Capt Pit Bull, according to your understanding of physics, falling bodies don't accelerate, because otherwise momentum wouldn't be conserved.
Well, that might be
your understanding of my understanding!
Falling body acceleration conserves momentum quite happily, you just need to consider the other mass involved in the gravitational attraction. You don't need any air around to explain it either.
Include gravity into your consideration of downwash, and you will find that momentum and energy are conserved.
Like I said, expand the view to a 3 dimensional model and conserving momentum become easy. Two vortices with equal and opposite angular momentum (for unaccelerated flight of course), job done.
pb