Quote:
Originally posted by BlueSky:
[QUOTE]Originally posted by Mobycat:
[qb]You guys are confusing the issue of what counteracts what to achieve flight.

Lift overcomes gravity.
Thrust overcomes drag.

BOTH must occur for flight. The wheels turning on a stationary surface allows the [b]thrust to overcome gravity
(the aircraft's weight).
[/b]
Uh, you first said "Thrust overcomes drag" and then later said "thrust overcomes gravity". I think you're confused.

The way I understand it is that there is no lift (which allows the plane to overcome gravity) until there is airflow over the wings.

In order for air to flow over the wings, the plane must be in motion RELATIVE TO THE AIR. Take the example of an airborne plane flying into a very strong headwind, with its engines thrusting at 50%. Let's say that the headwind is so strong, it produces enough drag on the aircraft that the plane appears to just hover over the ground.

Remember:
thrust <> drag (horizontal forces that occur in AIR)
lift <> gravity (vertical forces that also in AIR)

The plane's airspeed may be 100mph, but the headwind is also 100mph, so its groundspeed is 0mph. The plane doesn't move over the ground but as far as the air molecules are concerned, the plane is moving through them.

Now, let's say the pilot of this plane just gets sick of not making any progress because it's so windy. He could just position the ailerons to lower the lift produced over the wing, and let gravity bring the plane to the ground. (Of course, in order to prevent the plane from moving backward because of the headwind, he'd have to keep the engines thrusting at 50%.)

But, if he wanted to actually make some forward progress (i.e. groundspeed), he could just increase the throttle to 100%, which would allow the engine thrust to overcome the drag caused by the headwind.

OK, now: let's say that directly beneath the plane is a conveyor belt that's moving backward at 100mph, exactly the speed of the headwind. The plane is still in the air, thrusting away at 50%, hovering over the conveyor belt.

The pilot looks down and sees this racing conveyor belt and wants to set the plane down on it. His plane is equipped with special wheel motors that start to spin the tires really fast so that they don't skid when they hit the ground. (You've seen how plane tires always skid when planes land, right?) Let's say he operates these motors so that the tires are spinning at 100mph, exactly the speed of the conveyor belt, then disengages them as soon as the plane touches down.

(Remember, the plane is still in the air, fighting the headwind, hovering in place over the racing conveyor belt.)

Now, the plane is flying through air molecules, the tires are spinning, and the conveyor is racing backwards at the same speed the tires are spinning (which also happens to be the same speed of the headwind).

When the pilot moves the ailerons and sets the plane down on the conveyor ,the forward-spinning tires and the backward-moving conveyor speeds cancel each other out, so the plane neither moves forward nor backward. At this point, the tires are just freewheeling along the surface of the conveyor.

OK, remember that the engine thrust is at 50%. If the engine thrust is increased to 100%, the plane can overcome the drag produced by the headwind, and move forward. At this point, the pilot could either keep the plane on the ground with the ailerons and increase thrust to move forward, or he could make the plane ascend with the ailerons and increase thrust to move forward.

Either way, it doesn't matter whether the tires are in contact with the ground or conveyor, since the engine thrust is overcoming drag and the ailerons are increasing or decreasing lift over the wing.

If there's less (or zero) headwind, 50% thrust will just move the plane forward through the air, since there's less drag.
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