Teetering rotor

A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that 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 connects through a combination of drive shaft(s) and gearboxes along the tail boom. The blade pitch is typically controlled by the pilot using the helicopter flight controls. Helicopters are one example of rotary-wing aircraft (rotorcraft). The name is derived from the Greek words ''helix'', helik-, meaning spiral; and ''pteron'' meaning wing.

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 a Jesus nut) for the rotor blades called the hub. The rotor blades are then attached to the hub, and the hub can have 10-20 times the drag of the blade. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. There are three basic classifications: rigid, semirigid, and fully articulated, although some modern rotor systems use a combination of these classifications. A rotor is a finely tuned rotating mass, and different subtle adjustments reduce vibrations at different airspeeds. The rotors are designed to operate at a fixed RPMCroucher, Phil
Professional helicopter pilot studies
page 2-11. . Quote: otor speed"is constant in a helicopter". (within a narrow range of a few percent), but a few experimental aircraft used variable speed rotors.Datta, Anubhav et al.
Experimental Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios
' page 2. ''NASA'' ARC-E-DAA-TN3233, 2011
Header
Accessed: May 2014. Size: 26 pages in 2MB

Unlike the small diameter fans used in turbofan jet engines, the main rotor on a helicopter has a large diameter that lets it accelerate a large volume of air. This permits a lower downwash velocity for a given amount of thrust. 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,Paul Bevilaqua
The shaft driven Lift Fan propulsion system for the Joint Strike Fighter
page 3. Presented May 1, 1997. DTIC.MIL Word document, 5.5 MB. Accessed: 25 February 2012. 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.Johnson, Wayne
Helicopter theory
pp3+32, ''Courier Dover Publications'', 1980. Accessed: 25 February 2012. Wieslaw Zenon Stepniewski, C. N. Keys
Rotary-wing aerodynamics
p3, ''Courier Dover Publications'', 1979. Accessed: 25 February 2012. The hover efficiency ("figure of merit")Jackson, Dave.

''Unicopter'', 16 December 2011. Retrieved: 22 May 2015

on 26 November 2013. of a typical helicopter is around 60%. The inner third length of a rotor blade contributes very little to lift due to its low airspeed. Bensen, Igor.
How they fly - Bensen explains all
''Gyrocopters UK''. Accessed: 10 April 2014.

Blades

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 a glider for comparison). They generally contain a degree of washout that reduces the lift generated at the tips, where the airflow is fastest and vortex generation would be a significant problem. Rotor blades are made out of various materials, including aluminium, composite structure, and steel or titanium, with abrasion shields along the leading edge.

Rotorcraft blades are traditionally passive; however, some helicopters include active components on their blades. The Kaman K-MAX uses trailing edge flaps for blade pitch control and the Hiller YH-32 Hornet was powered by ramjets mounted on the blade ends. , research into active blade control through trailing edge flaps is underway.Mangeot et al
New actuators for aerospace
''Noliac''. Retrieved: 28 September 2010. Tips of some helicopter blades can be specially designed to reduce turbulence and noise and to provide more efficient flying. An example of such tips are the tips of the BERP rotors created during the British Experimental Rotor Programme.

Hub

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 main gearbox

Fully 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 for dissymmetry of lift. The flapping hinge may be located at varying distances from the rotor hub, and there may be more than one hinge. The vertical hinge, called the lead-lag hinge or drag hinge, allows the 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 drag hinge. The purpose of the drag hinge and dampers is to compensate for acceleration and deceleration caused by the difference in drag experienced by the advancing and retreating blades. Later models have switched from using traditional bearings to elastomeric bearings. Elastomeric bearings are naturally fail-safe and their wear is gradual and visible. The metal-to-metal contact of older bearings and the need for lubrication is eliminated in this design. The third hinge in the fully articulated system is called the feathering hinge about the feathering axis. This hinge is responsible for the change in pitch of rotor blades excited via pilot input to the collective or cyclic.

A variation of the fully articulated system is the ''soft-in-plane'' rotor system. This type of rotor can be found on several aircraft produced by Bell Helicopter, such as the OH-58D Kiowa Warrior. This system is similar to the fully articulated type in that each blade has the ability to lead/lag and hunt independently of the other blades. The difference between a fully articulated system and soft-in-plane system is that the soft-in-plane system utilises a composite yoke. This yoke is attached to the mast and runs through the blade grips between the blades and the shear bearing inside 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 maintenance time and cost are reduced.

Rigid

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, R
Lockheed CL-475"
Smithsonian National Air & Space Museum. Revised on 15 August 2002. Accessed at archive.org on 3 September 2007

. The rigid rotor system thus eliminates the danger of mast bumping inherent in semirigid rotors.Cox, Taylor
"Blades and Lift"
Helis.com. Retrieved: 10 March 2007.

Semirigid

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 or coning angle on the blades, minimizes variations in the radius of each blade's center of mass from the axis of rotation as the rotor turns, which in turn reduces the stress on the blades from lead and lag forces caused by the Coriolis effect. Secondary flapping hinges may also be used to provide sufficient flexibility to minimize bouncing. Feathering is accomplished by the feathering hinge at the blade root, which allows changes to the pitch angle of the blade.

Combination

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. Stanley Hiller arrived 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 of control rotor, and the paddles provided the added stability by damping the effects of external forces on the rotor.

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

In fly-by-wire helicopters or Remote Control (RC) models, a microcontroller with gyroscope sensors and a Venturi sensor can replace the stabilizer. This ''flybar-less'' design has the advantage of easy reconfiguration and fewer mechanical parts.

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 the speed of sound. To reduce the problem, the speed of rotation may be slowed, allowing the helicopter to fly faster.

To adjust the rotor lift at slower speeds, in a conventional design the rotor blades' angle of attack is reduced via a collective pitch control. Slowing the rotor instead can reduce drag during this phase of flight and thus improve fuel economy.

Rotor 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 that Igor Sikorsky settled on for his VS-300 helicopter, and it has become the recognized convention for helicopter design, although designs do vary. When viewed from above, most American helicopter rotors turn counter-clockwise; French and Russian helicopters turn clockwise.

Single main rotor

With a single main rotor helicopter, the creation of torque 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, some sort of antitorque control must be used with a sufficient margin of power available to allow the helicopter to maintain its heading and provide yaw control. The three most common controls used today are the tail rotor, Eurocopter's ''Fenestron'' (also called a ''fantail''), and MD Helicopters' ''NOTAR''.

Tail 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 the center of gravity allow it to develop thrust in a direction opposite of the main rotor's rotation to counter the torque effect created by the main rotor. Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, 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.

Ducted 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 model Aérospatiale SA 341 Gazelle. Besides Eurocopter and its predecessors, a ducted fan tail rotor was also used on the canceled military helicopter project, the United States Army's RAH-66 Comanche, as the FANTAIL.

NOTAR

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 the Coandă effect.Frawley 2003, p. 151. A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and is driven by the main rotor transmission. To provide the sideways force to counteract the clockwise torque produced by a counterclockwise-spinning main rotor (as seen from above the main rotor), the variable-pitch fan forces low 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 the amount of airflow from the rotorwash. This is augmented by a direct jet thruster which also provides directional yaw control, with the presence of a fixed-surface empennage near the end of the tail, incorporating vertical stabilizers.

Development of the NOTAR system dates back to 1975 when engineers at Hughes Helicopters began concept development work. In December 1981, Hughes flew an OH-6A fitted with NOTAR for the first time. A more heavily modified prototype demonstrator first flew in March 1986 and successfully completed an advanced flight-test program, validating the system for future application in helicopter design."The Boeing Logbook: 1983-1987"
Boeing.com. Retrieved: 25 February 2007. There are currently three production helicopters that incorporate the NOTAR design, all produced by MD Helicopters. This antitorque design also improves safety by eliminating the possibility of personnel walking into the tail rotor.

A predecessor (of sorts) to this system existed in the form of Great Britain's Cierva W.9 helicopter, a late 1940s aircraft using the cooling fan from its piston engine to push air through a nozzle built into the tailboom to counteract rotor-torque.

Tip 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. The Hiller YH-32 Hornet had good lifting capability but performed poorly otherwise. Other aircraft used auxiliary thrust for translational flight so that the tip jets could be shut down while the rotor autorotated. The experimental Fairey Jet Gyrodyne, 48-seat Fairey Rotodyne

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