(By Capt. H.S. Chandele, A.E.C.)

The idea behind the helicopter is a very old one. The conception of the helicopter appears in some of the old toys and devices like the Chinese top, the Australian bushman’s boomerang and the tiny tin propeller which is pushed vertically off a twisted rod. Nature has got its helicopter in the well-known humming bird which can hover in one place and fly backwards.

Leonardo da Vinci, the great Italian artist-inventor, is accredited with having given, in the sixteenth century, the fundamental idea of the helicopter. He had drawn in his note-book models of helicopters which had air-screw or flying windmill revolving in a horizontal plane. He used the Greek word helix, meaning spiral or twist in connection with the idea of flight. Much later a combination of helix with the word pteron, meaning wing, led to the term helicopter.

In 1907 the Frenchman Breguet made a 1000 1b rectangular helicopter which attained an altitude of 15 feet and flew a distance of 64 feet.

The first successful helicopters made were the Focke-Wulf FW-61 which appeared in Germany in 1936 and the VS-300 which was built and flown by the famous aeronautical engineer Igor Sikorsky in 1937.

The principle of the helicopter can be understood contrasting it with the airplane and the autogyro. In an airplane, the lift or the sustention is provided by the fixed wings; in the autogyro, a free rotor which is turned by the air-flow set up by the forward motion of the aircraft serve the same function. The forward motion or propulsion in both the airplane and the autogyro is obtained from the thrust of an ordinary propeller which absorbs the whole power of the engine. But in the helicopter both the sustention and the propulsion are provided by the engine-driven rotors. Thus basically the helicopter is an ordinary airplane with its fixed wings stripped off and its propeller shifted overhead in the form of longer rotor blades turning about a vertical axis which can be tilted forward to achieve propulsion.

One of the greatest difficulties in the design of the helicopter is the method of counteracting what is known as torque. It is the tendency of the machine to twist itself counter to its lifting rotor. Various configurations have been used to counteract this torque. One of the most common methods is to use an engine-driven small auxiliary rotor revolving along a horizontal axis and mounted at the tail of the aircraft. This tail rotor pushes the aircraft opposite to the normal direction of the torque. Their effects can be exactly balanced and a straight course is maintained by the machine. Such well known aircraft as the Sikorsky and Bell-Young helicopters have used this configuration. Rotors disposed on either side of the fuselage and rotating in opposite directions have been employed by Focke-Wulf in Germany and Platt-Le Page Aircraft in the United States. Two partly overlapping rotor discs on either side of the fuselage have been employed by Kellett Aircraft to cut down the total width of the helicopter and the air drag of the supporting trusses. The arrangement of two rotors in tandem is utilized in the PV Engineering Forum and Rotor-craft machines and possesses the advantages of providing a large central space for the passengers. Bendix and Hiller have used two coaxil rotors one over the other and turning in opposite direction. This arrangement has three advantages of eliminating (i) the loss of power, (ii) the complexities of a long transmission (iii) the danger to the public of the tail rotor.

Jet propulsion is yet another method which when applied to the helicopter will completely eliminate mechanical transmission and will add to the uses of the helicopter.