Prerotating the Rotor

Because an autogiro rotor is not powered, the rotor needs to be brought up to speed by some means before takeoff. In Cierva’s day this was initially done by taxiing the aircraft around for several minutes. Later, a “spinning top” method was used, where a rope was
wound around the rotor shaft on pegs mounted on the bottom of the blades, the rope being pulled quickly (either manually by ground crew or by taxiing away from an anchor) to start the rotor. Instead, the Cierva C-12 model of 1929 saw the use of an alternative aerodynamic method to prerotate the rotor. Here, a “Scorpion” type of biplane tail was used to deflect the propeller slipstream up toward the blade tips. This horizontal tail was deflected at a steep incidence angle and the machine was held on the brakes and the throttle opened until the deflected propeller slipstream caused the rotor to autorotate and come up to speed. The tail incidence was then reduced back to its normal pitch before takeoff. The Scorpion tail design, however, was a relatively heavy solution and suffered from higher parasitic drag compared

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airspeeds because the vertical stabilizer and rudders were outside the propeller slipstream. Yet, the Scorpion tail was used on several models of the Cierva’s Autogiros and seemed to offer a reasonably good compromise as an interim practical solution to starting the rotor. However, by 1930 engineers were to develop a more powerful, lightweight prerotator. This mechanical starter allowed the rotor to be brought quickly up to speed, solving finally the problem once and for all.