The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 377
Fig. 9.29 UAV with coaxial rotors and a rotationally symmetrical body can attain high speeds by near-axial
flight.
be used for tasks in which there is a significant risk. Pilotless aircraft are often called
drones and the term is also applied to pilotless helicopters.
Small remotely controlled helicopters can be used for reconnaissance and may escape
detection or prove very difficult targets if they are detected. Small helicopters have
also been developed for crop dusting, notably in Japan. However, the limitations of
the helicopter apply to small ones equally. Small fixed-wing drones can be launched
from a simple ramp and recovered by parachute and will have greater range than
the inefficient helicopter. Consequently drone helicopters are very rare indeed and fill
extremely specialized niches.
The drone helicopter may depend entirely on a remote pilot, or it may be entirely
autonomous so that it can complete an entire mission automatically. Early drones were
totally pilot controlled and had to stay within sight of the pilot. Subsequently the drone
could relay a television picture of the view from the cockpit so that the drone could
be flown out of sight. As electronics developed, it became possible to integrate the
helicopter controls with autopilot functions and control the whole with a navigation
system. This led to the UAV (unmanned air vehicle) concept that can be applied to the
helicopter as well as it can to the aeroplane.
In the absence of a pilot the UAV designer has much more freedom. The contrarotating
coaxial rotor is popular as it avoids the use of very small and delicate tail
rotors. Figure 9.29 shows a typical UAV having coaxial rotors. The hull is axially
symmetrical and high speed forward flight can be achieved by tilting the whole machine
over. Tilt-rotor UAVs have also been developed.
9.11 Radio control principles
Radio control is simply the use of radio signals to relay the position of control sticks
without any physical connection. In practical applications there will be a need to have
several control channels simultaneously available and each one will need to be proportional,
which means that small or large movements of the control stick will be remotely
reproduced by the movement of a servo.
The foundation of this technology was a patent by A.H. Reeves in the 1930s which
showed that any type of information could be carried by varying the width or distribution
of pulses. In a helicopter, the actual flying controls require four simultaneous