Comment by somat
1 day ago
It is probably obvious, so obvious that no one starts with it? but it took me an absurdly long time to put together that an airplane lifts by moving air down.
Admittedly there is an amazing amount of fluid-dynamic subtly on top of this simple Newtonian problem. But I am surprised that almost no one starts with "An airplane produces lift by moving air down, for steady flight it needs to move exactly as much air mass down as the plane weighs. here are the engineering structures that are used to do this and some mathematical models used to calculate it"
That was what I was taught 30 years ago in university.
To be more precise, we defined or made a shorthand of this downward force W. Originally it stood for weight but we knew it was the downward force that must be counteracted by an upward force called L for lift. Lift by convention was always an upward force.
These are conventions taught and used.
I should also remark the laminar fluid boundary layer only is true for subsonic flight. When you go over Mach one, a permanent shock wave forms in front of the wing. This shock wave disrupts the laminar boundary layer. Lift is achieved by Newtonian breakdown of the air hitting the wings underside. This is why supersonic planes require fly by wire. As computers are constantly issuing commands to ailerons and rudder to prevent stall.