Digital Displacement Hydrostatic Transmission for Rotorcraft and Distributed Propulsion

Abstract

This paper introduces a new series-hybrid digital transmission technology for distributed propulsion systems offering a lower cost, a lighter weight and more environmental tolerance than series-electric-hybrid powertrains. In particular, this concept can be used on large multicopters for applications requiring long range, heavy payloads and continuous hover, such as offshore search and rescue, surveillance and logistics. The concept, design principles and analysis are described to allow component sizing and calculation of operating point under steady state and dynamic conditions. A design is described for a quadcopter with maximum take-off weight of 380kg, including a Rotax 915 engine, a Digital Displacement® hydraulic pump and bent-axis motors. The components of the propulsion system are described and compared to an equivalent electric system, showing lower cost and weight, and similar efficiency. A computationally efficient model is described of the rotor behavior allowing rapid simulation with accurate rotor loads. A 6 DOF vehicle simulation is described, including dynamics of the proposed system, coupled to a flight controller. Results show that such a standard flight controller can fly the proposed system under a range of conditions including at the engine power limit. The expected flight endurance of the 380kg quadcopter is over 6 hours and range over 1000km depending on payload. Test rigs are described, including at full scale for the 380 kg study, which demonstrate stable control of hover. Other applications for distributed propulsion are discussed.