Slowed rotor

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Slowed rotor is a concept in designing and flying certain rotorcraft. Reducing the rotational speed of the rotor reduces the drag, enabling the aircraft to go faster and/or fly more economically.

Rotors of conventional helicopters are designed to operate at a fixed RPM (within just a few percent), causing suboptimal operation in large parts of the flight envelope.

Two main issues restrict the speed of rotorcraft:

These (and other) problems limit the practical speed of helicopters to around 160–200 knots (300–370 km/h). At the extreme, the theoretical top speed for a rotary winged aircraft is about 225 knots (259 mph; 417 km/h), just above the current official speed record for a conventional helicopter held by a Westland Lynx, which flew at 400 km/h (250 mph) in 1986 where its blade tips were nearly Mach 1.

For rotorcraft, advance ratio (or Mu, symbol ${\displaystyle \mu }$) is defined as the aircraft forward speed V divided by its relative blade tip speed. Upper mu limit is a critical design factor for rotorcraft, and the optimum for traditional helicopters is around 0.4.

The "relative blade tip speed" u is the tip speed relative to the aircraft (not the airspeed of the tip). Thus the formula for Advance ratio is

${\displaystyle \mu ={\frac {V}{u}}={\frac {V}{\Omega \cdot R}}}$ where Omega (Ω) is the rotor's angular velocity, and R is the rotor radius (about the length of one rotor blade)

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Wikipedia