The Art of the Helicopter John Watkinson - Karatunov.net

7.1 Introduction
7
Control
The physical control requirement in a helicopter is no more than the application of
appropriate cyclic, collective and tail rotor pitch settings. Implicit in this is the need for
some form of rotor speed governing. However, the physical control requirement is only
a small part of the overall process that is inevitably more complex. In order to perform
useful missions in safety, the pitch settings of the rotors must be such that at all times
the machine has, for example, the desired attitude, airspeed, altitude and geographical
location.
In order to control the flight, information must be gathered about the present conditions,
and these must be compared with the desired conditions. Any discrepancies must
result in changes to the physical controls that would tend to reduce discrepancies to
zero or at least as near as makes no difference. This is, of course, classical negative feedback
as was introduced in Chapter 2. The essential components of this process must
be information gathering, or sensing, information transmission, decision-making, or
information processing, all of which takes place with small amounts of power, followed
by a power amplification stage that allows the control information actually to operate
the controls.
Figure 7.1(a) shows the case of a simple light helicopter. It will have instruments
such as airspeed indicator, altimeter and compass whose output is purely visual. The
pilot looks at the instruments and out of the canopy in order to establish the actual
conditions, but has to perform all of the actions necessary to stabilize the machine and
hold it on a desired course. The machine itself may actually be unstable, in that left to
itself it would diverge from its original course, but the combination of the pilot and the
machine can be a stable system. In other words the pilot is working out the difference
between what is wanted and what is actually happening. As a result of a difference
of this kind the pilot then has to operate one or more of the controls in a sense that
reduces the difference. An unskilled pilot may overcontrol so that the machine oscillates
about the desired course, whereas the experienced pilot will have learned the dynamics
of the machine and applies just the right control inputs smoothly to bring the difference
to zero.
In Figure 7.1(a) the pilot is part of a feedback loop and is not only computing the
servo error, but is also mentally modelling the machine’s response to avoid instability.
The pilot may also be providing all of the control power with his own muscles. This
task must be performed for the duration of the flight, as the machine will diverge if
attention lapses for more than a few seconds.