Fuel Flow Reduction With Altitude (Constant Power)
A question arose during a flight the other day and I was unable to come up with a satisfactory explanation at the time. After some thought and several hours searching the internet (to include this forum) I think I now have a basic understanding of the phenomenon. However, I'm wondering if it's really as simple as it seems on the surface.
The question:
What causes fuel flow to decrease with altitude for a given torque setting in a turboshaft powered helicopter?
My understanding of the explanation:
As altitude increases, air density decreases, so the gas generator must spin faster in order to supply sufficient compressed air to satisfy the power demand and maintain equilibrium.
Typically, gas turbine engines operate most efficiently toward the high-power end of their operating spectrum. So, as the gas generator spools up the compressor and gas generator turbine approach their design points (optimum angle of attack) and become more efficient at producing that compressed air. That is, less power is required by the compressor to generate the required discharge air pressure, and the gas generator turbine becomes more efficient as well, thus reducing the fuel flow.
This increase in efficiency continues until the design point is passed or an operating limit is reached.
Additionally, with the increase in altitude comes a decrease in ambient temperature (within the altitude range where helicopters operate, at least). This increases the allowable temperature difference within the engine, increasing its thermal efficiency. However, it seems to me the contribution of this aspect would only be realized when the engine is operating near its temperature limit.
Have I got this right and what other factors are involved in this phenomenon?
-Stan-