Incorporating rotor blades with high inertia potential is desirable in helicopter design and is essential for jump takeoff gyroplanes. A rotor hub design allowing the rotor speed to exceed normal flight r.p.m. by over 50 percent is not found in helicopters, and predicates a rotor head design particular to the jump takeoff gyroplane, yet very similar to that of the helicopter.
PREROTATOR
Prior to takeoff, the gyroplane rotor must first achieve a rotor speed sufficient to create the necessary lift. This is accomplished on very basic gyroplanes by initially spinning the blades by hand. The aircraft is then taxied with the rotor disc tilted aft, allowing airflow through the system to accelerate it to flight r.p.m. More advanced gyroplanes use a prerotator, which provides a mechanical means to spin the rotor. Many prerotators are capable of only achieving a portion of the speed necessary for flight; the remainder is gained by taxiing or during the takeoff roll. Because of the wide variety of prerotation systems available, you need to become thoroughly familiar with the characteristics and techniques associated with your particular system.
MECHANICAL PREROTATOR
Mechanical prerotators typically have clutches or belts for engagement, a drive train, and may use a transmission to transfer engine power to the rotor. Friction drives and flex cables are used in conjunction with an automotive type bendix and ring gear on many gyroplanes. [Figure 18-3]
 
The mechanical prerotator used on jump takeoff gyroplanes may be regarded as being similar to the helicopter main rotor drive train, but only operates while the aircraft is firmly on the ground. Gyroplanes do not have an antitorque device like a helicopter, and ground contact is necessary to counteract the torque forces generated by the prerotation system. If jump takeoff capability is designed into a gyroplane, rotor r.p.m. prior to liftoff must be such that rotor energy will support the aircraft through the acceleration phase of takeoff. This combination of rotor system and prerotator utilizes the transmission only while the aircraft is on the ground, allowing the transmission to be disconnected from both the rotor and the engine while in normal flight.
HYDRAULIC PREROTATOR
The hydraulic prerotator found on gyroplanes uses engine power to drive a hydraulic pump, which in turn drives a hydraulic motor attached to an automotive type bendix and ring gear. [Figure 18-4] This system also requires that some type of clutch and pressure regulation be incorporated into the design.