though, which I think is what the initial question is worded to make you think is being suggested

Cheers,
Dave.

[Tunicates]

Friction in the bearings AND the rolling resistance of the tyres.

Not enough to prevent take off, but certainly enough to require some extra thrust.
Especially if the pilot was an elephant (a forgetful one) and he'd forgotten to pump up the tyres.

[uwila]

Originally Posted by Tunicates are Mad

Friction in the bearings AND the rolling resistance of the tyres.

Nope. The conveyor is being driven backwards and this will cancel out the tyre resistance.


Normal aircraft can take off vertically. The minimum forward speed required is 0 knots - all you need is a strong enough headwind. This happened at my gliding club last week. We share the field with a 737/757 etc 'MOT centre'. They have a static engine test area which someone parked a glider behind. When the jet engines were run up, the back-draft caused enough wind to lift the glider off the deck and deposit it (in an unusable state ) further down the airfield.

[Tunicates]

Originally Posted by David G

The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction.

David.

I would take opposite direction to mean 'not the direction that the plane is travelling'
I'm assuming that the wheels are going round twice as fast as they would normally.