
Originally Posted by
M
Wow, I would not have expected so many responses to this seemingly simple question. It turns out to be a good question after all.
As a side note, I am also intrigued by a certain (predictable) psychology that tends to play out with controversial problems (that alone is worth scanning through all the responses): When the controversy is strong enough to create a feeling of despair, people start nitpicking on the semantics of the original question or completely dismiss the hypothetical framework by calling it unrealistic. Let's not succumb to that practice, yet. The fact that boundary conditions are abstract or hypothetical does not invalidate an otherwise well-posed problem.
We all agree: to create lift, we need air motion over the wings. If the aircraft stands on the ground (no motion relative to the airport frame of reference) and if we had a strong enough wind in the right direction (against the aircraft's nose), we could simply turn on the engines to counter aerodynamic drag (the aircraft is still not moving relative to the airport), and make use of the lift created on the wings to counter gravity and take off vertically.
Usually we don't have that much wind. In the other extreme, with no wind at all, the air is quiessent in the airport frame of reference. To create air motion over the wings, the aircraft will have to move relative to the standing air (duh!) and in this case it just happens to also move relative to the airport.
In all the previous responses (unless I missed something) you guys have pretty much tied the question of "lift" or "no lift" to the question whether the airplane is moving relative to the airport or not. This implies that you are assuming the air does not move relative to the airport (no wind). Now here's the problem with that assumption: If the conveyor belt runway moves at significant speed (about the speed of a regular aircraft at take-off), there will be wind! Don't get upset, yet. I'll explain...
First, let's assume (hypothetically) that we can create a situation where the aircraft eninges are just being turned on, but the aircraft does not move relative to the airport frame of reference. According to Newton, the only way to do this is to have a perfect force balance in this reference frame. This means that the engine thrust must be compensated by a force of equal strength and opposite direction. In the case of zero wind, what would such a force be? Maybe the rolling friction imposed on the weels by the conveyor belt. Let's assume, hypothetically, that this is possible, and the aircraft doesn't move because of this force balance created by its engines and the friction of the very fast conveyor belt. I'd like to make the point that even in this case we will eventually have lift!
When air moves over a solid body (such as a wing, or the Earth's surface) there is something called a viscous "boundary layer" due to the viscosity of the air. If you have ever been on a high building, you probably know from experience that winds are usually much stronger at higher altitude, than they are on the ground. This is partially due to the fact that the wind is slowed down by trees and other things on the ground but it's even the case on a smooth surface. That's because, due to viscosity, the air "sticks" to the ground, i.e. right on the surface the relative velocity between ground and air is zero! If you measure a profile of the velocity as a function of altitude (at a steady wind), you'll see a more or less parabolic variation from zero velocity on the ground to a maximum velocity somewhere far above. I think you get the point of this lengthy but simplified explanation of what we call "boundary layer". Now back to the aircraft, standing in the airports frame of reference on a fast conveyor belt. No significant winds (at first).
Remember, the relative velocity between ground and air is always zero. This is also the case on the conveyor belt, meaning, the air sticks to the belt and is actually moving along! Given enough time (seconds), we will have a nice (inverse) boundary layer with air moving at a high speed on the ground, and somewhat slower but still at significant speed on the altitude of the wing, and even far above. We now have air moving over the wing and therefore lift! How much lift? Just as much as a regular aircraft running down the runway in quiessent air. This new situation is entirely equivalent to the airplane moving down a runway, except its happening in a different reference frame (kind of like an outdoor wind tunnel).
If you thought you could cheat on nature I have to disappoint you. You may (or not?) be able to use a conveyor belt to switch from one reference frame to another, but the forces will be pretty much the same, even in that (hypothetical) case.