Originally Posted by G SXTY
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Jet engines are most efficient at high power settings (say around 90-95% of maximum fan speed, or N1). For the best specific fuel consumption (i.e. miles per gallon) you would want to operate at an altitude where the aircraft maintains unaccelerated straight and level flight with the engines running at 90-95% N1...
G SXTY's got it. It's not that the
engine is more efficient at altitude, but that the engine is best matched to the
aircraft at altitude. The machine is bloody overpowered at takeoff, is working hardest at top of climb, and happiest in cruise.
