From RC Helicopter Wiki
The Bell-Hiller mixer is a linkage connecting the flybar, swashplate and the main blade grip. It is a combination of technologies designed to help stabilize and control the main rotor. Without some sort of stabilizing influence, the main rotor is very hard to control in any wind or in forward flight.
The Bell control system, invented by Arthur M. Young, and used on the original full-size Bell helicopters, consists essentially of a weighted flybar without any paddles (the stabilizer bar). Cyclic pitch inputs are applied to the main blades (causing the rotor disk to tilt), but they are applied, in effect, relative to the stabilizer bar. Subsequently, the stabilizer bar will begin to follow the tilt of the mast effected by the main blades, and the process repeats. This leads to extremely stable, but very slow continuous cyclic response. The damping to control how fast the stabilizer bar follows the main mast can be extremely critical.
In the Hiller control system, named by it's inventor Stanley Hiller Jr. as the Rotor-Matic, main blade cyclic pitch is controlled indirectly by the flybar. Cyclic pitch inputs control the cyclic pitch of the flybar paddles; the flybar would then tilt into a new plane, and the main rotor would follow. This retains much of the stability of the Bell control system, and continuous cyclic pitch control is better, but it loses the initial instant control response that the Bell system has, as cyclic pitch is always mediated by the flybar.
The Bell-Hiller control system combines the best of both these systems. By allowing cyclic pitch inputs directly through to the main blades, we keep the instant response of the Bell system, while the controllable flybar keeps the stability and continuous response of the Hiller system.
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