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Helicopter rotor The rotor head of a Sikorsky S-92 Helicopter rotor From Wikipedia, the free encyclopedia A helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight. Each main rotor is mounted on a vertical mast over the top of the helicopter, as opposed to a helicopter tail rotor, which is connected through a combination a drive shaft(s) and gearboxes along the tail boom. A helicopter's rotor is generally made up of two or more rotor blades. The blade pitch is typically controlled by a swashplate connected to the helicopter flight controls. Helicopter rotor diameters are relatively large, as this gives much better energy and propellant efficiency for the speeds at which helicopters fly. Contents 1 History and development 2 Design 2.1 Parts and functions 2.2 Swash plate 2.3 Fully articulated 2.4 Rigid 2.5 Semirigid 2.5.1 Stabilizer bar 2.6 Combination 3 Rotor configurations 3.1 Single main rotor 3.1.1 Tail rotor 3.1.2 Ducted fan 3.1.3 NOTAR 3.1.4 Tip jets 3.2 Dual rotors (counterrotating) 3.2.1 Tandem 3.2.2 Coaxial 3.2.3 Intermeshing 3.2.4 Transverse 3.2.5 Quadrotor Helicopter rotor - Wikipedia, the free encyclopedia http://en.wikipedia.org/w/index.php?title=Helicopter_rot... 1 of 12 31.03.11 21:00

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Helicopter rotor

The rotor head of a Sikorsky S-92

Helicopter rotorFrom Wikipedia, the free encyclopedia

A helicopter main rotor or rotor system isa type of fan that is used to generate boththe aerodynamic lift force that supports theweight of the helicopter, and thrust whichcounteracts aerodynamic drag in forwardflight. Each main rotor is mounted on avertical mast over the top of the helicopter,as opposed to a helicopter tail rotor, which isconnected through a combination a driveshaft(s) and gearboxes along the tail boom.A helicopter's rotor is generally made up oftwo or more rotor blades. The blade pitch istypically controlled by a swashplateconnected to the helicopter flight controls.

Helicopter rotor diameters are relativelylarge, as this gives much better energy and propellant efficiency for the speeds at whichhelicopters fly.

Contents1 History and development2 Design

2.1 Parts and functions2.2 Swash plate2.3 Fully articulated2.4 Rigid2.5 Semirigid

2.5.1 Stabilizer bar

2.6 Combination

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 Transverse3.2.5 Quadrotor

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Helicopter rotor of EngelbertZaschka, German masterengineer, 1931, image from theGerman Federal Archives

4 Blade design5 Limitations and hazards

5.1 Abrasion in sandy environments

6 References7 External links

History and developmentBefore the development of powered helicopters in themid 20th century, autogyro pioneer Juan de la Ciervaresearched and developed many of the fundamentals ofthe rotor. Cierva is credited with successfuldevelopment of multi-bladed, fully articulated rotorsystems. This type of system is widely used today inmany multi-bladed helicopters.

In the 1930s, Arthur Young improved the stability oftwo-bladed rotor systems with the introduction of astabilizer bar. This system was used in several Bell andHiller helicopter models. It is also used in many remotecontrol model helicopters.

DesignA helicopter rotor is powered by the engine, through the transmission, to the rotating mast.The mast is a cylindrical metal shaft which extends upward from—and is driven by—thetransmission. At the top of the mast is the attachment point for the rotor blades called thehub. The rotor blades are then attached to the hub. Main rotor systems are classifiedaccording to how the main rotor blades are attached and move relative to the main rotorhub. There are three basic classifications: rigid, semirigid, or fully articulated, althoughsome modern rotor systems use an engineered combination of these classifications.

Unlike the small diameter fans used in turbofan jet engines, the main rotor on a helicopterhas a quite large diameter, permitting a large volume of air to be accelerated. This permits alower downwash velocity for a given amount of thrust. As it is more efficient at low speedsto accelerate a large amount of air by a small degree than a small amount of air by a largedegree it greatly increases the aircraft's energy efficiency and this reduces the fuel use andpermits reasonable range.

Parts and functions

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The simple rotor of a RobinsonR22

Robinson R44 rotor head

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

The following are driven by the link rods from therotating part of the swashplate.

Pitch hinges, allowing the blades to twistabout the axis extending from blade root toblade tip.

Teeter hinge, allowing one blade to rise verticallywhile the other falls vertically. This motion occurswhenever translational relative wind is present, orin response to a cyclic control input.Scissor link and counterweight, carries the mainshaft rotation down to the upper swashplateRubber covers protect moving and stationaryshaftsSwashplates, transmitting cyclic and collectivepitch to the blades (the top one rotates)Three non-rotating control rods transmit pitchinformation to the lower swashplateMain mast leading down to main gearbox

Swash plate

Main article: Swashplate (helicopter)

The pitch of main rotor blades can be varied cyclically throughout its rotation in order tocontrol the direction of rotor thrust vector (the part of the rotor disc where the maximumthrust will be developed, front, rear, right side, etc.). Collective pitch is used to vary themagnitude of rotor thrust (increasing or decreasing thrust over the whole rotor disc at thesame time). These blade pitch variations are controlled by tilting and/or raising or loweringthe swash plate with the flight controls. The vast majority of helicopters maintain a constantrotor speed (RPM) during flight, leaving only the angle of attack of the blades as the solemeans of adjusting thrust from the rotor.

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 connectedto the individual blades through pitch links and pitch horns. The non-rotating plate isconnected to links which are manipulated by pilot controls, specifically, the collective andcyclic controls.

The swash plate can shift vertically and tilt. Through shifting and tilting, the non-rotatingplate controls the rotating plate, which in turn controls the individual blade pitch.

Fully articulated

Juan de la Cierva developed the fully articulating rotor for the autogyro, and it is the basis ofhis design that permitted successful helicopter development. In a fully articulated rotorsystem, each rotor blade is attached to the rotor hub through a series of hinges which allow

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the blade to move independently of the others. These rotor systems usually have three ormore blades. The blades are allowed to flap, feather, and lead or lag independently of eachother. The horizontal hinge, called the flapping hinge, allows the blade to move up anddown. This movement is called flapping and is designed to compensate for dissymmetry oflift. The flapping hinge may be located at varying distances from the rotor hub, and theremay be more than one hinge. The vertical hinge, called the lead-lag or drag hinge, allowsthe blade to move back and forth. This movement is called lead-lag, dragging, or hunting.Dampers are usually used to prevent excess back and forth movement around the draghinge. The purpose of the drag hinge and dampers is to compensate for the accelerationand deceleration caused by momentum conservation,[1] and not by Coriolis Effect. Eachblade can also be feathered, that is, rotated around its spanwise axis. Feathering the blademeans changing the pitch angle of the blade. By changing the pitch angle of the blades thethrust and direction of the main rotor disc can be controlled. An example of this type of rotorsystem is the Agusta AW109 series of aircraft; later models have switched from a traditionalbearing system to an Elastomeric bearing based system.

Rigid

The term "rigid rotor" usually refers to a hingeless rotor system with blades flexibly attachedto the hub. The two basic types of rigid rotor include the Reiseler-Kreiser feathering systemand the Lockheed flapping system. The Reiseler-Kreiser feathering rigid rotor wasdeveloped and tested on a series of gyroplanes sponsored by E.B. Wilford in Pennsylvania.Irven Culver of Lockheed developed one of the first flapping rigid rotors and was tested anddeveloped on a series of helicopters in the 1960s and 1970s. In a flapping rigid rotorsystem, each blade flaps, drags, and feathers (depending on the design) about flexiblesections of the root. The flapping rigid rotor system is mechanically simpler than the fullyarticulated rotor system. Loads from flapping and lead/lag forces are accommodated bybending rather than through hinges. By flexing, the blades themselves compensate for theforces which previously required rugged hinges. The result is a rotor system that has lesslag in the control response, because the rotor has much less oscillation.[2] The rigid rotorsystem also negates the danger of mast bumping inherent in semi-rigid rotors.[3] The rigidrotor can also be called a hingeless rotor.[4][5] Developed most notably for the XH-51 highspeed and AH-56 Cheyenne attack compound helicopter, the rotors simplified aerobaticmaneuvers at high speeds, but proved troublesome to perfect on the AH-56, and wouldnever be produced in large numbers or adopted by other helicopter makers.

However, to completely contradict the previous statement, flapping rigid rotors have longbeen standard equipment on the Bolkow series of helicopters, as well as models producedby Aerospatiale, AgustaWestland, and MD helicopters.

Semirigid

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Semirigid rotor system

The semirigid rotor can also be referred to as ateetering or seesaw rotor. This system is normallycomposed of two blades which meet just under acommon flapping, or teetering hinge at the rotor shaft.This allows the blades to flap together in oppositemotions like a seesaw. This underslinging of the bladesbelow the teetering hinge, combined with an adequatedihedral or coning angle on the blades, minimizesvariations in the radius of each blade's center of massfrom the axis of rotation as the rotor turns, which in turnreduces the stress on the blades from lead and lagforces caused by coriolis effect. Secondary flappinghinges may also be provided to provide sufficient flexibility to minimize bouncing.Feathering is accomplished by the feathering hinge at the blade root, which allows changesto the pitch angle of the blade. The most widespread implementations of this system are theBell 206/OH-58 series of aircraft and the Robinson R22 series.

Stabilizer bar

A number of engineers, among them Arthur M. Young in the U.S., and Dieter Schlüter inGermany, found that flight stability for helicopters could be achieved with a stabilizer bar orflybar. The stabilizer bar has weighted ends which cause the bar to stay relatively stable inthe plane of rotation. Through mechanical linkages, the stable rotation of the bar is mixedwith the swashplate movement so that internal (steering) as well as external (wind) forceson the rotor are dampened. This eases the workload of the pilot to maintain control of theaircraft. Stanley Hiller arrived at a similar method to improve stability by adding short stubbyairfoils, or paddles, at each end; However, Hiller's "Rotormatic" system was also used todeliver cyclic control inputs to the main rotor as a sort of control rotor, the paddles providedthe added stability by dampening the effects of external forces on the rotor.

In fly-by-wire helicopters or RC models, a microcontroller with gyroscopes and a venturisensor can replace the stabilizer. This flybar-less design has the advantage of easyreconfiguration and fewer mechanical parts.

Combination

Modern rotor systems may use the combined principles of the rotor systems mentionedabove. Some rotor hubs incorporate a flexible hub, which allows for blade bending (flexing)without the need for bearings or hinges. These systems, called "flextures",[6] are usuallyconstructed from composite material. Elastomeric bearings may also be used in place ofconventional roller bearings. Elastomeric bearings are bearings constructed from a rubbertype material and have limited movement that is perfectly suited for helicopter applications.Flextures and elastomeric bearings require no lubrication and, therefore, require lessmaintenance. They also absorb vibration, which means less fatigue and longer service lifefor the helicopter components. Examples include Bell 407, Bell 430, EurocopterA-Star(AS350)/Twin-Star(AS355) and arguably MD Helicopters (Formerly Hughes 500), thismodel has externally mounted lead-lag dampeners {which makes it more of a hingelessfully articulated hub}.

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Antitorque: Torque effect on ahelicopter

Tail rotor of an SA 330 Puma

Rotor configurationsMost helicopters have a single, main rotor but require a separate rotor to overcome torque.This is accomplished through a variable pitch, antitorque rotor or tail rotor. This is the designthat Igor Sikorsky settled on for his VS-300 helicopter and it has become the recognizedconvention for helicopter design, although designs do vary. When viewed from above, themain rotors of helicopter designs from Germany, United Kingdom and the United Statesrotate counter-clockwise, all others rotate clockwise. This can make it difficult whendiscussing aerodynamic effects on the main rotor between different designs, since theeffects may manifest on opposite sides of each aircraft.

Single main rotor

With a single main rotor helicopter, the creation oftorque as the engine turns the rotor creates a torqueeffect that causes the body of the helicopter to turn inthe opposite direction of the rotor. To eliminate thiseffect, some sort of antitorque control must be used,with a sufficient margin of power available to allow thehelicopter to maintain its heading and provide yawcontrol. The three most common controls used todayare the traditional tail rotor, Eurocopter's Fenestron(also called a fantail), and MD Helicopters' NOTAR.

Tail rotor

Main article: Tail rotor

The tail rotor is a smaller rotor mounted so that itrotates vertically or near-vertically at the end of the tailof a traditional single-rotor helicopter. The tail rotor'sposition and distance from the center of gravity allow itto develop thrust in a direction opposite of the mainrotor's rotation, to counter the torque effect created bythe main rotor. Tail rotors are simpler than main rotorssince they require only collective changes in pitch tovary thrust. The pitch of the tail rotor blades isadjustable by the pilot via the anti-torque pedals, whichalso provide directional control by allowing the pilot torotate the helicopter around its vertical axis (thereby

changing the direction the craft is pointed).

Ducted fan

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Fenestron on a EC 120B

Diagram showing the movementof air through the NOTAR system

Main article: Fenestron

Fenestron and FANTAIL[7] are trademarks for a ductedfan mounted at the end of the tail boom of the helicopterand used in place of a tail rotor. Ducted fans havebetween eight and 18 blades arranged with irregularspacing, so that the noise is distributed over differentfrequencies. The housing is integral with the aircraftskin and allows a high rotational speed, therefore aducted fan can have a smaller size than a conventionaltail rotor.

The Fenestron was used for the first time at the end of the 1960s on the secondexperimental model of Sud Aviation's SA 340, and produced on the later modelAérospatiale SA 341 Gazelle. Besides Eurocopter and its predecessors, a ducted fan tailrotor was also used on the canceled military helicopter project, the United States Army'sRAH-66 Comanche, as the FANTAIL.

NOTAR

Main article: NOTAR

NOTAR, an acronym for NO TAil Rotor, is a helicopteranti-torque system that eliminates the use of the tailrotor on a helicopter. Although the concept took sometime to refine, the NOTAR system is simple in theoryand works to provide antitorque the same way a wingdevelops lift using the Coandă effect.[8] A variable pitchfan is enclosed in the aft fuselage section immediatelyforward of the tail boom and driven by the main rotortransmission. This fan forces low pressure air throughtwo slots on the right side of the tailboom, causing thedownwash from the main rotor to hug the tailboom,producing lift, and thus a measure of antitorqueproportional to the amount of airflow from the rotorwash. This is augmented by a direct jetthruster (which also provides directional yaw control) and vertical stabilizers.

Development of the NOTAR system dates back to 1975 when engineers at HughesHelicopters began concept development work.[8] In December 1981 Hughes flew a OH-6Afitted with NOTAR for the first time.[9] A more heavily modified prototype demonstrator firstflew in March 1986 and successfully completed an advanced flight-test program, validatingthe system for future application in helicopter design.[10] There are currently threeproduction helicopters that incorporate the NOTAR design, all produced by MD Helicopters.This antitorque design also improves safety by eliminating the possibility of personnelwalking into the tail rotor.

Tip jets

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Boeing CH-47 Chinook

Main article: Tip jet

Another single main rotor configuration without a tail rotor is the tip jet rotor, where the mainrotor is not driven by the mast, but from nozzles on the rotor blade tips; which are eitherpressurized from a fuselage-mounted gas turbine or have their own turbojet, ramjet orrocket thrusters. Although this method is simple and eliminates torque, the prototypes thathave been built are less fuel efficient than conventional helicopters and produced morenoise. The Percival P.74 was underpowered and was not able to achieve flight, while theHiller YH-32 Hornet had good lifting capability but performed poorly otherwise. Other aircraftrelied on supplemental thrust so that the tipjets could be shut down and the rotor couldautorotate after the fashion of an autogyro. The experimental Fairey Jet Gyrodyne and40-seat Fairey Rotodyne passenger prototype were evaluated to have flown very well usingthis method. Perhaps the most unusual design of this type was the Rotary Rocket RotonATV, which was originally envisioned to take off utilizing a rocket-tipped rotor. No tip jetrotorcraft have ever entered into production.

Dual rotors (counterrotating)

Counterrotating rotors are rotorcraft configurations with a pair or more of large horizontalrotors turning in opposite directions to counteract the effects of torque on the aircraft withoutrelying on an antitorque tail rotor. This allows the power normally required to drive the tailrotor to be applied to the main rotors, increasing the aircraft's lifting capacity. Primarily, thereare three common configurations that use the counterrotating effect to benefit the rotorcraft.Tandem rotors are two rotors with one mounted behind the other. Coaxial rotors are tworotors that are mounted one above the other with the same axis. Intermeshing rotors aretwo rotors that are mounted close to each other at a sufficient angle to allow the rotors tointermesh over the top of the aircraft. Another configuration found on tiltrotors and someearlier helicopters is called transverse rotors where the pair of rotors are mounted at eachend of wing-type structures or outriggers.

Tandem

Main article: Tandem rotors

Tandem rotors are two horizontal main rotor assembliesmounted one behind the other. Tandem rotors achievepitch attitude changes to accelerate and decelerate thehelicopter through a process called differential collectivepitch. To pitch forward and accelerate, the rear rotorincreases collective pitch, raising the tail and the frontrotor decreases collective pitch, simultaneously dippingthe nose. To pitch upward while decelerating (or movingrearward), the front rotor increases collective pitch toraise the nose and the rear rotor decreases collectivepitch to lower the tail. Yaw control is developed through opposing cyclic pitch in each rotor;to pivot right, the front rotor tilts right and the rear rotor tilts left, and to pivot left, the frontrotor tilts left and the rear rotor tilts right. All of the rotor power contributes to lift, and it issimpler to handle changes in the center of gravity fore-aft. However, it requires the expenseof two large rotors rather than the more common one large main rotor and a much smaller

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

HH-43 Huskie

tail rotor. The Boeing CH-47 Chinook is the most common tandem rotor helicopter today.

Coaxial

Main article: Coaxial rotors

Coaxial rotors are a pair of rotors mounted one abovethe other on the same shaft and turning in oppositedirections. The advantage of the coaxial rotor is that, inforward flight, the lift provided by the advancing halvesof each rotor compensates for the retreating half of theother, eliminating one of the key effects of dissymmetryof lift: retreating blade stall. However, other designconsiderations plague coaxial rotors. There is anincreased mechanical complexity of the rotor systembecause it requires linkages and swashplates for tworotor systems. Add that each rotor system needs to be turned in opposite directions meansthat the mast itself is more complex, and provisions for making pitch changes to the upperrotor system must pass through the lower rotor system.

Intermeshing

Main article: Intermeshing rotors

Intermeshing rotors on a helicopter are a set of tworotors turning in opposite directions, with each rotormast mounted on the helicopter with a slight angle tothe other so that the blades intermesh without colliding.This configuration is sometimes referred to as asynchropter. Intermeshing rotors have high stability andpowerful lifting capability. The arrangement wassuccessfully used in Nazi Germany for a smallanti-submarine warfare helicopter, the Flettner Fl 282Kolibri. During the Cold War, the American company,Kaman Aircraft produced the HH-43 Huskie for theUSAF firefighting and rescue missions. The latest Kaman model, the Kaman K-MAX, is adedicated sky crane design.

Transverse

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Mi-12

De Bothezat Quadrotor, 1923

Transverse rotors are mounted on the end of wings oroutriggers, perpendicular to the body of the aircraft.Similar to tandem rotors and intermeshing rotors, thetransverse rotor also uses differential collective pitch.But like the intermeshing rotors, the transverse rotorsuse the concept for changes in the roll attitude of therotorcraft. This configuration is found on two of the firstviable helicopters, the Focke-Wulf Fw 61 and theFocke-Achgelis Fa 223, as well as the world's largesthelicopter ever built, the Mil Mi-12. It is also theconfiguration found on tiltrotors, such the Bell XV-15and the newer Bell-Boeing V-22 Osprey.

Quadrotor

Main article: quadrotor

A quadrotor helicopter has four rotors in an "X"configuration designated as front-left, front-right,rear-left, and rear-right. Rotors to the left and right arein a transverse configuration while those in the front andto the rear are in a tandem configuration.

The main attraction of quadrotors is their mechanicalsimplicity—a quadrotor helicopter using electric motors and fixed-pitch rotors uses only fourmoving parts.[11]

Blade designThe blades of a helicopter are long, narrow airfoils with a high aspect ratio, a shape whichminimises drag from tip vortices (see the wings of a glider for comparison). They generallycontain a degree of washout to reduce the lift generated at the tips, where the airflow isfastest and vortex generation would be a significant problem. Rotor blades are made out ofvarious materials, including aluminium, composite structure and steel or titanium withabrasion shields along the leading edge. Rotorcraft blades are traditionally passive, butresearch into active blade control trailing edge flaps is performed.[12]

Limitations and hazardsHelicopters with teetering rotors, for example the two-blade system on the Bell, Robinsonand others, must not be subjected to a low-g condition because such rotor systems do notcontrol the fuselage attitude. This can result in the fuselage assuming an attitude controlledby momentum and tail rotor thrust that causes the tail boom to intersect the main rotortip-path plane, or result in the blade roots contacting the main rotor drive shaft causing theblades to separate from the hub (mast bumping).

Abrasion in sandy environments

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See also: Brownout (aviation) — Sensory illusions

When operating in sandy environments, sand hitting the moving rotor blades erodes theirsurface. This can damage the rotors; the erosion also presents serious and costlymaintenance problems.[13]

The 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 is flown near to the groundin desert environments abrasion occurs from the sand striking the rotor blade. At night, thesand hitting the metal abrasion strip causes a visible corona or halo around the rotorblades. The corona effect is caused by the oxidation of eroded particles resulting in visiblecorona.[14][15]

In 2009, war correspondent Michael Yon referred to this corona effect as "Kopp-Etchellseffect", to honor Cpl. Benjamin Kopp, and Cpl. Joseph Etchells, recently fallen Americanand British soldiers, respectively.[16]

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

1.

^ 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) .

2.

^ Cox, Taylor. "Blades and Lift" (http://www.webcitation.org/5NFat7fwZ) . Helis.com. Retrieved:10 March 2007.

3.

^ Landis, Tony and Jenkins, Dennis R. Lockheed AH-56A Cheyenne - WarbirdTech Volume 27,p.5. Specialty Press, 2000. ISBN 1580070272.

4.

^ Model 286 (http://www.aeroengineer.net/history/lockheed/lockheed12.html)5.^ FAA Flight Standards Service 20016.^ Alpman, Emre and Long, Lyle N. "Understanding Ducted-Rotor Antitorque and DirectionalControl: Characteristics Part II: Unsteady Simulations." (http://www.kothmann.com/Papers/AIAAJournalAircraftPart2.pdf) Journal of Aircraft Vol. 41, No. 6, November–December 2004.

7.

^ a b Frawley 2003, p. 151.8.^ "NOTAR Fleet Marks 500,000 Flight Hours" (http://www.webcitation.org/5MvjUcNbi) .American Helicopter Society. Retrieved: 25 February 2007.

9.

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

10.

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

11.

^ 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.

12.

^ Jim Bowne, Public Affairs Office, U.S. Army Aviation and Missile Command (February 2004)."These boots are made for flying: Rotor blades get new protective shields"(http://www.rdecom.army.mil/rdemagazine/200402/itf_amrdec_boots.html) . RDECOMMagazine. U.S. Army Research, Development and Engineering Command (Provisional).http://www.rdecom.army.mil/rdemagazine/200402/itf_amrdec_boots.html. Retrieved2009-09-04. "The 'corona effect' is characterized by distinctive glowing rings along metal or

13.

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fiberglass rotor blades operating in desert conditions."^ Warren (Andy) Thomas; Shek C. Hong;, Chin-Jye (Mike) Yu, Edwin L. Rosenzweig(2009-05-27). "Enhanced Erosion Protection for Rotor Blades: Paper presented at theAmerican Helicopter Society 65th Annual Forum, Grapevine, Texas, May 27 – 29, 2009."(http://www.vtol.org/f65_bestPapers/productSupport.pdf) (pdf). American Helicopter Society.http://www.vtol.org/f65_bestPapers/productSupport.pdf. Retrieved 2009-09-02. "A secondaryconcern with the erosion of metal abrasion strips pertains to the visible signature that occurs ...causing a corona effect in sandy environments."

14.

^ (pdf) Office of Naval Research Broad Agency Announcement(BAA): Advanced HelicopterRotor Blade Erosion Protection (http://www.onr.navy.mil/02/baa/docs/BAA%2008-011_ONRBAA%2008-011.pdf) . United States Department of the Navy, Officeof Naval Research. p. 3. BAA 08-011. http://www.onr.navy.mil/02/baa/docs/BAA%2008-011_ONRBAA%2008-011.pdf. Retrieved 2009-09-02. "An equally importantproblem 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.".

15.

^ "The Kopp-Etchells Effect" (http://www.michaelyon-online.com/the-kopp-etchells-effect.htm) .http://www.michaelyon-online.com/the-kopp-etchells-effect.htm. Retrieved 2009-09-02.

16.

External linksRotor 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 flybar invention, 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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