Uncrewed Systems Technology 049 - April/May 2023

90 The ejector is typically fabricated and pressed in-house, using a combination of Inconel and titanium to endure the high exhaust gas temperatures. The exact material choices are determined by the specifics of the installation, particularly regarding weight and noise requirements. Titanium is more appropriate for ‘cooler’ parts of the ejector, while Inconel goes into the ‘hotter’ sections. Liquid-cooled housing and shaft The housing and eccentric shaft are still cooled by a water-glycol mix, as per the LCR and XE, but rather than still using an impeller to draw water through the shaft and then send it into the housing, the HC uses a pump with a bypass valve installed to apportion the water flow between the shaft and the housing. “The main failure mode of rotary engines is that, as they run at higher power, they heat up and near the tempering points of their metals, reducing their hardnesses, weakening bearings and so on. So getting fine control over the coolant flow, through repeated CFD analysis, has been a big focus of our activities in recent years,” Head says. “CFD showed us that our old approach of drawing coolant in from the back and then out at the back again caused some thermal issues, so now it flows diagonally across the engine. It goes in one side of the housing, around that housing, and through the centre plate before mirroring that journey through the other rotor housing and then out of the engine. “We’ve also designed a number of bleed apertures across the coolant flow of the engine, as well as larger waterway unions. Those, along with the bypass valve, help us control the thermal management via the ECU while generally increasing our cooling capacity.” Aluminium piping connects via rubber ends between the engine’s coolant ports and the end-user’s arrangements of tanks, pumps and radiators, to optimise reliability for installed mass. Ancillaries for the liquid cooling are a major focus of the customisation for each end-user, as parts such as radiators must be shaped differently to fit the fuselage of each aircraft, along with tailored parameters such as thinner cores over a larger area for greater heat rejection. “Fans for cooling the radiator also need to be positioned and sized appropriately on a case-by-case basis, as helicopters spend a lot of time in stationary air, and you can’t hang a radiator below their propeller downwash,” Head adds. “Fortunately though, we can now control the fans via the ECU as an automated function of engine bay temperature readings. That’s critical, because in some use cases upwards of 600 W is needed for sufficient heat dissipation, and is part of the reason more and more powerful generators are needed in helicopter UAVs.” Hybridisation As with the airbox, different alternators and electric powertrain configurations have been developed and supplied for different customers. “As standard, the HC comes with a 28 V AC, 1.4 kW alternator, but we’ve April/May 2023 | Uncrewed Systems Technology RT600-HC Wankel type Twin-rotor Unleaded 95 octane gasoline or Avgas 100LL Naturally aspirated Effective displacement: 600 cc Maximumpower output: 64 bhp Maximum continuous power: 45 bhp Maximum speed: 6800 rpm Specific fuel consumption: 0.26 kg/bhp per hour TBO: 250-1000 hours (depending on flight profile) Weight: 38.5 kg Some key suppliers Servos: Volz ECU: General Engine Management Systems Sensors: Bosch Electroplating: Poeton Precision grinding/EDM: A&M EDM Precision sand casting: Hadleigh Precision sand casting: Hayworth Bearings: Eriks Seals: Eriks Generators: Sullivan-Acutronic Radiators: Docking Engineering Radiator fans: Spal Coolant lines: SFS Performance Specifications A diagram depicting how Rotron’s exhaust ejector works to air-cool the RT600-HC’s rotor