Concept of Tiltrotor UAV Control System

Abstract Nowadays, civil UAV industry market grows rapidly. This expansion is followed by the new requirements and expectations against UAVs, which force their constructors to look for less typical solutions. Expected long time endurance and range are the typical examples of such expectations. Clients are often looking for UAV with VTOL ability and time of flight much greater than 30 minutes and long range. They want to inspect large areas, i.e. between major cities without need of paying for building and maintaining developed aircraft infrastructure. Example of UAV with low infrastructure requirements are multirotors. Major disadvantage of them is short flight time. Elongating time of flight is hard to achieve by classical multirotor with standard Li-Pol batteries available on the market. They have too low energy density in currently used technology. Alternative power solutions, like fuel cells, have low financially rewarding factor, which cause whole projects to be unprofitable. Foregoing circumstances force engineers to find less usual ways for improvement energy efficiency, which will cause extending the time and range of flight. One of them is a tiltrotor. Tiltrotors are hybrid solutions – they combine airplane and multirotor capabilities to achieve features, which exclude each other in classical constructions. Aircraft-like wing make it able to use its lift-to-drag ratio to achieve energy savings, higher top speed and extended range in comparison with multirotors. UAV is also equipped with multiple multirotor-style engines with additional capability to rotate itself in pitch. In horizontal engine position, vehicle behaves like classical multirotor – allowing pilot to hover and perform VTOL manoeuvres. When engines are tilted to vertical position, whole UAV get performance similar to airplane – high speed and flight endurance. In the other hand, practical implementation of tiltrotor solution can be problematic: simulation, steering and controlling such aircraft in transition state are complex tasks. Moreover, designed aircraft should follow major rule connected with multirotors: Should have as simple, robust mechanical design as it can. Proposed article will concentrate on concept and preliminary design of fly-by-wire steering system with unique properties for tiltrotor. One of such properties will be unification of steering method – which eliminates need for switch and setting initial conditions for control subsystems, when flight procedure requires changing flight mode. Second important improvement will be possibility to use transitional states as intermediate state between propeller driven fly and gliding – which allow achieving wide spectrum of flight speeds. Moreover, huge number degrees of freedom (at least 9) create new opportunities for steering optimization. Extensive thrust vectoring abilities of such UAV could not only implicate substantial efficiency improvement of multirotors, but also improve its manoeuvrability. The article will focus on basic concepts of kinematics, steering of such UAV and show proposition of energy-usage oriented optimization for its control trajectories. To let mechanical design be simple, all control and steering methods will be implemented in software, which will implicate complex structure of steering system. Overcoming complexity of software should be profitable in relation to expected improvements of UAV capabilities.