High endurance fixed-wing UAVs powered by solar arrays or hydrogen fuel cells carry energy systems that cost more than the airframe itself and a single motor failure can end a multi-hour mission. This study aims to compare twin, trimotor and quad propulsion on measured data to find where the engine-out redundancy of each end.
Static thrust stand data for a 6.5 kg, 3.3 m airframe are transferred to flight with advance ratio dependent propeller coefficients. On a 1000 Wh budget, engine-out cases are scored against a two-level criterion of stopped propeller drag, trim drag and rudder authority.
All three configurations cruise at 18 m.s-1 on 211–233 W and hold that airspeed after a motor loss with the tested motor and propeller combinations. What separates them is directional control. The quad and trimotor need under 12° of rudder and the trimotor loses its centerline motor with no yaw. The twin needs 73% of the rudder travel and cannot be trimmed below 12.8 m.s-1, marginal against the 12 m.s-1 minimum sustainable flight level.
For the tested components on this airframe, the twin-engine redundancy assumption inherited from manned practice does not carry over: the fin, sized for cruise stability, sets the redundancy of the twin and a minimum control speed after a motor loss belongs in the design case of such a layout.
