Back to the Gas Turbine, yes I slept well thank you
Gas Turbines for helicopters are, generally, small fixed spool compressor turbines optimised for a specific altitude. If the turbine is operated at this design altitude it will perform at its best. Remove it from its design criteria and the ISA conditions that the egg heads tested it on and it will generally perform worse. This is due to a variety of factors:
1. If the density, i.e. the pressure and the temperature of the air, is very thick, we are looking at cold and high pressure then the turbine will still under perform. The reason being that the compressor is inefficient at compressing the thick, sticky air. The combustion chamber will love it, all that cold thick air and the cooling will be excellent however the power required to turn the compressor will rise dramatically and therefore reduce the theoretical power available to the free power turbine.
2. Going the opposite way, if the air is very thin and very hot i.e High altitude and high temperature then the compressor is happy because it doesn.t have to work so hard to compress the thin air. But, by the time the air reaches the combustion chamber it has to be split into three flows:
a. Primary, the air available for combustion
b. Secondary, the air fed into the combustion chamber to prevent the flame from contacting the sides. Flame shaping
c. Tertiary cooling air, pumped around the combustion chamber to provide cooling.
If these flows are already thin and hot then it is obvious that the overall operating temperature of the engine will increase. There is no longer, physically, as much air to burn therefore the efficiency of the combustion chamber is reduced which leaves less ooomph for the power turbines and the free power turbine. The cooling is less efficient resulting in a high operating temperature for the engine.
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 (;-)), and all of the engine graphs are entered at pressure altitude and then corrected for temperature giving you, hey presto, density altitude. So irrespective of the pressure, if it's 1045mb and 50 degrees the performance would be questionable on ECU temperature grounds.
