Looking back at the pioneering period of aviation one is struck by the almost complete lack of mechanical components in the early aeroplanes. Besides the single primitive engine there were no more than about a dozen hinges on control surfaces which, together with a few pulleys and levers, formed the entire mechanical system of the aircraft. As flying speeds increased, however, the need for mechanisms grew; retracting gear for the undercarriage became necessary, wings had to be fitted with flap gear, and engines and propellers received reduction gears and pitch varying mechanisms. The advent of the turbine opened further avenues for development; thus for flight at great heights cabin pressurisation became necessary, and supersonic flight demanded powered controls and flight stabilising mechanisms. Finally, a large family of VTOL aircraft, including the helicopter, came into existence.