The Art of the Helicopter John Watkinson - Karatunov.net

312 The Art of the Helicopter
Fig. 7.48 In the Lockheed system the flybar is caused to precess by the application of couples. A double-spring
arrangement allows the precession without interference from the other cyclic axis. See text.
by a similar amount. The springs will balance each other over a significant range of
movement of the flybar. Thus if the flybar tilts it will not experience a restoring force
from the controls. However, if the controls are displaced from neutral, the spring
imbalance is upset and the result is a couple applied to the flybar.
Control is achieved as follows. If the pilot wishes to roll the machine, this can only be
done if the flybar is made to roll. As the flybar is gyroscopic, it can only be made to roll
by applying a couple in the pitch axis. Consequently the controls are rigged such that
the roll cyclic pushrod applies a pitch couple to the flybar through the spring system.
The flybar will sense this couple and correctly precess with a 90 ◦ lag to perform the
commanded roll. The flybar is allowed to precess in this way by the balanced springs
in the pitch pushrod.
The hingeless model 286 was aerobatic and performed loops and rolls with ease,
whilst remaining easy to fly. Essentially the same control approach was adopted in
the first models of the Lockheed Cheyenne compound helicopter (see Chapter 9). The
high speed of this machine increased the loads that the rotors fed into the gyro through
the pitch links, undermining gyro authority. The solution was the system shown in
Figure 7.49. Lockheed called this the AMCS (advanced mechanical control system).
Here the gyro has been moved inboard, below the transmission. The absence of an
external flybar reduces drag. The gyro is fitted with a swashplate through which it
controls the mechanical inputs to hydraulic actuators. These provide an irreversible
control over the cyclic pitch so that pitch link forces from the blades cannot reach the
gyro. The hydraulic actuators operate a spider that passes through the centre of the gyro
and the transmission and emerges above the rotor head where it operates pitch links.
The gyro is supplied with couples by the pilot’s cyclic control using the positive and
negative balanced spring system as before. These are not shown in the diagram. Having
eliminated pitch link loads with the hydraulic actuators, the gyro is now made to follow
the hull attitude using springs attached to the mast.
Lockheed’s AMCS represented the ultimate mechanical helicopter gyrostabilizer,
but even as it was being tested, systems were being developed using miniature gyros
and electronic amplification and the future of helicopter stabilization would lie in that
direction.
Flybars are still found in flying model helicopters. Models are harder to fly than the
full size and the flybar allows a welcome degree of stability for the pilot who won’t have