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45 acceleration/deceleration, turn rate or other actions may require flight test modes to be programmed into the autopilot; running simulations for each test can help identify where such programming is needed. Without that, it may be impossible to breach the constraints designed for users to abide by for safety purposes, and effectively establish the absolute limits of a UAV’s performance. Engine checks Power system tests for UAVs are less well-defined than those for manned aircraft engines, as they are far newer and less mature, with fewer records and established testing practices. In the case of battery-powered systems, monitoring current and voltage, and checking for appropriate charge, can suffice for pre- flight inspections. For the two-stroke heavy fuel engines typical of tactical-grade UAVs, however, additional challenges may lie in placing monitoring equipment, owing to space restrictions. They too are often custom- made, with less supporting design data than is common for manned aircraft engines, and using COTS propellers of sub-optimal aerodynamic efficiency. Other vital operating qualities such as levels of engine, coolant and gearbox oil should also be checked before each flight. For fuel as well as electric vehicles, it is vital to check key propulsion variables, for example that the motor is generating power consistent with its rated rpm, and that the propeller produces enough static thrust to enable take-off and ascent. For a prototype fresh off the assembly line, it may be recommended to run the motor at ground level, in a hangar or laboratory, in order to ‘break-in’ the system before attaching the propeller. Electromagnetic interference It is also worth testing the UAV on the ground for adequate EM shielding, as any defect here can cripple the control and comms feeds. Antennas, actuators, cable harnesses and other systems may be vulnerable if insufficient EM protection was added during the design stages. The use of a controlled environment capable of producing or simulating the kinds of frequencies and energy levels a UAV might experience when flying near ships, airports or other complexes that produce EM emissions can be key. In addition to emissions testing, critical EM issues can also arise between onboard systems. If ‘noise’ from an avionics or comms subsystem inadvertently enters the RF receiver, it will place extra demands on the power required for command and control signals to be received. There is an agreed set of tests for intra- system EMI detection, perhaps the most important of which is an EMC SOFT (electromagnetic compatibility safety of flight test) operation. Such tests may be centred on operating a UAV’s control surfaces at differing frequencies, transmitter power settings, antennas and engine revs (to account for engine system noise). Fluctuations in the movement of the ailerons or elevators, such as overshooting or undershooting the commanded degree of movement, may be evidence of EM interference, and should be eliminated before flight testing for the sake of safety. Payload testing Although comms and autopilots need close scrutiny in mission tests, it is payload testing that bears the brunt of attention in UAV systems. Of course, other tests form the bedrock on which payload tests sit – without accurate navigation data, for example, the targeting accuracy of systems that use GNSS might be insufficient for mission effectiveness. Information on altitude, pitch, yaw, roll and heading are also critical to supplement the limitations of GNSS coordinates in payload targeting. The update rate of all such data to the targeting computer should also be checked to satisfy that no lag can occur at some critical mission juncture. In the case of a typical sensor payload, such as an electro-optic/infrared (EO/ IR) system, ground checks such as checking that the telemetry feed operates effectively over the data connector linking the camera to the autopilot and data link hardware can be performed. All onboard systems should communicate with each other and the GCS effectively, particularly if the GCS operator is responsible for operating the payload during flight. In other regards, the testing of a UAV’s EO/IR payload is generally similar to that of a manned aircraft. On the ground and in the air, and in visual range and beyond it, the control sensitivity and Unmanned Systems Technology | February/March 2018 UAVs should be checked before every flight to ensure there is enough static thrust to ensure a steady ascent (Courtesy of Alaska Center for Unmanned Aircraft Systems Integration)