Helicopter


A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft and many forms of short take-off and landing or short take-off and vertical landing aircraft cannot perform without a runway.
The Focke-Wulf Fw 61 was the first successful, practical, and fully controllable helicopter in 1936, while in 1942, the Sikorsky R-4 became the first helicopter to reach full-scale production. Starting in 1939 and through 1943, Igor Sikorsky worked on the development of the VS-300, which over four iterations, became the basis for modern helicopters with a single main rotor and a single tail rotor.
Although most earlier designs used more than one main rotor, the configuration of a single main rotor accompanied by a vertical anti-torque tail rotor has become the most common helicopter configuration. However, twin-rotor helicopters, in either tandem or transverse rotors configurations, are sometimes in use due to their greater payload capacity than the monorotor design, and coaxial-rotor, tiltrotor and compound helicopters are also all flying today. Four-rotor helicopters were pioneered as early as 1907 in France, and along with other types of multicopters, have been developed mainly for specialized applications such as commercial unmanned aerial vehicles due to the rapid expansion of drone racing and aerial photography markets in the early 21st century, as well as recently weaponized utilities such as artillery spotting, aerial bombing and suicide attacks.

Etymology

The English word helicopter is adapted from the French word hélicoptère, coined by Gustave Ponton d'Amécourt in 1861, which originates from the Greek ', genitive ''helikos, "helix, spiral, whirl, convolution" and ' "wing". In a process of rebracketing, the word is often perceived by English speakers as consisting of heli- and -copter, leading to words like helipad and quadcopter''. English language nicknames for "helicopter" include "chopper", "copter", "heli", and "whirlybird". In the United States military, the common slang is "helo" pronounced /ˈhiː.loʊ/.

Design

A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more horizontally-spinning rotors. By contrast the autogyro and gyrodyne have a free-spinning rotor for all or part of the flight envelope, relying on a separate thrust system to propel the craft forwards, so that the airflow sets the rotor spinning to provide lift. The compound helicopter also has a separate thrust system, but continues to supply power to the rotor throughout normal flight. U.S. federal regulations state that "helicopter" means a rotorcraft that, for its horizontal motion, depends principally on its engine-driven rotors.

Rotor system

The rotor system, or more simply rotor, is the rotating part of a helicopter that 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 horizontal thrust to counteract torque from the main rotors. The rotor consists of a mast, hub and rotor blades.
The mast is a cylindrical metal shaft that extends upwards from the transmission. At the top of the mast is the attachment point for the rotor blades called the hub. Main rotor systems are classified according to how the rotor blades are attached and move relative to the hub. There are three basic types: hingeless, fully articulated, and teetering; although some modern rotor systems use a combination of these.

Anti-torque

Most helicopters have a single main rotor, but torque created by its aerodynamic drag must be countered by an opposed torque. The design that Igor Sikorsky settled on for his VS-300 was a smaller tail rotor. The tail rotor pushes or pulls against the tail to counter the torque effect, and this has become the most common configuration for helicopter design, usually at the end of a tail boom.
Some helicopters use other anti-torque controls instead of the tail rotor, such as the ducted fan and NOTAR. NOTAR provides anti-torque similar to the way a wing develops lift through the use of the Coandă effect on the tail boom.
File:Belgium Police 520N.jpg|thumb|MD 520N NOTAR
The use of two or more horizontal rotors turning in opposite directions is another configuration used to counteract the effects of torque on the aircraft without relying on an anti-torque tail rotor. This allows the power normally required to be diverted for the tail rotor to be applied fully to the main rotors, increasing the aircraft's power efficiency and lifting capacity. There are several common configurations that use the counter-rotating effect to benefit the rotorcraft:
  • Tandem rotors are two counter-rotating rotors with one mounted behind the other.
  • Transverse rotors are pair of counter-rotating rotors transversely mounted at the ends of fixed wings or outrigger structures. Now used on tiltrotors, some early model helicopters had used them.
  • Coaxial rotors are two counter-rotating rotors mounted one above the other with the same axis.
  • Intermeshing rotors are two counter-rotating rotors mounted close to each other at a sufficient angle to let the rotors intermesh over the top of the aircraft without colliding. An aircraft utilizing this is known as a synchropter.
  • Multirotors make use of three or more rotors. Specific terms are also used depending on the exact amount of rotors, such as tricopter, quadcopter, hexacopter and octocopter for three rotors, four rotors, six rotors and eight rotors respectively, of which quadcopter is the most common. Multirotors are primarily used on drones and use on aircraft with a human pilot is rare.
Tip jet designs let the rotor push itself through the air and avoid generating torque.

Engines

The number, size and type of engine used on a helicopter determines the size, function and capability of that helicopter design. The earliest helicopter engines were simple mechanical devices, such as rubber bands or spindles, which relegated the size of helicopters to toys and small models. For a half century before the first airplane flight, steam engines were used to forward the development of the understanding of helicopter aerodynamics, but the limited power did not allow for manned flight. The introduction of the internal combustion engine at the end of the 19th century became the watershed for helicopter development as engines began to be developed and produced that were powerful enough to allow for helicopters able to lift humans.
Early helicopter designs utilized custom-built engines or rotary engines designed for airplanes, but these were soon replaced by more powerful automobile engines and radial engines. The single, most-limiting factor of helicopter development during the first half of the 20th century was that the amount of power produced by an engine was not able to overcome the engine's weight in vertical flight. This was overcome in early successful helicopters by using the smallest engines available. When the compact, flat engine was developed, the helicopter industry found a lighter-weight powerplant easily adapted to small helicopters, although radial engines continued to be used for larger helicopters.
Turbine engines revolutionized the aviation industry; and the turboshaft engine for helicopter use, pioneered in December 1951 by the aforementioned Kaman K-225, finally gave helicopters an engine with a large amount of power and a low weight penalty. Turboshafts are also more reliable than piston engines, especially when producing the sustained high levels of power required by a helicopter. The turboshaft engine was able to be scaled to the size of the helicopter being designed, so that all but the lightest of helicopter models are powered by turbine engines today.
Special jet engines developed to drive the rotor from the rotor tips are referred to as tip jets. Tip jets powered by a remote compressor are referred to as cold tip jets, while those powered by combustion exhaust are referred to as hot tip jets. An example of a cold jet helicopter is the Sud-Ouest Djinn, and an example of the hot tip jet helicopter is the YH-32 Hornet.
Some radio-controlled helicopters and smaller, helicopter-type unmanned aerial vehicles, use electric motors or motorcycle engines. Radio-controlled helicopters may also have piston engines that use fuels other than gasoline, such as nitromethane. Some turbine engines commonly used in helicopters can also use biodiesel instead of jet fuel.
There are also human-powered helicopters.

Transmission

The transmission is a mechanical system that transmits power from the engine to the rotors. The transmission is a system of gears, bearings, clutches and shafts that performs several functions Translates the alignment of the drive shaft to match the alignment of the rotor shafts; Reduces the RPM of the drive shaft to the lower RPMs of the rotors; and Enables the engine to engage or disengage from the rotors. For helicopters with tail rotors, the transmission drivetrain forks into two paths: one leading to the main rotor, and one leading to the tail rotor.
The drive shafts of helicopter engines are typically not aligned with the rotor shafts, so the transmission must translate the alignment of the drive shaft to match the shafts of the rotors. Many engine drive shafts are aligned horizontally, yet the main rotor shaft is usually vertical, and the tail rotor shaft is often perpendicular to the engine's drive shaft. The transmission contains a series of gears, usually bevel gears, that translate the alignment of the drive shaft to the alignment of the rotor shafts.
The transmission also reduces the RPMs of the engine to the lower RPMs required by the rotors. The output drive shaft of the engine, before any gearing is applied, is typically between 3,000 and 50,000 RPM. The main rotor typically rotates between 300 and 600 RPM. The tail rotor, if present, usually rotates between 1,000 and 5,000 RPM. . The transmission contains a series of reduction gears to reduce the engine RPM to the rotor RPMs. Several types of reduction gears may be used, including bevel gears, planetary gears, helical gears, and spur gears. Most transmissions contain several reduction gears: the engine itself may contain reduction gears between the engine's internal shaft and the output drive shaft; the main rotor may have a reduction gear at its base ; and there may be reduction gears at the tail rotor, and on the shaft leading to the tail rotor.
The transmission often includes one or more clutches, which permit the rotors to engage or disengage from the engine. A clutch is required so the engine can start up and gain speed before taking the load of the rotors. A clutch is also required in the case of engine failure: in that situation, the rotors must disengage from the engine so that the rotors can continue spinning and perform autorotation. Helicopter clutches are usually freewheel clutches relying on centrifugal forces, but belt drive clutches are also used.