Helicopters are classified as rotary-wing aircraft in which lift force is supplied by horizontally spinning rotors. Unlike the more common fixed-wing aircraft, the helicopter is capable of direct vertical take-off and landing; it can also hover in a fixed position, and fly forward, backward, and laterally. These features render it ideal for use where space is limited or where the ability to hover over a precise area is necessary where fixed-wing aircraft and many forms of STOL (Short Takeoff and Landing) or STOVL (Short Takeoff and Vertical Landing) aircraft cannot perform without a runway. In 1942, the Sikorsky R-4 became the first helicopter to reach full-scale production.
Presently, helicopters are used in agriculture to dust crops, apply pesticides, also access remote areas for environmental work, deliver supplies to workers on remote maritime oil rigs, film movies, evacuate people trapped in inaccessible spots, and transport accident victims, and Firefighting. Besides, they have numerous military applications.
A helicopter is powered by either a piston engine or a gas turbine to provide thrust force. Recently a gas turbine is commonly used to power helicopters. That moves the rotor shaft, causing the rotor to turn. whilst a standard plane is lifted by pushing air behind its wing as it moves forward, the helicopter’s rotor achieves lift by pushing the air beneath it downward as it spins. Lift is proportional to the change in the air’s momentum (its mass times its velocity): the greater the momentum, the greater the lift.
Helicopter rotor systems consist of between two and six blades attached to a central hub. Usually long and narrow, the blades turn relatively slowly, because this minimizes the amount of power necessary to achieve and maintain lift, and also because it makes controlling the vehicle easier. While light-weight, general-purpose helicopters often have a two-bladed main rotor, the heavier craft may use a four-blade design or two separate main rotors to accommodate heavy loads.
