A Hybrid Propulsion System for a High-Endurance UAV: Configuration Selection and Aerodynamic Study

Abstract

In recent years, a renewed interest in the development of unmanned air vehicles (UAVs) led to a wide range of interesting applications in the fields of reconnaissance and surveillance. In these types of mission, the noise produced by propeller driven UAVs is a major drawback, which can be partially solved by installing an electric motor to drive the propeller. The evolution of high performance brushless motors makes electric propulsion particularly appealing, at least for small and medium size UAVs. All electric propulsion systems developed to date are though penalized by the limited range/endurance that can be provided by a reasonably sized battery pack. In this paper we propose a hybrid propulsion system based on a recently developed, high efficiency microturbine which can be used to power an electric generator, thus providing a significant range/mission time extension. The UMTG is undergoing operational testing in our Laboratory, to identify its most suitable configuration and to improve its performance: a new compact regenerative combustion chamber was developed and several tests were performed to reduce its weight and size so as to increase the vehicle payload. In a high range/endurance mission the ultramicro turbine drives the electrical motor that powers the propeller only during the cruise phase (the so-called “transfer to target”), while in the final approach, in which a quiet flight attitude is mandatory, a (smaller) battery pack drives the motor directly and the UMTG is turned off. The mission requirements considered for the preliminary design of the UAV consist of a long endurance (> 12 hours) step, with a cruise speed of 33.3 m/s and a dash speed of 45 m/s at an altitude of 5000 meters. The maximum take-off weight is 500 N, with a payload of 80 N. Under the above assumptions, a flying wing configuration for the UAV was defined, with a length of 1.6 meters and a span of 2.5 meters. A system of elevons assures the pitch and roll motion while a double vertical tail, in which a pusher propeller is lodged, guarantees the yaw stability and control.