Flybar equipped model helicopters are based on the pioneering work of Arthur M. Young of Bell Helicopters and Stanley Hiller Jr. of Hiller Helicopters.
In the late 1930s, Young created a helicopter control system which used a bar with weights on the ends mounted 90° to the main rotor to gyroscopically stabilize it. As cyclic is applied control rods tilt the bottom half of the swash plate and rods connected to the top half change the pitch of the blades. Because the stabilizer is a weighted bar airflow has no effect.
In 1944 Hiller - then a 19 year-old who had never seen a helicopter fly - designed, built and flew the first coaxial helicopter created in the the United States. He later created the Hiller-Matic Paddle System. Although similar in appearance to the Young/Bell stabilizer bar design it worked on a different principle. Hiller connected the pilot's control to a tilting flybar with airfoils on the ends so when the pilot moved the controls the aerodynamics of the paddles flew the bar into a different position with links from flybar-to-rotor changing the pitch of the rotor.
The Young/Bell stabilizer bar design is very responsive but requires more force and is less stable and self correcting than Hiller's design. The Hiller aerodynamic paddle design requires very little effort to change the cyclic pitch of the rotor and self-correcting, but there is lag between control input and the flybar changing the rotors. Both designs were utilized in full-scale helicopters but never used together. Young's rotor design led to the creation of the Bell-47: Wiki for the Bell-47Hiller's design was incorporated into the H-23 / U-12 flow in the Korean War and popularized by the TV series MASH: Wiki for the H-23
The first successful RC model Helicopters which appeared in the late 1960s were variations on Hiller's paddle design flown by by Dieter Schluter. In the mid-70s RC models began to use a combination of the two systems, grafting a heavier version Hiller's airfoil paddles to the gyroscopic stabilization bar concept of the Young/Bell design. The gyroscopic effect of the spinning weight of the paddles helps to stabilize hover, while the aerodynamics of the paddle with the paddles make control movement more predictable and require less servo force to initiate and maintain rotor pitch changes.
Blade models angle the flybar 45° from the main rotor -instead of 90° as in the original Young/Bell and Hiller designs - to further increase stability and the ability to self-correct. The flybar can be tuned for more stability or more responsiveness by adjusting the fly bar variables: size, weight, length of paddles, and other variables such as the angle of the control arms/flybar and main rotor. Overall responsiveness can also be increased by shortening the control arms which tilt the swash plate. Near the end of the cause and effect tutorial I include text from the first patent / patent application I could find which mention the use of flybar phasing of < 90°.
What about Sikorski?
Igor Sikorski is significant in the history of helicopter development because he was the first to solve the problem of main rotor torque by devising the boom mounted tail rotor which keeps the body of a helicopter from spinning opposite the direction of the main rotor. He doesn't factor in to the discussion of stabilizing dual-blade rotors because he solved the stability problem a different way in his early designs by using three blades spaced 120° apart.
Juan de la Cierva
Anyone flying a full-size or model helicopter also owes a debt of gratitude to Juan de la Cierva, whose pioneering work with Autogyros in the 1920s discovered and solved some the counter-intuitive puzzles of rotary flight.