Control Surface Failure
Assuming that the primary control system is functioning, the next area of concern to be considered is operation of the control surfaces - these are usually servo-operated flaps on the wings and empennage. Failure of the control surfaces generally arises because of servo or linkages failing (assuming there is sufficient electric power available to the control system). If a servo jams in a significantly deflected position, control of the aircraft can be severely affected. If high levels of resilience are required, it may be necessary to fit multiple control surfaces to deal with such situations: the 2SEAS aircraft, for example, has four main wing ailerons and four elevators so that if any one is jammed in an adverse position, the aircraft can still fly safely, see Figure 8.2.
Since the main point of UAVs is generally to fly under some sort of automated control, it is natural to fit an autopilot to the UAV. Such systems generally take over operation of all the control surfaces and navigate by using predefined global positioning system (GPS) way-points. The failure modes of these systems can be many and varied, ranging from simple mechanical problems to subtle software bugs. However, at the initial design stage the designer is generally seeking to just select an appropriate autopilot system so as to assess cost and communications requirements. Approvals to fly beyond the line of sight will require very significant discussions with the local aviation authority - lying outside the scope of this book - but they will heavily
Figure 8.2 2SEAS aircraft with redundant ailerons and elevators.
influence the final choice of system. Except for the very smallest UAVs, weight and power requirements are usually much less of a concern. Aside from selecting a high-quality system, delivering reliability requires ensuring that all aerials are well sited and that the gyros and accelerators are not exposed to significant levels of vibration. IC engines can prove to be quite troublesome in this regard, and we find great care has to be exercised in terms of engine and autopilot mountings to prevent vibration levels on the autopilot causing sensor difficulties. We typically fit multiple isolation mounts to our engines and suspend sensitive parts of the autopilot on dedicated and tuned antivibration systems, see Figures 8.3 and 8.4. Adding such treatments can take up significant amounts of space and add noticeably to the total weight.
Figure 8.3 Autopilot system on vibration test.
Figure 8.4 Treble isolated engine mounting.