Helicopter Rotor - Wikipedia, The Free Encyclopedia

Helicopter rotor

The rotor head of a Sikorsky S-92

Helicopter rotorFrom Wikipedia, the free encyclopedia

A helicopter main rotor or rotor system is thecombination of a rotary wing and a control system thatgenerates the aerodynamic lift force that supports theweight of the helicopter, and the thrust that counteractsaerodynamic drag in forward flight. Each main rotor ismounted on a vertical mast over the top of the helicopter,as opposed to a helicopter tail rotor, which connectsthrough a combination of drive shaft(s) and gearboxesalong the tail boom. A helicopter's rotor is generally madeof two or more rotor blades. The blade pitch is typicallycontrolled by a swashplate connected to the helicopterflight controls. Helicopters are one example ofrotary-wing aircraft.

Contents

1 History and development2 Design

2.1 Parts and functions2.2 Swash plate2.3 Fully articulated

2.3.1 Aircraft2.4 Rigid2.5 Semirigid

2.5.1 Flybar (stabilizer bar)2.5.2 Aircraft

2.6 Combination2.6.1 Aircraft

3 Rotor configurations3.1 Single main rotor

3.1.1 Tail rotor3.1.2 Ducted fan3.1.3 NOTAR3.1.4 Tip jets

3.2 Dual rotors (counterrotating)3.2.1 Tandem3.2.2 Coaxial3.2.3 Intermeshing3.2.4 Transverse

3.3 Quadcopter4 Blade design5 Limitations and hazards

5.1 Abrasion in sandy environments6 References7 External links

Helicopter rotor - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Helicopter_rotor

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A decorated Japanese taketombobamboo-copter. The toy consists ofa rotor attached to a stick.

Helicopter rotor of Engelbert Zaschka,German master engineer, 1927, imagefrom the German Federal Archives

History and development

See also: Bamboo-copter and Science and inventions of Leonardo da Vinci

The use of a rotor for vertical flight has existed since 400 BC in the formof the bamboo-copter, an ancient Chinese toy.[1][2] The bamboo-copteris spun by rolling a stick attached to a rotor. The spinning creates lift,and the toy flies when released.[1] The philosopher Ge Hong's book theBaopuzi (Master Who Embraces Simplicity), written around 317,describes the apocryphal use of a possible rotor in aircraft: "Some havemade flying cars [feiche 飛車] with wood from the inner part of thejujube tree, using ox-leather (straps) fastened to returning blades so as toset the machine in motion."[3] Leonardo da Vinci designed a machineknown as an "aerial screw" with a rotor based on a water screw. TheRussian polymath Mikhail Lomonosov developed a rotor based on theChinese toy. The French naturalist Christian de Launoy constructedhis rotor out of turkey feathers.[1] Sir George Cayley, inspired by theChinese toy in his childhood, created multiple vertical flightmachines with rotors made of tin sheets.[1] Alphonse Pénaud wouldlater develop coaxial rotor model helicopter toys in 1870, poweredby rubber bands. One of these toys, given as a gift by their father,would inspire the Wright brothers to pursue the dream of flight.[4]

Before development of powered helicopters in the mid 20th century,autogyro pioneer Juan de la Cierva researched and developed manyof the fundamentals of the rotor. Cierva is credited with successfuldevelopment of multi-bladed, fully articulated rotor systems. Thissystem, in its various modified forms, is the basis of most multi-bladed helicopter rotor systems.

In the 1930s, Arthur Young improved the stability of two-bladed rotor systems with the introduction of astabilizer bar. This system was used in several Bell and Hiller helicopter models. It is also used in manyremote control model helicopters.

Design

The helicopter rotor is powered by the engine, through the transmission, to the rotating mast. The mast is acylindrical metal shaft that extends upward from—and is driven by—the transmission. At the top of the mastis the attachment point for the rotor blades called the hub. The rotor blades are then attached to the hub.Main rotor systems are classified according to how the main rotor blades are attached and move relative tothe main rotor hub. There are three basic classifications: rigid, semirigid, or fully articulated, although somemodern rotor systems use an engineered combination of these classifications. The rotors are designed tooperate in a narrow range of RPM.[5][6][7][8][9]

Unlike the small diameter fans used in turbofan jet engines, the main rotor on a helicopter has a largediameter that lets it accelerate a large volume of air. This permits a lower downwash velocity for a givenamount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degreethan a small amount of air by a large degree,[10] a low disc loading (thrust per disc area) greatly increases theaircraft's energy efficiency, and this reduces the fuel use and permits reasonable range.[11][12]

Parts and functions


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The simple rotor of a Robinson R22

Robinson R44 rotor head

The simple rotor of a Robinson R22 showing (from the top):

The following are driven by the link rods from the rotating partof the swashplate.

Pitch hinges, allowing the blades to twist about the axisextending from blade root to blade tip.

Teeter hinge, allowing one blade to rise vertically while theother falls vertically. This motion occurs whenevertranslational relative wind is present, or in response to a cycliccontrol input.Scissor link and counterweight, carries the main shaft rotationdown to the upper swashplateRubber covers protect moving and stationary shaftsSwashplates, transmitting cyclic and collective pitch to theblades (the top one rotates)Three non-rotating control rods transmit pitch information tothe lower swashplateMain mast leading down to main gearbox

Swash plate

Main article: Swashplate (helicopter)

Controls vary the pitch of the main rotor blades cyclically throughout rotation. The pilot uses this to controlthe direction of the rotor thrust vector, which defines the part of the rotor disc where the maximum thrustdevelops. Collective pitch varies the magnitude of rotor thrust by increasing or decreasing thrust over thewhole rotor disc at the same time. These blade pitch variations are controlled by tilting, raising, or loweringthe swash plate with the flight controls. The vast majority of helicopters maintain a constant rotor speed(RPM) during flight, leaving the angle of attack of the blades as the sole means of adjusting thrust from therotor.

The swash plate is two concentric disks or plates. One plate rotates with the mast, connected by idle links,while the other does not rotate. The rotating plate is also connected to the individual blades through pitchlinks and pitch horns. The non-rotating plate is connected to links that are manipulated by pilot controls—specifically, the collective and cyclic controls. The swash plate can shift vertically and tilt. Throughshifting and tilting, the non-rotating plate controls the rotating plate, which in turn controls the individualblade pitch.

Fully articulated

Juan de la Cierva developed the fully articulating rotor for the autogyro. The basis of his design permittedsuccessful helicopter development. In a fully articulated rotor system, each rotor blade is attached to therotor hub through a series of hinges that let the blade move independently of the others. These rotor systemsusually have three or more blades. The blades are allowed to flap, feather, and lead or lag independently ofeach other. The horizontal hinge, called the flapping hinge, allows the blade to move up and down. Thismovement is called flapping and is designed to compensate for dissymmetry of lift. The flapping hinge maybe located at varying distances from the rotor hub, and there may be more than one hinge. The verticalhinge, called the lead-lag or drag hinge, allows the blade to move back and forth. This movement is calledlead-lag, dragging, or hunting. Dampers are usually used to prevent excess back and forth movement aroundthe drag hinge. The purpose of the drag hinge and dampers is to compensate for the acceleration anddeceleration caused by momentum conservation,[13] and not by Coriolis effect. Later models have switchedfrom using traditional bearings to elastomeric bearings.[citation needed] A variation of the fully articulatedsystem is the "soft-in-plane" rotor system. This type of rotor can be found on several aircraft produced by


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Semirigid rotor system

Bell Helicopter, such as the OH-58D Kiowa Warrior. This system is similar to the fully articulated type inthat each blade has the ability to lead/lag and hunt independent of the other blades. The difference betweena fully articulated system and soft-in-plane system is that the soft-in-plane system utilizes a composite yoke.This yoke is attached to the mast and runs through the blade grips between the blades and the shear bearinginside the grip. This yoke does transfer some movement of one blade to another, usually opposing blades.While this is not fully articulated, the flight characteristics are very similar and maintenace time and cost arereduced.

Aircraft

AgustaWestland AW109

Rigid

The term "rigid rotor" usually refers to a hingeless rotor system[14][15] with blades flexibly attached to thehub. Irven Culver of Lockheed developed one of the first rigid rotors, which was tested and developed on aseries of helicopters in the 1960s and 1970s. In a rigid rotor system, each blade flaps and drags about flexiblesections of the root. A rigid rotor system is mechanically simpler than a fully articulated rotor system. Loadsfrom flapping and lead/lag forces are accommodated through rotor blades flexing, rather than throughhinges. By flexing, the blades themselves compensate for the forces that previously required rugged hinges.The result is a rotor system that has less lag in control response, because the rotor has much lessoscillation.[16] The rigid rotor system also negates the danger of mast bumping inherent in teeteringrotors.[17] Example:MBB Bo 105

Semirigid

The semirigid rotor can also be referred to as a teetering or seesawrotor. This system is normally composed of two blades that meet justunder a common flapping or teetering hinge at the rotor shaft. Thisallows the blades to flap together in opposite motions like a seesaw.This underslinging of the blades below the teetering hinge, combinedwith an adequate dihedral or coning angle on the blades, minimizesvariations in the radius of each blade's center of mass from the axisof rotation as the rotor turns, which in turn reduces the stress on theblades from lead and lag forces caused by the Coriolis Effect.Secondary flapping hinges may also be provided to provide sufficientflexibility to minimize bouncing. Feathering is accomplished by thefeathering hinge at the blade root, which allows changes to the pitchangle of the blade.

Flybar (stabilizer bar)

A number of engineers, among them Arthur M. Young in the U.S. and Dieter Schlüter in Germany, found thatflight stability for helicopters could be achieved with a stabilizer bar or flybar. The flybar has a weight orpaddle (or both for added stability on smaller helicopters) at either end. These keep the bar relatively stablein the plane of rotation and reduce crosswind thrust on rotors. Through mechanical linkages, the stablerotation of the bar mixes with the swashplate movement to damp internal (steering) as well as external(wind) forces on the rotor. This makes it easier for the pilot to maintain control of the aircraft. Stanley Hillerarrived at a similar method to improve stability by adding short stubby airfoils, or paddles, at each end.However, Hiller's "Rotormatic" system also delivered cyclic control inputs to the main rotor as a sort ofcontrol rotor, and the paddles provided the added stability by dampening the effects of external forces on therotor.


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Antitorque: Torque effect on ahelicopter

The Lockheed rotor system used a control gyro, similar in principle to that of the Bell stabilizer bar, butdesigned for both hands-off stability and rapid control response of the hingeless rotor system.

In fly-by-wire helicopters or RC models, a microcontroller with gyroscope sensors and a Venturi sensor canreplace the stabilizer. This flybar-less design has the advantage of easy reconfiguration and fewermechanical parts.

Aircraft

Bell 206/OH-58Robinson R22

Combination

Modern rotor systems may use the combined principles of the rotor systems mentioned above. Some rotorhubs incorporate a flexible hub, which allows for blade bending (flexing) without the need for bearings orhinges. These systems, called "flexures",[18] are usually constructed from composite material. Elastomericbearings may also be used in place of conventional roller bearings. Elastomeric bearings are constructedfrom a rubber type material and provide limited movement that is perfectly suited for helicopter applications.Flexures and elastomeric bearings require no lubrication and, therefore, require less maintenance. They alsoabsorb vibration, which means less fatigue and longer service life for the helicopter components.

Aircraft

Bell 407Bell 430Eurocopter AS350

Rotor configurations

Most helicopters have a single main rotor but require a separate rotor to overcome torque. This isaccomplished through a variable-pitch antitorque rotor or tail rotor. This is the design that Igor Sikorskysettled on for his VS-300 helicopter, and it has become the recognized convention for helicopter design,although designs do vary. When viewed from above, the main rotors of helicopter designs from Germany,United Kingdom, The United States and Canada rotate counter-clockwise, while nearly all others rotateclockwise. This can make it difficult when discussing aerodynamic effects on the main rotor betweendifferent designs, since the effects may manifest on opposite sides of each aircraft.

Single main rotor

With a single main rotor helicopter, the creation of torque as theengine turns the rotor creates a torque effect that causes the body ofthe helicopter to turn in the opposite direction of the rotor. Toeliminate this effect, some sort of antitorque control must be usedwith a sufficient margin of power available to allow the helicopter tomaintain its heading and provide yaw control. The three mostcommon controls used today are the traditional tail rotor,Eurocopter's Fenestron (also called a fantail), and MD Helicopters'NOTAR.

Tail rotor


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Tail rotor of an SA 330 Puma

Fenestron on an EC 120B

Diagram showing the movement of airthrough the NOTAR system

Main article: Tail rotor

The tail rotor is a smaller rotor mounted so that it rotates vertically ornear-vertically at the end of the tail of a traditional single-rotorhelicopter. The tail rotor's position and distance from the center ofgravity allow it to develop thrust in a direction opposite of the mainrotor's rotation to counter the torque effect created by the main rotor.Tail rotors are simpler than main rotors since they require onlycollective changes in pitch to vary thrust. The pitch of the tail rotorblades is adjustable by the pilot via the anti-torque pedals, which alsoprovide directional control by allowing the pilot to rotate thehelicopter around its vertical axis, thereby changing the direction thecraft is pointed.

Ducted fan

Main article: Fenestron

Fenestron and FANTAIL[19] are trademarks for a ducted fanmounted at the end of the tail boom of the helicopter and used inplace of a tail rotor. Ducted fans have between eight and eighteenblades arranged with irregular spacing so that the noise is distributedover different frequencies. The housing is integral with the aircraftskin and allows a high rotational speed; therefore, a ducted fan canhave a smaller size than a conventional tail rotor.

The Fenestron was used for the first time at the end of the 1960s onthe second experimental model of Sud Aviation's SA 340 andproduced on the later model Aérospatiale SA 341 Gazelle. Besides Eurocopter and its predecessors, a ductedfan tail rotor was also used on the canceled military helicopter project, the United States Army's RAH-66Comanche, as the FANTAIL.

NOTAR

Main article: NOTAR

NOTAR, an acronym for NO TAil Rotor, is a helicopter anti-torquesystem that eliminates the use of the tail rotor on a helicopter.Although the concept took some time to refine, the NOTAR system issimple in theory and provides antitorque the same way a wingdevelops lift by using the Coandă effect.[20] A variable pitch fan isenclosed in the aft fuselage section immediately forward of the tailboom and is driven by the main rotor transmission. This fan forceslow pressure air through two slots on the right side of the tailboom,causing the downwash from the main rotor to hug the tailboom,producing lift and thus a measure of antitorque proportional to theamount of airflow from the rotorwash. This is augmented by a directjet thruster which also provides directional yaw control, and verticalstabilizers.

Development of the NOTAR system dates back to 1975 when engineers at Hughes Helicopters beganconcept development work.[20] In December 1981, Hughes flew an OH-6A fitted with NOTAR for the firsttime.[21] A more heavily modified prototype demonstrator first flew in March 1986 and successfully


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Boeing CH-47 Chinook

completed an advanced flight-test program, validating the system for future application in helicopterdesign.[22] There are currently three production helicopters that incorporate the NOTAR design, all producedby MD Helicopters. This antitorque design also improves safety by eliminating the possibility of personnelwalking into the tail rotor.

Tip jets

Main article: Tip jet

The main rotor may be driven by tip jets. Such a system may be powered by high pressure air provided by acompressor. The air may or may not be mixed with fuel and burnt in ram-jets, pulse-jets, or rockets. Thoughthis method is simple and eliminates torque reaction, prototypes that have been built are less fuel efficientthan conventional helicopters. Except for tip jets driven by unburnt compressed air, very high noise levels isthe single most important reason why tip jet powered rotors have not gained wide acceptance. However,research into noise suppression is ongoing and may help make this system viable.

There are several examples of tip jet powered rotorcraft. The Percival P.74 was under-powered and couldnot fly. The Hiller YH-32 Hornet had good lifting capability but performed poorly otherwise. Other aircraftused auxiliary thrust for translational flight so that the tip jets could be shut down while the rotorautorotated. The experimental Fairey Jet Gyrodyne, 48-seat Fairey Rotodyne passenger prototypes andMcDonnell XV-1 compound gyroplanes flew well using this method. Perhaps the most unusual design of thistype was the Rotary Rocket Roton ATV, which was originally envisioned to take off using a rocket-tippedrotor. The French Sud-Ouest Djinn used unburnt compressed air to drive the rotor, which minimized noiseand helped it become the only tip jet driven rotor helicopter to enter production.

Dual rotors (counterrotating)

Counterrotating rotors are rotorcraft configurations with a pair or more of large horizontal rotors that turn inopposite directions to counteract the torque effect on the aircraft without relying on an antitorque tail rotor.This lets the aircraft apply the power that would have driven a tail rotor to the main rotors, increasing liftingcapacity. Primarily, three common configurations use the counterrotating effect on rotorcraft. Tandem rotorsare two rotors—one mounted behind the other. Coaxial rotors are two rotors mounted one above the otheron the same axis. Intermeshing rotors are two rotors mounted close to each other at a sufficient angle to letthe rotors intermesh over the top of the aircraft. Another configuration—found on tiltrotors and some earlyhelicopters—is called transverse rotors, where a pair of rotors are mounted at each end of a wing-typestructure or outrigger.

Tandem

Main article: Tandem rotors

Tandem rotors are two horizontal main rotor assemblies mounted onebehind the other. Tandem rotors achieve pitch attitude changes toaccelerate and decelerate the helicopter through a process calleddifferential collective pitch. To pitch forward and accelerate, the rearrotor increases collective pitch, raising the tail, and the front rotordecreases collective pitch, simultaneously dipping the nose. To pitchupward while decelerating (or moving rearward), the front rotorincreases collective pitch to raise the nose and the rear rotordecreases collective pitch to lower the tail. Yaw control developsthrough opposing cyclic pitch in each rotor. To pivot right, the frontrotor tilts right and the rear rotor tilts left. To pivot left, the front rotortilts left and the rear rotor tilts right. All rotor power contributes to lift, and it is simpler to handle changes in


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Kamov Ka-50 of the Russian AirForce, with coaxial rotors

HH-43 Huskie

Mi-12

the center of gravity fore-aft. However, it requires the expense of two large rotors rather than the morecommon one large main rotor and a much smaller tail rotor. The Boeing CH-47 Chinook is the most commontandem rotor helicopter.

Coaxial

Main article: Coaxial rotors

Coaxial rotors are a pair of rotors mounted one above the other onthe same shaft and turning in opposite directions. The advantage ofthe coaxial rotor is that, in forward flight, the lift provided by theadvancing halves of each rotor compensates for the retreating half ofthe other, eliminating one of the key effects of dissymmetry of lift:retreating blade stall. However, other design considerations plaguecoaxial rotors. There is an increased mechanical complexity of therotor system because it requires linkages and swashplates for tworotor systems. Also, because the rotors must rotate in oppositedirections, the mast is more complex, and control linkages for pitch

changes to the upper rotor system must pass through the lower rotor system.

Intermeshing

Main article: Intermeshing rotors

Intermeshing rotors on a helicopter are a set of two rotors turning inopposite directions with each rotor mast mounted on the helicopterwith a slight angle to the other so that the blades intermesh withoutcolliding. This configuration is sometimes referred to as asynchropter. Intermeshing rotors have high stability and powerfullifting capability. The arrangement was successfully used in NaziGermany for a small anti-submarine warfare helicopter, the FlettnerFl 282 Kolibri. During the Cold War, an American company, KamanAircraft, produced the HH-43 Huskie for USAF firefighting andrescue missions. The latest Kaman model, the Kaman K-MAX, is adedicated sky crane design.

Transverse

Transverse rotors are mounted on the end of wings or outriggersperpendicular to the body of the aircraft. Similar to tandem rotorsand intermeshing rotors, the transverse rotor also uses differentialcollective pitch. But like the intermeshing rotors, the transverserotors use the concept for changes in the roll attitude of therotorcraft. This configuration is found on two of the first viablehelicopters, the Focke-Wulf Fw 61 and the Focke-Achgelis Fa 223,as well as the world's largest helicopter ever built, the Mil Mi-12. It isalso the configuration found on tiltrotors such as the Bell-BoeingV-22 Osprey and the AgustaWestland AW609.

Quadcopter

Main article: Quadcopter


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de Bothezat helicopter, 1923 photo

A quadcopter has four rotors in an "X" configuration designated asfront-left, front-right, rear-left, and rear-right. Rotors to the left andright are in a transverse configuration while those in the front and tothe rear are in a tandem configuration.

The main attraction of quadcopters is their mechanical simplicity,since a quadcopter using electric motors and fixed-pitch rotors hasonly four moving parts.[23]

Blade design

The blades of a helicopter are long, narrow airfoils with a high aspect ratio, a shape that minimizes drag fromtip vortices (see the wings of a glider for comparison). They generally contain a degree of washout thatreduces the lift generated at the tips, where the airflow is fastest and vortex generation would be a significantproblem. Rotor blades are made out of various materials, including aluminium, composite structure, and steelor titanium, with abrasion shields along the leading edge.

Rotorcraft blades are traditionally passive, but research into active blade control through trailing edge flapsis underway.[24] Tips of some helicopter blades can be specially designed to reduce turbulence and noise andto provide more efficient flying. An example of such tips are the tips of the BERP rotor.

Limitations and hazards

Helicopters with teetering rotors—for example the two-blade system on the Bell, Robinson andothers—must not be subjected to a low-g condition because such rotor systems do not control the fuselageattitude. This can result in the fuselage assuming an attitude controlled by momentum and tail rotor thrustthat causes the tail boom to intersect the main rotor tip-path plane or result in the blade roots contacting themain rotor drive shaft, causing the blades to separate from the hub (mast bumping).[25]

Abrasion in sandy environments

See also: Brownout (aviation) — Sensory illusions

When operating in sandy environments, sand hitting the moving rotor blades erodes their surface. This candamage the rotors and presents serious and costly maintenance problems.[26]

Abrasion strips on helicopter rotor blades are made of metal, often titanium or nickel, which are very hard,but less hard than sand. When a helicopter flies low to the ground in desert environments, sand striking therotor blade can cause erosion. At night, sand hitting the metal abrasion strip causes a visible corona or haloaround the rotor blades. The effect is caused by the pyrophoric oxidation of eroded particles.[27][28]

In 2009, war correspondent Michael Yon referred to this corona effect as the "Kopp-Etchells effect" tohonor Cpl. Benjamin Kopp and Cpl. Joseph Etchells, recently fallen American and British soldiers,respectively.[29]

References

^ a b c d Leishman, J. Gordon. Principles of Helicopter Aerodynamics. Cambridge aerospace series, 18.Cambridge: Cambridge University Press, 2006. ISBN 978-0-521-85860-1. pp. 7-9. Web extract(http://terpconnect.umd.edu/~leishman/Aero/history.html)

1.

^ Taking Flight: Inventing the Aerial Age, from Antiquity Through the First World War2.


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(http://books.google.com/books?id=YRqV_PayIKIC&pg=PA22). Oxford University Press. 8 May 2003.pp. 22–23. ISBN 978-0-19-516035-2.^ Joseph Needham (1965), Science and civilisation in China: Physics and physical technology, mechanicalengineering Volume 4, Part 2, page 583-587.

3.

^ John D. Anderson (2004). Inventing Flight: The Wright Brothers & Their Predecessors(http://books.google.com/books?id=IYnl_XPggZYC&pg=PA353). JHU Press. p. 35. ISBN 978-0-8018-6875-7.

4.

^ Croucher, Phil. Professional helicopter pilot studies (http://books.google.com/books?id=AovdKRWSqJAC&printsec=frontcover&dq=%22Professional+Helicopter+Pilot+Studies%22&hl=da&ei=LYZ4TdmcDMjRsgbj56TyBw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CD8Q6AEwAA#v=onepage&q&f=true) page 2-11. ISBN 978-0-9780269-0-5. Quote: [Rotor speed] "isconstant in a helicopter".

5.

^ Johnson, Pam. Delta D2 (http://www.michaeljohnsonmp.com/pdf/Pacific_wings_P42-49_Delta_v4_-_bill_whitney.pdf) page 44 Pacific Wings. Accessed: 2 January 2010

6.

^ "Helicopters." (http://www.helicoptervietnam.com/history.htm) Helicopter Vietnam. Retrieved: 16 February2011.

7.

^ The UH-60 permits 95–101% rotor RPM UH-60 limits (http://www.usarmyaviation.com/studyguides/index.php?folder=Documents/UH-60BlackhawkSpecific&download=Uh60limits.doc) US Army Aviation. Accessed: 2 January 2010

8.

^ John M. Seddon, Simon Newman. Basic Helicopter Aerodynamics (http://books.google.dk/books?id=X_X3nOODGLgC&printsec=frontcover&hl=da) p216, John Wiley and Sons, 2011. Accessed: 25February 2012. ISBN 1-119-99410-1. Quote: The rotor is best served by rotating at a constant rotor speed

9.

^ Paul Bevilaqua : The shaft driven Lift Fan propulsion system for the Joint Strike Fighter (http://www.dtic.mil/dticasd/sbir/sbir032/n184.doc) page 3. Presented May 1, 1997. DTIC.MIL Word document, 5.5 MB. Accessed:25 February 2012.

10.

^ Johnson, Wayne. Helicopter theory (http://books.google.dk/books?id=SgZheyNeXJIC&hl=da&source=gbs_navlinks_s) pp3+32, Courier Dover Publications, 1980. Accessed: 25 February 2012. ISBN0-486-68230-7

11.

^ Wieslaw Zenon Stepniewski, C. N. Keys. Rotary-wing aerodynamics (http://books.google.dk/books?id=PawbFeAAllIC&printsec=frontcover&hl=da) p3, Courier Dover Publications, 1979. Accessed: 25February 2012. ISBN 0-486-64647-5

12.

^ Rotary Wing Forum [1] (http://www.rotaryforum.com/forum/showthread.php?t=961&page=2&highlight=Coriolis). Retrieved: 28 September 2010.

13.

^ Landis, Tony and Jenkins, Dennis R. Lockheed AH-56A Cheyenne - WarbirdTech Volume 27, p.5. SpecialtyPress, 2000. ISBN 1-58007-027-2.

14.

^ Model 286 (http://www.aeroengineer.net/history/lockheed/lockheed12.html)15.^ Connor, R. Lockheed CL-475" (http://web.archive.org/web/20080101194953/http://www.nasm.si.edu/research/aero/aircraft/lockheed_cl475.htm). Smithsonian National Air & Space Museum. Revised on 15 August 2002.Accessed at archive.org on 3 September 2007. original link (http://www.nasm.si.edu/aircraft/lockheed_cl475.htm).

16.

^ Cox, Taylor. "Blades and Lift" (http://www.webcitation.org/5NFat7fwZ). Helis.com. Retrieved: 10 March2007.

17.

^ FAA Flight Standards Service 200118.^ Alpman, Emre and Long, Lyle N. "Understanding Ducted-Rotor Antitorque and Directional Control:Characteristics Part II: Unsteady Simulations." (http://www.kothmann.com/Papers/AIAAJournalAircraftPart2.pdf) Journal of Aircraft Vol. 41, No. 6, November–December 2004.

19.

^ a b Frawley 2003, p. 151.20.^ "NOTAR Fleet Marks 500,000 Flight Hours" (http://www.webcitation.org/5MvjUcNbi). American HelicopterSociety. Retrieved: 25 February 2007.

21.

^ "The Boeing Logbook: 1983-1987" (http://www.webcitation.org/5Mvnymw0Y). Boeing.com. Retrieved: 25February 2007.

22.

^ Markus Waibel. "Quadcopter, Hexacopter, Octocopter ... UAVs" (http://spectrum.ieee.org/automaton/robotics/robotics-software/quadcopter-hexacopter-octocopter-uavs). IEEE Spectrum, 2010-02-19.

23.

^ Mangeot et al. New actuators for aerospace (http://www.noliac.com/Files/Billeder/05%20News/Diamond_actuator_for_aerospace.pdf) Noliac. Retrieved: 28 September 2010.

24.

^ Rotorcraft Flying Handbook (http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf). U.S.Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2000. pp. 11–10.ISBN 1-56027-404-2. FAA-8083-21.

25.

^ Jim Bowne, Public Affairs Office, U.S. Army Aviation and Missile Command (February 2004). "These bootsare made for flying: Rotor blades get new protective shields" (http://www.rdecom.army.mil/rdemagazine/200402/itf_amrdec_boots.html). RDECOM Magazine. U.S. Army Research, Development and Engineering Command

26.


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(Provisional). Retrieved 2009-09-04. "The 'corona effect' is characterized by distinctive glowing rings alongmetal or fiberglass rotor blades operating in desert conditions."^ Warren (Andy) Thomas; Shek C. Hong;, Chin-Jye (Mike) Yu, Edwin L. Rosenzweig (2009-05-27). "EnhancedErosion Protection for Rotor Blades: Paper presented at the American Helicopter Society 65th Annual Forum,Grapevine, Texas, May 27 – 29, 2009." (http://www.vtol.org/f65_bestPapers/productSupport.pdf) (pdf).American Helicopter Society. Retrieved 2009-09-02. "A secondary concern with the erosion of metal abrasionstrips pertains to the visible signature that occurs ... causing a corona effect in sandy environments."

27.

^ Office of Naval Research Broad Agency Announcement(BAA): Advanced Helicopter Rotor Blade ErosionProtection (http://www.onr.navy.mil/02/baa/docs/BAA%2008-011_ONRBAA%2008-011.pdf) (pdf). UnitedStates Department of the Navy, Office of Naval Research. p. 3. BAA 08-011. Retrieved 2009-09-02. "Anequally important problem with Ti protection is that a visible corona or halo is generated around the rotor bladesat night from the sand impacting the Ti leading edge and causing Ti to spark and oxidize."

28.

^ "The Kopp-Etchells Effect" (http://www.michaelyon-online.com/the-kopp-etchells-effect.htm). Retrieved2009-09-02.

29.

External links

Rotor Analysis - Blade Element Momentum Theory (http://www.aerodyndesign.com/ANALYSIS/ANALYSIS.htm)Helicopter Rotorhead Close-up Image Gallery (http://www.b-domke.de/AviationImages/Rotorhead.html)"Helicopter Aircraft" (http://www.google.com/patents?id=PYReAAAAEBAJ&pg=PA1&source=gbs_selected_pages&cad=2#v=onepage&q=&f=false). US Patent 2,368,698, for flybarinvention, by Arthur Young"The Kopp-Etchells Effect" (http://www.michaelyon-online.com/the-kopp-etchells-effect.htm).Dispatch by Michael Yon, war correspondent

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