Hybrid thermo-electric propulsion system for climbing performance enhancement of lightweight fixed-wing UAVs

In conventional lightweight Unmanned Aerial Vehicles (UAVs), climbing performances are often limited by the maximum allowable temperature of the cylinder head of internal combustion engines. Before achieving the cruise altitude, UAVs typically alternate the climb phase and the levelled flight to let the engine cool down, but the resulting rate and gradient of climb are far from satisfactory. This paper aims to evaluate the feasibility of converting the conventional propulsion system architectures of lightweight UAVs into reconfigurable hybrid systems, integrating the internal combustion engine with its electric generator working as booster motor. The basic idea is that during the climb phase the three-phase BLDC machine is supplied by the Li-Po battery pack and controlled 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) manufactured 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 vehicle flight dynamics. The system 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.