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Helicopter

A helicopter is an aircraft which is lifted and propelled by one or two 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).


Robinson Helicopter Company R44.
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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 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.

Helicopters have many uses, both military and civil, including 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.

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, a helicopter can travel to any location, and land anywhere with a clearing a rotor disk and a half in diameter.

Table of contents

Generating lift

A conventional aircraft is able to fly because its forward motion forces air to pass rapidly above and below the wings, which are shaped and angled in such a way that an area of lower air pressure is created above the wing, and this "sucks" the aircraft up: it generates lift. 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 is simply some 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 nozzle 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.

Controlling flight

Useful flight requires that an aircraft be controlled in all three dimensions. In a fixed-wing aircraft, this is easy: small movable surfaces are adjusted to change the aircraft's shape so that the air rushing past pushes it in the desired direction. In a helicopter, however, there often isn't enough airspeed for this method to be practical.

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 controls the collective pitch, or angle, of the helicopter blades together. When the angle increased, the blades cut the air more deeply, producing more lift. The collective 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 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.

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 helicopter goes faster. (The vibration before this happens will be so extreme that there probably won't be much of an aircraft left.) The collective can be too extreme for the throttle setting. The cyclic can stall just on the side that's supposed to generate the extra lift.

Older piston-powered helicopters were rather underpowered, and required a careful touch to avoid rotor stall. Modern turbine-powered helicopters usually have power to spare; in fact, most of them omit the throttle on the collective and automatically regulate the throttle based on collective pitch.

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.

  • When the helicopter is at rest, the outer tips of the rotor travel at a speed determined by the length of the blade and the RPM. 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 velocity. The airspeed of the forward-going rotor is much higher than that the helicopter itself. It is very easy for this blade to exceed the speed of sound, and thus produce vastly increased drag and vibration. It is theoretically possible to have spiraling rotors, similar in principle to variable-pitch swept wings, which could exceed the speed of sound, but no presently known materials are light enough, strong enough, and flexible enough to construct them.
  • Rotors are not entirely rigid. Because the advancing blade has higher airspeed than the retreating blade, a fully rigid blade would generate more lift on that side and tip the aircraft over. In consequence, 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, thus producing less lift than a rigid blade would. 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 it stalls.

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 mesurement 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 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.



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