Paul said:
Have just checked the operating data manual for the S-61N, which incidentally includes Inlet Guide Vanes to assist with mass flow during startup due to the fixed compressor design and thick, slow moving air (;-)),
The IGVs on an S61N do not move until Ng exceeds 63% - well above the ground idle speed. There is an air dump valve in the compressor that is opened during the start sequence to allow the engine to start without hanging or stalling.
I am not sure what you mean by "fixed compressor design". The IGVs and the first 3 stages of guide vanes move (between 63 and 95% Ng). The CT58-140-2 Engine has a free power turbine.... Are there engines that have moveable compressor rotor blades or are there engines that have the compressor spools disconnected from their turbine spools?
As for aircraft performance graphs, I was under the impression that these had as much to do with the blade aerodynamics as engine performance. Density altitude affects the amount of lift a blade can generate and therefore the amount of drag and hence the torque required, which then determines how much power the engine needs to supply. Agreed the engine would also be affected, but I doubt it is as much as the blades are. Besides I seem to remember that the thermodynamic cycle the gas turbine engine is based on is a Temperature cycle not a volume or pressure cycle.