110 UVD | CycloTech Bumblebee 2.0 the hub. In this way, rotation of the hub either pulls or pushes on the aft pivot on the blade to change the pitch angle with respect to the outer rim. Hofreither emphasises that the cyclogyro’s combination of 360° thrust vectoring with very quick control responses results from running the rotor at a constant speed and changing only the pitch of the blades. It also makes for a much more compact vehicle design, enabling missions for which comparable air vehicles are too large. With a pair of rotors at each end, spinning on axes parallel to the aircraft’s pitch axis, Bumblebee 2.0 can be manoeuvred around and along its pitch, roll and yaw axes, giving it six degrees of freedom. The aircraft can also turn without rolling, keeping the fuselage horizontal through 360° turns in either direction if necessary. Asked about the technical challenges to be faced in the development of the technology, Hofreither says the toughest task is achieving the necessary thrustto-weight ratio, along with sufficient rotor efficiency to realise a battery powered eVTOL machine using mainly off-the-shelf drivetrain components, apart from the rotors. The system’s most highly loaded components are the rotor blades and the pitch-change mechanism, so the safety of the system overall had to be considered carefully. As with other eVTOL machines, however, redundancy is ensured though distributed electric propulsion. “With a sufficient number of CycloRotors on a vehicle, if one or two failures occur, the remaining rotors can safely control the vehicle,” Hofreither adds. Powertrain and flight control Each rotor is powered by an off-the-shelf electric motor from Lehner-MotorenTechnik combined with an in-house developed reduction gearbox and an electric speed controller from MGM COMPRO, which also supplies the battery management system (BMS) and bus contactors. In total, the propulsion system draws 40 kW peak power and 30 kW continuous power in the hover. The battery itself is an in-house design, based on pouch cells that combine high power density with a “reasonable” energy density. The complete battery system weighs 18.7 kg and there is no onboard charger. The pitch-change mechanism at the heart of the thrust vectoring capability consists of a set of rods connected to the blades and to a hub that surrounds the main axis of rotation but is free to move eccentrically in either direction around it. The company chose to develop its own flight control system because offthe-shelf options could not handle the CycloRotor’s unique capabilities. “Almost all flight control software currently on the market is made for either conventional aeroplanes or drones,” says Lukas Kinast, chief engineer, rotor. “None of them has a propulsion system capable of generating thrust through 360° and changing this thrust rapidly. In order to make the most of the cyclogyro rotor, the flight control system must be aware of that unique characteristic.” While the Bumblebee 2.0 does not have an autopilot and is flown remotely by a human pilot, it does have an onboard controller that translates the pilot’s commands into movements of the pitch-change mechanism. This controller runs CycloTech’s own flight control algorithm, which is based on the well-proven Inner Loop Nonlinear Dynamic Inversion (INDI) method. In flight control systems, the inner loop refers to the low-level control loop used to stabilise the aircraft and track commanded flight trajectories. It typically operates at high frequency to deal with the fast dynamics of the aircraft. The outer loop operates at a lower frequency and focuses on higher-level flight control tasks, such as trajectory planning, altitude control, navigation and mission management. The dynamics of aircraft, particularly of unconventional VTOL machines, tend to be inherently nonlinear due, for example, to complex interactions between aerodynamic phenomena, April/May 2024 | Uncrewed Systems Technology A rotating set of blades with a pitch-change mechanism, based on rods connected to an eccentric hub rotating independently of the blade set and collected to the blades via rods, is the heart of the thrust vectoring capability (Image courtesy of CycloTech)
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