Specifications

The left/right servo is directly connected to the lower swashplate and controls the left/right tilt of
the swashplate.
The pitch servo is not connected to the swashplate and controls the main blade pitch through a
separate mechanical connection.
From the transmitter's point of view, the mechanical CCPM and non-CCPM control systems
are identical, because both have independent servos for collective pitch, fore/aft cyclic, and
left/right cyclic.
On the Zoom 400, the main shaft is hollow, and there is a rod which runs through the main ro-
tor shaft and controls the main blade pitch.
On the Piccolo Pro, the there is a hollow tube which goes around the main shaft and inside the
swashplate and controls the main blade pitch.
30.1.4. From the upper swashplate to the rotor head
There are three systems which are widely used to control the rotor head from the upper
swashplate:
• Bell control system
• Hiller control system
• Bell-Hiller control system
30.1.4.1. The Bell control system
For a Bell control system, the upper swashplate is directly connected to the main blade grips.
Usually there are two main rotor blades and these are directly connected to two control balls
on the upper swashplate. This is sometimes called a "flybarless" control system.
One advantage of the Bell control system is very quick cyclic response. The control system dir-
ectly controls the main blade pitch, so the system is very sensitive to swashplate changes.
One disadvantage of the Bell control system is the lack of stability. The system is very sensit-
ive to minor gusts. It also stresses the control linkages because they control the heavy rotor
blades directly and therefore strong servos must be used.
The early helicopters using Bell control systems had a "stabilizer bar" which was perpendicular
to the main rotor blades, which had heavy weights attached to the ends (similar to a flybar but
with weights instead of paddles and no tilting). This added stability to the system.
There are no popular helicopters which implement a pure Bell control system.
30.1.4.2. The Hiller control system
The Hiller control system was invented by Stanley Hiller in the 1940s. This was originally called
the "Rotormatic" control system, and was so stable the first prototypes could be hovered
hands-off for minutes at a time.
For a Hiller control system, the upper swashplate does not directly control the main blade
pitch. Instead, it indirectly controls the main blade pitch by changing the pitch of the flybar
paddles. As the flybar seesaws up and down, it changes the main blade pitch.
Technical Appendix
169