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OR:

A helicopter is a type of
rotorcraft A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Interna ...
in which
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...
and
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that s ...
are supplied by horizontally spinning
rotor Rotor may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...
s. 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 A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are disti ...
and many forms of
STOL A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh cond ...
(Short TakeOff and Landing) or
STOVL A short take-off and vertical landing aircraft (STOVL aircraft) is a fixed-wing aircraft that is able to take off from a short runway (or take off vertically if it does not have a heavy payload) and land vertically (i.e. with no runway). The ...
(Short TakeOff and Vertical Landing) aircraft cannot perform without a
runway According to the International Civil Aviation Organization (ICAO), a runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft". Runways may be a man-made surface (often asphalt, concrete ...
. In 1942, the Sikorsky R-4 became the first helicopter to reach full-scale
production Production may refer to: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, the act of making products (goods and services) * Production as a stat ...
.Munson 1968.Hirschberg, Michael J. and David K. Dailey
"Sikorsky"
. ''US and Russian Helicopter Development in the 20th Century'', American Helicopter Society, International. 7 July 2000.
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 The tail rotor is a smaller rotor mounted vertically or near-vertically at the tail of a traditional single-rotor helicopter, where it rotates to generate a propeller-like horizontal thrust in the same direction as the main rotor's rotation. ...
(i.e. unicopter, not to be confused with the single-blade
monocopter A monocopter or gyropter is a rotorcraft that uses a single rotating blade. The concept is similar to the whirling helicopter seeds that fall from some trees. The name gyropter is sometimes applied to monocopters in which the entire aircraft rota ...
) has become the most common helicopter configuration. However, twin-main rotor helicopters (bicopters), in either
tandem Tandem, or in tandem, is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction. The original use of the term in English was in ''tandem harness'', which is used for two ...
or
transverse rotors Transverse rotor aircraft have two large horizontal rotor assemblies mounted side by side. Single-rotor helicopters (unicopters) need an additional tail rotor or tail exhaust to neutralize the reactional angular momentum produced by the ...
configurations, are sometimes in use due to their greater payload capacity than the monorotor design, and coaxial-rotor,
tiltrotor A tiltrotor is an aircraft which generates lift and propulsion by way of one or more powered rotors (sometimes called ''proprotors'') mounted on rotating shafts or nacelles usually at the ends of a fixed wing. Almost all tiltrotors use a ...
and compound helicopters are also all flying today.
Quadrotor A quadcopter or quadrotor is a type of helicopter with four rotors. Although quadrotor helicopters and convertiplanes have long been flown experimentally, the configuration remained a curiosity until the arrival of the modern UAV or drone. ...
helicopters (
quadcopter A quadcopter or quadrotor is a type of helicopter with four rotors. Although quadrotor helicopters and convertiplanes have long been flown experimentally, the configuration remained a curiosity until the arrival of the modern UAV or drone. ...
s) were pioneered as early as 1907 in France, and along with other types of
multicopter A multirotor or multicopter is a rotorcraft with more than two lift-generating rotors. An advantage of multirotor aircraft is the simpler rotor mechanics required for flight control. Unlike single- and double-rotor helicopters which use complex ...
s, have been developed mainly for specialized applications such as
drone Drone most commonly refers to: * Drone (bee), a male bee, from an unfertilized egg * Unmanned aerial vehicle * Unmanned surface vehicle, watercraft * Unmanned underwater vehicle or underwater drone Drone, drones or The Drones may also refer to: ...
s.

# Etymology

The English word ''helicopter'' is adapted from the French word , coined by Gustave Ponton d'Amécourt in 1861, which originates from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
' () "helix, spiral, whirl, convolution" and ' () "wing". For various reasons, the word is often erroneously, from an etymological point of view, analysed by English speakers into ''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 The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territor ...
military, the common slang is "helo" pronounced with a long "e".

# Design characteristics

A helicopter is a type of
rotorcraft A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Interna ...
in which lift and thrust are supplied by one or more horizontally-spinning rotors. By contrast the
autogyro An autogyro (from Greek and , "self-turning"), also known as a ''gyroplane'', is a type of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. Forward thrust is provided independently, by an engine-driven propeller. ...
(or gyroplane) and
gyrodyne A gyrodyne is a type of VTOL aircraft with a helicopter rotor-like system that is driven by its engine for takeoff and landing only, and includes one or more conventional propeller or jet engines to provide forward thrust during cruising fligh ...
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.

## Rotor system

The rotor system, or more simply ''rotor'', is the rotating part of a helicopter that generates
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...
. 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 In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...
must be countered by an opposed torque. The design that
Igor Sikorsky Igor Ivanovich Sikorsky (russian: И́горь Ива́нович Сико́рский, p=ˈiɡərʲ ɪˈvanəvitʃ sʲɪˈkorskʲɪj, a=Ru-Igor Sikorsky.ogg, tr. ''Ígor' Ivánovich Sikórskiy''; May 25, 1889 – October 26, 1972)Fortie ...
settled on for his
VS-300 The Vought-Sikorsky VS-300 (or S-46) is an American single-engine helicopter designed by Igor Sikorsky. It had a single three-blade rotor originally powered by a 75 horsepower (56  kW) engine. The first "free" flight of the VS-300 was on 13 ...
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 In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing (VTOL) applicati ...
(called ''
Fenestron A Fenestron (sometimes alternatively referred to as a fantail or a "fan-in-fin" arrangementLeishman 2006, p. 321.) is an enclosed helicopter tail rotor that operates like a ducted fan. The term ''Fenestron'' is a trademark of multinational hel ...
'' or ''FANTAIL'') and
NOTAR NOTAR ("no tail rotor") is a helicopter system which avoids the use of a tail rotor. It was developed by McDonnell Douglas Helicopter Systems (through their acquisition of Hughes Helicopters). The system uses a fan inside the tail boom to buil ...
. NOTAR provides anti-torque similar to the way a wing develops lift through the use of the Coandă effect on the tail boom.Frawley 2003, p. 151. 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 Tandem, or in tandem, is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction. The original use of the term in English was in ''tandem harness'', which is used for two ...
are two counter-rotating rotors with one mounted behind the other. *
Transverse rotors Transverse rotor aircraft have two large horizontal rotor assemblies mounted side by side. Single-rotor helicopters (unicopters) need an additional tail rotor or tail exhaust to neutralize the reactional angular momentum produced by the ...
are pair of counter-rotating rotors transversely mounted at the ends of fixed wings or outrigger structures. Now used on
tiltrotor A tiltrotor is an aircraft which generates lift and propulsion by way of one or more powered rotors (sometimes called ''proprotors'') mounted on rotating shafts or nacelles usually at the ends of a fixed wing. Almost all tiltrotors use a ...
s, some early model helicopters had used them. *
Coaxial rotors Coaxial rotors or coax rotors are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions ( contra-rotating). This rotor configuration is a feature of helico ...
are two counter-rotating rotors mounted one above the other with the same axis. *
Intermeshing rotors Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding. T ...
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. Aircraft utilizing this is known as a synchropter. *
Multirotor A multirotor or multicopter is a rotorcraft with more than two lift-generating rotors. An advantage of multirotor aircraft is the simpler rotor mechanics required for flight control. Unlike single- and double-rotor helicopter A helico ...
s make use of three or more rotors. Specific terms are also used depending on the exact amount of rotors, such as tricopter,
quadcopter A quadcopter or quadrotor is a type of helicopter with four rotors. Although quadrotor helicopters and convertiplanes have long been flown experimentally, the configuration remained a curiosity until the arrival of the modern UAV or drone. ...
, 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 A tip jet is a jet nozzle at the tip of some helicopter rotor blades, used to spin the rotor, much like a Catherine wheel firework. Tip jets replace the normal shaft drive and have the advantage of placing no torque on the airframe, thus not re ...
designs let the rotor push itself through the air and avoid generating torque.

## Engines

The number, size and type of engine(s) 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 An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
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 engine The rotary engine is an early type of internal combustion engine, usually designed with an odd number of cylinders per row in a radial configuration. The engine's crankshaft remained stationary in operation, while the entire crankcase and its ...
s 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 A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, ...
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 jet A tip jet is a jet nozzle at the tip of some helicopter rotor blades, used to spin the rotor, much like a Catherine wheel firework. Tip jets replace the normal shaft drive and have the advantage of placing no torque on the airframe, thus not re ...
s. 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 helicopter A radio-controlled helicopter (also '' RC helicopter'') is model aircraft which is distinct from a RC airplane because of the differences in construction, aerodynamics, and flight training. Several basic designs of RC helicopters exist, of ...
s and smaller, helicopter-type
unmanned aerial vehicle An unmanned aerial vehicle (UAV), commonly known as a drone, is an aircraft without any human pilot, crew, or passengers on board. UAVs are a component of an unmanned aircraft system (UAS), which includes adding a ground-based controlle ...
s, use
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate fo ...
s or motorcycle engines. Radio-controlled helicopters may also have
piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fe ...
s that use fuels other than gasoline, such as
nitromethane Nitromethane, sometimes shortened to simply "nitro", is an organic compound with the chemical formula . It is the simplest organic nitro compound. It is a polar liquid commonly used as a solvent in a variety of industrial applications such as in ...
. Some turbine engines commonly used in helicopters can also use biodiesel instead of jet fuel. There are also human-powered helicopters.

## Flight controls

A helicopter has four flight control inputs. These are the cyclic, the collective, the anti-torque pedals, and the throttle. The cyclic control is usually located between the pilot's legs and is commonly called the ''cyclic stick'' or just ''cyclic''. On most helicopters, the cyclic is similar to a joystick. However, the
Robinson R22 The Robinson R22 is a two-seat, two-bladed, single-engine light utility helicopter manufactured by Robinson Helicopter Company. It was designed in 1973 by Frank D. Robinson, and has been in production since 1979. Development The majority of fl ...
and
Robinson R44 The Robinson R44 is a four-seat light helicopter produced by Robinson Helicopter Company since 1992. Based on the company's two-seat Robinson R22, the R44 features hydraulically assisted flight controls. It was first flown on 31 March 1990 a ...
have a unique teetering bar cyclic control system and a few helicopters have a cyclic control that descends into the cockpit from overhead. The control is called the cyclic because it changes cyclic pitch of the main blades. The result is to tilt the rotor disk in a particular direction, resulting in the helicopter moving in that direction. If the pilot pushes the cyclic forward, the rotor disk tilts forward, and the rotor produces a thrust in the forward direction. If the pilot pushes the cyclic to the side, the rotor disk tilts to that side and produces thrust in that direction, causing the helicopter to hover sideways. The collective pitch control or ''collective'' is located on the left side of the pilot's seat with a settable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e. all at the same time) and independently of their position. Therefore, if a collective input is made, all the blades change equally, and the result is the helicopter increasing or decreasing in altitude. A swashplate controls the collective and cyclic pitch of the main blades. The swashplate moves up and down, along the main shaft, to change the pitch of both blades. This causes the helicopter to push air downward or upward, depending on the
angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is ...
. The swashplate can also change its angle to move the blades angle forwards or backwards, or left and right, to make the helicopter move in those directions. The anti-torque pedals are located in the same position as the
rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adverse ...
pedals in a fixed-wing aircraft, and serve a similar purpose, namely to control the direction in which the nose of the aircraft is pointed. Application of the pedal in a given direction changes the pitch of the tail rotor blades, increasing or reducing the thrust produced by the tail rotor and causing the nose to yaw in the direction of the applied pedal. The pedals mechanically change the pitch of the tail rotor altering the amount of thrust produced. Helicopter rotors are designed to operate in a narrow range of
RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimension ...
.Croucher, Phil
Professional helicopter pilot studies
page 2-11. . Quote: otor speed"is constant in a helicopter".
Johnson, Pam
Delta D2
page 44 ''Pacific Wings''. Retrieved 2 January 2010
John M. Seddon, Simon Newman
Basic Helicopter Aerodynamics
p216, ''
John Wiley and Sons John Wiley & Sons, Inc., commonly known as Wiley (), is an American multinational publishing company founded in 1807 that focuses on academic publishing and instructional materials. The company produces books, journals, and encyclopedias, ...
'', 2011. Retrieved 25 February 2012. . Quote: "The rotor is best served by rotating at a constant rotor speed"
The throttle controls the power produced by the engine, which is connected to the rotor by a fixed ratio transmission. The purpose of the throttle is to maintain enough engine power to keep the rotor RPM within allowable limits so that the rotor produces enough lift for flight. In single-engine helicopters, the throttle control is a motorcycle-style
twist grip A twistgrip is a handle that can be twisted to operate a control. It is commonly found as a motorcycle's right handlebar grip to control the throttle, but is sometimes found elsewhere, such as on a bicycle as a gearshift, and in helicopters. His ...
mounted on the collective control, while dual-engine helicopters have a power lever for each engine.

## Compound helicopter

A compound helicopter has an additional system for thrust and, typically, small stub fixed wings. This offloads the rotor in cruise, which allows its rotation to be slowed down, thus increasing the maximum speed of the aircraft. The Lockheed AH-56A Cheyenne diverted up to 90% of its engine power to a
pusher propeller In an aircraft with a pusher configuration (as opposed to a tractor configuration), the propeller(s) are mounted behind their respective engine(s). Since a pusher propeller is mounted behind the engine, the drive shaft is in compression in no ...
during forward flight.

# Flight

There are three basic flight conditions for a helicopter: hover, forward
flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This c ...
and the transition between the two.

## Hover

Hovering is the most challenging part of flying a helicopter. This is because a helicopter generates its own gusty air while in a hover, which acts against the
fuselage The fuselage (; from the French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraft ...
and flight control surfaces. The end result is constant control inputs and corrections by the pilot to keep the helicopter where it is required to be. Despite the complexity of the task, the control inputs in a hover are simple. The cyclic is used to eliminate drift in the horizontal plane, that is to control forward and back, right and left. The collective is used to maintain altitude. The pedals are used to control nose direction or
heading Heading can refer to: * Heading (metalworking), a process which incorporates the extruding and upsetting processes * Headline, text at the top of a newspaper article * Heading (navigation), the direction a person or vehicle is facing, usually s ...
. It is the interaction of these controls that makes hovering so difficult, since an adjustment in any one control requires an adjustment of the other two, creating a cycle of constant correction.

## Transition from hover to forward flight

As a helicopter moves from hover to forward flight it enters a state called translational lift which provides extra lift without increasing power. This state, most typically, occurs when the airspeed reaches approximately , and may be necessary for a helicopter to obtain flight.

## Forward flight

In forward flight a helicopter's flight controls behave more like those of a fixed-wing aircraft. Applying forward pressure on the cyclic will cause the nose to pitch down, with a resultant increase in airspeed and loss of altitude. Aft cyclic will cause the nose to pitch up, slowing the helicopter and causing it to climb. Increasing collective (power) while maintaining a constant airspeed will induce a climb while decreasing collective will cause a descent. Coordinating these two inputs, down collective plus aft cyclic or up collective plus forward cyclic, will result in airspeed changes while maintaining a constant altitude. The pedals serve the same function in both a helicopter and a fixed-wing aircraft, to maintain balanced flight. This is done by applying a pedal input in whichever direction is necessary to center the ball in the
turn and bank indicator In aviation, the turn and slip indicator (T/S, a.k.a. turn and bank indicator) and the turn coordinator (TC) variant are essentially two aircraft flight instruments in one device. One indicates the rate of turn, or the rate of change in the aircr ...
.

# Uses

Due to the operating characteristics of the helicopter—its ability to take off and land vertically, and to hover for extended periods of time, as well as the aircraft's handling properties under low
airspeed In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: * Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system; * Calibrat ...
conditions—it has proved advantageous to conduct tasks that were previously not possible with other aircraft, or were time- or work-intensive to accomplish on the ground. Today, helicopter uses include
transport Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land ( rail and road), water, cable, pipe ...
ation of people and cargo, military uses, construction, firefighting,
search and rescue Search and rescue (SAR) is the search for and provision of aid to people who are in distress or imminent danger. The general field of search and rescue includes many specialty sub-fields, typically determined by the type of terrain the search ...
,
tourism Tourism is travel for pleasure or business; also the theory and practice of touring, the business of attracting, accommodating, and entertaining tourists, and the business of operating tours. The World Tourism Organization defines tourism ...
, medical transport, law enforcement, agriculture,
news News is information about current events. This may be provided through many different media: word of mouth, printing, postal systems, broadcasting, electronic communication, or through the testimony of observers and witnesses to events. ...
and
media Media may refer to: Communication * Media (communication), tools used to deliver information or data ** Advertising media, various media, content, buying and placement for advertising ** Broadcast media, communications delivered over mass ...
, and
aerial observation Aerial reconnaissance is reconnaissance for a military or strategic purpose that is conducted using reconnaissance aircraft. The role of reconnaissance can fulfil a variety of requirements including artillery spotting, the collection of imag ...
, among others. A helicopter used to carry loads connected to long cables or slings is called an aerial crane. Aerial cranes are used to place heavy equipment, like radio transmission towers and large air conditioning units, on the tops of tall buildings, or when an item must be raised up in a remote area, such as a radio tower raised on the top of a hill or mountain. Helicopters are used as aerial cranes in the logging industry to lift trees out of terrain where vehicles cannot travel and where environmental concerns prohibit the building of roads. These operations are referred to as longline because of the long, single sling line used to carry the load. In military service helicopters are often useful for delivery of outsized slung loads that would not fit inside ordinary cargo aircraft: artillery pieces, large machinery (field radars, communications gear, electrical generators), or pallets of bulk cargo. In military operations these payloads are often delivered to remote locations made inaccessible by mountainous or riverine terrain, or naval vessels at sea. The largest single non-combat helicopter operation in history was the disaster management operation following the 1986 Chernobyl nuclear disaster. Hundreds of pilots were involved in
airdrop An airdrop is a type of airlift in which items including weapons, equipment, humanitarian aid or leaflets are delivered by military or civilian aircraft without their landing. Developed during World War II to resupply otherwise inaccessibl ...
and observation missions, making dozens of sorties a day for several months. "
Helitack Helitack crews are teams of wildland firefighters who are transported by helicopter to wildfires. Helicopters provide rapid transport, enabling helitack crews to quickly respond and assess a wildfire situation. Helitack crews may land near a w ...
" is the use of helicopters to combat
wildland fires A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identif ...
.Butler, Bret W. et al
"Appendix A: Glossary: Fire Behavior Associated with the 1994 South Canyon Fire on Storm King Mountain, Colorado research paper"
''U.S. Dept. of Agriculture, Forest Service'', September 1998. Retrieved 2 November 2008.
The helicopters are used for
aerial firefighting Aerial may refer to: Music * ''Aerial'' (album), by Kate Bush * ''Aerials'' (song), from the album ''Toxicity'' by System of a Down Bands * Aerial (Canadian band) * Aerial (Scottish band) * Aerial (Swedish band) Performance art * Aerial si ...
(water bombing) and may be fitted with tanks or carry helibuckets. Helibuckets, such as the Bambi bucket, are usually filled by submerging the bucket into lakes, rivers, reservoirs, or portable tanks. Tanks fitted onto helicopters are filled from a hose while the helicopter is on the ground or water is siphoned from lakes or reservoirs through a hanging snorkel as the helicopter hovers over the water source. Helitack helicopters are also used to deliver firefighters, who
rappel Abseiling ( ; ), also known as rappelling ( ; ), is the controlled descent of a steep slope, such as a rock face, by moving down a rope. When abseiling the person descending controls their own movement down the rope, in contrast to low ...
down to inaccessible areas, and to resupply firefighters. Common firefighting helicopters include variants of the Bell 205 and the Erickson S-64 Aircrane helitanker. Helicopters are used as
air ambulance Air medical services is a comprehensive term covering the use of air transportation, aeroplane or helicopter, to move patients to and from healthcare facilities and accident scenes. Personnel provide comprehensive prehospital and emergency and cri ...
s for emergency medical assistance in situations when an
ambulance An ambulance is a medically equipped vehicle which transports patients to treatment facilities, such as hospitals. Typically, out-of-hospital medical care is provided to the patient during the transport. Ambulances are used to respond to medi ...
cannot easily or quickly reach the scene, or cannot transport the patient to a medical facility in time. Helicopters are also used when patients need to be transported between medical facilities and air transportation is the most practical method. An air ambulance helicopter is equipped to stabilize and provide limited medical treatment to a patient while in flight. The use of helicopters as air ambulances is often referred to as "
MEDEVAC Medical evacuation, often shortened to medevac or medivac, is the timely and efficient movement and en route care provided by medical personnel to wounded being evacuated from a battlefield, to injured patients being evacuated from the scene of a ...
", and patients are referred to as being "airlifted", or "medevaced". This use was pioneered in the
Korean War {{Infobox military conflict , conflict = Korean War , partof = the Cold War and the Korean conflict , image = Korean War Montage 2.png , image_size = 300px , caption = Clockwise from top:{ ...
, when time to reach a medical facility was reduced to three hours from the eight hours needed in
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...
, and further reduced to two hours by the
Vietnam War The Vietnam War (also known by other names) was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vie ...
. In naval service a prime function of rescue helicopters is to promptly retrieve downed aircrew involved in crashes occurring upon launch or recovery aboard aircraft carriers. In past years this function was performed by destroyers escorting the carrier, but since then helicopters have proved vastly more effective. Police departments and other law enforcement agencies use helicopters to pursue suspects. Since helicopters can achieve a unique aerial view, they are often used in conjunction with police on the ground to report on suspects' locations and movements. They are often mounted with lighting and heat-sensing equipment for night pursuits. Military forces use
attack helicopter An attack helicopter is an armed helicopter with the primary role of an attack aircraft, with the offensive capability of engaging ground targets such as enemy infantry, military vehicles and fortifications. Due to their heavy armament th ...
s to conduct aerial attacks on ground targets. Such helicopters are mounted with missile launchers and
minigun The M134 Minigun is an American 7.62×51mm NATO six-barrel rotary machine gun with a high rate of fire (2,000 to 6,000 rounds per minute). It features a Gatling-style rotating barrel assembly with an external power source, normally an electri ...
s. Transport helicopters are used to ferry troops and supplies where the lack of an
airstrip An aerodrome ( Commonwealth English) or airdrome (American English) is a location from which aircraft flight operations take place, regardless of whether they involve air cargo, passengers, or neither, and regardless of whether it is for pub ...
would make transport via fixed-wing aircraft impossible. The use of transport helicopters to deliver troops as an attack force on an objective is referred to as "
air assault Air assault is the movement of ground-based military forces by vertical take-off and landing (VTOL) aircraft—such as the helicopter—to seize and hold key terrain which has not been fully secured, and to directly engage enemy forces behind ...
". Unmanned aerial systems (UAS) helicopter systems of varying sizes are developed by companies for military
reconnaissance In military operations, reconnaissance or scouting is the exploration of an area by military forces to obtain information about enemy forces, terrain, and other activities. Examples of reconnaissance include patrolling by troops ( skirmishe ...
and
surveillance Surveillance is the monitoring of behavior, many activities, or information for the purpose of information gathering, influencing, managing or directing. This can include observation from a distance by means of electronic equipment, such as ...
duties. Naval forces also use helicopters equipped with
dipping sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects on ...
for
anti-submarine warfare Anti-submarine warfare (ASW, or in older form A/S) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, or destroy enemy submarines. Such operations are ty ...
, since they can operate from small ships. Oil companies charter helicopters to move workers and parts quickly to remote drilling sites located at sea or in remote locations. The speed advantage over boats makes the high operating cost of helicopters cost-effective in ensuring that
oil platform An oil platform (or oil rig, offshore platform, oil production platform, and similar terms) is a large structure with facilities to extract and process petroleum and natural gas that lie in rock formations beneath the seabed. Many oil platfo ...
s continue to operate. Various companies specialize in this type of operation.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...
is developing the Mars Helicopter, a helicopter to be launched to survey Mars (along with a rover) in 2020. Given that the Martian atmosphere is 100 times thinner than that of Earth's, its two blades will spin at close to 3,000 revolutions a minute, approximately 10 times faster than that of a terrestrial helicopter. In
electronic news-gathering Electronic news-gathering (ENG) or electronic journalism (EJ) is usage of electronic video and audio technologies by reporters to gather and present news instead of using film cameras. The term was coined during the rise of videotape tech ...
, helicopters have provided aerial views of some major news stories, and have been doing so, from the late 1960s. Helicopters have also been used in films, both in front and behind the camera.

## Birth of an industry

Cierva Autogiro Company The Cierva Autogiro Company was a British firm established in 1926 to develop the autogyro. The company was set up to further the designs of Juan de la Cierva, a Spanish engineer and pilot, with the financial backing of James George Weir, a Sco ...
, which according to Frank Kingston Smith Sr., included "the fully controllable cyclic/collective pitch hub system". In return, Cierva Autogiro received a cross-license to build the Focke-Achgelis helicopters. Focke designed the world's first practical transverse twin-rotor helicopter, the
Focke-Wulf Fw 61 The Focke-Wulf Fw 61 is often considered the first practical, functional helicopter, first flown in 1936. It was also known as the Fa 61, as Focke began a new company— Focke-Achgelis—in 1937. Design and development Professor Henrich Fo ...
, which first flew in June 1936. The Fw 61 had flown higher than at speeds of . Autogiro development was now being bypassed by a focus on helicopters. During World War II,
Nazi Germany Nazi Germany (lit. "National Socialist State"), ' (lit. "Nazi State") for short; also ' (lit. "National Socialist Germany") (officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945) was ...
used helicopters in small numbers for observation, transport, and medical evacuation. The Flettner Fl 282 ''Kolibri''
synchropter Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding. T ...
—using the same basic configuration as
Anton Flettner Anton Flettner (November 1, 1885 – December 29, 1961) was a German aviation engineer and inventor. Born in Eddersheim (today a district of Hattersheim am Main), Flettner made important contributions to airplane, helicopter, vessel, and autom ...
's own pioneering Fl 265—was used in the Mediterranean, while the Focke Achgelis Fa 223 ''Drache'' twin-rotor helicopter was used in Europe. Extensive bombing by the Allied forces prevented Germany from producing any helicopters in large quantities during the war. In the United States, Russian-born engineer
Igor Sikorsky Igor Ivanovich Sikorsky (russian: И́горь Ива́нович Сико́рский, p=ˈiɡərʲ ɪˈvanəvitʃ sʲɪˈkorskʲɪj, a=Ru-Igor Sikorsky.ogg, tr. ''Ígor' Ivánovich Sikórskiy''; May 25, 1889 – October 26, 1972)Fortie ...
and Wynn Laurence LePage competed to produce the U.S. military's first helicopter. LePage received the
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A ...
rights to develop helicopters patterned after the Fw 61, and built the XR-1.Francillon 1997 Meanwhile, Sikorsky settled on a simpler, single rotor design, the
VS-300 The Vought-Sikorsky VS-300 (or S-46) is an American single-engine helicopter designed by Igor Sikorsky. It had a single three-blade rotor originally powered by a 75 horsepower (56  kW) engine. The first "free" flight of the VS-300 was on 13 ...
, which turned out to be the first practical single lifting-rotor helicopter design. After experimenting with configurations to counteract the torque produced by the single main rotor, Sikorsky settled on a single, smaller rotor mounted on the tail boom. Developed from the VS-300, Sikorsky's R-4 was the first large-scale mass-produced helicopter, with a production order for 100 aircraft. The R-4 was the only Allied helicopter to serve in World War II, primarily for
search and rescue Search and rescue (SAR) is the search for and provision of aid to people who are in distress or imminent danger. The general field of search and rescue includes many specialty sub-fields, typically determined by the type of terrain the search ...
(by the
USAAF The United States Army Air Forces (USAAF or AAF) was the major land-based aerial warfare service component of the United States Army and ''de facto'' aerial warfare service branch of the United States during and immediately after World War II ...
1st Air Commando Group) in the Burma campaign; in Alaska; and in other areas with harsh terrain. Total production reached 131 helicopters before the R-4 was replaced by other Sikorsky helicopters such as the R-5 and the R-6. In all, Sikorsky produced over 400 helicopters before the end of World War II.Day, Dwayne A
"Igor Sikorsky – VS 300"
''Centennial of Flight Commission'', 2003. Retrieved 9 December 2007.
While LePage and Sikorsky built their helicopters for the military,
Bell Aircraft The Bell Aircraft Corporation was an American aircraft manufacturer, a builder of several types of fighter aircraft for World War II but most famous for the Bell X-1, the first supersonic aircraft, and for the development and production of many ...
hired Arthur Young to help build a helicopter using Young's two-blade teetering rotor design, which used a weighted stabilizer bar placed at a 90° angle to the rotor blades. The subsequent Model 30 helicopter showed the design's simplicity and ease of use. The Model 30 was developed into the
Bell 47 The Bell 47 is a single-rotor single-engine light helicopter manufactured by Bell Helicopter. It was based on the third Bell 30 prototype, which was the company's first helicopter designed by Arthur M. Young. The 47 became the first h ...
, which became the first helicopter certified for civilian use in the United States. Produced in several countries, the Bell 47 was the most popular helicopter model for nearly 30 years.

## Turbine age

In 1951, at the urging of his contacts at the Department of the Navy, Charles Kaman modified his K-225
synchropter Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding. T ...
—a design for a twin-rotor helicopter concept first pioneered by
Anton Flettner Anton Flettner (November 1, 1885 – December 29, 1961) was a German aviation engineer and inventor. Born in Eddersheim (today a district of Hattersheim am Main), Flettner made important contributions to airplane, helicopter, vessel, and autom ...
in 1939, with the aforementioned Fl 265 piston-engined design in Germany—with a new kind of engine, the
turboshaft A turboshaft engine is a form of gas turbine that is optimized to produce shaftpower rather than jet thrust. In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust ...
turbine engine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
provided a large amount of power to Kaman's helicopter with a lower weight penalty than piston engines, with their heavy engine blocks and auxiliary components. On 11December 1951, the
Kaman Kaman may refer to: * Kaman (surname) * Kamein (Kaman), an ethnic group in Burma * Kaman Aircraft, an American aerospace company and helicopter manufacturer * Kaman Music Corporation, a company of several musical instrument manufacturers * Kama ...
K-225 became the first turbine-powered helicopter in the world. Two years later, on 26 March 1954, a modified Navy HTK-1, another Kaman helicopter, became the first twin-turbine helicopter to fly. However, it was the
Sud Aviation Sud Aviation (, ''Southern Aviation'') was a French state-owned aircraft manufacturer, originating from the merger of Sud-Est (SNCASE, or ''Société nationale des constructions aéronautiques du sud-est'') and Sud-Ouest (SNCASO or ''Société ...
Alouette II that would become the first helicopter to be produced with a turbine-engine. 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 A turboshaft engine is a form of gas turbine that is optimized to produce shaftpower rather than jet thrust. In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust ...
engines in the second half of the 20th century led to the development of larger, faster, and higher-performance helicopters. While smaller and less expensive helicopters still use piston engines, turboshaft engines are the preferred powerplant for helicopters today.

# Safety

## Maximum speed limit

There are several reasons a helicopter cannot fly as fast as a fixed-wing aircraft. When the helicopter is hovering, the outer tips of the rotor travel at a speed determined by the length of the blade and the rotational speed. In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational speed. The airspeed of the advancing rotor blade is much higher than that of the helicopter itself. It is possible for this blade to exceed the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as ...
dissymmetry of lift Dissymmetry of lift (also known as asymmetry of lift or asymmetric lift) in rotorcraft aerodynamics refers to an unequal amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters and autogyro ...
, rotor blades are designed to "flap" – lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift. At high speeds, the force on the rotors is such that they "flap" excessively, and the retreating blade can reach too high an angle and stall. For this reason, the maximum safe forward airspeed of a helicopter is given a design rating called VNE, ''velocity, never exceed''. In addition, it is possible for the helicopter to fly at an airspeed where an excessive amount of the retreating blade stalls, which results in high vibration, pitch-up, and roll into the retreating blade.

## Noise

During the closing years of the 20th century designers began working on helicopter noise reduction. Urban communities have often expressed great dislike of noisy aviation or noisy aircraft, and police and passenger helicopters can be unpopular because of the sound. The redesigns followed the closure of some city
heliport A heliport is a small airport suitable for use by helicopters and some other vertical lift aircraft. Designated heliports typically contain one or more touchdown and liftoff areas and may also have limited facilities such as fuel or hangars. ...
s and government action to constrain flight paths in
national parks A national park is a natural park in use for conservation purposes, created and protected by national governments. Often it is a reserve of natural, semi-natural, or developed land that a sovereign state declares or owns. Although individua ...
and other places of natural beauty.

## Vibration

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is ...
to counter the vibration. Adjustment can be difficult in part because measurement of the vibration is hard, usually requiring sophisticated accelerometers mounted throughout the airframe and gearboxes. The most common blade vibration adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or coloured reflectors on the underside of the rotor blades. The traditional low-tech system is to mount coloured chalk on the rotor tips, and see how they mark a linen sheet. Health and Usage Monitoring Systems (HUMS) provide vibration monitoring and rotor track and balance solutions to limit vibration. Gearbox vibration most often requires a gearbox overhaul or replacement. Gearbox or drive train vibrations can be extremely harmful to a pilot. The most severe effects are pain, numbness, and loss of tactile discrimination or dexterity.

## Loss of tail-rotor effectiveness

For a standard helicopter with a single main rotor, the tips of the main rotor blades produce a vortex ring in the air, which is a spiraling and circularly rotating airflow. As the craft moves forward, these vortices trail off behind the craft. When hovering with a forward diagonal crosswind, or moving in a forward diagonal direction, the spinning vortices trailing off the main rotor blades will align with the rotation of the tail rotor and cause an instability in flight control. When the trailing vortices colliding with the tail rotor are rotating in the same direction, this causes a loss of thrust from the tail rotor. When the trailing vortices rotate in the opposite direction of the tail rotor, thrust is increased. Use of the foot pedals is required to adjust the tail rotor's angle of attack, to compensate for these instabilities. These issues are due to the exposed tail rotor cutting through open air around rear of the vehicle. This issue disappears when the tail is instead ducted, using an internal impeller enclosed in the tail and a jet of high pressure air sideways out of the tail, as the main rotor vortices can not impact the operation of an internal impeller.

## Critical wind azimuth

For a standard helicopter with a single main rotor, maintaining steady flight with a crosswind presents an additional flight control problem, where strong crosswinds from certain angles will increase or decrease lift from the main rotors. This effect is also triggered in a no-wind condition when moving the craft diagonally in various directions, depending on the direction of main rotor rotation. This can lead to a loss of control and a crash or hard landing when operating at low altitudes, due to the sudden unexpected loss of lift, and insufficient time and distance available to recover.

## Transmission

Conventional rotary-wing aircraft use a set of complex mechanical gearboxes to convert the high rotation speed of gas turbines into the low speed required to drive main and tail rotors. Unlike powerplants, mechanical gearboxes cannot be duplicated (for redundancy) and have always been a major weak point in helicopter reliability. In-flight catastrophic gear failures often result in gearbox jamming and subsequent fatalities, whereas loss of lubrication can trigger onboard fire. Another weakness of mechanical gearboxes is their transient power limitation, due to structural fatigue limits. Recent EASA studies point to engines and transmissions as prime cause of crashes just after pilot errors. By contrast, electromagnetic transmissions do not use any parts in contact; hence lubrication can be drastically simplified, or eliminated. Their inherent redundancy offers good resilience to single point of failure. The absence of gears enables high power transient without impact on service life. The concept of electric propulsion applied to helicopter and electromagnetic drive was brought to reality by Pascal Chretien who designed, built and flew world's first man-carrying, free-flying electric helicopter. The concept was taken from the conceptual
computer-aided design Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve c ...
model on 10 September 2010 to the first testing at 30% power on 1 March 2011 – less than six months. The aircraft first flew on 12 August 2011. All development was conducted in Venelles, France.

## Hazards

As with any moving vehicle, unsafe operation could result in loss of control, structural damage, or loss of life. The following is a list of some of the potential hazards for helicopters: * Settling with power is when the aircraft has insufficient power to arrest its descent. This hazard can develop into vortex ring state if not corrected early. *
Vortex ring A vortex ring, also called a toroidal vortex, is a torus-shaped vortex in a fluid; that is, a region where the fluid mostly spins around an imaginary axis line that forms a closed loop. The dominant flow in a vortex ring is said to be toroidal, ...
state is a hazard induced by a combination of low airspeed, high power setting, and high descent rate. Rotor-tip vortices circulate from the high pressure air below the rotor disk to low pressure air above the disk, so that the helicopter settles into its own descending airflow. Adding more power increases the rate of air circulation and aggravates the situation. It is sometimes confused with settling with power, but they are aerodynamically different. *
Retreating blade stall Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack, causing a stall and loss of lift. ...
is experienced during high speed flight and is the most common limiting factor of a helicopter's forward speed. * Ground resonance is a self-reinforcing vibration that occurs when the lead/lag spacing of the blades of an articulated rotor system becomes irregular. * Low-G condition is an abrupt change from a positive G-force state to a negative G-force state that results in loss of lift (unloaded disc) and subsequent roll over. If aft cyclic is applied while the disc is unloaded, the main rotor could strike the tail causing catastrophic failure. * Dynamic rollover in which the helicopter pivots around one of the skids and 'pulls' itself onto its side (almost like a fixed-wing aircraft ground loop). *
Powertrain A drivetrain (also frequently spelled as drive train or sometimes drive-train) is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components ...
failures, especially those that occur within the shaded area of the height-velocity diagram. * Tail rotor failures which occur from either a mechanical malfunction of the tail rotor control system or a loss of tail rotor thrust authority, called "loss of tail-rotor effectiveness" (LTE). * Brownout in dusty conditions or whiteout in snowy conditions. * Low rotor RPM, is when the engine cannot drive the blades at sufficient RPM to maintain flight. * Rotor overspeed, which can over-stress the rotor hub pitch bearings (brinelling) and, if severe enough, cause blade separation from the aircraft. * Wire and tree strikes due to low altitude operations and take-offs and landings in remote locations. *
Controlled flight into terrain In aviation, a controlled flight into terrain (CFIT; usually ) is an accident in which an airworthy aircraft, under pilot control, is unintentionally flown into the ground, a mountain, a body of water or an obstacle. In a typical CFIT scenario ...
in which the aircraft is flown into the ground unintentionally due to a lack of situational awareness. * Mast bumping in some helicoptersFAA RFH, page 11-10

# References

## Bibliography

* Chiles, James R. ''The God Machine: From Boomerangs to Black Hawks: The Story of the Helicopter''. New York: Bantam Books, 2007. . * Cottez, Henri. ''Dictionnaire des structures du vocabulaire savant''. Paris: Les Usuels du Robert. 1980. . * Francillon, René J. ''McDonnell Douglas Aircraft since 1920: Volume II''. London: Putnam, 1997. . * Frawley, Gerard. ''The International Directory of Civil Aircraft, 2003–2004''. Fyshwick, Canberra, Act, Australia: Aerospace Publications Pty Ltd., 2003, p. 155. . * Munson, Kenneth. ''Helicopters and other Rotorcraft since 1907''. London: Blandford Publishing, 1968. .
''Rotorcraft Flying Handbook''
Washington: Skyhorse Publishing, Inc., 2007. .
''Rotorcraft Flying Handbook: FAA Manual H-8083-21''
Washington, D.C.: Federal Aviation Administration (Flight Standards Division), U.S. Dept. of Transportation, 2001. . * Thicknesse, P. ''Military Rotorcraft'' (Brassey's World Military Technology series). London: Brassey's, 2000. . * Watkinson, John. Art of the Helicopter. Oxford: Elsevier Butterworth-Heinemann, 2004. * Wragg, David W. ''Helicopters at War: A Pictorial History''. London: R. Hale, 1983. . * Zaschka, Engelbert. ''Drehflügelflugzeuge. Trag- und Hubschrauber''. Berlin-Charlottenburg: C. J. E. Volckmann Nachf. E. Wette, 1936. .

"www.helicopterpage.com – How Helicopters Work"
Complete site explaining different aspects of helicopters and how they work.
"Planes That Go Straight Up"
1935 article about early development and research into helicopters.
"Flights — of the Imagination"
1918 article on helicopter design concepts.
"Twin Windmill Blades Fly Wingless Ship"
''Popular Mechanics'', April 1936
Silent (Russian-language intertitled) video about the Cheremukhin/Yuriev TsAGI 1-EA pioneer helicopter

American Helicopter Society
* {{Authority control Aircraft configurations Articles containing video clips