The plane has weight that has to be overcome. To do that, it has to move down the runway.

The air - the engines are not pushing the rest of the plane. The air is being sucked through the engines and pushing it out the other end. The air over the wings is NOT moving.

If the ground is solid, the plane is moving down the runway - THEN the air over the wings is moving in relation to the plane. Otherwise, it's not.

By the way - the rubber band plane would probably take off, because the propeller IS pushing air over the wings.

But we are talking about a jet engine, no?

A prop or turbine does not push air over wings (maybe a little, but not enough to matter). It takes air from in front and pushes it to the back. Because the air wants to keep doing what it's doing (sitting still or blowing in whatever direction it was already going) the prop/turbine meets resistance. The resistance makes the prop/turbine pull forward, and hence, pull the plane forward. Just like rowing a boat, but you're pushing water instead of air to get moving.

Because you're relying on your ability to move air to create forward motion, the surface can work against you and it won't matter, as long as you have a way to reduce friction with the surface. Something like wheels.

Lets invert the problem. Say you have a conveyor suspended over water, directly over your power boat. Your boat is on calm water and has a set of wheels overhead which remain in contact with the conveyor. [The conveyor in this example would work against the direction of the boat, just like the other works against the direction of the plane] If you try to accelerate the boat, and the conveyor spins against it, will you move forward?

Hint- the answer is yes.

The boat doesn't need lift. The propeller pushes it.

Water isn't a gas like air. The molecules are closer together.

Moby, planes don't fly because their propellers or engines push air over the wings. There are lots of winged objects that don't have propellers. Gliders and birds, for example.

Sure, the boat doesn't need lift, but you said the propellor pushes it, and you're right. The plane doesn't need lift to move forward, just to fly. It can move forward, though, and increase its airspeed enough to attain lift, no matter what the conveyor does.

::edited for grammar::

Right. I'm just saying a prop plane would be more likely to be able to obtain lift.

Right. But it can't move forward. Not if the conveyor is matching *exactly* the speed.

On a somewhat related note - jets on aircraft carriers do NOT take off with the wind. They take off against it. If they take off with the wind, they may not have enough lift.

And why is that?

Why not? You said that the boat would move forward, but the plane wouldn't? Why is that?

Put that same boat in a river. Trying to go UPstream. If the stream is matching the speed of the propeller...no movement.

Because it's pushing air over the wings. Albeit, very little...*probably* not enough to give it lift...but if the prop were large enough, it might.

Think of it this way.

You have a podium connected to the wing. Stand on it. You are not going to have wind blowing on you. With no wind, there is no lift on the wings.

We are assuming the air is calm around the plane, right?

So what would be enough to give it lift?

In a single engine prop plane, the prop wash to which you are referring, is confined to the area where the wings are joined to the fuselage and would not produce enough lift by itself to lift the aircraft.

And how does the plane get speed?

Liz- I've been noticing you've been changing the thread title. It's pretty funny. Keep it up!

Ah, but that is an erroneous analogy. The overhead conveyor plays exactly the same role as the conveyor in the first problem. You said yourself earlier (essentially) that planes like to take off into the wind (with a headwind), so assuming that there would be, say, ten knots headwind or headwater. That value being constant, any equal and opposite force would keep a craft (air or water) stationary. That would mean ten knots of airspeed or waterspeed. once the power of the craft overcomes the headwind/water, the craft moves forward, regardless of what the conveyor might do.

Exactly. For that, the prop would have to be as long as (or close) to the wingspan. Plus have enough force to put enough air over the wings.

Moby- how do you think a glider can become airborne?

But we are not assuming a headwind. We are assuming calm air. If the plane overcomes the headwind/water (conveyor belt), then the puzzle is flawed - because that would mean the conveyor belt has NOT matched the speed of the wheels.

It has to have wind over the wings.

A stationary plane on a conveyor belt in calm air does NOT have wind going over the wings.