The slowed rotor principle is used in the design of some

helicopters A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribute ...

. On a conventional helicopter the

rotational speed Rotational frequency (also known as rotational speed or rate of rotation) of an object rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second (cps), etc. The symbol fo ...

of the

rotor Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...

is constant; reducing it at lower flight speeds can reduce fuel consumption and enable the aircraft to fly more economically. In the

compound helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...

and related aircraft configurations such as the

gyrodyne A gyrodyne is a type of VTOL aircraft with a helicopter rotor-like system that is driven by its engine for takeoff and landing only, and includes one or more conventional propeller or jet engines to provide forward thrust during cruising flig ...

and winged

autogyro An autogyro (from Greek and , "self-turning"), also known as a ''gyroplane'', is a type of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. Forward thrust is provided independently, by an engine-driven propeller. ...

, reducing the rotational speed of the rotor and offloading part of its lift to a

fixed wing A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinc ...

reduces

drag Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street adj ...

, enabling the aircraft to fly faster.

Introduction

Traditional helicopters get both their

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

and

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...

from the main rotor; by using a dedicated propulsion device such as a

propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...

jet engine A jet engine is a type of reaction engine discharging a fast-moving jet (fluid), jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include Rocket engine, rocket, Pump-jet, water jet, and ...

, the rotor burden is lessened.Chandler, Jay.
Advanced rotor designs break conventional helicopter speed restrictions (page 1)

. ''ProPilotMag'', September 2012. Accessed: 10 May 2014

/ref> If

wing A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is exp ...

s 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 helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...

s 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.Johnson HT, p. 325 Varying the rotor speed may induce severe vibrations at specific resonance frequencies.

Contra-rotating Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque. Examples include some aircraft propellers, re ...

rotors (as on the

Sikorsky X2 The Sikorsky X2 is an experimental high-speed compound helicopter with coaxial rotors, developed by Sikorsky Aircraft, that made its first flight in 2008 and was officially retired in 2011. Design and development Sikorsky developed the X2 heli ...

) 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.Robert Beckhusen.
Army Dumps All-Seeing Chopper Drone
''

Wired ''Wired'' (stylized as ''WIRED'') is a monthly American magazine, published in print and online editions, that focuses on how emerging technologies affect culture, the economy, and politics. Owned by Condé Nast, it is headquartered in San Fran ...

'' June 25, 2012. Accessed: 12 October 2013. "''for standard choppers ... the number of revolutions per minute is also set at a fixed rate''" This introduces limitations in areas of the

flight envelope In aerodynamics, the flight envelope, service envelope, or performance envelope of an aircraft or spacecraft refers to the capabilities of a design in terms of airspeed and load factor or atmospheric density, often simplified to altitude. The t ...

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

/ref>Harris 2003, page 7 * 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,Robb 2006, page 31Silva 2010, page 1. and blade stability suffers as the blade reaches its limits for flapping.Johnson HT, p. 323 *

Transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transoni ...

drag near the rotor blade tip. The faster-moving advancing blade tip may begin to approach the

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as ...

, 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)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 .Wise, Jeff.
The Rise of Radical New Rotorcraft
''

Popular Mechanics ''Popular Mechanics'' (sometimes PM or PopMech) is a magazine of popular science and technology, featuring automotive, home, outdoor, electronics, science, do-it-yourself, and technology topics. Military topics, aviation and transportation o ...

'', 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 The Westland Lynx is a British multi-purpose twin-engined military helicopter designed and built by Westland Helicopters at its factory in Yeovil. Originally intended as a utility craft for both civil and naval usage, military interest led to ...

, which flew at in 1986 where its blade tips were nearly

Mach Mach may refer to Mach number, the speed of sound in local conditions. It may also refer to: Computing * Mach (kernel), an operating systems kernel technology * ATI Mach, a 2D GPU chip by ATI * GNU Mach, the microkernel upon which GNU Hurd is bas ...

Slowed rotors and aircraft speed

Simulated drag curve for compound helicopter

Venn diagram of slowed rotor aircraft in high speed mode

For rotorcraft, advance ratio (or Mu, symbol

\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.Filippone, Antonio (2000). "Data and performances of selected aircraft and rotorcraft" pages 643-646. Department of Energy Engineering,

Technical University of Denmark The Technical University of Denmark ( da, Danmarks Tekniske Universitet), often simply referred to as DTU, is a polytechnic university and school of engineering. It was founded in 1829 at the initiative of Hans Christian Ørsted as Denmark's fir ...

/ Progress in Aerospace Sciences, Volume 36, Issue 8. Accessed: 21 May 2014.
Abstract
/ref> 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

\mu = \frac  = \frac

where Omega (Ω) is the rotor's

angular velocity In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...

, 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

on 21 October 2014. When the rotor blade is

perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...

to the aircraft and advancing, its tip airspeed V_t is the aircraft speed plus relative blade tip speed, or V_t=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 V_t=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 simulation Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...

s 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

.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. 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.

Fuel economy of slowed rotors

Conventional helicopters have constant-speed rotors and adjust lift by varying the blade angle of attack or

collective pitch A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor bl ...

. 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

.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 In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under ...

(L/D).

Early development

When

Juan de la Cierva Juan de la Cierva y Codorníu, 1st Count of la Cierva (; 21 September 1895 in Murcia, Spain – 9 December 1936 in Croydon, United Kingdom) was a Spanish civil engineer, pilot and a self taught aeronautical engineer. His most famous accomplish ...

developed the

through the 1920s and 30s, it was found that the tip speeds of the advancing rotor blade could become excessive. Designers such as he and

Harold F. Pitcairn Harold Frederick Pitcairn (June 20, 1897 – April 23, 1960) was an American aviation inventor and pioneer. He played a key role in the development of the autogyro and founded the Autogiro Company of America. He patented a number of innovation ...

developed the idea of adding a conventional wing to offload the rotor during high-speed flight, allowing it to rotate at slower speeds. The

Pitcairn PCA-2 The Pitcairn PCA-2 was an autogyro (designated as "autogiro" by Pitcairn) developed in the United States in the early 1930s.Taylor 1989, p.735 It was Harold F. Pitcairn's first autogyro design to sell in quantity. It had a conventional design fo ...

of 1932 had a maximum speed of 20-, ''μ'' = 0.7, and L/D = 4.8

NACA The National Advisory Committee for Aeronautics (NACA) was a United States federal agency founded on March 3, 1915, to undertake, promote, and institutionalize aeronautical research. On October 1, 1958, the agency was dissolved and its assets ...

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.

Postwar projects

Fairey Aviation The Fairey Aviation Company Limited was a British aircraft manufacturer of the first half of the 20th century based in Hayes in Middlesex and Heaton Chapel and RAF Ringway in Cheshire. Notable for the design of a number of important military ...

in the UK developed a series of experimental tip-jet driven

s in the late 1940s and 1950s. They culminated in the Fairey Rotodyne, a VTOL passenger aircraft with a single main rotor supplemented by wings and twin turboprop engines. In forward flight the power to the rotor was reduced to about 10%. .Anders, Frank. (1988)
The Fairey Rotodyne
(excerpt) ''Gyrodyne Technology (Groen Brothers Aviation)''. Retrieved: 17 January 2011
Archived
26 February 2014 0.6. 120 to 140 60% \ 40%.Braas, Nico.
Fairey Rotodyne
''Let Let Let Warplanes,'' 15 June 2008. Accessed: April 2014
Archived
on 30 September 2013 At the same time, the US Air Force was investigating fast VTOL aircraft. McDonnell developed what became the

McDonnell XV-1 The McDonnell XV-1 is an experimental Convertiplane developed by McDonnell Aircraft for a joint research program between the United States Air Force and the United States Army to explore technologies to develop an aircraft that could take off a ...

, the first of the V-designated types, which flew in 1955. It was a tip-jet driven gyrodyne, which turned off rotor thrust at high airspeeds and relied on a pusher propeller to maintain forward flight and rotor autorotation. Lift was shared between the rotor and stub wings. It established a rotorcraft speed record of . 0.95.Anderson, Rod.
The CarterCopter and its legacy
Issue 83, '' Contact Magazine'', 30 March 2006. Accessed: 11 December 2010
Mirror
!--primary source--> 180-410 (50%). 85% \ 15%. 6.5 (Wind tunnel tests at 180 RPM with no propeller.) The Lockheed AH-56 Cheyenne arose out of Lockheed's ongoing research programme into rigid rotors, which began with the CL-475 in 1959. Stub wings and a thrust turbojet to offload the rotor were first added to an XH-51A and in 1965 this allowed the craft to achieve a world speed record of . The Cheyenne flew just two years later, obtaining its forward thrust from a pusher propeller. However it did not enter production. . 0.8. .. \ 20%. The Piasecki 16H Pathfinder project similarly evolved an initially conventional design into a compound helicopter through the 1960s, culminating in the 16H-1A Pathfinder II which flew successfully in 1965. Thrust was obtained via a ducted fan at the tail. The Bell 533 of 1969 was a compound jet helicopter. . File:McDonnell_XV-1_on_the_ground_1954.jpg,

. Optionally powered rotor, pusher propeller, wings. File:In_conventional_flight_mode.jpg, Fairey Rotodyne. Optionally powered rotor, tractor propellers, wings. File:YAH-56A "Cheyenne" (front) 4017.jpg, Lockheed AH-56 Cheyenne. Powered rotor, pusher propeller, wings. File:Bell_533a.jpg, Bell 533. Powered rotor, jets, wings.

Modern developments

The compound helicopter has continued to be studied and flown experimentally. In 2010 the

flew with

coaxial rotor Coaxial rotors or coax rotors are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions (contra-rotating). This rotor configuration is a feature of helicopt ...

s. . 0.8. 360 to 446.Datta, page 2.Jackson, Dave.
Coaxial - Sikorsky ~ X2 TD
''Unicopter''. Accessed: April 2014. No wings.D. Walsh, S. Weiner, K. Arifian, T. Lawrence, M. Wilson, T. Millott and R. Blackwell.
High Airspeed Testing of the Sikorsky X2 Technology Demonstrator
'' Sikorsky'', May 4, 2011. Accessed: October 5, 2013. In 2013 the

Eurocopter X3 Airbus Helicopters SAS (formerly Eurocopter Group) is the helicopter manufacturing division of Airbus. It is the largest in the industry in terms of revenues and turbine helicopter deliveries. Its head office is located at Marseille Provence ...

flew. .Thivent, Viviane.
Le X3, un hélico à 472 km/h
''

Le Monde ''Le Monde'' (; ) is a French daily afternoon newspaper. It is the main publication of Le Monde Group and reported an average circulation of 323,039 copies per issue in 2009, about 40,000 of which were sold abroad. It has had its own website si ...

'', 11 June 2013. Accessed: 10 May 2014
Possible mirror
/ref>X3 Helicopter Sets Speed Record At Nearly 300 MPH

310 minus 15%.Nelms, Douglas.
Aviation Week Flies Eurocopter’s X3
''

Aviation Week & Space Technology ''Aviation Week & Space Technology'', often abbreviated ''Aviation Week'' or ''AW&ST'', is the flagship magazine of the Aviation Week Network. The weekly magazine is available in print and online, reporting on the aerospace, defense and aviatio ...

'', 9 July 2012. Accessed: 10 May 2014
Alternate linkArchived
on 12 May 2014 40-80% \.Norris, Guy.
Eurocopter X-3 Targets U.S. Market
''

Aviation Week ''Aviation Week & Space Technology'', often abbreviated ''Aviation Week'' or ''AW&ST'', is the flagship magazine of the Aviation Week Network. The weekly magazine is available in print and online, reporting on the aerospace, defense and aviatio ...

'', 28 February 2012. Accessed: 1 March 2012
Mirror
The compound autogyro, in which the rotor is supplemented by wings and thrust engine but is not itself powered, has also undergone further refinement by Jay Carter Jr. He flew his CarterCopter in 2005. . 1. 50%. By 2013 he had developed its design into a

personal air vehicle A personal air vehicle (PAV) is a proposed type of aircraft providing on-demand aviation services. The emergence of this alternative to traditional ground transport methods has been enabled by unmanned aerial vehicle technologies and electric p ...

, the Carter PAV. . 1.13. 105Warwick, Graham.
Carter Hopes To Demo SR/C Rotorcraft To Military
''Aviation Week'', 5 February 2014. Accessed: 19 May 2014
Archived on 19 May 2014
/ref> to 350.Moore, Jim.
Carter seeks factory
''

Aircraft Owners and Pilots Association The Aircraft Owners and Pilots Association (AOPA) is a Frederick, Maryland-based American non-profit political organization that advocates for general aviation. AOPA's membership consists mainly of general aviation pilots in the United St ...

'', 21 May 2015. Accessed: 28 May 2014
Archived
on 22 May 2015. The potential of the slowed rotor in enhancing fuel economy has also been studied in the Boeing A160 Hummingbird UAV, a conventional helicopter. . 140 to 350.Hambling, David.
The Rise of the Drone Helicopter - A160T Hummingbird
''

''. Accessed: April 2014. File:Boeing A160 Hummingbird VTOL-UAS.jpg, Boeing A160 Hummingbird
No wings, no propeller. File:Sikorsky X2 World Record Speed Demonstrator RSideFront MacDill AirFest 5Oct2011 (14513000689).jpg,

Powered rotor, pusher propeller, no wings. File:Eurocopter X3 F-ZXXX ILA 2012 2.jpg,

Powered rotor, tractor propellers, wings. File:CarterPAV.jpg, Carter PAV
Unpowered rotor, pusher propeller, wings.

References

Citations

Bibliography

* Berry, Ben & Chopra, Inderjit.
Slowed Rotor Wind Tunnel Testing at UMD
'

University of Maryland The University of Maryland, College Park (University of Maryland, UMD, or simply Maryland) is a public university, public Land-grant university, land-grant research university in College Park, Maryland. Founded in 1856, UMD is the Flagship un ...

, 19 February 2014. Size: 3MB. * Bowen-Davies, Graham M.
Performance and loads of variable tip speed rotorcraft at high advance ratios
' (dissertation)

, 25 June 2015
Header
DOI:10.13016/M2N62C . Size: 313 pages in 7MB * Datta, Anubhav et al. (2011).
Experimental Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios
' ''

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

'' ARC-E-DAA-TN3233, 2011
Header
Accessed: April 2014. Size: 26 pages in 2MB * DuBois, Cameron J. (2013).
Flow Control on an Airfoil Under Reversed Flow Conditions Using Nanosecond Dielectric Barrier Discharge Actuators
' (dissertation abstract) ''

Ohio State University The Ohio State University, commonly called Ohio State or OSU, is a public land-grant research university in Columbus, Ohio. A member of the University System of Ohio, it has been ranked by major institutional rankings among the best pu ...

''. Accessed: 4 December 2014. Size: 86 pages in 6MB * Floros, Matthew W. & Wayne Johnson (2004).
Stability Analysis of the Slowed-Rotor Compound Helicopter Configuration
' (1MB).

Defense Technical Information Center The Defense Technical Information Center (DTIC, pronounced "Dee-tick") is the repository for research and engineering information for the United States Department of Defense (DoD). DTIC's services are available to DoD personnel, federal govern ...

, 2004
Alternate version
8MB * Gustafson, F. B.
Effect on helicopter performance of modifications in profile-drag characteristics of rotor-blade airfoil sections
'

, August 1944. * Harris, Franklin D. (2003).
An Overview of Autogyros and the McDonnell XV–1 Convertiplane
'

/CR—2003–212799, 2003
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Size: 284 pages in 13MB * Harris, Franklin D. (2008).
Rotor Performance at High Advance Ratio: Theory versus Test
'

/CR—2008–215370, October 2008
HeaderMirror
Accessed: 13 April 2014. Size: 521 pages in 5MB * * * Khoshlahjeh, Maryam & Gandhi, Farhan (2013).
Helicopter Rotor Performance Improvement with RPM Variation and Chord Extension Morphing
' American Helicopter Society. Accessed: 9 June 2014. * Kottapalli, Sesi et al. (2012).
Performance and loads correlation of a UH-60A slowed rotor at high advance ratios
''

ARC-E-DAA-TN4610, June 2012
Header
Size: 30 pages in 2MB * Landis, Tony and Jenkins, Dennis R. ''Lockheed AH-56A Cheyenne – WarbirdTech Volume 27'', Specialty Press, 2000. . * Munson, Kenneth (1973); ''Helicopters: And Other Rotorcraft Since 1907'', London, Blandford, Revised edition 1973. * Rigsby, James Michael (2008).
Stability and control issues associated with lightly loaded rotors autorotating in high advance ratio flight
' (dissertation abstract)

Georgia Institute of Technology The Georgia Institute of Technology, commonly referred to as Georgia Tech or, in the state of Georgia, as Tech or The Institute, is a public research university and institute of technology in Atlanta, Georgia. Established in 1885, it is part of ...

, December 2008. Size: 166 pages in 3MB * Robb, Raymond L. (2006).
Hybrid helicopters: Compounding the quest for speed
', Vertiflite. Summer 2006. American Helicopter Society. Size: 25 pages in 2MB * Seddon, John M. (& Simon Newman).
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' John Wiley and Sons, 2011. * Silva, Christopher ; Yeo, Hyeonsoo ; Johnson, Wayne. (2010)
Design of a Slowed-Rotor Compound Helicopter for Future Joint Service Missions
'

ADA529322
Mirror
Size: 17 pages in 4MB * D. Walsh, S. Weiner, K. Arifian, T. Lawrence, M. Wilson, T. Millott and R. Blackwell.
High Airspeed Testing of the Sikorsky X2 Technology Demonstrator
' Sikorsky, May 4, 2011. Accessed: October 5, 2013. Size: 12 pages in 3MB *

External links

{{ external media , align=width=250px , image1
Some previous attempts at high-speed VTOL
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