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

61 the ECU on our two-strokes and our rotary but it’s largely the same platform, with the same CAN communication.” The company intends to continue using CAN comms for the foreseeable future. “If some customer-requested powertrains require additional components to be added to the engine, or to the [hybrid system] e-drive, then we’ll configure those parts to communicate by CAN, to ensure smooth functionality throughout the whole system,” Seidl explains. Direct F-E drive A key part of the hybrid concept is the company’s ‘direct F-E drive’, whereby the crankshaft directly drives the MGU. The inverter controlling the MGU is typically limited to a 48 V, 350 A system, and is air-cooled, although this depends on the unit’s position in the aircraft. Likewise, a battery and a supercapacitor are integrated into the hybrid system. Rather than being limited to the functionality of a starter-generator, Sky Power has sought to integrate larger and more powerful MGUs, with the end-goal being the enabling of five key operational modes for the engines incorporating this hybrid set-up. The first mode involves simply starting the engine with electricity from the generator. In the second mode, the generator provides a charging mechanism for the battery. “In the third mode, we can combine engine power – 20 kW from our rotary with 15 kW of electrical power from the generator – use that as a motor and apply them both to propulsion for a short time,” Seidl reports. The duration of this ‘boost mode’ depends on the capacity of the battery or the supercapacitor, which is also integrated. “This mode is especially important for take-off. In flight mode the maximum power is usually no more than 70% of the engine’s maximum power output.” In the fourth mode, in the event of an engine failure the generator can again be used as a motor, to drive the propeller by itself with the battery providing a short- term power source. This would constitute an ‘emergency mode’. In the fifth mode, if a certain measure of the engine’s power is not needed for propulsion then the battery can be charged using the excess, while the supercapacitor can be used to drive the MGU as a range extender. As Seidl explains, “That means you can have 15 kW of electrical power from the supercapacitor, delivered via our battery management board to the motor- generator, and the battery just serves as a back-up power source in this mode.” Battery and supercapacitor Since 200-300 A could potentially be necessary for a range-extender mode, Sky Power’s hybrid model combines a battery and a supercapacitor, the latter of which can drive very high currents for short amounts of time, making it ideal for range extension. The ability of this hybrid set-up to combine engine power and MGU power means weight is used more effectively. The power management board is separate from the ECU, and is connected to the battery and the supercapacitor. “The board for battery management is tuned for each customer; it is not productionised in the same way that our ECU and ignition systems are,” Seidl points out. “Instead, it is a complex system geared towards particular parameters for software functionality, for both the battery management system and the comms for power management.” Sky Power’s SP-180 SRE rotary was the first product from the company to be launched with this hybrid functionality, which is now also being applied to its existing portfolio of two-stroke engines. “In a two-stroke hybrid system, you Sky Power hybrid system | Dossier We can combine engine power – 20 kW from our rotary with 15 kW from the generator – and apply them both to propulsion Unmanned Systems Technology | April/May 2019 Block diagram of Sky Power’s hybrid propulsion solution