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This question showed that rotary wing is less efficient than fixed wing, but the answers don't really address why.

Why are rotary wing aircraft less efficient than their fixed-wing counterparts?

It seems to me that the most important thing for efficiency is lift-to-drag ratio, so what makes this ratio much worse in rotary wing? Presumably it can use the same airfoil shape as fixed wing aircraft, so I could imagine a similar lift to drag ratio.

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  • $\begingroup$ Presumably it can use the same airfoil shape as fixed wing aircraft No, your "presumably" is incorrect. The answers below identify a few reasons why. $\endgroup$
    – KorvinStarmast
    Jun 3, 2017 at 17:51

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Quick answer:

  1. The rotary wing is moving at different speeds over the radius, and on the left and right side in forward flight, and since there can be only one optimum speed, most of the rotor is working at less than optimum speeds, lowering the lift-to-drag ratio overall.
  2. The slim rotor blade needs a reflex airfoil, an airfoil with low pitching moment, which is less efficient at creating high lift coefficients. If it would use a regular wing airfoil, it would twist horribly.
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    $\begingroup$ Nice concise answer. The only thing I would add is that the speed is not only different over the radius, but also differs between the forward moving blade and the backward moving blade when the helicopter has forward speed. The tip of the forward moving blade can be get close to Mach 1, while the tip of the backward going blade is at Mach 0.6. The inner section of the backward going blade can experience reverse airflow. $\endgroup$
    – DeltaLima ♦
    Jun 26, 2014 at 6:38
  • $\begingroup$ @Delta Lima: Correct and very valid comment. I modified the answer to include this aspect. $\endgroup$
    – Peter Kämpf
    Jun 26, 2014 at 21:39
  • $\begingroup$ Setting aside the difficulties of actually achieving such a thing, could efficiency be improved by having separate pitch control swash plates for the inner and outer edges of the rotor? $\endgroup$
    – supercat
    Mar 4, 2015 at 0:50
  • $\begingroup$ @supercat: Yes, any way to locally improve efficiency would help. You could even segment the blade and make the segments change pitch independently, for example by an actively controlled flap at the trailing edge. $\endgroup$
    – Peter Kämpf
    Mar 4, 2015 at 12:56
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Very good question. Lower efficiency is exactly the same thing as lower lift-to-drag ratio.

Helicopters have a lot of extra drag that fixed wing planes don't have:

  • Complex interference drag from the rotor/fuselage etc. interaction.

  • In forward flight: higher induced drag, due to the disk shaped lifting surface having a much lower aspect ratio than fixed wings.

  • Part of this disk being pretty ineffective: the inner bit, around the swash plate, has very low linear velocity and does not effectively contribute to the lift, but does to the drag.

  • The two factors named by @Peter Kämpf play a role.

  • The tail rotor burning fuel mentioned by @Simon at another question does not help fuel efficiency either.

The rotor having to generate forward thrust as well as vertical thrust is not that much a factor per se. Tilt the rotor 10 deg forward and the lift only reduces by cos 10 = 1.5%, while generating forward thrust of sin 10 = 17%

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