Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

28 the aerofoil design ensures a consistent stress factor from the root to the tip, maximising the overall thrust production along the entire length. Ducts ducked The most immediately noticeable thing about the Hummingbird XRP’s outboard rotors is that they are no longer in ducts. Despite being quieter and having an efficiency advantage of between 10 and 14% in vertical flight over open rotors – thanks to their suppression of vortex formation – aerodynamic problems in transition and forward flight meant the ducts had to go. The issue is the angle at which air enters the ducts when the vehicle pitches forward to accelerate and adopts a nose-down attitude in forward flight. (The autopilot limits pitch and roll angles to a maximum of 20° to avoid losing lift and, consequently, altitude.) The airflow delaminates on the lower edge of the duct, while air that impinges on the curve of the upper edge generates aerodynamic burble through the duct. Furthermore, some of the air is deflected up and out, never making it into the duct. All of that is bad for efficiency, Bishop explains. “The air that does go in becomes a burbling air mass that goes into the rotors,” he says. “Not only do we lose the calculated efficiency of the duct, we lose more efficiency overall because the rotors can’t maximise their thrust capability. So we dispensed with the external ducts.” The arms on which the outboard rotors are mounted emerge from the sides of the central duct at an upward angle, creating dihedral that adds stability because it tends to return the aircraft to level flight after a disturbance. “We wanted an extremely solid platform for imaging such as Lidar, which is very intolerant of shakes and bumps,” he says. “I wanted an aircraft that could still be a good imaging platform in relatively rough weather.” The rotor configuration in the central duct also contributes to stability, as counter-rotation cancels out the yaw moment that a single rotor would produce and would require a separate anti-torque system to eliminate. Bishop considered using a single-rotor plus a stator system to straighten out the airflow within the duct, as used in some other ducted fan vehicles, but rejected that owing to weight and reliability concerns. “Every pound of plastic or metal we put on the aircraft is a pound of fuel we don’t have. Second, it’s a mechanical device with pivots, bearing surfaces and servos, and they all become single points of failure,” he says. Each rotor in the duct is driven by its own electric motor; the generator is the only device that is mechanically driven by the engine. Less-than-ideal aerodynamics can be tolerated in the central duct because airflow through it is more important for cooling the engine, the generator and the power electronics than it is for thrust. However, the duct does contribute about 50 lb of thrust, and it still works reasonably efficiently in a hover. “If there were two rotors and nothing else inside the duct, we would be closer to 70 lb,” Bishop says. “We lose about August/September 2019 | Unmanned Systems Technology Lidar is intolerant of shakes and bumps; I wanted an aircraft that could still be a good imaging platform in fairly rough weather The Hummingbird XRP’s major components conform to the vehicle’s general annular theme, including the fuel cell, girdle, central duct and electronics bays and covers (Courtesy of Reference Technologies)