The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 7.42 An electro-hydraulic valve. Small electrical signals can control very powerful actuators with this
mechanism.
7.23 Stability augmentation
Early helicopters were beset by weight and stability problems where solving one often
made the other worse. A further issue is that by definition there was not initially a pool
of experienced pilots to fly the first helicopters and any extra stability would be very
welcome.
The flybar was an appropriate and elegant solution at the time and played a pivotal
role in the development of the helicopter, but subsequent developments have made it
obsolete in the full size, although it is still found in most model helicopters.
Early helicopter blades were relatively lightly built because power was limited. This
resulted in a rotor with low inertia having a high following rate in response to a cyclic
control input or an external disturbance. Making the blades heavier would reduce the
following rate, but the weight could not be accepted. The flybar essentially stabilizes
the machine by reducing the rate at which the rotor can change its attitude. Although
the flybar works, it raises the drag and complexity of the rotor head and causes a
performance penalty. Later systems were developed which sensed the machine’s attitude
using miniature gyroscopes. Correcting signals were then fed into the powered control
system. In a sense the flybar was miniaturized and put in a black box.
7.24 The Bell bar
Figure 7.43(a) shows how the Bell flybar system works. This was developed by Arthur
Young, using initially electrically driven models. It was subsequently used on a number
of full-sized machines including the Bell 47 and the Huey. Use is restricted to two-bladed
rotors. The flybar is mounted across the rotor on a bearing parallel to the feathering
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