Design principles
Overview
The helicopter rotor is powered by the engine, through the transmission, to the rotating mast. The mast is a cylindrical metal shaft that extends upward from—and is driven by—the transmission. At the top of the mast is the attachment point (colloquially called aBlades
The blades of a helicopter are long, narrow airfoils with a high aspect ratio, a shape that minimizes drag from tip vortices (see the wings of aHub
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. ** Pitch hinges, allowing the blades to twist about the axis extending from blade root to blade tip. * Teeter hinge, allowing one blade to rise vertically while the other falls vertically. This motion occurs whenever translational relative wind is present, or in response to a cyclic control input. * Scissor link and counterweight, carries the main shaft rotation down to the upper swashplate * Rubber covers protect moving and stationary shafts * Swashplates, transmitting cyclic and collective pitch to the blades (the top one rotates) * Three non-rotating control rods transmit pitch information to the lower swashplate * Main mast leading down to mainFully articulated
Juan de la Cierva developed the fully articulating rotor for the autogyro. The basis of his design permitted successful helicopter development. In a fully articulated rotor system, each rotor blade is attached to the rotor hub through a series of hinges that let the blade move independently of the others. These rotor systems usually have three or more blades. The blades are allowed to flap, feather, and lead or lag independently of each other. The horizontal hinge, called the flapping hinge, allows the blade to move up and down. This movement is called flapping and is designed to compensate forRigid
The term ''rigid rotor'' usually refers to a hingeless rotor system with blades flexibly attached to the hub. Irv Culver of Lockheed developed one of the first rigid rotors, which was tested and developed on a series of helicopters in the 1960s and 1970s. In a rigid rotor system, each blade flaps and drags about flexible sections of the root. A rigid rotor system is mechanically simpler than a fully articulated rotor system. The aerodynamic and mechanical loads from flapping and lead/lag forces are accommodated through rotor blades flexing, rather than through hinges. 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 of the large hub moment typically generated.Connor, RSemirigid
The semirigid rotor can also be referred to as a teetering or seesaw rotor. This system is normally composed of two blades that meet just under a common flapping or teetering hinge at the rotor shaft. This allows the blades to flap together in opposite motions like a seesaw. This underslinging of the blades below the teetering hinge, combined with an adequate dihedral orCombination
Modern rotor systems may use the combined principles of the rotor systems mentioned above. Some rotor hubs incorporate a flexible hub, which allows for blade bending (flexing) without the need for bearings or hinges. These systems, called ''flexures'',FAA Flight Standards Service 2001 are usually constructed from composite material. Elastomeric bearings may also be used in place of conventional roller bearings. Elastomeric bearings are constructed from 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 also absorb vibration, which means less fatigue and longer service life for the helicopter components.Swash plate
Controls vary the pitch of the main rotor blades cyclically throughout rotation. The pilot uses this to control the direction of the rotor thrust vector, which defines the part of the rotor disc where the maximum thrust develops. Collective pitch varies the magnitude of rotor thrust by increasing or decreasing thrust over the whole rotor disc at the same time. These blade pitch variations are controlled by tilting, raising, or lowering the 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 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 connected to the individual blades through pitch links 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. Through shifting and tilting, the non-rotating plate controls the rotating plate, which in turn controls the individual blade pitch.Flybar (stabilizer bar)
A number of engineers, among them Arthur M. Young in the U.S. and radio-control aeromodeler Dieter Schlüter in Germany, found that flight stability for helicopters could be achieved with a stabilizer bar, or flybar. The flybar has a weight or paddle (or both for added stability on smaller helicopters) at each end to maintain a constant plane of rotation. Through mechanical linkages, the stable rotation 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.Slowed rotor
Most helicopter rotors spin at constant speed. However slowing the rotor in some situations can bring benefits. As forward speed increases, the advancing rotor tip speed soon approaches theRotor configurations
Most 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 design thatSingle main rotor
With a single main rotor helicopter, the creation ofTail rotor
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 tail rotor's position and distance from theDucted fan
Fenestron and FANTAIL are trademarks for a ducted fan mounted at the end of the tail boom of the helicopter and used in place of a tail rotor. Ducted fans have between eight and eighteen blades arranged with irregular spacing so that the noise is distributed over different frequencies. The housing is integral with the aircraft skin and allows a high rotational speed; therefore, a ducted fan can have a smaller size than a conventional tail rotor. 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 and produced on the later modelNOTAR
NOTAR, an acronym for ''no tail rotor'', is a helicopter anti-torque system that eliminates the use of the tail rotor on a helicopter. Although the concept took some time to refine, the NOTAR system is simple in theory and provides antitorque the same way a wing develops lift by using theTip jets
The main rotor may be driven by tip jets. Such a system may be powered by high pressure air provided by a compressor. The air may or may not be mixed with fuel and burnt in ram-jets, pulse-jets, or rockets. Though this method is simple and eliminates torque reaction, prototypes that have been built are less fuel efficient than conventional helicopters. Except for tip jets driven by unburnt compressed air, very high noise levels is the 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 could not fly. TheTwin rotors
Twin rotors turn in opposite 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 lifting capacity. Primarily, three common configurations use the counterrotating effect on rotorcraft. ''Tandem rotors'' are two rotors—one mounted behind the other. ''Coaxial rotors'' are two rotors mounted one above the other on the same axis. ''Intermeshing rotors'' are two rotors mounted close to each other at a sufficient angle to let the rotors intermesh over the top of the aircraft. Another configuration—found on tiltrotors and some early helicopters—is called transverse rotors, where a pair of rotors are mounted at each end of a wing-type structure or outrigger.Tandem
Tandem rotors are two horizontal main rotor assemblies mounted one behind the other. Tandem rotors achieve pitchCoaxial
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, in forward flight, the lift provided by the advancing halves of each rotor compensates for the retreating half of the other, eliminating one of the key effects of dissymmetry of lift:Intermeshing
Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions with each rotor mast mounted on the helicopter with a slight angle to the other so that the blades intermesh without colliding. This configuration is sometimes referred to as a synchropter. Intermeshing rotors have high stability and powerful lifting capability. The arrangement was pioneered inTransverse
Quad rotor
Etienne Oehmichen, Paris, France, 192Limitations and hazards
Abrasion in sandy environments
When operating in sandy environments, sand hitting the moving rotor blades erodes their surface. This can damage the rotors and presents serious and costly maintenance problems. Abrasion strips on helicopter rotor blades are made of metal, oftenHistory
The use of a rotor for vertical flight has existed since 400 BC in the form of the bamboo-copter, an ancient Chinese toy.Leishman, J. Gordon. ''Principles of Helicopter Aerodynamics''. Cambridge aerospace series, 18. Cambridge:References
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