User:Born2flie/Helicopter

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Main article: Helicopter

Helicopter
The Bell 206 of Canadian Helicopters

A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors consisting of two or more rotor blades. Helicopters are classified as rotorcraft to distinguish them from fixed-wing aircraft because the helicopter derives its source of lift from the rotor blades rotating around a mast. In fact, the word 'helicopter' originates from the Greek words elikoeioas (helical or spiral) and pteron (wing or feather).^[1]

The primary advantages of the helicopter are due to its rotor, which provides lift in a vertical direction, giving it the ability to take off and land vertically and to maintain a steady hover in the air over a single point on the ground. This allows the helicopter to land and take off from pinnacles and confined areas that airplanes are not able to take off from, including heliports in the middle of busy cities and rugged terrain in remote areas. The helicopter is used for rescue, medical evacuation and as an observation platform. Other operations that involve the use of helicopters are fire fighting, tours, as an aerial crane, logging, personnel transport, electronic news gathering, law enforcement, military and for pleasure.

Although helicopters were developed and built during the first half century of flight, some even reaching limited production, it wasn't until 1942 that a helicopter designed by Igor Sikorsky became the first helicopter to enter full-scale production,^[2] totalling over 400 copies. Even though most previous designs utilized more than one main rotor, it was the single main rotor with antitorque tail rotor configuration of this design that would come to be recognized worldwide as the helicopter.

Contents 1 History 2 Helicopter configurations 2.1 Antitorque 2.1.1 Tail rotor 2.1.2 Fenestron 2.1.3 NOTAR 2.2 Dual rotors (contra-rotating) 2.2.1 Tandem 2.2.2 Coaxial 2.2.3 Intermeshing 2.2.4 Transverse 3 Rotor 3.1 Semirigid 3.2 Fully articulated 3.3 Rigid 3.4 Combination 4 Use 5 References 6 External links 7 See also 8 Gallery

[edit] History

Since 400 BC the Chinese had a bamboo flying top that was used as a children's toy. Eventually, this flying top toy made it to Europe,^{[citation needed]} and is depicted in a 1463 European painting.^{[citation needed]} Pao Phu Tau (???) was a 4th-century book in China describing some of the ideas inherent to rotary wing aircraft.^{[citation needed]} Around 1490, Leonardo da Vinci first conceived the semi-practical, manned helicopter.^{[citation needed]}

The word "helicopter" (hélicoptère) was coined in 1861 by Gustave de Ponton d'Amécourt,^[1] a french inventor who demonstrated a small steam-powered model. In 1907, the French inventor Paul Cornu made a helicopter that used two 20-foot (6-meter) counter-rotating rotors driven by a 24-hp (18-kW) Antoinette engine. It lifted its inventor to about five feet (1.5 meters) and remained aloft one minute.

In the early 1920s, Raúl Pateras de Pescara, an Argentinian working in Europe, demonstrated one of the first, successful applications of cyclic pitch.^[3] His coaxial, contra-rotating, biplane rotors were able to be warped to cyclically increase and decrease the lift they produced and the rotor hub could also tilt, both allowing the aircraft to move laterally without a separate propeller to push or pull it. Pescara is also credited with demonstrating the principle of autorotation, the method by which helicopters land safely after engine failure. By January 1924, Pescara's helicopter No. 3 was capable of flights up to 10 minutes. One of Pescara's contemporaries, a Frenchman Etienne Oemichen, set the first helicopter world record recognized by the Fédération Aéronautique Internationale on 14 April 1924, flying his helicopter 360 meters (1,181 feet). On 18 April 1924, Pescara beat Oemichen's record, flying for a distance of 736m (nearly a half mile) in 4 minutes and 11 seconds (about 8 mph, 13 km/h) maintaining a height of six feet.^[4] Not to be outdone, Oemichen reclaimed the world record on 4 May when he flew his No.2 machine again for a 14-minute flight covering 5,550 feet (1.05 mi, 1.692 km) while climbing to a height of 50 feet (15 meters).^[4] Oemichen also set the 1-km closed-circuit record at 7 minutes 40 seconds.^[5]

During this time that Juan de la Cierva was developing and introducing the first practical autogyro. In 1923, the rotorcraft that became the basis for the modern helicopter began to take shape, in the form of an autogyro.^[6] Cierva discovered aerodynamic and structural deficiencies in his early designs that could caus his autogyros to flip over after takeoff. The flapping hinges Cierva designed allowed the rotor to develop lift equally on the left and right halves of the rotor disk. A crash in 1927 led to the development of the drag hinge.^[6] These two developments allowed for a stable rotor system, not only in a hover, but in forward flight.

In 1922, Albert Gillis von Baumhauer, a Dutch aeronautical engineer, started studying VTOL rotor craft. His first prototype 'flew' ('hopped' and hovered really) on September 24, 1925, with Dutch Army-Air arm Captain Floris Albert van Heijst at the controls. The controls that Captain van Heijst used were Von Baumhauer's inventions, the cyclic and collective. Patents were granted Von Baumhauer by the British ministry of aviation on January 31, 1927, under number 265,272.

Soviet aeronautical engineers Boris Yuriev and Alexei Cheremukhin began experiments with the TsAGI 1-EA helicopter in 1931. A single rotor helicopter, with forward and aft anti-torque rotors, it reached an altitude of 605 meters (1,984 ft) on August 14, 1932 with Cheremukhin at the controls.^[7]

The German Focke-Wulf FW 61 was the first viable helicopter first flying in 1936. The FW-61 broke all of the helicopter world records in 1937. Nazi Germany used helicopters in small numbers during World War II. Models such the Flettner FL 282 Kolibri were used in the Mediterranean Sea, while the Focke-Achgelis Fa 223 Drache was used in Europe.

Mass production of the military version of the Sikorsky XR-4 began in May 1942 for the United States Army and was used over Burma for rescue duties.^[8] It was also used by the Royal Air Force, the first British military unit to be equipped with helicopters being the Helicopter Training School, formed in January 1945 at RAF Andover with nine R-4B Hoverfly I helicopters.

The Bell Model 47 designed by Arthur Young became the first helicopter, in March 1946, to be certified for civilian use in the United States. Two decades later the Bell 206 became the most successful commercial helicopter ever built with more hours and more industry records than any other aircraft in the world.

Reliable helicopters capable of stable hover flight were developed decades after fixed wing aircraft. This is largely due to higher engine power density requirements than fixed wing aircraft. Improvements in fuels and engines during the first half of the 20th century were a critical factor in helicopter development. The availability of lightweight turboshaft engines in the second half of the 20th century led to the development of larger, faster, and higher performance helicopters. Turboshaft engines are the preferred powerplant for all but the smallest and least expensive helicopters today.

[edit] Helicopter 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, designs from Germany, United Kingdom and the United States are said to rotate counter-clockwise, all others are said to rotate clockwise. This can make it difficult when discussing aerodynamic effects on the main rotor between different designs, since the effects may manifest on opposite sides of each aircraft.

[edit] Antitorque

Antitorque
Torque effect on a helicopter

With a single main rotor helicopter, the creation of torque as the engine turns the rotor creates a torque effect which 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 traditional tail rotor, Eurocopter's Fenestron™ (also called a fantail), and NOTAR®.

[edit] Tail rotor

The tail rotor is a smaller rotor mounted vertically or near-vertical on the tail of a traditional single-rotor helicopter. The tail rotor either pushes or pulls against the tail to counter the torque. The tail rotor drive system consists of a drive shaft powered from the main transmission and a gearbox mounted at the end of the tail boom. The drive shaft may consist of one long shaft or a series of shorter shafts connected at both ends with flexible couplings. The flexible couplings allow the drive shaft to flex with the tail boom. The gearbox at the end of the tailboom provides an angled drive for the tail rotor and may also include gearing to adjust the output to the optimum RPM for the tail rotor. On some larger helicopters, intermediate gearboxes are used to transition the tail rotor drive shaft from along the tailboom or tailcone to the top of the tail rotor pylon which also serves as a vertical stabilizing airfoil to alleviate the power requirement for the tail rotor in forward flight. It may also serve to provide limited antitorque within certain airspeed ranges in the event that the tail rotor or the tail rotor flight controls fail.

[edit] Fenestron

The term Fenestron™ is a trademark of Eurocopter.^[9]

A Fenestron (or Fantail) is a ducted fan mounted on the tail boom of the helicopter and used in place of a tail rotor. It's housing is integral with the tail skin, and while conventional tail rotors typically possess a maximum of 5 rotor blades, Fenestrons have between 8 and 18 blades. These are arranged in varying distance, so that the noise is distributed over different frequencies and thus appears quieter. The housing allows a higher rotational speed than a conventional rotor and therefore it can have a smaller size than a conventional rotor.

The Fenestron tail rotor was used for the first time at the end of the 1960s on the second experimental model of the SA 340, and on the later model Aérospatiale SA 341 Gazelle. Other than Eurocopter and its predecessors, a ducted fan tail rotor was also used on the US military helicopter project RAH-66 Comanche, which was cancelled in 2004.

[edit] NOTAR

The term NOTAR® is a registered trademark of McDonnell Douglas Helicopter Company.^[10]

NOTAR, an acronym for NO TAil Rotor, is a relatively new helicopter anti-torque system developed by Hughes Helicopters and currently produced by MD Helicopters which eliminates the use of the tail rotor on a helicopter.

Although the concept, which uses the Coandă effect, took some time to refine, the NOTAR system is simple in theory and works to provide antitorque the same way a wing develops lift.^[11] A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor transmission.

This 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) and vertical stabilizers.

Development of the NOTAR system dates back to 1975 when engineers at Hughes Helicopters began concept development work.^[11] In December 1981 Hughes flew a OH-6A fitted with NOTAR for the first time.^[12] 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.^[13]

There are three production helicopters that utilize the NOTAR system, all produced by MD Helicopters:

• MD 520N - a NOTAR variant of the Hughes/MD500 series helicopter is seen in the movie Mission: Impossible
• MD 600N - a larger version of the MD 520N, is seen in the James Bond film Die Another Day
• MD Explorer - a twin-engine, 8-seat light helicopter.

[edit] Dual rotors (contra-rotating)

Contra-rotating 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. Primarily, there are three common configurations that utilize the contra-rotating effect to benefit the rotorcraft; tandem rotors are two rotors with one mounted behind the other, coaxial rotors are two rotors that are mounted one above the other with the same axis, and intermeshing rotors are two rotors that are mounted close to each other at enough angle to allow the rotors to intermesh over the top of the aircraft. Another configuration found on tiltrotors and some earlier helicopters is called transverse rotors where the pair of rotors is mounted at each end of a wing-type structure or outriggers.

[edit] Tandem

Tandem rotors are two horizontal main rotor assemblies mounted one behind the other with the rear rotor mounted slightly higher than the front rotor. Tandem rotors achieve pitch attitude changes to accelerate and decelerate the helicopter through a process called differential collective pitch. To pitch forward and accelerate, the rear rotor increases collective pitch, raising the tail and the front rotor decreases collective pitch, simultaneously dipping the nose. To pitch upward while decelerating (or moving rearward), the front rotor increases collective pitch to raise the nose and the rear rotor decreases collective pitch 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 front rotor tilts left and the rear rotor tilts right.

[edit] Coaxial

Coaxial rotors are a pair of rotors turning in opposite directions, but mounted on a mast, with the same axis of rotation, one above the other. 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; retreating blade stall. However, other design considerations plague coaxial rotors. There is an increased mechanical complexity of the rotor system because it requires linkages and swashplates for two rotor systems. Add that each rotor system needs to be turned in opposite directions means that the mast itself is more complex, and provisions for making pitch changes to the upper rotor system must pass through the lower rotor system.

[edit] 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 successfully used in Nazi Germany for a small anti-submarine warfare helicopter, the Flettner Fl 282 Kolibri. During the Cold War, the American company, Kaman Aircraft produced the HH-43 Huskie for the USAF firefighting and rescue missions. The latest Kaman model, the Kaman K-MAX, is a dedicated sky crane design.

[edit] Transverse

Transverse rotors are mounted on the end of wings or outriggers, perpendicular to the body of the aircraft. Similar to tandem rotors and intermeshing rotors, the transverse rotor also utilizes differential collective pitch. But like the intermeshing rotors, the transverse rotors use the concept for changes in the roll attitude of the rotorcraft. This configuration is found on two of the first viable helicopters, 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 is also the configuration found on tiltrotors, such as Bell's XV-15 and the newer V-22 Osprey.

[edit] Rotor

The rotor system, or more simply rotor, is the rotating part of a rotorcraft which generates lift. A rotor system may be mounted horizontally as main rotors are, providing lift vertically, or it may be mounted vertically, such as a tail rotor, to provide lift horizontally as thrust to counteract torque effect. In the case of tiltrotors, the rotor is mounted on a nacelle that rotates at the edge of the wing to transition the rotor from a horizontal mounted position, providing lift horizontally as thrust, to a vertical mounted position providing lift exactly as a helicopter. Main rotor systems are classified according to how the main rotor blades move relative to the main rotor hub. There are three basic classifications: semirigid, rigid, or fully articulated, although some modern rotor systems use an engineered combination of these types.

[edit] Semirigid

A semirigid rotor system allows for two different movements, flapping and feathering. This system is normally comprised of two blades, which are rigidly attached to the rotor hub. The hub is then attached to the rotor mast by a trunnion bearing or teetering hinge and is free to tilt with respect to the main rotor shaft. This allows the blades to see-saw or flap together. As one blade flaps down, the other flaps up. Feathering is accomplished by the feathering hinge, which changes the pitch angle of the blade. Since there is no vertical drag hinge, lead-lag forces are absorbed through blade bending.

Helicopters with semi-rigid rotors are vulnerable to a condition known as mast bumping which can cause the rotor flap stops to shear the mast. Mast bumping is normally encountered during low-G maneuvers, so it is written into the operator's handbook to avoid any low-G conditions.

[edit] Fully articulated

In a fully articulated rotor system, each rotor blade is attached to the rotor hub through a series of hinges, which allow the blade to 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 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 the acceleration and deceleration caused by Coriolis Effect. Each blade can also be feathered, that is, rotated around its spanwise axis. Feathering the blade means changing the pitch angle of the blade. By changing the pitch angle of the blades you can control the thrust and direction of the main rotor disc.

[edit] Rigid

In a rigid rotor system, the blades, hub, and mast are rigid with respect to each other. The rigid rotor system is mechanically simpler than the fully articulated rotor system. There are no vertical or horizontal hinges so the blades cannot flap or drag, but they can be feathered. Operating loads from flapping and lead/lag forces must be absorbed by bending rather than through hinges. By flexing, the blades themselves compensate for the forces which previously required rugged hinges. The result is a rotor system that has less lag in the control response, because the rotor has much less oscillation.^[14] The rigid rotor system also negates the danger of mast bumping inherent in semi-rigid rotors.^[15]

[edit] 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 flextures, are usually constructed from composite material. Elastomeric bearings may also be used in place of conventional roller bearings. Elastomeric bearings are bearings constructed from a rubber type material and have limited movement that is perfectly suited for helicopter applications. Flextures 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.

[edit] Use

• Aerial crane (helicopter)
• Air ambulance
• Search and Rescue
• Military helicopters

[edit] References

• Thicknesse P, Jones A et al, Military Rotorcraft, 2nd edition, 2000, Brassey's World Military Technology series, Shirvenham UK, xvi + 160pp, ISBN 1-85753-325-9
• Wragg D, Helicopters at War: A pictorial history, 1983, Robert Hale Ltd, London UK, 283pp, ISBN 0-7090-0858-9

[edit] External links

• American Helicopter Society
• Helicopter Association International
• Helicopter history
• All the World's Rotorcraft
• Image of a Chinese flying top
• Gyrodyne Helicopter
• Rotorfly
• Kamov Helicopters
• Kumertau Helicopters
• MD Helicopters
• Boeing PAV
• Rotor Analysis - Blade Element Momentum Theory

[edit] See also

• U.S. Patent 1848389  : "Aircraft, especially aircraft of the direct lift amphibian type and means of construction and operating the same"
• Rotorcraft
• Attack Helicopter
• Autogyro
• Autorotation
• Gyrodyne
• Helicopter rotor
• Helicopter pilotage
• Military helicopters
• Phase lag (rotorcraft)
• Rotorcraft
• Tiltrotor
• Transverse Flow Effect

[edit] Gallery

The major components of a helicopter.	Bell 206B-3 JetRanger III.	MD 600N (Helicopters of America).	Controls of an Alouette III.
Enstrom 280FX Shark.	Kamov Ka-50 with contra-rotating co-axial rotors.	Sikorsky S-92.	HAL Dhruv.
Westland Scout AH.1 (XV134).	Bell 407.	SH-60 prepares to lift off from the flight deck.	The Bristol Type 192 Belvedere.
Sikorsky S-65.	HAL Dhruv helicopters.

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*Helicopters Category:Firefighting equipment

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