Thread: Theory on lift
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28th Jul 2018, 18:05
#365 (permalink)
pattern_is_full

Join Date: Jan 2008
Location: Denver
Posts: 1,068
No problem with a little revival.

My take is - one can fly a barn door (or a paper airplane) with no camber and no significant "Bernoulli" lift, simply with enough AoA and power (or gliding force from gravity).

But the process is much more efficient using every possible physical effect one can. Whether one is the Wright Bros. with a pitiful little 12-hp engine, or today's commercial jets trying to save evey penny on fuel while going fast.

Lift is a lot of little pieces, which in the final analysis, together push down (or in the case of Bernoulli, pull down) a mass of air equal to the mass of the aircraft, holding it up against the force of gravity (Newton's 3rd). Actually, probably slightly more than the weight of the aircraft, due to losses to tip vortices and such, but with a net vector "down" that is equal to aircraft weight. Newton, Bernoulli, Coanda, Kutta-Joukowski - they all identified things that contribute something to lift, or rearranged the pieces to look at them from a different angle.

But unless there is room and time to write a thick textbook covering all those pieces, any "popular" explanation of lift has to be abbreviated, and thus only partly correct. It is a bit like "Is light waves or particles?" - the correct answer is "both" or "all of the above."

In theory, one could just put a tube full of people on top of a 12,000m girder, and move it around on wheels. The weight of the craft (mass x G) is supported by the girder, and eventually transferred to the wheels and thus the ground. Lift does the same thing, and someone has even detected the "weight" of an aircraft flying in the flight levels, as a tiny increase in local baro pressure (tiny because the weight is distributed over of billions of square inches, in a wide, spreading "footprint" below the aircraft).