From RC Helicopter Wiki
The flybar (or stabilizer bar) is a component of the helicopter's main rotor system that acts to stabilize the main rotor by automatically changing the cyclic pitch, reducing the effect of wind and turbulence and making the helicopter more controllable. The flybar is able to tilt separately from the mast and main rotor disk as it passes through the seesaw.
The flybar can be used in one of three different ways:
- Bell control system
- In its simplest form the flybar is a rod with weights on the end that is mounted perpendicular to the main blades and connected via a mechanical mixer arm to the swashplate and main blade grips. If any force (such as a gust of wind, or the pilot) tries to change the plane in which the rotor is spinning, the flybar will change the cyclic pitch to try to oppose the change. If the force persists, and the rotor is tilted, then over the course of a few seconds, the flybar will follow the mast and therefore the main rotor. This does severely limit the cyclic control authority. When used like this, the flybar is often called a balance bar. This sort of flybar can often be found on the top rotor of a coaxial helicopter (though it is usually at a strange angle, either due to interaction between the two rotors or so that the helicopter can fit into a small box assembled).
- Hiller control system
- In the Hiller control system, all cyclic pitch control is mediated by the flybar. The flybar has a small airfoil paddle on each end, and is able to 'twist' to alter the pitch of the paddles cyclically in response to cyclic inputs. As the paddle pitch changes, the flybar will move to rotate in a different plane to the main rotor, but as it does so, it will change the cyclic pitch of the main blades, causing the main rotor to follow the plane of the flybar. As the flybar paddles have little aerodynamic loading (as they are not carrying the weight of the helicopter), they will maintain their plane of rotation well, even if the main rotor is disturbed by wind or turbulence. The Hiller control system can often be found on fixed pitch helicopters.
- Bell-Hiller control system
- In the Bell-Hiller system, the pitch of the blades is controlled both directly from the swashplate and indirectly by the flybar (which is also controlled by cyclic inputs). This allows the rotor to keep the instant response of the Bell system with the good cyclic authority of the Hiller system. It is used almost exclusively on collective pitch helicopters.
The Hiller (and to a lesser extent Bell-Hiller) control system can also use the flybar to provide additional power to control the helicopter's cyclic pitch, allowing weaker servos to be used.
In general, increasing paddle weight will reduce the cyclic response speed, while increasing the flybar length will increase the cyclic response speed (while also increasing the flybar's stabilizing effect). An easy way to temporarily increase the paddle weight (to slow down the cyclic for beginners) is to attach wheel colletts to the flybar near the paddles.
Electronics stabilization systems, which use gyros to measure any disturbance, and uses the cyclic servos to correct it, are becoming popular in a number of disciplines, including:
- Scale flying, as it permits the use of multi-bladed rotors.
- 3D flying, as it permits much higher cyclic rates than flybarred heads.
- Electric helicopters, as flybarless flight is more efficient.
(It is important to understand that by 'stable' in this context, we mean the ability for the main rotor to maintain it's plane of rotation in the presence of outside forces. It does not mean that the rotor will return to horizontal and the helicopter will hover.)
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