The flybar both adds balance and indirectly controls the direction of a helicopter. At the each end of the flybar is a "flybar paddle" which is what gives the flybar its ability to tilt as you apply cyclic. To understand this a little better, turn your helicopter on with the motor unplugged and apply forward cyclic. Now spin the rotors slow, by hand, and pay close attention to the flybar paddles. You should see them pitching up and down as they spin. When one pitches up in relation to the rotation, the other will pitch down.
This pitching motion causes the flybar to tilt in whatever direction you have indicated. As the flybar tilts it causes the main rotor to tilt and the helicopter to travel in the direction of the tilt. Using the flybar to tilt the main rotor means that the servos don't have to directly control the main rotor disc angle. In flybarless helicopters, the servos have to directly control the rotor disc and have to provide higher torque and be more durable.
Flybars of coaxial helicopters, such as those shown in the photo at right, rely on weights for stability.
Modifying your flybar for more stability
One method of making your helicopter more stable is to add weight to the flybar. This is normally done by sliding weights onto the flybar and using a grub screw to secure the weight. However, some flybar paddles have a hole specifically designed to hold weights.
The extra weight of the flybar aids in the gyroscopic stability of the helicopter and makes it more resistant to a change in axis.
Underslung and Overslung Flybars
The location of the flybar can differ between models. Some models place the flybar above the main rotor disc - this is considered an "Overslung" flybar. Others place the flybar under the main rotor - this is considered an "underslung" flybar.
There is much discussion on which method is best.
The overslung flybar group argues that, with the main rotor disc being close to the helicopters vertical center of gravity, the helicopter is more responsive. On the other hand, however, with the rotor disc closer to the center of gravity (and thus closer to the tail boom), you have more of a chance of getting a boom strike.