(Edit May 2012-- my thoughts on this matter have now changed significantly-- I'm still convinced that the earth "feels" a downward push from the wing of an aircraft in flight, equal in magnitude to the weight of the aircraft, which is also equal to the upward gravitational attraction that the aircraft exerts on the earth, but I no longer believe that this downward force need involve any specific amount of downward momentum of the air (downwash). For more, see posts 58, 61, and 72. End edit.)
Originally Posted by HerkS
Momentum is conserved in a closed system - so if you fly your helicopter inside a box there is no net force on the system. Not so in an open system - an aircraft in flight is an open system,
The purpose of this particular thread is to consider an aircraft in flight as a closed system. The planet and the atmosphere are part of the closed system. We may admit solar radiation into the system to provide power to over come drag. That doesn't change the fundamental conservation of (vertical) momentum issues. I think that there is a legitimate subject to discuss here.
Helicopter in a closed box-- gravity is pulling down on the helicopter from outside the box. If the box is on a scale, the weight of the helicopter will register on a scale even when the helicopter is in flight within the box. This can only happen if the helicopter's downwash is hitting the bottom of the box with a force equal to the helicopter's weight. Surely we all agree on that.
This thread argues that the same is true of an airplane in flight over the earth. The earth "feels" the aircraft's weight. That must be due to some sort of downward momentum reaching the earth's surface. Which could be measured by means of a (large enough) scale. Unlike the pressure increase from a balloon in flight-- this occurs on a global not a local scale, and could be measured with a (sensitive enough) barometer at any point on the earth's surface, but not with a scale located somewhere under the balloon no matter how large the scale is. I gave a more detailed argument for this in an earlier post, based on global mass distribution-- the center of mass of the earth/ balloon system is located at the center of the earth, but the center of mass of the earth/ airplane system is not located at the center of the earth. See items 4) and 6) in post #1.