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Wiring Looms

As has already been noted in previous chapters, the automated manufacture of customized wiring looms is generally not affordable for low-cost, low-volume UAV manufacture. Even if a fully geometrically and functionally detailed CAD representation is prepared, assembly still requires lengths of wire to be cut and soldered to connectors, plug, sockets, and so on, by hand. This can be aided in a number of ways, however. First, a fully detailed logical wiring diagram should be prepared, which notes the color and thickness/grade of each wire in the loom. This should also show terminations and plug breaks, See, for example, Figure 18.14. Then a full-scale printed plan view drawing of the aircraft showing all parts to be connected should be printed and glued to a plywood baseboard: the so-called iron bird, see Figure 18.15. At this point, wires can be cut and compared for length to the iron bird before connectors are soldered or crimped in place. Each wire should be labeled or color-coded during manufacture.

It is good practice to use only connectors that have an auxiliary locking mechanism to prevent disconnections being caused by accident or vibration. This can take the form of extra safety clips or connectors with built-in locks. If soldering is to be adopted, good-quality fluxed solder should be used, and the technician carrying this out should be provided with a dedicated soldering station with suitable cable holding clamps, see Figure 18.16. If crimped connectors are to be adopted, proper professional-grade crimping pliers should be used to form the crimps. In all cases, terminations should have shrink-wraps placed over exposed conductors and strain reliefs added where bulkheads will be penetrated or other chaffing or stress raisers might occur. Once terminated, each wire or group of wires can be added to the iron bird and the prototype loom gradually built up. When complete, the harness should be functionally tested by connection to all the avionics items to be fitted to the aircraft. We carry out interference and soak tests with spring-loaded servos and motor-driven generators and ignition systems on the iron bird, for example. If all is well, the loom can then be covered in cable wrap and test-fitted in the aircraft to see if any cable lengths need adjusting. At this point, either the test loom can be used to specify wire lengths and bundles for subsequent professional loom manufacture or the test loom can be finished off for flight use with permanent wraps and heat-shrink termination.

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