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.
Introduction 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.
Design principles
Speed limits of aircraft rotors
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:
Retreating blade stall. As forward speed of the helicopter increases, the airflow over the retreating blade becomes relatively slower, while the airflow over the advancing blade is relatively faster, creating more lift. If not counteracted by flapping, this would cause dissymmetry of lift and eventually retreating blade stall, and blade stability suffers as the blade reaches its limits for flapping. Transonic drag near the rotor blade tip. The faster-moving advancing blade tip may begin to approach the speed of sound, where transonic drag begins to rise steeply, and severe buffeting and vibration effects can occur. This effect prevents any further increase in speed, even if the helicopter has surplus power remaining, and even if it features a highly streamlined fuselage. A similar effect prevents propeller-driven aircraft from achieving supersonic speeds, although they can achieve higher speeds than a helicopter since the propeller blade isn't advancing in the direction of travel. 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.
Slowed rotors and aircraft speed
For rotorcraft, advance ratio (or Mu, symbol μ {\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
μ = V u = V Ω ⋅ R {\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) When the rotor blade is perpendicular to the aircraft and advancing, its tip airspeed Vt is the aircraft speed plus relative blade tip speed, or Vt=V+u. At mu=1, V is equal to u and the tip airspeed is twice the aircraft speed. At the same position on the opposite side (retreating blade), the tip airspeed is the aircraft speed minus relative blade tip speed, or Vt=V-u. At mu=1, the tip airspeed is zero. At a mu between 0.7 and 1.0, most of the retreating side has reverse airflow. Although rotor characteristics are fundamental to rotorcraft performance, little public analytical and experimental knowledge exists between advance ratios of 0.45 to 1.0, and none is known above 1.0 for full-size rotors. Computer simulations are not capable of adequate predictions at high mu. The region of reverse flow on the retreating blade is not well understood, however some research has been conducted, particularly for scaled rotors. The US Army Aviation Applied Technology Directorate runs a supporting program in 2016 aiming at developing transmissions with a 50% rotor speed reduction. The profile drag of a rotor corresponds to the cube of its rotational speed. Reducing the rotational speed is therefore a significant reduction of rotor drag, allowing higher aircraft speed A conventional rotor such as the UH-60A has lowest consumption around 75% rpm, but higher aircraft speed (and weight) requires higher rpm. A rotor disk with variable radius is a different way of reducing tip speed to avoid compressibility, but blade loading theory suggests that a fixed radius with varying rpm performs better than a fixed rpm with varying radius.
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