Airframe Avionics and Systems

Even without considering payloads, the avionics in all but the simplest unmanned air vehicle (UAV) can become fearsomly complex. Figures 6.1 and 6.2 show the outline avionics diagram for the SPOTTER aircraft and the level of complexity potentially present in a UAV with a maximum take-off weight (MTOW) of «40 kg. In this chapter, we step through the main building blocks of such systems and explain the rationale behind our designs.

Primary Control Transmitter and Receivers

Although the whole point of UAVs is that they are not flown directly by a pilot operating the control surfaces with manual inputs, we always equip our systems with a primary receiver that can be operated by a pilot with a standard aero-modeler hand-held transmitter. We now almost always use Futaba products for this purpose since we find them to be highly robust, well built, and with good short-range communications. Such systems are typically effective only out to «1km from the pilot since their range is limited: anyway, at any greater ranges, the ground-based pilot is unable to see what the aircraft is doing sufficiently well to be able to fly it safely. This primary control system is used for initial test flights of new aircraft and for overriding autopilot control if the flight team thinks this best (e.g., in difficult cross-wind landings or to force-ditch the aircraft).

The systems we use typically make use of between 6 and 14 channels. Table 6.1 sets out a typical set of assignments, in this case for a system with twin engines and dual receivers. We commonly fit dual receivers on our more complex aircraft, both bound to the same transmitter, with a switch-over system that allows the second receiver to take control if the first loses stable connection to the transmitter. Figure 6.3 shows a typical avionics fit with dual receivers and a switch-over system. Note that because each channel has its own independent wiring using neutral, power, and signal lines, this requires a very significant quantity of cables to connect the two receivers, switch-over unit, and autopilot together. The incoming signals pass from the receiver, via the switch-over unit, through the autopilot before heading on to the relevant servos connected to the control surfaces, throttle, and so on.

Small Unmanned Fixed-wing Aircraft Design: A Practical Approach, First Edition. Andrew J. Keane, Andras Sobester and James P. Scanlan.

©2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.

Outline avionics diagram for SPOTTER UAV

Figure 6.1 Outline avionics diagram for SPOTTER UAV.

Outline avionics diagram for SPOTTER UAV (detail) - note switch-over unit linking dual receivers and dual autopilots

Figure 6.2 Outline avionics diagram for SPOTTER UAV (detail) - note switch-over unit linking dual receivers and dual autopilots.

Table 6.1 Typical primary transmitter/receiver channel assignments.




Normal position

Fail-safe position




As demanded

2°-3° port




As demanded



Throttle 1


As demanded





As demanded




Switch (flaps)




Throttle 2

Mixed to throttle 1

As demanded



Auto-pilot engage

Switch (AUX1)


Mission dependent


Redundant receiver system


Signal < 1.5 ms

Signal > 1.5 ms

Typical avionics boards

Figure 6.3 Typical avionics boards. Note the use of MilSpec connectors (the Futaba receivers are marked 1, the switch-over unit 2, the SC2 autopilot and GPS antenna 3, and the avionics and ignition batteries 4 and 5, respectively).

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