Originally Posted by ShoeDLG
You asked for clarification and I offered a straightforward accounting of the forces acting on the air. Newton's Laws indicate very clearly that the air's net rate of vertical momentum change must be zero. If you believe Newton's Laws, there's no getting around that.
The motion of the rotor is certainly adding downward momentum to some of the air, and that local momentum exchange does matter. However, despite this local momentum exchange, the rotor never changes the net vertical momentum of the air. For every element of air displaced downward by a rotor in ground effect, another element of must be displaced upward by the same amount. This corresponding displacement doesn't happen at some time in the future, it happens simultaneously. If this were not the case, the air would collect below the rotor in the same way the BB's collect on the scale in the example in post #901. The air doesn't collect below the rotor. Instead it circulates outward and then upward once outboard of the rotor.
Suppose you neglected the air going upward outboard of the rotor and focused only on the down-going column of air right below the rotor. Is the rate at which the rotor adds downward momentum to that column of air equal to the weight of the helicopter? Suppose we carefully measured it and found that it was. If we changed the altitude of the helicopter, we would change the velocity of the air at any point below the rotor (note that this is not the case for a stream of independent BB's). The change in velocity means that we would find a different value for the rate of momentum change at every altitude. This is problematic if you are suggesting the rate of downward momentum change is always equal to the weight of the helicopter.
Nice story, but as scientists, we must stick to the facts. Rotors kick air down, not up. The force added to that air is measurable with a scale. In a hover, the force matches the weight of the chopper. Therefore, there's only one conclusion to draw - NASA draws the right conclusion - why can't you?.
Lift is a reaction to shoving air downwards. It even adds up - the weight of the chopper hovering over a scale to the weight shown on the scale from the choppers air hitting it. They'll pretty well match, depending on the skill of the pilot doing the hovering.
Put yourself in NASA's shoes for once, and see lift not as some mysterious thing requiring wierd incantations about net momentum not changing, but as something simple enough, anyone with a fan in their hands can understand it. Cause it is, that simple.
Now, trying to design a wing requires math - but that's the same for missiles and rockets, where the lift is more obvious then with planes (but not choppers). Choppers have an obvious similarity with missiles and rockets, in that they all visually shove something down, in order to lift. But,.a chopper's rotors are it's wings, and you have to use that, to get your head around wings - they also react to shoving air down, with lift.