Does the height at which a helicopter is flying affect its fuel consumption? Which factors are influencing its fuel consumption?
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1$\begingroup$ Yes, altitude affects fuel burnt. See other elements page 24 of the HAI Guide For The Presentation Of Helicopter Operating Cost Estimates $\endgroup$– minsCommented Aug 15, 2016 at 1:42
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The lift equation shows that as air density decreases, as it does with altitude, in order to maintain lift, the speed of the blades must increase or, the coefficient of lift (CL) must increase. Since the characteristics of the blades do not change, in order to increase the CL, the angle of attack must be increased.
Increasing either of these increases drag so, in order to prevent the blades from slowing down, the engine must produce more power to counteract the drag.
Most helicopters use constant speed rotors so in practice, the angle of attack (the pitch of the blades) is the only variable. In order to maintain speed of the blades, the pilot, or a throttle governor, must increase power from the engine.
Therefore, a helicopter uses more fuel as it's altitude increases.
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$\begingroup$ what helis don't have constant speed rotors? $\endgroup$– rbpCommented Aug 16, 2016 at 0:05
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$\begingroup$ @rbp The A160 for example, but I'm jut covering my behind lest some smart Alec says "what about the Acme Wing Flinger 40?" $\endgroup$– SimonCommented Aug 16, 2016 at 7:07
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$\begingroup$ H260 not collective-controlled you can definitely fly a heli with low rotor rom but it's not a normal operations $\endgroup$– rbpCommented Aug 16, 2016 at 13:25
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3$\begingroup$ @rbp Of course, I was just trying to keep it simple for the lay person leading to collective pitch and engine power being the variables. My worst experience ever, lesson learned and never repeated, was pulling collective before I'd joined the needles at the end of a power recovery auto. Good job my instructor was on the ball. I personally learned how much below NrMin you can go in an R22 :( $\endgroup$– SimonCommented Aug 16, 2016 at 15:18