Originally Posted by
compressor stall
.............but uplinker, by that analogy you need the same number of molecules passing around the wing to create the lift wouldn't you? So the same number must be present at any given time?
I am no physicist, but you seem to be suggesting that the number of air molecules passing over the wing per unit time needs to be constant to provide constant lift? If that were true, IAS would always be the same as TAS no matter what altitude you flew at, but it isn’t as we all know.
The lift equation is 1/2 x air density x velocity(sq) x wing area x coeficient of lift.
At high altitude, the air density is much less, so to provide the same lift, the velocity of the molecules has to be much more, and that term is squared. So the reduced lift owing to many fewer air molecules passing over the wing is compensated for by the greater force imparted by each molecule resulting from the increased speed (squared) of those molecules.
If we think of the air molecules as ping pong balls* then the difference in noise caused by flying at low level through the maximum density of ping pong balls, (each one touching their neighbours), to flying through many fewer ping pong balls at altitude, (albeit faster), then the reduction in noise will be apparent?
*a bit kinder to the aircraft structure than marbles !!
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