Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Avionics systems also need to be extensively tested before committing the airframe to flight. If the avionics malfunctions, it is highly likely that the aircraft will crash. In many jurisdictions, there is a mandatory set of ground tests the avionics have to be put through before every flight; we assume that a competent and qualified pilot will be in charge and thus such tests will routinely be carried out and no flight is commenced unless all is satisfactory. Even so, it is not good practice to wait until reaching the airfield before testing the on-board systems. Indeed, we would not commit to final avionics build without first carrying some preliminary experimental testing of the intended configurations in the lab. To do this, we construct a full-scale plan view drawing of the airframe and attach this to a rigid plywood baseboard: this is termed the “iron-bird”, see Figure 16.20. To this, we attach all the avionics we propose to fly and wire it up with appropriate length cables laid out as we expect the final harnesses to lie. This system can then be extensively soak-tested for reliability and any interference problems. To do this, we energize any ignition systems to make sure that spark generation does not cause any problems. If there are generator systems to be included in the design, we include these and power them by dedicated electric motors. Sensitive items of equipment will be subjected to vibration test, also while powered up.
Figure 16.20 SPOTTER iron-bird being used to test a complete avionics build-up: note motors to spin generators in a realistic manner.
When carrying out vibration tests on components, it is important that any mounting does not significantly change the natural frequencies of the parts being tested as compared to their behavior in flight conditions. For wings, clamping at the spar mounting point, as in static structural load tests, is normal practice. However, for small components and on-board instrumentation, achieving realistic mounting can be very difficult in practice, and often one has to accept simple free-free mounting conditions as simulated by supporting components in soft springs or elastic bands, see Figure 16.21.
Figure 16.21 Avionics board under vibration test. Note the free-free mounting simulated by elastic band supports. In this case, a force transducer has been placed between the shaker and the long connecting rod that stimulates the board. The in-built accelerometer in the flight controller is used to register motions.
Figure 16.22 Typical Servo test equipment: (front left to right) simple low-cost tester, large servo, motor speed tester with in-built power meter, and servo control output; (rear) avionics battery and standard primary receiver.
Having tested all the avionics on the “iron-bird”, components can next be tested in the airframe during construction. So, as each part is installed, it should be checked for correct functionality before proceeding. Many elements of the wiring are tedious to remove from a completed airframe and so should be thoroughly checked before being “buried” by further assembly operations. Careful attention should be paid to servos during this stage; they should not be coupled up without first being tested for adequate movement. Thus the linkage to the control surface or other item being moved should be disconnected and the servo powered up with an avionics battery and servo tester or receiver/transmitter combination (see, e.g., Figure 16.22). Servo testers are simpler to use in the lab, but ultimately all servo testing must, of course, be carried out using the intended primary transmitter and receiver system. Mechanical adjustments can then be made to linkages during build to ensure that full movement of the servo does not result in the device being “stalled” by a limited range in the mechanical movement of the components being driven.
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