Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

20 In conversation | Janek Biskupski Fluid dynamics The Catalyst is designed as a reverse- flow turboprop, meaning that air enters through a front-facing manifold but is then channelled directly to an intake port at the rear of the engine. The compression, combustion and exhaust stages then occur from back to front, with exhaust gas expelled from outlets at the front end. “It’s a single air inlet, although we could use any customer’s preferred inlet packaging, even two manifolds if needed, since that part will correspond closely with the shape of the UAV,” Biskupski explains. “Either way, we’ll use our latest 3D CAE aerodynamic simulation tools to ensure that the outer inlet shape streamlines into the circular engine inlet, to minimise distortions in air pressure and temperature. “That’s very important for sustaining our 16:1 pressure ratio – which as far as I know is 50% better than that of our nearest competitor.” A metal net is also installed in the port to stop debris or particles entering the initial four-stage axial compressor. This section, Biskupski notes, constitutes the critical emissions reduction quality of the engine, as it is through intense compression that the Catalyst achieves far higher pressure and hence thermodynamic efficiency than turboprop engines of the past. To prevent stalling, some of the compressor’s stator vanes can change their angle hydraulically (using the fuel as an actuation medium, to avoid carrying any other liquids), adjusting the airflow to prevent it dropping too much. From the compressor, air enters the combustor, which is an S-shaped chamber. An axial dual-reversing design enables packing of sufficient ‘length’ for thorough burning of fuel and air within a compact overall space, a key requirement for GE Aviation in order to maximise fuel efficiency and power output within the size and weight parameters it wanted for the engine. Advanced thermal management “We’ve also worked hard on the cooling inherent in the design,” Biskupski says. “For instance, both the combustor and subsequent turbine section have to operate in temperatures very close to the melting points of their metals, so without proper cooling this engine would melt within minutes. “While the exact choices of metals are sensitive information, I can tell you our stages use a great deal of titanium, as well as ‘blisks’ – that is, combined rotor blade-disks – to achieve better lightness and aerodynamics than conventional designs. They’ve become more common in modern, smaller turbomachines but are very rare in this class.” The compressor features inlet guide vanes, which are designed to direct the warmest air to points of very low temperature, to prevent icing that could block the inlet. Thermal barrier coatings are used sparingly in the subsequent sections. Rather than depending entirely on coatings, the CFD and CAD design for the Catalyst has focused on forming a ‘film’ of cold air along the walls of the combustor, to separate flame from metal. “We’ve therefore designed thousands of tiny holes in the sides of the chamber to draw a flow of cold air that directs the combustion away from the walls,” Biskupski explains. “This system was also devised from a blank sheet, but tests have proven our analytical predictions to be in line with what happens in the combustor, so we’re really very proud of this aspect.” He adds that the fuel injectors use ‘swirler’ devices to create a stable flame – one that does not oscillate and spiral outwards towards the walls in the first place – to further ensure thermal overload is avoided. Multiple heat exchangers have also been integrated throughout the engine between various media – oil, fuel and air – to maximise thermal efficiency throughout. Turbine design Regarding the design of the turbine, Biskupski says, “Clean Sky 2 helped greatly with sponsoring our experiments on the combustor and turbine to make sure they had the desired profile and cooling operations.” As well as the aforementioned cooling passages, the two-stage high-pressure turbine also uses single-crystal blades. This is a high-strength material type December/January 2022 | Unmanned Systems Technology Additive manufacturing has been used to a large extent to optimise parts such as the propeller gearbox for weight and geometry