Thanks gents, that is the explanation I was looking for. Probably explains why we consider thrust specific fuel consumption for jet engines and brake SFC for piston-props.
Agaricus bisporus,
The only on-line reference to it that I can find is here, pg5, bottom paragraph.
ftp://ftp.seu.edu.cn/Pub2/EBooks/Boo...on26/Ch177.PDF
I vaguely remember reference to thrust- and power-producers from my degree, but that was a while ago.