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.Chandler, Jay. "Advanced rotor designs break conventional helicopter speed restrictions (page 1) " Page 2 Page 3 . ProPilotMag, September 2012. Accessed: 10 May 2014. Archive 1 Archive 2 Archive 3 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.Dubois, Thierry. "Researchers Look at Single-engine Cruise Ops on Twins" AINonline, 14 February 2015. Accessed: 19 February 2015.Perry, Dominic. "Airbus Helicopters promises safe single-engine operations with Bluecopter demonstrator" Flight Global, 8 July 2015. ArchivePerry, Dominic. "Turbomeca eyes flight tests of 'engine sleep mode'" Flight Global, 25 September 2015. Archive
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,Rigsby, page 3 mainly because the increased complexities have not been justified by military or civilian markets.Johnson HT, p. 325 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.Walsh 2011, page 3
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.Robert Beckhusen. "Army Dumps All-Seeing Chopper Drone" Wired June 25, 2012. Accessed: 12 October 2013. "for standard choppers ... the number of revolutions per minute is also set at a fixed rate"The UH-60 permits 95–101% rotor RPM UH-60 limits US Army Aviation. Retrieved 2 January 2010 This introduces limitations in areas of the flight envelope where the optimal speed differs.Khoshlahjeh
In particular, it limits the maximum forward speed of the aircraft. Two main issues restrict the speed of rotorcraft:Lombardi, Frank. "Optimizing the Rotor" Rotor&Wing, June 2014. Accessed: 15 June 2014. Archived on 15 June 2014Harris 2003, page 7Chiles, James R. "Hot-Rod Helicopters" Page 2 Page 3 Air & Space/Smithsonian, September 2009. Accessed: 18 May 2014.
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,"Blade flapping" Dynamic Flight this would cause dissymmetry of lift and eventually retreating blade stall,Robb 2006, page 31Silva 2010, page 1."Helicopter Limitations " Challis Heliplane"Retreating blade stall" Dynamic Flight and blade stability suffers as the blade reaches its limits for flapping.Johnson HT, p. 323 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.Prouty, Ray. "Ask Ray Prouty: Compound Helicopters, Compressibility (archived)" Rotor&Wing, 1 May 2005. Accessed: 11 Dec 2019."Nomenclature: Transonic drag rise " NASA
These (and other)Beare, Glenn. "Why can't a Helicopter fly faster than it does ?" helis.com . Accessed: 9 May 2014.Krasner, Helen. "Why Can’t Helicopters Fly Fast?" Decoded Science, 10 December 2012. Accessed: 9 May 2014. problems limit the practical speed of a conventional helicopter to around .Majumdar, Dave. "DARPA Awards Contracts in Search of a 460 MPH Helicopter" United States Naval Institute, 19 March 2014. Accessed: 9 May 2014.Wise, Jeff. "The Rise of Radical New Rotorcraft" Popular Mechanics, 3 June 2014. Accessed: 19 June 2014. Archive Quote: "This aerodynamic principle limits conventional helicopters to about 200 mph." At the extreme, the theoretical top speed for a rotary winged aircraft is about , just above the current official speed record for a conventional helicopter held by a Westland Lynx, which flew at in 1986"Rotorcraft Absolute: Speed over a straight 15/25 km course ". Fédération Aéronautique Internationale (FAI). Note search under E-1 Helicopters and "Speed over a straight 15/25 km course". Accessed: 26 April 2014. where its blade tips were nearly Mach 1.
Slowed rotors and aircraft speed
For rotorcraft, advance ratio (or Mu, symbol ) is defined as the aircraft forward speed V divided by its relative blade tip speed."Nomenclature: Mu " NASADefinition of Advance ratio"Flapping Hinges" Aerospaceweb.org. Accessed: 8 May 2014. Upper mu limit is a critical design factor for rotorcraft, and the optimum for traditional helicopters is around 0.4.Filippone, Antonio (2000). "Data and performances of selected aircraft and rotorcraft" pages 643-646. Department of Energy Engineering, Technical University of Denmark / Progress in Aerospace Sciences, Volume 36, Issue 8. Accessed: 21 May 2014. Abstract
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)Jackson, Dave. "Tip Speed Ratio (Advance Ratio)" Unicopter, 6 September 2013. Retrieved: 22 May 2015. Archived on 21 October 2014.
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."Helicopter Flying Handbook", Chapter 02: Aerodynamics of Flight (PDF, 9.01 MB), Figure 2-33 page 2-18. FAA-H-8083-21A, 2012. Accessed: 21 May 2014. 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.Berry, page 3-4 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,Harris 2008, page 13 little public analytical and experimental knowledge exists between advance ratios of 0.45 to 1.0,Berry, page 25 and none is known above 1.0 for full-size rotors.Harris 2008, page 25Kottapalli, page 1 Computer simulations are not capable of adequate predictions at high mu.Harris 2008, page 8Bowen-Davies, page 189-190 The region of reverse flow on the retreating blade is not well understood,Harris 2008, page 14Bowen-Davies, page 198 however some research has been conducted,DuBois 2013 particularly for scaled rotors.Bowen-Davies, page 216 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.Gustafson, page 12Johnson RA, page 251. 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.Bowen-Davies, page 97-99
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.Bowen-Davies, page 101
Fuel economy of slowed rotors Conventional helicopters have constant-speed rotors and adjust lift by varying the blade angle of attack or collective pitch. The rotors are optimised for high-lift or high-speed flight modes and in less demanding situations are not as efficient.
The profile drag of a rotor corresponds to the cube of its rotational speed.Gustafson, page 12Johnson RA, page 251. Reducing the rotational speed and increasing the angle of attack can therefore give a significant reduction in rotor drag, allowing lower fuel consumption.
History
Technical parameters given for each type listed: maximum speed. μ, the ratio of forward airspeed to rotational tip speed. Rotor lift as a percentage of total lift, at full speed. Lift-to-drag ratio (L/D).
Early development
When Juan de la Cierva developed the autogyro through the 1920s and 1930s, it was found that the tip speeds of the advancing rotor blade could become excessive. Designers such as he and Harold F. Pitcairn developed the idea of adding a conventional wing to offload the rotor during high-speed flight, allowing it to rotate at slower speeds.
The 1932 Pitcairn PCA-2 autogyro had a maximum speed of 20-,Harris 2003, page A-40 μ of 0.7,Harris 2008, page 19 and L/D of 4.8
NACA engineer John Wheatley examined the effect of varying advance ratios up to about 0.7 in a wind tunnel in 1933 and published a landmark study in 1934. Although lift could be predicted with some accuracy, by 1939 the state of the art theory still gave unrealistically low values for rotor drag.Harris (2008) pp.35-40.
Postwar projects Fairey Aviation in the UK worked on gyrodynes in the late 1940s and 1950s developing tip-jet propulsion which eliminated the need for countertorque. They culminated in the Fairey Rotodyne, the prototype for a VTOL passenger aircraft, which could combine the vertical landing of a helicopter with the speed of a fixed wing aircraft. The Rotodyne had a single 90 ft diameter main rotor supplemented by a 46 ft wide wing with forward thrust provided by twin turboprop engines. In forward flight the power to the rotor was reduced to about 10%. Its maximum speed was a record set in 1959."FAI Record ID #13216 - Rotodyne, Speed over a closed circuit of 100 km without payload " Fédération Aéronautique Internationale. Record date 5 January 1959. Accessed: April 2014.Anders, Frank. (1988) "The Fairey Rotodyne" (excerpt) Gyrodyne Technology (Groen Brothers Aviation). Retrieved: 17 January 2011. Archived 26 February 2014 0.6.Rigsby, page 4 Rotor speed was 120 (high speed cruising flight as an autogyro) to 140 (flare out while landing as a helicopter) rpm During forward flight 60% of the lift came from the wings and 40% from the rotor.Braas, Nico. "Fairey Rotodyne" Let Let Let Warplanes, 15 June 2008. Accessed: April 2014. Archived on 30 September 2013