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Many new helicopers have rotor blades with swept tip. Why? Acoustic? e.g.

enter image description here

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Helicopter rotor design is, among many other things, a balancing act of retreating blade stall and transonic effects on the advancing blade.

Helicopter blade tip devices usually serve to reduce transonic effects on the advancing blade. The Ka-50's blade tip is rather simple (only swept) as the coaxial design is affected less from these transonic effects, as they happen on both sides. For the same reason, retreating blade stall is also a lesser problem, so the designer can get away with a slower blade tip speed.

Helicopters with a single main rotor don't have this luxury, so many of them have complex blade tip devices with sweep, larger chord and sometimes different airfoils. I believe a good example is the Westland (now merged, AgustaWestland) Lynx.

Westland Lynx blade tip

Blade leading edge is top of picture. Sweep, increased chord and droop on end are evident.

For my Bachelors thesis, I was looking for blade tips with the highest drag divergence Mach number, the Lynx blade tip's was M 0,91. I believe this is an important reason why the Lynx is the fastest conventional helicopter.

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Among other thing for the KA-50 in the picture is to-

The rotor blade double-swept-tip geometry can effectively improve the aerodynamic and aeroacoustic behaviors of a rotor

Source:

Open Access Aeromechanical Stability of a Bearingless Rotor Helicopter with Double-Swept Blades

A review of helicopter rotor blade tip shapes

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  • $\begingroup$ A major part of this shape/design is to prevent the tips of the blades from going Supersonic. Allowing the helicopter to fly faster in a straight line. Your answer does cover this, but I think this is the main reason. $\endgroup$
    – Kaddyuk
    Commented Dec 4, 2020 at 11:57
  • $\begingroup$ The cutaway drawing depicts a KA-50, not AH-64 $\endgroup$
    – DeepSpace
    Commented Dec 4, 2020 at 15:06
  • $\begingroup$ Thanks @DeepSpace, fixed $\endgroup$
    – Rsf
    Commented Dec 4, 2020 at 15:51

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