Aircraft engine propulsion and a little thermodynamics

[WaltBJ <waltbj01@xxxxxxxxxxxxxx>]

Ah, but more efficiency is reached with a greater mass at lower
velocity. Mv=Mv, and to get twice the velocity requires four times the
energy in the gas stream. Even worse, as I recall, (it's been 10 years
since I last taught this stuff), the compressor on an axial flow engine
is but a series of little propeller blades, and thus its power
requirement rises as the cube of the rpm. So if we use a big slow
turning fan(s) to push a LOT of air out the back end we get more fuel
efficiency than if we sent less out faster to attain equal thrust. J75
vs CFM56, for example.

Walt BJ