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67 through Turbotech’s Electronic Engine Control (EEC), which has a similar design to the full-authority digital engine control systems seen on larger turbine engines. It can also be used to control a variable- pitch propeller. A number of auxiliary components are also under development in order to cater for the engine’s ancillary needs, including oil pumps and filters to be integrated into the gearbox, where two of the TP-R90’s three bearings are located (the other is at the bulkhead separating the turbine chamber and compressor chamber). Also, a compact fuel pump will be integrated under the engine, and stainless steel tubing will be used for the fuel lines. Turbotech’s CTO of turbomachinery, Jean-Michel Guimbard, says, “Sealing the oil was a bigger issue for us than delivering it. We opted for labyrinth seals owing to their low friction and long life, so we’ve calculated and modelled the airflow around those seals to design them in such a way that no oil will leak out of the engine.   “The bearings we use are actually ceramic roller bearings, so oil isn’t really needed to help reduce friction at all; 90% of its purpose is to cool the bearings, with the other 10% being lubrication. The oil tank will also be integral to the gearbox for the TP-R90.”  Air delivery Initial rotary motion of the TP-R90’s turbine shaft – and thus its compressor and turbine wheels – comes from a starter/alternator, while the TG-R55 is started using battery power. As the compressor wheel spins, it sucks in air through an annular inlet vent in front of its chamber. The blades are designed to funnel this air into a ring- shaped air chamber, which encircles the combustion chamber and has several thousand openings in a connecting plate at the back. The openings connect to a series of Inconel microtubes, each about 300 mm long. They serve as the medium of heat exchange, with the air being channelled back and forth through them in the rear section of either system. Fauvet notes, “The TP-R90 creates less aircraft drag, as the air is ejected rearwards after being taken in by the engine. That compares with naturally aspirated reciprocating engines, where air intake can increase overall drag by up to 20%, mainly owing to the cooling airflow requirements of the engine.” The microtubes are divided into two series. The first runs backwards in a straight line from the air chest along the inner walls of the heat exchanger’s outer hull. They then exit into a connecting plate that forms the front bulkhead of a secondary air chest at the back of the engine, which can be accessed for maintenance by removing an end-plate that acts as the chest’s rear enclosure.  This secondary air chamber reverses the direction of the airflow, with the first set of outer microtubes breathing air into it and the second, inner ones, drawing air into them. Accordingly, the second chest’s connecting plate has about as many perforations as the first. The second series of tubes then runs forwards, closer to the central axis of the heat exchanger (and to the flow of exhaust gas), connecting the rear air Unmanned Systems Technology | April/May 2020 Turbotech is based in Toussus-Le-Noble in the outskirts of Paris. It was founded in 2017 (and has been headed since) by a team of four engineers who had all formerly worked for the French aerospace and defence giant Safran Group. “We had all worked on large aerospace engines in various capacities, departments and contracts,” the company’s CEO Damien Fauvet recalls. “Before founding Turbotech, I had begun working on a personal project aimed at developing a small turbine engine with low fuel consumption. This was a side project, separate from my job at Safran, so to start with I just worked on it at home. Within his limited free time, Fauvet developed a rough early proof-of-concept engine from COTS parts, which combined a turbine with a heat exchanger that would serve to lower the former’s fuel consumption. When the results of testing his creation were indeed found to validate his predictions of improved fuel efficiency for a given power output, he decided it was time to create a company for managing and developing the engine for commercial and defence applications. He initially sought potential partners among his colleagues at Safran who had expressed an interest in helping to make his project a reality. Upon leaving Safran to form Turbotech, Fauvet along with his new CTO of turbomachinery, Jean-Michel Guimbard, his COO Baptiste Guerin, and CTO of mechatronics Marc Nguyen began a round of fundraising.  This saw Safran invest a significant minority stake in Turbotech, with French private equity firm GoCapital giving similar funding. Additional development grants came from the Ile-de-France Regional Council as well as France’s civil aviation authority, the Direction Generale de l’Aviation Civile. At the time of writing, the first working prototypes of the TP-R90 and TG-R55 examined in these pages have been run for more than 100 hours and over 400 cycles. Their second prototypes are now complete, with the company planning to continue ground cycles before starting flight tests this summer.  The company plans to make both systems commercially available in summer 2021, and have them fully certified within the next two or three years. Company and project origins