I INTRODUCTION Helicopter has many definitions some of them very unflattering One individual defined helicopter as a vehicle that generates lift by beating air into submission A very precise definition of the helicopter is given by Wayne Johnson as a vehicle that depends primarily on its rotor to generate lift thrust and control forces The evolution of helicopter may be classified into three phases early developments autogiro and modern helicopters See Johnson Chapter 1 for chronological discussions The early devices were simple rotor attached to a mast with no significant method of control The Chinese top is a prime example of such an early device Some of these rotors were powered by springs or even tethered to a steam engine Because of the lack of light weight power plants and controls for operating these early devices they did not substantially impact the development of modern helicopters The next machines were autogiros These were airplanes with a rotor on top The rotor was not powered by an engine Instead it was allowed to windmill as a result of the forward motion of the aircraft The windmilling produces enough lift to keep the aircraft aloft Thus only the lift was produced by the rotor while the thrust and control were provided by conventional components An autogiro requires forward speed however small to operate It can operate at lower forward speeds than a conventional airplane and was used in applications that requires a very low speed flight both military and civilian Autogiros incorporated many of the features found in modern helicopter rotors a The blades were allowed to pitch up and down Modern propellers and rotors do so called feathering in propellers This changed the thrust i e lift generated by the rotor permitting climb descent or level flight b Cierva noticed that in forward flight the autogiro made of rigid blades had a tendency to roll over This caused by the increased amount of thrust produced on the advancing side of the rotor disk where the relative velocity between the rotor blades and the oncoming stream is high Blades with flexible palm wood did not have this problem He realized that the primary purpose of the blade is to provide lift not a rolling moment He modified his rotors with a flap hinge which allowed the blade to freely flap up and down At the hinge the lift was transferred to the vehicle but not a moment c Flapping of the rotor blades as they rotate created Coriolis forces Their magnitude is proportional to V x where V is the velocity of the blade These Coriolis forces have significant components that lie along the chordwise direction This caused high stresses particularly near the root of the blade This was cured by the use of lead lag hinges that allow the blade to move in the plane of the rotor d The combination of pitch links flap hinges lead lag hinges produces what is known as a fully articulated rotor Autogiros played a significant role in the perfection of these concepts A fully articulated rotor has too many parts and is very draggy See figure 1 4 of Johnson Today helicopter designers are going to the old idea of Cierva and are considering the use of flex beams soft root sections that allows the blade to align itself in a moment free manner Helicopters are the final step in the evolution from early concepts to autogiros to helicopters For a period helicopter design proceeded in parallel with autogiros But the lack of light weight power plants low horsepower engines etc kept these machines small Modern advancements in materials reciprocating engines and finally turboshaft engines gradually brought these machines from novelties to practical machines By the early 1950s the evolution of helicopter was essentially complete