Unmanned Systems Technology 020 | Alpha 800 I Additive Manufacturing focus I USVs insight I Pegasus GE70 I GuardBot I AUVSI Xponential 2018 show report I Solar Power focus I CUAV Expo Europe 2018 show report

56 proven Desert Aircraft DA70 two-stroke married to a starter/generator rather than directly driving a propeller. The DA70 is a smaller version of the DA150, which was fully described in UST 15 (August/ September 2017). The stock engine has an extended tapering nose to its crankcase, at the end of which is an additional – third – main bearing, outrigged thus close to the propeller drive hub. Pegasus replaces that taper with a tubular extension around which runs its generator, retaining the concept of an outrigged third bearing. In this case the third bearing is close to the drive for the generator’s rotor. The rotor is external to the generator unit; internally the stator is mounted around the tubular extension. For the crankshaft, which has thereby passed through the stator to provide the drive to the rotor, the latter item forms a rotating bell which is attached to the crankshaft nose, while the stator is mechanically attached to the crankcase. The centre of the bell is attached to the crankshaft nose via a tapered connection that keeps it true, in the manner of the fixing of a propeller. The face of the bell incorporates a fan to draw in air, to help cool the stator within. “Keeping electronic components cool makes them more efficient,” notes McRoberts. “We are using a permanent magnet radial flux generator, which outputs three-phase AC. “That power is taken via a three-phase power connector that is attached to a clip mounted on the crankcase. The UAV requires DC power, so the GCU takes that AC and rectifies it into nice clean DC power.” While based on the DA70, both the ICE’s crankcase and its crankshaft are Pegasus productions, to suit this marriage with the generator. The cylinders are modified in a switch from air to water cooling. With Pegasus’ own ignition and fuelling systems, little other than the piston/con rod assembly remains stock. The all-aluminium engine is a true boxer, each con rod having its own crankpin arranged at 180 º such that both cylinders are at TDC at the same time and fire simultaneously. The two cylinders share a common crankcase, which is fed by a single central throttle body/reed valve assembly. “We didn’t want to change the porting geometry or combustion chamber,” explains McRoberts. “We did want to re- engineer the bottom end to integrate with a generator though. “The DA70 is designed to swing a large, heavy propeller, and mechanically has to be sound enough to fulfil that role, whereas we just spin a generator. So there is a lot of room for re-engineering. “You can buy very well balanced propellers but as a propeller wears over time it can become unbalanced and cause issues. I believe that is why they have a much more heavily reinforced crankcase – their tapered spigot is particularly good at counteracting those types of loads. “For us, we have a very balanced generator rotor that doesn’t wear, so that simplifies the balancing and vibration aspects. Our generator is very compact compared to a large propeller. That affects the inertia of the system, so that is a different requirement in terms of the way the engine is built. “With a propeller they have a thrust load. We don’t have any thrust load, which again allowed us to take a different direction in terms of the crankcase. “Our crankcase accepts a different output shaft [third] bearing to what would be used with a propeller – it is a sealed needle roller versus a deep-groove ball bearing. We also did our own crankshaft web counterbalancing but, having said that, it is pretty much the same as stock. “The [five-piece] crankshaft construction is the same, but even its length is different. Although the two main bearings sandwiching the webs June/July 2018 | Unmanned Systems Technology A comparison of the twin-cylinder based GE70 with its single-cylinder GE35 sister; the two share the same ancillary components