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76 since the exit is linked to the exhaust, the energy of the exhaust flow is cleverly exploited to create suction to draw cooling air through the rotor. On top of that, Rotron devised water cooling for the eccentric shaft, which is hollowed out to accommodate it. An impeller fitted to the non-drive end sends water into and out of the shaft, flowing close to the rotor bearing. That approach of water cooling the heart of the engine is unique among these UST -examined rotaries. We saw that AIE uses an innovative Self-Pressurising Air Rotor Cooling System (SPARCS). The concept of SPARCS is to circulate a mixture of blow- by gas and oil mist on a loop that takes it through the centre of the rotor and through a gas-to-liquid heat exchanger that rejects its heat to the engine’s external water-cooling system. The Aixro UAV engine eschews the complication of fan delivery, of water- cooling the eccentric shaft and of a SPARCS-type system. It has its own approach to the concept of using the charge to cool the rotor, which is based on the approach of Fichtel & Sachs. The other three engines all have peripheral intake ports, whereas the Aixro’s unique combination of peripheral and side intake ports is the key to this approach. The Aixro’s charge air exits the carburettor into a manifold that bifurcates. One branch links to the rotor housing, where it feeds a peripheral port. The other links to one of the hollow end plates. From that end plate the charge travels across to the other, in the process passing through the rotor. The ‘exit’ plate sends the charge it receives to the engine’s side intake port. The charge always feeds to the side port, with the flow through the more direct route to the peripheral port controlled by a butterfly valve. Clearly, the flow to the side port is heated by its passage through the rotor, reducing its density and thus its potential for power. However, at 70% throttle and above, a lever opens the control valve to let unheated charge take a short cut directly into the combustion chamber through the peripheral port. “There have been previous Wankel designs that tried split ports, but all the charge went through the rotor first, so you’d always get that power loss,” explains Woelfle Jnr. “In our system, the [peripheral port] ‘bypass’ means some of the air doesn’t need to go through the rotor and get heated.” Opening the peripheral port gives the engine peak power of 26 kW (35 bhp) at approximately 6500 rpm. Using only the side port, the output at 70% throttle is around 20 kW. Respective torque figures are 35 Nm peak and 25-30 Nm. We are told that helicopter customers typically run their XH40s at a constant speed of about 6000 rpm, while fixed-wing customers usually work their XP40s and XF40s from 5000 to 6000 rpm in cruise. “You can’t use the peripheral port bypass continuously,” explains Woelfle Jnr, “because in the medium term the engine will start to get warm internally, as some of the charge is taking a short cut and the engine is producing more power and hence heat. That’s also why we run the Aixro UAV engines at 6000-6500 rpm, and not up to 10,500 rpm as on a kart.” So it is that the Aixro UAV’s internal Wankel cooling solution is less complex than others UST has investigated. One can argue that simpler inherently implies less expensive, logistically less of a burden and, with fewer elements to fail, potentially greater reliability. However, that third consideration is countered by the fact that the more complex solutions are specifically designed to enhance cooling. While the Aixro solution is not greatly less complex, Woelfle Jnr highlights October/November 2020 | Unmanned Systems Technology Power and torque curves for the Aixro UAV engine Previous Wankel designs tried split ports but all the charge went through the rotor first, so you’d always get a loss of power