Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Avionics Power Supplies
At the most basic level, we supply the electrical power needed by the avionics from one or more battery packs. Since it is quite common for the same power supply that feeds the primary receiver and autopilot to also feed the control surface servos, the batteries need to have sufficient capacity to deal with any servo torques that may be needed in flight. The forces on large flaps can be significant and lead to large current draws by their servo systems, for example. Since the avionics supply is so critical to the safety of the aircraft, it is common to have multiple batteries with some form of fail-over system to switch between them. In addition, the operators should assess over what flight time any battery goes from 100% to 50% capacity in typical flying conditions so that they can estimate likely battery condition directly by timing the flight. Although it is possible to use telemetry systems to monitor battery voltages from the ground, this should never be the sole mechanism for assessing battery health.
We also commonly fit LED strips to our airframes that are visible from the outside so that there is a clear and unambiguous voltage status visible before takeoff, see, for example, Figure 6.4.
If batteries do not offer sufficient endurance for the UAV under consideration, some form of engine-powered generation system will be needed. This can either be in the form of a generator fixed to the main propulsion engine(s) or from a dedicated power generation system. If a sufficiently large power generation capacity is provided, it can even be possible to use electric motors for the main form of propulsion, supplied from the power generator to yield a hybrid system where batteries then only augment the operations of the airframe. When such generators are large enough, they can also be configured to allow for in-flight (re)starting of the prime mover given a suitably sized battery. Gasoline-powered engine systems with attached generators are commercially available, see, for example, those sold by the UAV Factory, Figure 6.5. We have also built our own systems based on brushless motors, see, for example, Figure 6.6.
Figure 6.4 Fuselage with externally visible LED voltage monitor strips. Here, one is for the avionics system and the second for the ignition system.
Figure 6.5 Aircraft with twin on-board, belt-driven generators as supplied by the UAV Factory and a close-up of UAV Factory system.
Figure 6.6 On-board, belt-driven brushless motor used as generators.
Figure 6.7 Aircraft with a Sullivan pancake starter-generator system.
As already noted, if sufficiently large systems are used, they can generate enough torque to provide on-board engine starting, as seen in Figure 6.7, although the losses in such large systems often create significant reductions in the propulsive power available, even when not generating any useful electrical power.
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