Olin Lathrop wrote: > 'William Chops" Westfield ' >> The physics says that a plane must leave a *net* downdraft behind it. >>=20 >> So does a strict Newtonist explain the action of airfoils as >> something like 'redirecting air that passes over the top surface in >> a downward direction, due to surface effects' or something like >> that, rather than the "longer path =3D reduced pressure =3D lift" ? >=20 > When looking at the airplane as a whole, the physics says nothing > about how wings or any other structure accomplishes the pushing down > of air, only that there must be a certain net downward momentum per > time imparted on the air.=20 "Momentum per time" is what exactly, physically speaking? I don't think you wrote this with your physicist's hat on. Which one of Newton's rules says that air must move down to keep a plane in steady height? And please, don't put it in fuzzy and easy to misunderstand (both in the writer's and in the reader's mind) words -- put it in clear formulae, using proper terms used in physics. Gerhard --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .