Climbing performance enhancement of small fixed-wing UAVs via hybrid electric propulsion

The climb rate and climb gradient of small fixed-wing Unmanned Aerial Vehicles (UAVs), characterized by extremely compact and lightweight design, are typically limited by the maximum allowable temperature of the engine cylinder head, especially in hot environment conditions. The problem is often overcome by alternating climb and levelled flight phases to let the engine cool down, but the resulting performances are far from satisfactory. This paper aims to evaluate the feasibility of a reconfigurable hybrid propulsion system based on the integration of the UAV internal combustion engine with its electric generator, temporarily (during climb) converted into motor and supplied by the battery pack, to maintain/boost the propeller thrust while reducing the combustion engine temperature. With reference to the lightweight surveillance UAV Rapier X-25 (maximum take-off weight up to 25 kg) by Sky Eye Systems (Italy), a detailed nonlinear model of the reconfigurable hybrid propulsion system is developed and coupled with the models of the propeller and the vehicle flight dynamics. The system dynamic performances during severe climb manoeuvres are thus characterized, by demonstrating that the proposed solution can both improve the UAV climb rate and, by reducing the combustion engine power request, limit the temperature increase of the cylinder head.