A helicopter is an aircraft which is lifted and propelled by one (or more) horizontal rotors (propellers), the rotary motion of which generates lift. They are known as Rotary wing aircraft to distinguish them from conventional fixed wing aircraft. The word helicopter is derived from the Greek words helix (spiral) and pteron (wing).
Turning the rotor applies a force that would make the vehicle spin (by inducing a "torque" on it). The usual way to counteract this torque is to have a smaller propeller mounted on a tail, called a tail rotor. If the rotor is a fan in a vertical fin, it's called a fenestron.
Other helicopters use the "Notar" design that blows air through a nozzle to counter the torque induced by a single main rotor. Notar is an acronym meaning NO TAil Rotor.
A realistic alternative to the weight of a tail boom and rotor is to use two horizontal rotors turning in opposite directions. One example is the Boeing CH-47 Chinook, another being the Kamov Ka-50.
All of these antitorque schemes are used to control the helicopter's direction (yaw). Tail rotors are sped and slowed by transmissions. Dual-rotor helicopters have a differential between the two rotors' transmissions that's usually turned by an electric or hydraulic motor to make differential torques to turn the helicopter.
Yaw controls are usually controlled with rudder pedals.
A helicopter cannot fly as fast as a plane because the forward-going edge of its rotor moves much faster than the helicopter. It is very easy for this edge to exceed the speed of sound. When supersonic, conventional rotors begin to get increasing drag. It is theoretically possible to have spiraling rotors, similar in principle to variable-pitch swept wings. These could exceed the speed of sound, but no materials are strong enough and flexible enough to construct them.
Although helicopters cannot fly as fast as planes, they can hover, land and take off vertically. They can therefore reach more places than planes.
Helicopters maneuver with three flight controls besides the "yaw" controls described above, that use torque.
The "collective" controls the collective pitch, or angle, of the helicopter blades. When the angle increased, the blades cut the air more deeply, producing more lift. The collective is usually a lever at the plot's left side, near his leg. Increasing the collective, and the throttle, causes a helicopter to rise.
The "throttle" controls the absolute power to the rotor. It is usually a twist grip on the collective.
The "cyclic" changes the pitch of the blades cyclically, causing the lift to vary across the width of the rotor disk. This is how the pilot causes the helicopter to tilt, and therefore to move. the cyclic is usually controlled by the stick in front of the pilot.
A very peculiar feature of the cyclic is that the lift is made to occur 90 degrees of rotation before the direction of tilt. This is because when one tries to tilt a spinning object (like a rotor), it tilts at right angles to the direction of the force. This is called "precession" and it is the reason why gyroscopes stay upright.
It took inventors many years to recognize precession, and how to arrange the cyclic's control system to overcome it.
If the blades cut too deeply, they have too much drag and "stall." This can happen in any of four ways. The blades on the forward going rotor can be going too fast, causing "tip stall." Tip stall usually causes the helicopter to pitch forward as the copter goes faster. The collective can be too extreme. The cyclic can stall just on the side that's supposed to generate the extra lift.
The helicopter has many uses, both military and civil. For example, troop transportation, infantry support, firefighting, business transportation, casualty evacuation (including MEDEVAC, and air/sea/mountain rescue), police and civilian surveillance, carrying goods (some helicopters can carry a slung load, which allows them to carry extremely awkward loads), or as a mount for still, film or television cameras.
Helicopters are powered aircraft, but they can still fly without power by using the momentum in the rotors and using downward motion to force air through the rotors forcing them to turn. This is known as autorotation, and will give the helicopter a few precious seconds to quickly find a landing spot in the event of an engine failure.
The idea of the helicopter was first conceived by Leonardo da Vinci in the 16th century, but it was not until after the invention of the powered aeroplane in the 20th century that actual models were first produced. Developers such as Louis Breguet, Juan de la Cierva and Igor Sikorsky pioneered this type of aircraft. A flight of the first fully controllable helicopter Focke-Wulf Fw 61 was demonstrated by Hanna Reitsch 1936 in Berlin, Germany.
During the closing years of the 20th century helicopter designers began design features to reduce the noise. Urban communities have often expressed great dislike of noisy aircraft, and the police and passenger helicopters have been widely disliked.
The redesigns followed the closure of some city heliports and government action to constrain flight paths in national parks and other places of natural beauty.
Helicopters are made quieter basically by reducing the tip speed below supersonic speeds, so that they don't create little sonic booms. The basic tricks are to increase the numbers blade of blades to keep the load per rotor area low. The blade tips are reprofiled to be sharper, and shed the shockwave more efficiently. The rotation rate is reduced. And a fenestron or Notar eliminates the sonic booms of a tail rotor's tips.
In identifying conventional helicopters during flight it is helpful to realise that when viewed from below the rotor of a French, Russian, Soviet or Ukrainian designed helicopter rotates anti-clockwise, whilst a helicopter completed in the UK or USA rotates clockwise.
Some companies, notably Schweizer in the USA, are developing remotely-controlled variants of light helicopters for use in future battlefields.
US Army helicopters:
- AH-1 Cobra
- AH-64 Apache
- CH-47 Chinook (nicknamed the "Wokka Wokka")
- OH-55 Kiowa
- UH-1 Iroquois (nicknamed the "Huey")
- UH-60 Blackhawk
large gatherings of small US Army helicopters have been nicknamed "chocolate mice".
Other military helicopters include:
- Aerospatiale Puma
- Kamov Ka-25
- Kamov Ka-50
- Kaman SH-2 Seasprite
- Mil-24 Hind
- Sikorsky HH-53 (nicknamed the "Jolly Green Giant")
- Westland Lynx
- Westland Sea King a derivative of the Sikorsky H-3 made famous during the recovery of early US spacecraft from the ocean.
Popular civil helicopters include the:
- Bell model 47 (made famous in the 1950s television programme "Whirlybirds")
- Bell 206 Jet Ranger and Long Ranger
- Enstrom 28, 280 and 480 models
- Eurocopter Squirrel or Twin Star
- Hiller 12
- Robinson 22 and 44 series
Hybrid types that combine features of helicopters and fixed wing designs include the Fairey Rotordyne and the Bell Boeing Osprey. The latter is under development with the US Marine Corps and may become the first mass produced tilt-rotor to see active military service.
See also autogyro, a historical predecessor of the helicopter.