34 Dossier | Schiebel Camcopter a measure of similarity with that of Rotron’s RT600-HC). “But also, after trying a couple of COTS rotor bearings, we ended up designing a bearing that was perfect for the specific forces, loads and thermal characteristics of our application,” Hecht adds. “It is manufactured to our spec by a specialist Austrian company, much as several other parts in the engine have been made or consulted on by companies and university departments with great knowledge of the problems we’ve had to solve, especially of materials and coatings.” He adds that the rotor’s cooling fins have been extensively designed to maximise thermal transfer from the internal air-cooling system. Aspiration is performed naturally to prioritise simplicity and weight over the addition of turbo- or superchargers (although Hecht and his team have experimented at length with these, as with various approaches to direct injection and other potentially useful modifications). The housing, meanwhile, is liquid-cooled using a water-glycol jacket approach typical of 50-60 hp engines. “Though we still sell some S1 AvGas engines, our long-term engineering focus is on the S2. Heavy fuel is really where the future of the S-100’s powertrain lies. Given the money and time to allocate, we may optimise it as a multi-fuel engine too,” Hunter notes. From shaft to blade tip The engine and rotors run at fixed rpms in operation, with the blade pitches adjusted by the flight controller to govern between ascent, descent, hover and forward acceleration or deceleration, with the adjustment optimised over the years for minimal power consumption. “We’re a 200 kg platform, so in coarse wind conditions we can provide a lot of tail rotor thrust. Conversely, in forward flight, we barely need any,” Hecht notes. “The engine’s output shaft connects to a freewheel assembly, which, as mentioned, is how we can autorotate, and the upper side of that assembly goes directly to the input shaft of the main gearbox, which functions as the central distribution system for mechanical power across the aircraft.” One output shaft atop the main gearbox runs into the main rotor head. Schiebel has opted for a rigid rotor in its main rotor configuration, being mechanically simpler than most, and giving faster control response due to its light weight and inertia. “A part can’t fail if you don’t use it, so from early on we tried to strip out everything that wasn’t absolutely necessary with the rotor head, meaning all the stabilisers, hinges and so on that historical doctrine insists are necessary in rotor hubs,” Hecht says. “Rigid rotor designs are aggressive and difficult to control from an engineering point of view, and you need a lot of software to stabilise them, but once you have them under control, they’re very stable and really robust against CoG movements. We have asymmetrically mounted 25 kg payloads on the side, and it’s been no issue for the flight control – only once have we ever needed to add a kilo or two of ballast.” With the main rotor head being hingeless, each of the two blades is affixed to the hub at two points, with the blade material and variable pitch of the rotors taking care of aspects of flight dynamics that are typically trusted to hinges, stabilisers and a human pilot. Much of those parts are typically needed for a human to feel the helicopter’s behaviour, but the flight software senses changes in movement accurately enough through the IMU to react in milliseconds by altering acceleration, pitch, roll or yaw. “The head is made from titanium, so it is extremely strong and it’s hollow, so we could put a sensor in the rotor head to feed data back to the autopilot. We’ve even put a SATCOM in there before with good results,” Hunter notes. The proprietary blade design targets a medium between torsional stiffness and flexibility, partially to compensate for the lack of hinges, and the blades are mostly manufactured in-house, with some thirdparty facilities being leveraged when orders exceed capacity. A standard, prepreg, carbon/fibreglass material August/September 2024 | Uncrewed Systems Technology Metal additive printers such as this one are used to manufacture the S-100’s titanium rotor head
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