page_title: Slowed rotor
section_title: Slowed rotors and aircraft speed
hierarchical_section_title: Slowed rotor / Design principles / Slowed rotors and aircraft speed
caption_attribution_description: English: Diagram of parameters for Aspect rato in rotorcrafts. Mu=V/oR
context_page_description: 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 also 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 the drag, enabling the aircraft to fly faster.
context_section_description: For rotorcraft, advance ratio (or Mu, symbol ) 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
    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 have 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.
caption: Rotorcraft Aspect ratio (mu) diagram
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