Uncrewed Systems Technology 049 - April/May 2023

86 assembly sub-components and eventually thewhole engine, all through processes we’ve developed ourselves.” Other aspects of the engine have been kept largely unchanged over the past 7 years. For instance, the eccentric shaft still runs in two bearings, with a tapered spherical roller bearing at the output end and a standard roller bearing at the other. Internal r&d has investigated alternative arrangements such as central bearings and split bearings, but ultimately the simplicity of a two-bearing design means this architecture has stood the test of time. The apex seals are also largely unchanged, except that they are now produced in ingots and then cut to specified tolerances, rather than being investment cast, as theywere in the beginning. “That ensures the processing, measurement and passing requirements of the apex seals can be matured, but more importantly they can be controlled to zero in on their working tolerances relative to the dimensions of the rotor, springs and housing,” Head says. Modular intake The throttle and airbox largely differ for each customer, and Rotron uses an interface control document with about 40 different potential interfaces between the engine and aircraft to define the air intake componentry and the key parts around it. The customer typically provides a shielding that should lead from a NACA duct on the outside of their fuselage, which interfaces with a rubber seal onto the airbox. The seal is designed by Rotron as needed for the end-user’s packaging and volume requirements, and if feasible it integrates an air filter and moisture separator to clean the air before it flows into a set of throttle bodies. “We’ll also change the aperture sizes of the throttles,” Head says. “High- altitude applications for instance need more air, but if say you’re not at high altitude and need only 58 bhp, then a smaller aperture could yield great benefits in terms of controllability. “Our ECU also acts as the governor, so we can tailor different percentages of throttle opening ranges to certain percentages of available shaft horsepower. “We use a Volz servo for throttle control, which gives a lot of feedback for fine control and real-time diagnostics, and it can be pre-programmed in a number of ways tomitigate failuremodes that different use cases might induce.” A twin-pump fuel system has been developed in-house for a redundant fuel supply to the injectors. Fuel is sprayed after the throttle discs to prevent icing problems from condensation on moving components, an increasingly pressing issue as organisations seek to fly their UAVs higher and in extreme climates below -20 oC (despite helicopters typically not being safe to operate at such temperatures). The charge is sent down a set of pipes – some titanium, some aluminium, depending on the installation – directly into the inlet port. Rotron says it has experimented with direct port injection, high-pressure direct injection and other approaches, but has found that port injection high in the intake stream, just below the throttle, provides very good atomisation and mixing thanks to the length and wave energy of the charge. Enginemanagement Rotron borders on calling its current ECU a FADEC (full authority digital engine control) but points out that fallback control by a remote operator April/May 2023 | Uncrewed Systems Technology The throttle and airbox are typically designed differently for each customer’s integration and use case