The Art of the Helicopter John Watkinson - Karatunov.net

262 The Art of the Helicopter
Fig. 7.3 With an autopilot, attitude, altitude, airspeed and heading can be controlled automatically, but the
pilot must still select the course. With an autopilot coupled to a flight director, navigational inputs can control
the autopilot so that a course can be held.
understand the dynamic response of the helicopter in order to avoid any overshooting
or oscillation when correcting the attitude. The attitude of the machine is still controlled
by the pilot, but the stability of the machine is improved by using attitude signals from
the artificial horizon.
In Figure 7.3 the process has been taken one stage further. Here the augmented
stability achieved in Figure 7.1(b) makes it possible for the machine to be controlled
automatically. Inputs to the signal processor from the altimeter, the airspeed indicator
and from a compass allow the machine to hold a heading without any action from the
pilot. This system is generally called an automatic pilot or just an autopilot.
The system of Figure 7.3 relieves the pilot of the need to fly the machine continuously,
but can only hold a heading and would not compensate for a change in wind strength
or direction. However, with further inputs from a navigation system or from a flight
director the machine could hold a course determined by a GPS receiver, VOR beacon
or an instrument landing system (ILS). In this case the flight director creates a further
feedback loop. The actual course is computed by navigational equipment and compared
with the desired course. Any error is then fed to the autopilot as a modified heading
reference.
In early helicopters, the provision of stability augmentation and autopilot systems
was frequently optional, whereas in modern machines such systems are often designed
in from the outset. One modern trend is away from instruments with a direct visual
readout. Instead instruments may be used which have only an electrical output and are
best called sensors or transducers. These outputs are supplied not only to the autopilot,
but also to a display processor that produces a virtual instrument panel on a computer
graphics-type display. This is known colloquially as ‘glass cockpit’ technology. One
advantage is that the instruments themselves can be located anywhere convenient and
do not have to be in the instrument panel.
In helicopters, the stability augmentation and autopilot capabilities may be extended
into the hover. In this case altitude and airspeed information from atmospheric pressure
is not good enough and instead RADAR will be used to measure height and groundspeed.
In flying cranes and in search-and-rescue helicopters the ability to remain at a