Helicopter rotorFrom Wikipedia, the free encyclopedia This article is about helicopter main rotor systems. For antitorque control, see tail rotor. Helicopter rotor The rotor head of a Sikorsky S-92 A helicopter main rotor or rotor system is the combination of a rotary wing and a control system that is used to generate both the aerodynamic liftforce 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 of 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 aswashplate connected to the helicopter flight controls. Helicopters are one example of rotary-wing aircraft. Contents [hide] 1 History and development 2 Design o o o 2.1 Parts and functions 2.2 Swash plate 2.3 Fully articulated o 2.3.1 Aircraft 2.4 Rigid 5 Semirigid o 2.3 NOTAR 3.1 Abrasion in sandy environments 6 References 7 External links [edit]History and development Helicopter rotor of Engelbert Zaschka.2.1.1 Flybar (stabilizer bar) 2.2.2 Coaxial 3.2.1 Tandem 3.2.6. 1931.4 Tip jets 3.1.4 Transverse 3.5.6 Combination 2.1 Single main rotor o 3. German master engineer.3 Quadrotor 4 Blade design 5 Limitations and hazards o 5.2 Ducted fan 3.o 2.3 Intermeshing 3. image from the German Federal Archives .1.1.2 Dual rotors (counterrotating) o 3.1 Aircraft 3 Rotor configurations o 3.5.2 Aircraft 2.1 Tail rotor 3. through the transmission. This system was used in several Belland Hiller helicopter models.Before the development of powered helicopters in the mid 20th century. The rotors are designed to operate in a narrow range of RPM. fully articulated rotor systems. This permits a lower downwash velocity for a given amount of thrust. or fully articulated. Cierva is credited with successful development of multi-bladed. is the basis of most multi-bladed helicopter rotor systems. This system. In the 1930s. It is also used in manyremote control model helicopters. autogyro pioneer Juan de la Ciervaresearched and developed many of the fundamentals of the rotor. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree. although some modern rotor systems use an engineered combination of these classifications. The mast is a cylindrical metal shaft which extends upward from—and is driven by—the transmission. semirigid. to the rotating mast.[1][2][3][4][5] Unlike the small diameter fans used in turbofan jet engines. the main rotor on a helicopter has a quite large diameter. [edit]Design The helicopter rotor is powered by the engine. in its various modified forms. The rotor blades are then attached to the hub. There are three basic classifications: rigid.[7][8] [edit]Parts and functions The simple rotor of a Robinson R22 . At the top of the mast is the attachment point for the rotor blades called the hub. permitting a large volume of air to be accelerated. Arthur Young improved the stability of two-bladed rotor systems with the introduction of a stabilizer bar.[6] a low disc loading(thrust per disc area) greatly increases the aircraft's energy efficiency and this reduces the fuel use and permits reasonable range. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. or in response to a cyclic control input. The non-rotating plate is connected to links which are manipulated by pilot controls. The rotating plate is also connected to the individual blades through pitch links and pitch horns. right side. connected by idle links. etc. The vast majority of helicopters maintain a constant rotor speed (RPM) during flight. Pitch hinges. while the other does not rotate. allowing one blade to rise vertically while the other falls vertically. Collective pitch is used to vary the magnitude of rotor thrust (increasing or decreasing thrust over the whole rotor disc at the same time). front. the non-rotating plate controls the rotating plate.). which in turn controls the individual blade pitch. . The swash plate is two concentric disks or plates. Teeter hinge. leaving only the angle of attack of the blades as the sole means of adjusting thrust from the rotor. one plate rotates with the mast. The swash plate can shift vertically and tilt. carries the main shaft rotation down to the upper swashplate Rubber covers protect moving and stationary shafts Swashplates. allowing the blades to twist about the axis extending from blade root to blade tip. Through shifting and tilting. the collective and cyclic controls. This motion occurs whenever translational relative wind is present. specifically. These blade pitch variations are controlled by tilting and/or raising or lowering the swash plate with the flight controls. transmitting cyclic and collective pitch to the blades (the top one rotates) Three non-rotating control rods transmit pitch information to the lower swashplate [edit]Swash Main mast leading down to main gearbox plate Main article: Swashplate (helicopter) The pitch of main rotor blades can be varied cyclically throughout its rotation in order to control the direction of rotor thrust vector (the part of the rotor disc where the maximum thrust will be developed.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 part of the swashplate. Scissor link and counterweight. rear. [edit]Aircraft [edit]Rigid AgustaWestland AW109 The term "rigid rotor" usually refers to a hingeless rotor system[10][11] with blades flexibly attached to the hub. This yoke does transfer some movement of one blade to another. the blades themselves compensate for the forces which previously required rugged hinges. This movement is called flapping and is designed to compensate for dissymmetry of lift. This system is similar to the fully articulated type in that each blade has the ability to lead/lag and hunt independent of the other blades. because the rotor has much less oscillation. each blade flaps and drags about flexible sections of the root. The result is a rotor system that has less lag in the control response. usually opposing blades. The purpose of the drag hinge and dampers is to compensate for the acceleration and deceleration caused by momentum conservation. called the flapping hinge. The horizontal hinge. which was tested and developed on a series of helicopters in the 1960s and 1970s. and there may be more than one hinge. This type of rotor can be found on several aircraft produced by Bell Helicopter. These rotor systems usually have three or more blades. In a fully articulated rotor system. the flight characteristics are very similar and maintenace time and cost are reduced.[9] and not by Coriolis effect.[edit]Fully articulated Juan de la Cierva developed the fully articulating rotor for the autogyro. Later models have switched from using traditional bearings to elastomeric bearings. Loads from flapping and lead/lag forces are accommodated through rotor blades flexing. A rigid rotor system is mechanically simpler than a fully articulated rotor system. This movement is called lead-lag. such as the OH-58D Kiowa Warrior.[why?][citation needed] A variation of the fully articulated system is the "soft-in-plane" rotor system. This yoke is attached to the mast and runs through the blade grips between the blades and the shear bearing inside the grip. feather. or hunting. called the lead-lag or drag hinge. The vertical hinge. dragging. Irven Culver of Lockheed developed one of the first rigid rotors. The difference between a fully articulated system and soft-in-plane system is that the soft-in-plane system utilizes a composite yoke. and it is the basis of his design that permitted successful helicopter development. The blades are allowed to flap. In a rigid rotor system.[13] Example:MBB Bo 105 [edit]Semirigid . each rotor blade is attached to the rotor hub through a series of hinges which allow the blade to move independently of the others. While this is not fully articulated. By flexing.[12] The rigid rotor system also negates the danger of mast bumping inherent in teetering rotors. rather than through hinges. and lead or lag independently of each other. allows the blade to move up and down. The flapping hinge may be located at varying distances from the rotor hub. Dampers are usually used to prevent excess back and forth movement around the drag hinge. allows the blade to move back and forth. (November 2010) Semirigid rotor system The semirigid rotor can also be referred to as a teetering or seesaw rotor. Young in the U.. This allows the blades to flap together in opposite motions like a seesaw.S. found that flight stability for helicopters could be achieved with a stabilizer bar or flybar. among them Arthur M. but designed for both hands-off stability and rapid control response of the hingeless rotor system. combined with an adequate dihedral or coning angle on the blades. . or paddles. Unsourced material may be challenged and removed. minimizes variations in the radius of each blade's center of mass from the axis of rotation as the rotor turns. This underslinging of the blades below the teetering hinge. which in turn reduces the stress on the blades from lead and lag forces caused by coriolis effect. The flybar has a weight or paddle (or both for added stability on smaller helicopters) at either end which cause the bar to stay relatively stable in the plane of rotation and reduces crosswind thrust on rotors. similar in principle to that of the Bell stabilizer bar. Feathering is accomplished by the feathering hinge at the blade root. the paddles provided the added stability by dampening the effects of external forces on the rotor. The Lockheed rotor system used a control gyro. Secondary flapping hinges may also be provided to provide sufficient flexibility to minimize bouncing. Through mechanical linkages. the stable rotation of the bar is mixed with the swashplate movement so that internal (steering) as well as external (wind) forces on the rotor are damped. Please help improve this section by adding citations to reliable sources. However.This section does not cite any references or sources. and Dieter Schlüter in Germany. [edit]Flybar (stabilizer bar) A number of engineers. This eases the workload on the pilot to maintain control of the aircraft. This system is normally composed of two blades which meet just under a common flapping. at each end. which allows changes to the pitch angle of the blade. Stanley Hiller arrived at a similar method to improve stability by adding short stubby airfoils. or teetering hinge at the rotor shaft. Hiller's "Rotormatic" system was also used to deliver cyclic control inputs to the main rotor as a sort of control rotor. [edit]Single main rotor . This is accomplished through a variable pitch.[14] are usually constructed from composite material. Flextures and elastomeric bearings require no lubrication and. Elastomeric bearings may also be used in place of conventional roller bearings. the main rotors of helicopter designs from Germany. called "flextures". therefore. antitorque rotor or tail rotor. which allows for blade bending (flexing) without the need for bearings or hinges. require less maintenance. This can make it difficult when discussing aerodynamic effects on the main rotor between different designs. When viewed from above. Some rotor hubs incorporate a flexible hub. [edit]Aircraft [edit]Combination Bell 206/OH-58 Robinson R22 Modern rotor systems may use the combined principles of the rotor systems mentioned above. These systems. This is the design that Igor Sikorsky settled on for his VS-300 helicopter and it has become the recognized convention for helicopter design. which means less fatigue and longer service life for the helicopter components. This flybar-less design has the advantage of easy reconfiguration and fewer mechanical parts. Elastomeric bearings are bearings constructed from a rubber type material and have limited movement that is perfectly suited for helicopter applications. a microcontroller with gyroscope sensors and a venturi sensor can replace the stabilizer. since the effects may manifest on opposite sides of each aircraft. United Kingdom. They also absorb vibration.In fly-by-wire helicopters or RC models. The United States and Canada rotate counter-clockwise. main rotor but require a separate rotor to overcome torque. although designs do vary. all others rotate clockwise. [edit]Aircraft [edit]Rotor Bell 407 Bell 430 Eurocopter AS350 configurations Most helicopters have a single. [edit]Ducted fan . Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. with a sufficient margin of power available to allow the helicopter to maintain its heading and provide yaw control. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals. The three most common controls used today are the traditional tail rotor. [edit]Tail rotor Main article: Tail rotor Tail rotor of an SA 330 Puma The tail rotor is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. the creation oftorque as the engine turns the rotor creates a torque effect that causes the body of the helicopter to turn in the opposite direction of the rotor. To eliminate this effect.Antitorque: Torque effect on a helicopter With a single main rotor helicopter. some sort of antitorque control must be used. The tail rotor's position and distance from the center of gravity allow it to develop thrust in a direction opposite of the main rotor's rotation. Eurocopter's Fenestron(also called a fantail). to counter the torque effect created by the main rotor. and MD Helicopters' NOTAR. which also provide directional control by allowing the pilot to rotate the helicopter around its vertical axis (thereby changing the direction the craft is pointed). and produced on the later model Aérospatiale SA 341 Gazelle. as the FANTAIL. The housing is integral with the aircraft skin and allows a high rotational speed.Fenestron on an EC 120B Main article: Fenestron Fenestron and FANTAIL[15] are trademarks for aducted fan mounted at the end of the tail boom of the helicopter and used in place of a tail rotor. therefore a ducted fan can have a smaller size than a conventional tail rotor. Ducted fans have between eight and 18 blades arranged with irregular spacing. the NOTAR system is simple in theory and works to provide antitorque the same way a wing develops lift using the Coandă effect. an acronym for NO TAil Rotor. Besides Eurocopter and its predecessors. Although the concept took some time to refine. a ducted fan tail rotor was also used on the canceled military helicopter project. the United States Army's RAH-66 Comanche. is a helicopter anti-torque system that eliminates the use of the tail rotor on a helicopter.[16] A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor . [edit]NOTAR Main article: NOTAR Diagram showing the movement of air through the NOTAR system NOTAR. The Fenestron was used for the first time at the end of the 1960s on the second experimental model of Sud Aviation's SA 340. so that the noise is distributed over different frequencies. 74 which was underpowered and not able to achieve flight. and successfully completed an advanced flight-test program. ram-jets. This is augmented by a direct jet thruster (which also provides directional yaw control) and vertical stabilizers. causing the downwash from the main rotor to hug the tailboom. all produced by MD Helicopters.the Hiller YH-32 Hornet had good lifting capability but performed poorly otherwise. Coaxial rotors are two rotors that are mounted one above the other with the same axis. increasing the aircraft's lifting capacity. The French Sud-Ouest Djinn utilized unburnt compressed air to drive the rotor which minimized noise and enabled it to become the only tip-jet driven rotor helicopter to enter production. however research into noise suppression is ongoing with the objective making this system viable. This fan forces low pressure air through two slots on the right side of the tailboom. Other aircraft utilized auxiliary thrust for translational flight so that the tipjets could be shut down while the rotor autorotated. rockets. when engineers at Hughes Helicoptersbegan concept development work.[18] There are currently three production helicopters that incorporate the NOTAR design. Except for tip-jets driven by unburnt compressed air. This antitorque design also improves safety by eliminating the possibility of personnel walking into the tail rotor.transmission.[17] A more heavily modified prototype demonstrator first flew in March 1986. [edit]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 a compressor and which may or may not be mixed with fuel and burnt.[16] In December 1981. 48-seat Fairey Rotodynepassenger prototypes and McDonnell XV-1 compound gyroplanes flew very well using this method. Tandem rotors are two rotors with one mounted behind the other. [edit]Dual rotors (counterrotating) Counterrotating rotors are rotorcraft configurations with a pair or more of large horizontal rotors turning in opposite directions to counteract the effects of torque on the aircraft without relying on an antitorque tail rotor. producing lift. the prototypes that have been built are less fuel efficient than conventional helicopters. validating the system for future application in helicopter design. and thus a measure of antitorque proportional to the amount of airflow from the rotorwash. there are three common configurations that use the counterrotating effect to benefit the rotorcraft. Examples of tip-jet powered rotorcraft include the Percival P. Development of the NOTAR system dates back to 1975. The experimental Fairey Jet Gyrodyne. Primarily. This allows the power normally required to drive the tail rotor to be applied to the main rotors. Intermeshing rotors are two rotors that are . Although this method is simple and eliminates torque reaction. pulse-jets. which was originally envisioned to take off utilizing a rocket-tipped rotor. Hughes flew an OH-6A fitted with NOTAR for the first time. very high noise levels is the single most important reason why tip-jet powered rotors have not gained wide acceptance. Perhaps the most unusual design of this type was the Rotary Rocket Roton ATV. To pitch forward and accelerate. [edit]Tandem Boeing CH-47 Chinook Main article: Tandem rotors Tandem rotors are two horizontal main rotor assemblies mounted one behind the other. it requires the expense of two large rotors rather than the more common one large main rotor and a much smaller tail rotor. All of the rotor power contributes to lift. Yaw control is developed through opposing cyclic pitch in each rotor. the front rotor increases collective pitch to raise the nose and the rear rotor decreases collective pitch to lower the tail. simultaneously dipping the nose. The Boeing CH-47 Chinook is the most common tandem rotor helicopter today. and it is simpler to handle changes in the center of gravity fore-aft. Another configuration found on tiltrotors and some earlier helicopters is called transverse rotors where the pair of rotors are mounted at each end of wing-type structures or outriggers. Tandem rotors achieve pitch attitude changes to accelerate and decelerate the helicopter through a process called differential collective pitch. the rear rotor increases collective pitch. To pitch upward while decelerating (or moving rearward). the front rotor tilts right and the rear rotor tilts left. [edit]Coaxial . to pivot right. and to pivot left. raising the tail and the front rotor decreases collective pitch.mounted close to each other at a sufficient angle to allow the rotors to intermesh over the top of the aircraft. the front rotor tilts left and the rear rotor tilts right. However. Intermeshing rotors have high stability and powerful lifting capability. There is an increased mechanical complexity of the rotor system because it requires linkages andswashplates for two rotor systems. eliminating one of the key effects of dissymmetry of lift: retreating blade stall. This configuration is sometimes referred to as a synchropter. in forward flight. The latest Kaman model. Add that each rotor system needs to be turned in opposite directions means that the mast itself is more complex. The arrangement was successfully used in Nazi Germany for a small anti-submarine warfare helicopter. [edit]Intermeshing HH-43 Huskie Main article: Intermeshing rotors Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions.Kamov Ka-50 of the Russian Air Force. with each rotor mast mounted on the helicopter with a slight angle to the other so that the blades intermesh without colliding. is a dedicated sky crane design. [edit]Transverse . and provisions for making pitch changes to the upper rotor system must pass through the lower rotor system. the lift provided by the advancing halves of each rotor compensates for the retreating half of the other. the Kaman K-MAX. an American company. with coaxial rotors Main article: Coaxial rotors Coaxial rotors are a pair of rotors mounted one above the other on the same shaft and turning in opposite directions. The advantage of the coaxial rotor is that. During the Cold War. the Flettner Fl 282 Kolibri.Kaman Aircraft. However. produced the HH-43 Huskie for theUSAF firefighting and rescue missions. other design considerations plague coaxial rotors. [edit]Quadrotor De Bothezat Quadrotor. the transverse rotors use the concept for changes in the roll attitude of the rotorcraft.[19] [edit]Blade design The blades of a helicopter are long. It is also the configuration found on tiltrotors. front-right. such the Bell-Boeing V-22 Osprey and the AgustaWestland AW609. and rear-right. Rotor blades are made out of various materials. as well as the world's largest helicopter ever built. the Mil Mi-12. narrow airfoils with a high aspect ratio. Rotors to the left and right are in a transverse configuration while those in the front and to the rear are in a tandem configuration. 1923 Main article: Quadrotor A quadrotor helicopter has four rotors in an "X" configuration designated as front-left. the transverse rotor also uses differential collective pitch. Similar to tandem rotors and intermeshing rotors. This configuration is found on two of the first viable helicopters.Mi-12 Transverse rotors are mounted on the end of wings or outriggers. But like the intermeshing rotors. a shape which minimises drag from tip vortices (see the wings of a glider for comparison). rear-left. where the airflow is fastest and vortex generation would be a significant problem. perpendicular to the body of the aircraft. The main attraction of quadrotors is their mechanical simplicity—a quadrotor helicopter using electric motors and fixed-pitch rotors has only four moving parts. They generally contain a degree ofwashout to reduce the lift generated at the tips. composite structure and steel . the Focke-Wulf Fw 61 and theFockeAchgelis Fa 223. including aluminium. 5. 4. Accessed: 25 February 2012. the erosion also presents serious and costly maintenance problems. ISBN 1-119-99410-1. This can damage the rotors. Benjamin Kopp. the sand hitting the metal abrasion strip causes a visible corona or halo around the rotor blades. ISBN 978-09780269-0-5. but research into active blade control trailing edge flaps is performed. This can result in the fuselage assuming an attitude controlled by momentum and tail rotor thrust that causes the tail boom to intersect the main rotor tip-path plane. Accessed: 2 January 2010 ^ John M. Pam. ^ The UH-60 permits 95–101% rotor RPM UH-60 limits US Army Aviation. Rotorcraft blades are traditionally passive. often titanium or nickel. 3. respectively. Quote: [Rotor speed] "is constant in a helicopter".or titanium with abrasion shields along the leading edge. but less hard than sand. war correspondent Michael Yon referred to this corona effect as "Kopp-Etchells effect". 2011. [edit]Limitations and hazards Helicopters with teetering rotors. Retrieved: 16 February 2011. John Wiley and Sons. Phil. must not be subjected to a low-g condition because such rotor systems do not control the fuselage attitude.[21] [edit]Abrasion in sandy environments See also: Brownout (aviation) — Sensory illusions When operating in sandy environments. An example of such tips are the tips of the BERP rotor. and Cpl. Simon Newman. Quote: The rotor is best served by rotating at a constant rotor speed . When a helicopter is flown near to the ground in desert environments abrasion occurs from the sand striking the rotor blade. for example the two-blade system on the Bell. recently fallen American and British soldiers. ^ Johnson. ^ Croucher. Joseph Etchells. which are very hard.[25] [edit]References 1. or result in the blade roots contacting the main rotor drive shaft causing the blades to separate from the hub (mast bumping). 2.[20] The tips of some blades found in helicopters can be especially designed to reduce turbulence and thus also noise (and also allow the helicopter to fly more efficiently). At night. Basic Helicopter Aerodynamics p216. Robinson and others. The effect is caused by the pyrophoric oxidation of eroded particles. Accessed: 2 January 2010 ^ "Helicopters. Delta D2 page 44 Pacific Wings.[23][24] In 2009.[22] The abrasion strips on helicopter rotor blades are made of metal. sand hitting the moving rotor blades erodes their surface. Seddon." Helicopter Vietnam. Professional helicopter pilot studies page 2-11. to honor Cpl. Retrieved: 28 September 2010.S. Courier Dover Publications. ^ Alpman. Tony and Jenkins. ^ Jim Bowne. ^ a b Frawley 2003. ^ "The Boeing Logbook: 1983-1987". "Quadcopter. Specialty Press. DTIC. 17. original link. 22. Helis. pp." Journal of Aircraft Vol. Retrieved: 25 February 2007. Washington D.org on 3 September 2007. Boeing.com. 7. UAVs". Army Aviation and Missile Command (February 2004). 11. Lockheed CL-475". Accessed: 25 February 2012.S. 41. Courier Dover Publications. 2010-02-19. Government Printing Office. Taylor. ^ Rotorcraft Flying Handbook. "Blades and Lift". R. 16.S. p."These boots are made for flying: Rotor blades get new .. ^ Markus Waibel.5.000 Flight Hours". Public Affairs Office. ^ Wieslaw Zenon Stepniewski. 19. ^ Johnson. 2000. ^ Rotary Wing Forum [1]. N. ^ Mangeot et al. Presented May 1. ^ Cox. Octocopter . ^ "NOTAR Fleet Marks 500. Accessed: 25 February 2012. ISBN 0-486-64647-5 9. U. 14. Retrieved: 25 February 2007. Accessed at archive. ISBN 0-486-68230-7 8. 18. "Understanding Ducted-Rotor Antitorque and Directional Control: Characteristics Part II: Unsteady Simulations. New actuators for aerospace Noliac. 21. 1979. 20. 10. Lyle N. Wayne.C. ^ Connor. U. C.com. 6. Dennis R. 1997. p. Revised on 15 August 2002. Keys. Retrieved: 10 March 2007.. Retrieved: 28 September 2010. 11-10.5 MB.MIL Word document. Accessed: 25 February 2012. ^ Landis. 151. ^ Paul Bevilaqua : The shaft driven Lift Fan propulsion system for the Joint Strike Fighter page 3. 2000. 5. ISBN 1-56027404-2. 13. Hexacopter. ^ FAA Flight Standards Service 2001 15. November–December 2004. Smithsonian National Air & Space Museum. ISBN 1-58007-027-2. Lockheed AH-56A Cheyenne WarbirdTech Volume 27. ^ Model 286 12.: U. IEEE Spectrum. Helicopter theory pp3+32. 1980. Emre and Long. FAA-8083-21. No. Rotary-wing aerodynamics p3.6. American Helicopter Society. Federal Aviation Administration. [edit]External links Rotor Analysis . Retrieved 2009-09-04. Shek C. Rosenzweig (2009-05-27). Dispatch by Michael Yon. 3. Grapevine. US Patent 2. Hong. Office of Naval Research.S. Retrieved 2009-09-02. by Arthur Young View page ratings "The Kopp-Etchells Effect". Retrieved 2009-09-02.. "The 'corona effect' is characterized by distinctive glowing rings along metal or fiberglass rotor blades operating in desert conditions. war correspondent Rate this page What's this? Trustworthy Objective Complete Well-written I am highly knowledgeable about this topic (optional) ." 23. causing a corona effect in sandy environments. Chin-Jye (Mike) Yu. U. Army Research. United States Department of the Navy. Development and Engineering Command (Provisional)..Blade Element Momentum Theory Helicopter Rotorhead Close-up Image Gallery "Helicopter Aircraft". for flybar invention. May 27 – 29. ^ (pdf) Office of Naval Research Broad Agency Announcement(BAA): Advanced Helicopter Rotor Blade Erosion Protection." 24. ^ "The Kopp-Etchells Effect"." (pdf). Edwin L. p. 2009. RDECOM Magazine.. Retrieved 2009-09-02." 25. American Helicopter Society. BAA 08-011.protective shields"."Enhanced Erosion Protection for Rotor Blades: Paper presented at the American Helicopter Society 65th Annual Forum.368. ^ Warren (Andy) Thomas. "An equally important problem with Ti protection is that a visible corona or halo is generated around the rotor blades at night from the sand impacting the Ti leading edge and causing Ti to spark and oxidize.698. Texas. "A secondary concern with the erosion of metal abrasion strips pertains to the visible signature that occurs . 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