The Art of the Helicopter John Watkinson - Karatunov.net

fixed point in the hover despite external disturbances is extremely valuable. In advanced
search-and-rescue helicopters the pilot simply presses a button as he flies directly over
the victim and the helicopter will automatically perform a 360 ◦ turn and come to the
hover at exactly the same place.
A helicopter with a suitable degree of automation does not need a pilot on board
if the commands he would have given the autopilot can be transmitted by radio. This
makes possible a range of devices from the simple radio-controlled helicopter, which
must remain within the pilot’s view, to the autonomous drone that can undertake an
entire mission without human intervention.
The provision of various control, stabilization or autopilot systems depends upon a
number of fundamental technologies. These include the signalling of control positions
from one place to another, power operation of controls, attitude sensing, altitude and
airspeed sensing, parameter signalling and feedback control. The subject of safety must
also arise. What happens if any of these mechanisms go wrong? These concepts will be
considered in this chapter.
7.2 Flight sensors
There are four main categories of flight instrument into which virtually any device can
be placed. These will be contrasted prior to a detailed discussion of each one.
(a) Heading sensing devices to display information relating to the direction the helicopter
is pointing in. The most important of these is the magnetic compass that
displays the magnetic heading or direction with respect to the earth’s magnetic field.
The direction indicator (DI) is a gyroscopic device arranged to display the same
heading as the compass. It is less affected by manoeuvres than the compass and is
easier to read. The rate of turn indicator, usually just called the turn indicator, is a
gyroscopic device that displays the rate at which the heading is changing and the
direction of the change.
(b) Height sensing indicators display the vertical distance between the helicopter and
some reference. The altimeter is a pressure-sensing device whose display is calibrated
in feet or metres. It is important to use the appropriate reference or the
display will be misleading. The RADAR altimeter is an electronic device which
times the reflections of radio waves emitted downwards from the helicopter and so
displays the height above ground. The vertical speed indicator measures the rate
of change of air pressure and displays rate of altitude change and direction, usually
in hundreds of feet per minute. Height information is also available from GPS
receivers.
(c) The airspeed indicator (ASI) is a device that measures the dynamic pressure of the
air caused by forward motion of the helicopter. The scale is calibrated in knots or
mph. It is also possible to measure ground velocity using Doppler RADAR and
this will allow the helicopter’s track to be established. In this way the heading can
be adjusted to cancel the effects of wind.
(d) Attitude-sensing instruments display the attitude of the helicopter in pitch and roll
with respect to the earth’s gravitational field. These are commonly gyroscopic and
include the artificial horizon, which displays pitch and roll on one instrument, and
the simpler rate of turn display.
The above classification is of most use to the pilot since in flight one is more concerned
with the readings themselves than how the instruments work internally. As well as
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