Please help solve our stalemate

[HidingInTheCellar]

Hi All,

Long time lurker with a vein hope you can help my situation.

My better half and I have been having a futile argument for the last few weeks and neither of us are shifting on our stand point.

Things have gotten so desperate that we have taken to such childish endeavours such as withholding sexual rights, hiding the toilet rolls and removing the fuses from all the electrical appliances in order to get the other to concede.

The argument is basically this;

If an aeroplane was using a conveyor belt as a runway and the conveyor was moving in the opposite direction to the planes direction of travel, would the plane take off?

I say it will but my partner says it won't.

Please please please please help. I'm dying for some nookie!

No, it wouldn't take off - because what matters is the air speed over the wings. The conveyor belt 'uses up' some of the energy that the engines have generated - if the conveyor belt were to stop suddenly, the aircraft would begin to move forward more quickly.

The conveyor belt doesn't "use up" any energy that the engines have generated (bar a bit of friction loss in the wheel bearings). Once the aeroplanes engines start, the aeroplane will start moving forwards regardless of what the conveyor belt is doing.

Think about it a slightly different way - imagine that the aeroplane doesn't have wheels that hold it up while on the ground but, instead, it's sitting on a hovercraft that is keeping the aeroplane hovering a few inches above the conveyor. What would happen then?

Whether there are wheels or it's hovering then if the speed needed for the aircraft to take off is normally X miles per hour (to have sufficient flow of air over the upper wing surface) then the plane will need to be moving at that speed relative to a fixed point.

Let's say the plane needs to travel at 200 MPH on a fixed runway. Now let's assume the conveyor belt runs at 100 MPH (OK, difficult to expect this, but being able to carry a 747 is just as hypothetical).

The plane needs to run at 200 MPH relative to a fixed point on the ground (eg control tower).

If it moves forward at 200 MPH and the conveyor runs at 100 MPH the speed relative to the control tower is only 100 MPH and insufficient lift would be achieved.

So the plane would need to move at 200 MPH + to achieve the lift required to safely take off.

If the conveyor was at 100 MPH, the plane would need to move forward at 200 + 100 MPH (ie 300 MPH).

Once off the ground, the plane would perhaps lift significantly faster than usual because it would no longer have the effect of the conveyor slowing it down.

As for using a hovercraft, clearly if it just 'hovered' without using any forward thrust, it would not sit still but move with the conveyor belt at 100 MPH. For the plane to take off 'comfortably' without undue effects of higher speed once off the ground, you could get the hovercraft to use forward thrust at 100 MPH and the plane's thrust to push another 200 MPH so once off the support, the plane would be in free space at 200 MPH and have the same amount of lift as normal.

omg you two sound crackers, love it, will especialy remember hiding the loo roll, although can't help with the plane thing

"would only not take off if it was moving at the same speed as the conveyer belt? The minute it accelerated and was moving faster"

Just "moving faster" isn't enough - you need a significant speed to get sufficient lift. I suppose you might argue that once off the ground the plane is no longer being slowed by the belt, so it's like a drag racer hitting the button for nitrous oxide to give that 'boost' and it's time to hold tight and keep fingers crossed concerning not hitting anything / going off the track.

Very different with the plane on runway with a big fan in front. As long as you don't hit the fan there's a good chance of taking off and then (once out of the air stream from the fan) falling back to earth as the plane may not have achieved sufficient speed to provide forward movement and therefore there's insufficience airflow to maintain lift. Result: crash.

If you've got a hoverplane, then why would it pay any attention to what's happening to the conveyor belt below it? It's not even touching it. If you've got a hovercraft hovering just above a stationary conveyor belt, and then the belt starts moving, the hovercraft is just going to stay where it is.

"The plane needs to run at 200 MPH relative to a fixed point on the ground (eg control tower)"

Ah, no, that's not correct. The aeroplane needs to be moving at 200MPH relative to the air surrounding it. Wings work on the air, they don't give a damn about fixed points on the ground. If you've got a fan blowing a 200MPH wind at the plane, and the plane's engines are running at 200MPH, then the plane will rise off the ground even though it's not moving forwards or backwards an inch. It's moving at 200MPH relative to the air around it and that's all that matters.

troll