The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 139
Fig. 4.18 The rotor control arrangement used by Enstrom. The spider is below the transmission and pushrods
travel inside the shaft to rockers on the top of the rotor head.
Fig. 4.19 Servo tabs are small control surfaces hinged on the trailing edge of the blade operated by pushrods
running inside.
the axis about which the swashplate turns is the control axis. In fact the control cranks
could be designed so that the swashplate stays parallel to the cyclic stick.
In multi-bladed heads this situation is seldom found. Such heads will be articulated
or flexural and their flapping frequency will not be identical to the rotational frequency.
As a result their phase lag will not be precisely 90 ◦ and the control system will need to
compensate by arranging to tilt the swashplate in a slightly different direction to the
cyclic stick.
Although an advance angle of 90 ◦ is easily obtained in a two-bladed teetering rotor,
with articulated heads it becomes impossible because the end of the pitch-operating
arm must terminate on the flapping axis to avoid the δ3 effect (see Figure 4.9(b)). In
multi-blade heads the end of the pitch-operating arm must remain clear of the next
blade root. When the advance angle of the pitch operating arms is less than 90 ◦ , this is
corrected by advancing the stationary part of the swashplate. Figure 4.20 shows that