34 Dossier | ACC Thunder Wasp UAV gearboxes are the most sensitive part in helicopters, really prone to overheating and breaking down, but for anyone in disbelief at how our driveline can function without gearboxes, we’re happy to have them over for a visit, so they can see it for themselves. We just don’t release anything about it publicly for now.” Constant speed rotors ACC did not want four electric rotors, as per conventional quadrotor designs, due to physics-related limitations on scaling up to the Thunder Wasp GT’s lifting capacity. To change thrust, the propeller on an electric motor must accelerate or decelerate, but in a heavy-lifting UAV, there is significant propeller and motor mass that needs to be accelerated or decelerated, which translates into slower control response times and inefficiencies. Instead, the Thunder Wasp runs on variable-pitch props at a constant rpm. The blades’ pitches respond with near-instantaneous timing to autopilot commands, able to agilely increase their angle of attack for lifting, flattening it for hover and decreasing it for descent. The pitch of each carbon-composite blade pair is collectively controlled via a dedicated, electromechanical servo, ACC forgoing swashplates and similar components that can be troublesome in helicopter drivetrains, and would have interfered with the goal of mechanical simplicity. A single pitch servo is bolted atop each of the four pylons’ outer edges, with stainless-steel gear servos from Hitec used in this UAV (such gear types being critical for servo longevity and consistency), with four of the company’s SG33BLTs providing pitch control via their 1441.2 N/cm of max torque output. “Each pitch servo acts on a rod end bearing, which runs up to a plate sleeve, which mechanically links to and drives two further rods extending upwards. Each of those connects at their tops to a respective link for one of the rotor blades,” Max says. “When you push or pull the rod end bearing, the transmission pushes or pulls the two upper rods to crank and hence rotate the blade links in opposing directions, thereby increasing or decreasing both blades’ pitches in equal measure. If we’d gone with a swashplate, we could adjust prop pitch in any direction freely, but swashplates have to be constantly checked, adjusted and overhauled frequently.” Rudders in the air Each propeller has its own rudder, which sits below and downstream of the rotor blades, to catch on their airflow and adjust the attitude of the whole vehicle. “After conceiving them, we designed them, built them and integrated control subroutines for them into the software, all in less than three weeks,” Claes says. “They allow the UAV to pitch, roll and yaw as gently or sharply as we might need.” The team discovered afterwards through testing that, given how the rudders adjust the UAV’s attitude with ‘used air’ from the propellers, rather than needing to drive some rotors harder (requiring a thrust headroom of around 20% on each rotor) for attitude changes, as in conventional multirotors, the power efficiency and lifting capacity of the Thunder Wasp remains comparatively unchanged during operation. “When you combine the fast bladepitch response with the rudders, you can see from the videos on our website that December/January 2025 | Uncrewed Systems Technology Each rotor’s blades are actuated for pitch angle by a SG33BLT servo from Hitec via mechanical linkages, as visible here Each rudder is actuated by a Hitec SG50BL, enabling nimble roll, pitch and yaw, even amid strong winds and gusts
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