Uncrewed Systems Technology 049 - April/May 2023

83 aircraft, although it has a lot in common architecturally with the HC. Both are twin-rotor gasoline and heavy fuel Wankel engines with a 600 cc displacement, and with geometric similarities to the famed rotary design that was based by motorcycle manufacturer Norton on the Fichtel & Sachs KM914. Key differences between the HC and XE are power, weight and general performance. The XE is a 44.5 kg engine (including all ancillaries) with a 100 bhp peak output and 70 continuous bhp, while the HC weighs 39 kg (again including all ancillaries), with a peak power of 64 bhp and maximum continuous output of 46.5 bhp. These figures are more suitable for the power requirements of helicopters during their various flight modes, as well as the stricter weight limits imposed by the absence of fixed-wing lift. Looking at the power and torque curves of both engines as mapped across their speeds shows that when the HC is delivering its maximum continuous power, at about 4000 rpm, it generates 58 lb-ft (79 Nm) of torque, only slightly less than its peak of 59 lb-ft. This is a desirable property in starting and driving a helicopter’s top rotor, and the HC has a broadly higher torque-to-power ratio than the XE. The HC is Rotron’s leading commercial success story at the moment, hence our focus on it here. However, the company has become agnostic regarding the type of engine it is asked to supply, and now often operates as something of a Tier 2 provider rather than aiming to work as a bulk manufacturer of COTS engines. “We’ve found that theUAVmarket is either tiny start-upswhowant just one of our engines, or aerospace primes like Leonardowho have a detailed set of requirements and need continual support for the engine technology during integration and use,” says Alex Head, managing director at Rotron Power. “For the latter, the core rotary engine technology is really only as good as the ancillary systems and the ongoing support.” As a result, Head notes, while Rotron does supply COTS engines, the reality is that every unit needs to be customised, as dictated by their missions and installations. “Helicopter integrations in particular are a really difficult use case for an engine, because it’s high power and low airspeed, so there isn’t much cooling,” he says. “However, sincewe last spokewith you, we’ve gone on to integrate our engines into many such platforms. Leonardo’s AWHERO for instance is theworld’s firstmilitary- certified rotary UAS in the 200 kg class, and it incorporates their version of the RT600- HC. There are other certifiedWankels out there, but only for motorised gliders.” The certification has provided Rotron with new levels of insight into fault hazard assessment, reliability, safety and other qualities that are key to maturing engine systems, as well as a range of improvements across its engine subsystems. At the same time, the company has invested a lot of r&d in achieving a ‘design for manufacture’ approach for its engines. Manufacturing changes Many small changes have been made across the RT600’s core components (along with a few larger ones to essential ancillaries). The improvements have been driven by the aim of providing the longer TBOs and lower maintenance costs often demanded by modern UAV integrators. As Head explains, “I look at engines in terms of brake mean effective pressure – that is, cylinder pressure, and howmuch work the engine’s doing. The RT600 has a large cubic capacity, but when we use it in rotorcraft it’s rated at a lower duty cycle for maximum continuous power, so what we’ve sought to bridge with the RT600-HC was a gap in durability for rotary engines. “Some might compare our engine with others and find there have been 300 cc Wankels that output 55 bhp on their data sheets, but in reality they can’t do that for thousands of hours. That makes them more expensive to run, and complicates their supply chain logistics. “Instead, we’ve opted for a 64 bhp peak distributed across two rotors that operate within their thermal capacity. Even at that peak, there’s no flexing of the apex seals, no wearing of springs, none of those issues that can plague rotary designs that sacrifice durability for power.” That has enabled Rotron to Rotron Power RT600-HC | Dossier Uncrewed Systems Technology | April/May 2023 One of Rotron’s highest-profile customers is Leonardo and its AWHERO uncrewed helicopter. Its RT600-HC can be seen here in the engine bay behind the nose