A helicopter is an aircraft which is lifted and propelled by one or more large horizontal rotors (propellers). Helicopters are classified 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).
Since around 400 BC the Chinese had a flying top that was used as a children's toy. Incidentally, the Wright brothers were given this toy as kids and were very much fascinated by it. This toy eventually made its way to Europe via trade and it has been depicted in a 1463 painting. "Pao Phu Tau" was 4 century AD book in China that described some of the ideas in a rotary wing aircraft.
The first somewhat practical idea of a human carrying helicopter was first conceived by Leonardo da Vinci in the 15th century, but it was not until after the invention of the powered aeroplane in the 20th century that actual models were produced. Developers such as Louis Breguet, Paul Cornu, Juan de la Cierva, Emile Berliner, 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.
Helicopters have many uses, both military and civil, including troop transportation, infantry support, firefighting, shipboard operations (http://www.tropicaled.com/helicopter2.htm), business transportation, casualty evacuation (including MEDEVAC, and air/sea/mountain rescue), police and civilian surveillance, carrying goods (some helicopters can carry slung loads, accommodating awkwardly shaped items), or as a mount for still, film or television cameras.
Compared to conventional fixed-wing aircraft, helicopters are much more complex, more expensive to buy and operate, relatively slow, have poor range and restricted payload. The compensating advantage is maneuverability: helicopters can hover in place, reverse, and above all take off and land vertically. Subject only to refueling facilities and load/altitude limitations, a helicopter can travel to any location, and land anywhere with a clearing a rotor disk and a half in diameter.
A conventional aircraft is able to fly because the forward motion of its angled wings forces air downwards, creating an opposite reaction called lift that forces the wings upwards. A helicopter uses exactly the same method, except that instead of moving the entire aircraft, only the wings themselves are moved. The helicopter's rotor can simply be regarded as rotating wings.
Turning the rotor generates lift but it also applies a reverse force to the vehicle, that would spin the helicopter in the opposite direction to the rotor. The most common way to counteract this torque is to have a smaller vertical propeller mounted at the rear of the aircraft called a tail rotor. If the rotor is shrouded (i.e., a fan embedded in the vertical tail) it is called a fenestron. Other helicopters use a "Notar" design: they blow air through a long slot along the tail boom, utilizing the coanda effect to produce forces to counter the torque. Notar is an acronym meaning no tail rotor.
Another alternative, which saves the weight of a tail boom and rotor but adds its own complexities, is to use two large horizontal rotors which turn in opposite directions. An example is the Boeing CH-47 Chinook or the Kamov Ka-50.
For left-right horizontal direction (yaw) the antitorque system is used. Varying the pitch of the tail rotor alters the sideways thrust produced. Dual-rotor helicopters have a differential between the two rotor transmissions that can be adjusted by an electric or hydraulic motor to transmit differential torque and thus turn the helicopter. Yaw controls are usually operated with anti-torque pedals, on the floor in the same place as a fixed-wing aircraft's rudder pedals.
For pitch (tilting forward and back) or roll (tilting sideways) the angle of the main rotor is altered.
Helicopters maneuver with three flight controls besides the pedals. The collective pitch control lever controls the collective pitch, or angle of attack, of the helicopter blades together, that is, equally throughout the 360 degree plane-of-rotation of the main rotor system. When the angle-of-attack is increased, the blade produces more lift. The collective control is usually a lever at the pilot's left side, near his leg. Increasing the collective and adding power with throttle causes a helicopter to rise.
The throttle controls the absolute power produced by the engine that is connected to the rotor by a transmission. In piston-powered helicopters it is usually a twist grip on the collective. The pilot manipulates the throttle to maintain rotor RPM and therefore regulates the effect of drag on the rotor system. Turbine engined helicopters use servo-feedback loop in their engine controls to maintain rotor RPM and relieves the pilot of routine responsibility for that task.
The cyclic changes the pitch of the blades cyclically, causing the lift to vary across the plane of the rotor disk. This is how the pilot causes the rotor system to tilt, and the helicopter to move. The cyclic is usually controlled by the stick in front of the pilot.
As a helicopter moves forward, the rotor blades on one side move at rotor tip speed plus the aircraft speed and is called the advancing blade. As the blade swings to the other side of the helicopter, it moves at rotor tip speed minus aircraft speed and is called the retreating blade. To compensate for the added lift on the advancing blade and the decreased lift on the retreating blade - lift being a function of an airfoils angle-of-attack and its relative airspeed - the angle-of-attack of the blades is regulated by the geometry of the rotor blade control system and mechanisms that allow the blades to flap up and down. This fact of advancing and retreating blades defines the speed limitations of the helicopter.
If the angle-of-attack of any wing, including rotor blades, is too high, the airflow above the wing separates causing instant loss of lift and increase in drag. This condition is called aerodynamic stall. On a helicopter, this can happen in any of three ways. 1. As helicopter speed increases, the advancing blades approach the speed of sound and generate shock waves that disrupt the airflow over the blade causing loss of lift. 2. As helicopter speeds increase, the retreating blade experiences lower relative airspeeds and the controls compensate with higher angle-of-attack. With a low enough relative airspeed and a high enough angle-of-attack, aerodynamic stall is inevitable. This is called retreating blade stall. 3. Any low rotor RPM flight condition accompanied by increasing collective pitch application will cause aerodynamic stall.
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. The rotors act like a "windmill" and turn. This technique is known as autorotation, and will give the helicopter a few precious seconds to quickly find a landing spot if its engine fails.
Helicopters are always designed so that even if the engines fail, autorotation will power the tail rotor or torque differential. Helicopters retain all flight controls when unpowered.
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 moves at right angles to the direction of the force. This is called "gyroscopic precession." So control forces on the rotor are rotated 90 degrees before the desired motion. For example, forward motion requires less lift at the front of the disk and more lift at the rear of the disk, so the pilot pushes the cyclic forward. The helicopter's control linkages rotate the pitching forces 90 degrees backwards against the rotor spin, to push on the sides of the rotor rather than its front and back.
It took inventors many years to recognize precession, and to learn how to arrange the cyclic's control system to overcome it.
Limitations of rotary-wing flight
The single most obvious limitation of the helicopter is its slow speed. There are several reasons why a helicopter cannot fly as fast as a fixed wing aircraft.
During the closing years of the 20th century helicopter designers began working on noise reduction. Urban communities have often expressed great dislike of noisy aircraft, and police and passenger helicopters can be unpopular. 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 vibrate. An unadjusted helicopter can easily vibrate so much that it will shake itself apart. To reduce vibration, all helicopters have rotor adjustments for height and pitch. Most also have vibration dampers for height and pitch. Some also use mechanical feedback systems to sense and counter vibration. Usually the feedback system uses a mass as a "stable reference" and a linkage from the mass operates a flap to adjust the rotor's angle of attack to counter the vibration. Adjustment is difficult in part because measurement of the vibration is hard. The most common adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or colored reflectors on the underside of the rotor blades. The traditional low-tech system is to mount colored chalk on the rotor tips, and see how they mark a linen sheet.
Helicopter Models and Identification
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 Italy, 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.
Popular civil helicopters include the:
US Army helicopters:
Large gatherings of small US Army helicopters have been nicknamed "chocolate mice".
Other military helicopters include:
Hybrid types that combine features of helicopters and fixed wing designs include the experimental Fairey Rotordyne of the 1950s and the Bell Boeing Osprey, which is on order by the US Marine Corps and is the first mass produced tilt-rotor to enter service.
See also autogyro, a historical predecessor of the helicopter, which gains lift from an unpowered rotor.
Other names for Helicopter
A helicopter is sometimes known as a "copter", "chopper", or "whirlybird".
UK offshore oil industry also uses "paraffin budgie", as in this collection of poetry "Flight of the Paraffin Budgie: A North Sea Chronicle" by John F. Fyvie
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