Hello Dick,
Oh my God! I didn't mean to 'flame' you, in fact I'm confident that throughout all of my posts I've never sought to flame anybody! My sincere apologies!
I use emboldened print to highlight important points. Blame my lawyer for the red ink, she sends back my drafts with alterations required in
RED, and she's my sister!
Now to work............ (and being nit picky / pedantic) ........
"EAS is the same as TAS at ISA sea level DENSITY and is "equal" to CAS only at speeds where comprssibilty is not a factor".
Compressibility is ALWAYS a factor, even at 1 knot. It is also a significant factor at ISA / Sea Level, but instruments are calibrated at ISA / Sea Level to remove it from display.
"reduced compressibility" - As this discussion is about constant TAS with changeing altitude, EAS, CAS, and Mach Number will be lower, hence
REDUCED compressibility. (Note use of bold to highlight a point).
"I never meant the question to wander off into the stratosphere, and in any case when I said thrust would reduce I meant thrust at a given RPM, not drag".
Stratosphere has naught to do with Vmd (although it's in the region where Mmd may come into play). You can fly above or below Vmd at Sea Level.
If we consider the engine in question to be a modern generation very high bypass fan, most of the thrust comes from the fan. Thrust production from a fan is closely related to propeller characteristics. Thus, at the same RPM with increasing altitude and decreasing density, thrust will reduce. It will reduce linearly as does the density (rho). The drag for the aircraft will also be reduced by linearly reducing rho. So far it seems that constant fan speed will maintain the same TAS with flight above Vmd, but here comes the spanner in the works... Coefficient of Drag at reducing EAS (above Vmd) serves to further reduce the drag, so, hey presto,
LESS engine fan speed will be required.
Lots of generalisations and assumptions, but with a TAS at EAS above Vmd, and a jet engine that produces most of it's thrust from the fan, LOWER N1 will be required to maintain the same TAS with increasing altitude!
It's now my turn to stand by to be flamed
Regards,
Old Smokey