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Likely Failure Modes

It is a fundamental (and often legal) duty of any engineer to ensure that products are safe and fit for purpose (in that order). Given that there are no crew or passengers to be concerned with, here safety means that of other aircraft and people and structures on the ground. This directly leads to two areas of failure that the UAV designer must consider from the outset: first, how to sense and avoid other aircraft, and, second, how to avoid uncontrolled descent and ditching (it being assumed that normal ground maneuvers take place in strictly controlled areas where the safety of operators and spectators can be ensured). In what follows, we consider how things might go wrong and compromise the safety of operators or others who find themselves in the vicinity of the UAV.

Aerodynamic and Stability Failure

Aircraft flight is now so commonplace that it is worth recalling that it is only a little over a 100 years ago that powered flight was first achieved. One of the most important aspects of early flights was learning to deal with the subjects of aerodynamics and control. Just because an aircraft has wings and control surfaces, it will not necessarily fly safely, or indeed at all. The wings must be large enough to generate sufficient lift to carry the UAV, while drag must not be so great as to prevent flight given the installed power. Aerodynamic stability must be assured during flight, most obviously in pitch and roll, though there are numerous ways that aircraft can exhibit unstable flight behavior, including those linked to structural flexibility. This must allow for any changes in fuel or payload weight or their effects on the longitudinal CoG.

At the earliest stages of design, simply estimating likely wing area and installed power from similar aircraft for the desired landing and operational speeds will provide a start point. For conventional layouts, ensuring sufficient dihedral or a high wing design combined with a CoG at or in front of the main wing quarter chord point will go a long way toward ensuring stable flight. Refinements then include ensuring that the control surfaces are correctly placed and sufficiently large to allow typical maneuvers and that the main spars or equivalent structure will prevent divergence and flutter. Again, working from previous designs will help here, though it must be noted that UAV landing speeds are often much lower than for conventional manned aircraft, and this tends to mean that wings, empennage, and control surfaces may need to be on the large side - it is obviously better to have too much stability and control authority than too little.

If the aircraft is underpowered, it will not be able to take off (or if catapult-launched, maintain flight). Although this will be a very grave problem for the designer, it rarely places anyone in danger. This is a failure of function, not of safety. If the longitudinal pitch stability is insufficient, it is likely that the aircraft will stall and crash soon after takeoff. Given a suitably long test runway, again this will be unlikely to hurt anything but the designer’s pride. The inability to provide adequate control once airborne in the face of gusts or any desired maneuver is a much more serious issue. It is also far from simple to guarantee adequate maneuver authority by direct calculation using the laws of physics. Instead, the designer must rely on following successful similar aircraft and the design guides established by others, wind tunnel tests (if a suitable tunnel is available), and flight trials. It is thus an almost mandatory requirement when producing any new aircraft that it be put through a controlled set of flight trials before it is declared safe for use. Such trials start with low speed and simple activities, proceeding through the entire flight envelope until the test pilot is sure that all is behaving as expected. Any failure of control during test flying may lead to an unplanned ditching of the aircraft. Tests should thus be conducted only over a suitably controlled test field where access is strictly limited to those involved in the flight program. Typical details of suitable test programs are detailed in Chapter 20.

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