Specifications
Two advantages of the Hiller system are:
• It places less stress on the swashplate servos because they only control the pitch of the fly-
bar paddles.
• The flybar paddles dampen pitch and roll which improves stability.
One disadvantage of the Hiller system is the lag in control response. The flybar paddles must
change their plane of rotation in order to change the main blade pitch.
Two popular helis which implement the Hiller control system are the Piccolo ECO/Fun and the
GWS Dragonfly.
30.1.4.3. The Bell-Hiller control system
The Bell-Hiller control system is a hybrid of both the Bell and Hiller control systems. The key
component of this system is the Bell-Hiller mixer which mechanically mixes both the flybar tilt
and the swashplate tilt.
There are two basic types of Bell-Hiller control systems:
• Nonmovable flybar systems
• Movable Flybar systems
In a Bell-Hiller system with a nonmoving flybar, there are usually four control balls on the upper
swashplate.
Two of the control balls on the upper swashplate are connected to the washout unit which is a
mechanical isolation device which allows the flybar to tilt with the swashplate, but not move up
and down.
The other two control balls are connected directly to the Bell-Hiller mixer. The tilt of the flybar
also affects the Bell-Hiller mixer. The output of the Bell Hiller mixer controls the main blade
pitch.
So, in a Bell-Hiller control system, the swashplate controls the flybar pitch and the flybar tilt af-
fects the main blade pitch. This is the Hiller component of the Bell-Hiller control system.
The swashplate tilt also affects the main blade pitch through the Bell-Hiller mixer. This is the
Bell component of the Bell-Hiller control system.
Two popular helis which use a Bell-Hiller system with a nonmoving flybar are the ECO 8 and
Logo 10.
The Bell-Hiller system with a moving flybar works exactly the same as the nonmoving flybar
system except the washout unit is eliminated and the flybar is allowed to move up and down
with the swashplate.
Advantages of the Bell-Hiller system include:
• Less control force required (as in the Hiller system)
• More stable than a Bell system (as in the Hiller system)
• Faster cyclic than the Hiller system due to some direct swashplate input
Technical Appendix
170