The slowed rotor principle is used in the design of some helicopters. On a conventional helicopter the rotational speed of the rotor is constant; reducing it at lower flight speeds can reduce fuel consumption and enable the aircraft to fly more economically. In the compound helicopter and related aircraft configurations such as the gyrodyne and winged autogyro, reducing the rotational speed of the rotor and offloading part of its lift to a fixed wing reduces drag, enabling the aircraft to fly faster.

Traditional helicopters get both their propulsion and lift from the main rotor; by using a dedicated propulsion device such as a propeller or jet engine, the rotor burden is lessened. If wings are also used to lift the aircraft, the rotor can be unloaded (partially or fully) and its rotational speed further reduced, enabling higher aircraft speed. Compound helicopters use these methods, but the Boeing A160 Hummingbird shows that rotor-slowing is possible without wings or propellers, and regular helicopters may reduce turbine RPM (and thus rotor speed) to 85% using 19% less power. Alternatively, research suggests that twin-engine helicopters may decrease fuel consumption by 25%-40% when running only one engine, given adequate height and velocity well inside the safe areas of the height–velocity diagram.

As of 2012, no compound or hybrid wing/rotor (manned) aircraft had been produced in quantity, and only a few had been flown as experimental aircraft, mainly because the increased complexities have not been justified by military or civilian markets. Varying the rotor speed may induce severe vibrations at specific resonance frequencies.

Contra-rotating rotors (as on the Sikorsky X2) solve the problem of lift dissymmetry by having both left and right sides provide near equal lift with less flapping. The X2 deals with the compressibility issue by reducing its rotor speed from 446 to 360 RPM to keep the advancing blade tip below the sound barrier when going above 200 knots.

The rotors of conventional helicopters are designed to operate at a fixed speed of rotation, to within a few percent. This introduces limitations in areas of the flight envelope where the optimal speed differs.

In particular, it limits the maximum forward speed of the aircraft. Two main issues restrict the speed of rotorcraft:

These (and other) problems limit the practical speed of a conventional helicopter 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.