Aircraft engine propulsion and a little thermodynamics

[Don Stauffer <stauffer@xxxxxxxxxxxx>]

> 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.

Your comment on compressor blades and propellers reminds me of GE's 
"unducted fan" engines.  Seems like they'd go to any lengths to keep 
from calling them propellers :-)

When is a propeller an unducted fan?