Helicopters

NOTAR (NO TAil Rotor) System. (Photo credit: Wikipedia)

Helicopters are lifted into the air by the force of one or two whirling rotors. These rotors, which are actually wings that rotate parallel to the ground, resemble the blades of a fan. The first known documents describing the concept of a helicopter-type aircraft are Chinese, and date from about 320 A.D. In 1784, the first model helicopter that could fly was developed. It was not until the early 1900's that gasoline powered helicopters made manned helicopter flight possible. By the 1930s, the control systems, strength, and safety of helicopters improved to a point that helicopter travel was a practical option.

Helicopters have many advantages over airplanes. For example, helicopters do not need runways for take off and landing. They also have the ability to fly forward, backward, straight up, and straight down. Helicopters can also hover in one position or travel at very slow speeds. However, helicopters do use more fuel and travel at slower speeds than airplanes, and so are a less efficient method for traveling long distances. The smallest helicopters can only seat one person. The largest helicopters can transport two trucks in their cargo hold.

A helicopter's characteristics make them especially suited for rescue missions, aerial observation, transportation and construction work, agricultural and forestry work, and military operations. A typical rescue mission may require that a helicopter hover over the scene of an accident. Ropes and harnesses can be lowered from helicopters in order to rescue victims. Patients can then be flown to nearby hospitals that are usually equipped with a landing pad.

Single-rotor helicopters are the most common type of helicopter. These helicopters use a single main rotor with anywhere from 2 to 8 blades to provide lifting power. Single-rotor helicopters actually have a second, smaller rotor mounted on the tail of the aircraft. This rotor, which contains 2 to 13 vertical blades, provides stability and direction control for the helicopter. Without the tail rotor, helicopters would spin around in a direction opposite that of the main rotor.

Twin-rotor helicopters have two main rotors that spin in opposite directions. This provides more lifting power and eliminates the need for a tail rotor. There are two configurations of twin-rotor helicopters. Tandem-rotor helicopters have one rotor mounted at the front and a second rotor mounted in the rear of the helicopter. Coaxial-rotor helicopters have the rotors mounted one on top of the other.

The shape of the rotor blades is what enables a helicopter to lift off the ground. Each rotor blade is curved along the top surface and flat on the bottom. As the rotor turns, air travels above and below the blade. Because the top of the rotor blade is curved, air takes longer to pass over the wing than under the wing. This difference in air speed actually creates more air pressure below the blade and less pressure above. As air pushes with greater force below the wing, lift is created. An airplane wing, which is stationary, works under the same principle.

Helicopter pilots can control a helicopter's speed, direction, and altitude by manipulating the main and tail rotors. Cockpit controls vary the angle, or pitch, which the rotors make with ground. For example, if the pitch of all rotors is increased, more lift is created and the helicopter climbs. A smaller pitch allows the helicopter to descend. A helicopter moves forward, backward, and sideways by varying the pitch of the rotors from front to back or side to side. This means that at any given moment, some rotors have higher pitches than others do. The tail rotor overcomes the force that makes the helicopter spin while also pointing the aircraft in the desired direction.

Recent developments in helicopter technology have led to higher speeds and simplified operation. Some new single-rotor helicopters have done away with the need for a tail rotor. Others use jet or propeller systems, in addition to traditional rotors, to increase the speed of a helicopter.