Unmanned Systems Technology 025 | iXblue DriX I Maintenance I UGVs I IDEX 2019 I Planck Aero Shearwater I Sky Power hybrid system I Delph Dynamics RH4 I GCSs I StreetDrone Twizy I Oceanology Americas 2019

68 Dossier | Sky Power hybrid system undisclosed alloy has an inherently low coefficient of friction. Seidl says, “This approach was critical for ensuring the mechanical interactions between the rotor and the trochoid do not change, even within the controlled environment temperatures the SP-180 SRE is designed to sustain. Removing a coating means you no longer have to worry about it dissolving, owing to wide temperature changes, or the risk of processing errors during production.” As well as exploiting innovative materials, Sky Power is focused on CNC machining the SP-180’s components, rather than casting them. Seidl points out that CNC machines can easily be programmed to accommodate changes, providing the flexibility to accommodate different customer requests and special projects. The Sky Power team also determined that Wankel rotor/eccentric shaft mechanisms need cooling, particularly towards the three apexes of the rotor. “While thermal management of combustion chambers is easy enough with a water cooling system, inside we would need a rotor cooling system probably using oil,” Seidl notes. Typically, Wankel rotors do not incorporate any form of liquid cooling. Seidl notes, “For an aircraft that requires several high-power phases throughout flight, without some form of cooling, the rotor material could become excessively hot during sustained high-power operations, which can cause the main rotor bearing to overheat and fail.” Cooling the rotor To counteract that, the SP-180 SRE incorporates an oil-based internal rotor cooling system. “We pump oil through the rotor, which conducts heat away from its tips, so we maintain the rotor surfaces at a temperature similar to the trochoid inner surfaces.” Much of the design work for the rotor cooling placed emphasis on minimising the volume of oil needed to sufficiently reduce the temperature at the apex grooves. “We’ve constructed a specialised system that uses no pump; it just has internal oil channels that exploit the movement of the rotor,” says Schudt. Seidl adds, “The special design within the rotor controls the flow of oil to maximise cooling with minimum oil flow. Keeping the volume of oil low was important, as this is still a UAV engine design: every millilitre of added oil means more weight for the aircraft to carry.” Sky Power has developed a specialised portable test rig for cycling the SP-180 SRE, which demonstrates its thermal management system. As Seidl explains, “When running the engine in this test rig, there is only a 10-20 C gap in temperatures between the oil and the water running through our thermal management system, to reduce temperature disparity and thermal expansion stresses on the tolerances of our materials.” In January 2019 the SP-180 SRE was rated to consume 340 g/kW at maximum load at 6000 rpm. At the time of writing, however, with the integration and testing of a new ECU and ignition system (see sidebar), Sky Power has been able to reduce this, with 280 g/kW now consumed at 6000 rpm. As Seidl notes, a hallmark of the Wankel is the long distance that fuel must travel around the circumference of the trochoid chamber before combustion, which can contribute to fuel losses, even with an effective sealing system. Therefore, near the outset of the design process, the team determined that this could be circumvented by integrating a high-pressure fuel injector almost adjacent to the spark plugs (although manifold injection is the standard option). Intended for special applications where customers have a particularly strong emphasis on maximising fuel efficiency, this aspect of the engine architecture keeps the fuel distribution close within the compression ‘chamber’, and halves the distance that fuel travels around the trochoid, leaving less opportunity for losses. It is characteristic of the benefits that can be gained from Sky Power’s approach to hybridisation. April/May 2019 | Unmanned Systems Technology Map of the SP-180 SRE’s specific fuel consumption in g/kW, according to engine load and speed