Zapletal Writes Mark Your last sentence pretty much nails it in terms of this simplified billiard ball model For an even simpler version consider an aerofoil moving at constant horizontal velocity through a vacuum and above a large massive body such as the Earth The gravity force between Earth and foil pulls them together and gives them both equal time rate of change of inwards momenta ie d m V Of course the lighter foil has the higher dV here so follows a more curved path roughly parabolic concave down Now picture something like a football bouncing up and down elastically between the two bodies Each time it hits the Earth and bounces back up it exchanges twice its vertical linear momentum with the Earth and gives it some downward dP but of very tiny dV Similarly each time it hits the underside of the foil and bounces back down it gives the foil the same upward momentum dP So answer to your original question is indeed YES End result is that the foil travels along a path a bit like mmm The bouncing ball in this model is roughly equivalent to Lanchester s pillar from page 9 of his book ie As a whole the fluid in the previous section does not gain or lose momentum any more than does a cast iron pillar supporting a load Adding more detail to the model has many more balls bouncing every which way which add in Continuum Mechanics terms an ambient pressure everywhere But for this BB model to work there MUST be a time averaged increase in the number of these balls bouncing between each other in the zone between the Earth and foil This increased pressure zone MUST ALSO travel steadily with the foil much as the single football does above or as does Lanchester s cast iron pillar End Zapletal