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As I understand it helicopter rotor blades keep a constant aerofoil section along their length, and are not twisted. Other rotating aerodynamic devices such as propellors, turbofan fans and wind turbines thicken and get a steeper angle towards the hub due to the lower incident airspeed (in the direction of rotation).

I'd guess it'd have something to do with variation of airspeed on the advancing and retreating sides maybe?

Edit: some are pointing out that this may in fact not be true, and that rotors can in fact have twist, just that it's less visually apparent. The chord at least would still appear to be constant

Edit 2: Since posting this I've taken a module in helicopter aerodynamics at uni, and the question is indeed wrong, blade twist is most definitely a feature of helicopter design.

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    $\begingroup$ Very interesting question. Can you please point out to any references for no-twist and single airfoil rotors? It’s a surprise for me to hear that a helicopter rotor has no twist. $\endgroup$ – Gürkan Çetin Feb 13 '18 at 18:33
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    $\begingroup$ I will suggest that you heard incorrectly. When you assume something, and then ask "Why is it this way?" you end up asking an an invalid question. It's a bit liked asking "have you stopped beating your wife?" -- it assumes an answer in the asking. A better question is "do helicopter rotors have a constant section and angle of attack or are they twisted?' That way you get an answer to "what you heard' rather than assuming that it is so. I'll also point out that each helicopter's rotor blade is optimized for that helicopter ... $\endgroup$ – KorvinStarmast Feb 13 '18 at 19:58
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    $\begingroup$ Random model (Sikorsky S-76): "The main rotor blades use an SC1095 airfoil and have a non-linear twist to load the blades evenly in a hover. Tip caps are swept to reduce noise and vibration", source.. As the question is based on a wrong assumption, it will be difficult to answer properly. $\endgroup$ – mins Feb 14 '18 at 7:30
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Helicopter blades do have twist. The relative air speed increases from blade root to blade tip, and therefore on an untwisted blade lift would increase quadratically from root to tip. Or going the other way, lift would reduce quadratically from tip to root. Blade twist fixes this: it recuperates the lift generated on the inboard section of the rotor.

From Ray Prouty, Helicopter Aerodynamics Vol. 1., page 248:

By twisting the blades to reduce the lift at the tip, the flow pattern can be made more uniform and the hover figure of merit can be improved by up to about 5%. Since this may represent a 20% increase in payload and since twisted blades are about as easy to manufacture as untwisted blades, there is a good reason to use twist - and all modern helicopters do.

Due to the aerodynamic lift equation, ideal blade twist would be non-linear: again, would vary quadratically with blade span. But this ideal twist is only ideal in the hover situation, and definitely not ideal in the factory when the blade needs to be produced.

Pouty's article is very much worth reading, and further mentions:

  • High twist produces vibrations by the centre of pressure traveling a lot during rotation.
  • Negative blade twist (tip angle lower) is good for retreating blade stall but bad for autorotation, which would be helped by positive blade twist.
  • Optimum twist differs for hovering In Ground Effect and Out of Ground Effect.
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