Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

26 Dossier | Hummingbird XRP produced,” he says. “So while the world was working on a better compact hybrid power source, I ventured into designing the vehicle.” The initial concept coalesced in his mind at the start of a meal in a restaurant: the trigger was the cylindrical form of a water tumbler placed before him on the table. “On a napkin I drew a cylinder and put out six arms and then put propellers – little Xs– on the end of each arm, drew some landing gear with the curvature you see on the Hummingbird, then drew two propellers inside the main duct.” The first series hybrid proof-of-concept vehicle was just called the Hummingbird, and took the form of a hexacopter with ducted 17 in outboard rotors and a 24 in diameter central duct, which housed a two-cylinder two-stroke engine. That was coupled to a Sullivan starter/alternator and power system running KDE Direct electronic speed controllers (ESCs) and brushless DC motors for the outboard rotors as well as the counter-rotating rotors in the duct. “That allowed us to do a number of experiments and show that a compact power system was capable of producing the thrust we needed,” Bishop says. In this form, the Hummingbird did the rounds of the trade shows. Within a year, potential customers were asking for more range, endurance and payload, he says. That drove the development of the current, larger vehicle with the eXtended Range and Payload that gives it its XRP moniker. This has a central duct 30 in in diameter with twin, counter-rotating 30 in rotors and eight 27 in diameter outboard rotors that are no longer in ducts. Power is provided by a Sky Power (formerly 3W International) 180 cc, 20 kW SP-180 rotary engine with an integral starter- generator, supplemented by a lithium- polymer battery pack. Achieving 400 lb of thrust from this power plant required aerodynamic optimisation work beyond Bishop’s abilities as a self-taught aerospace engineer, so he turned to the University of Colorado, hiring interns to do the calculations that would allow him to minimise the power required to generate each pound of thrust. Getting the rotor design right was crucial. “Initially we tried our own design,” he recalls. “We did a lot of 3D printing using carbon fibre-infused plastics, and we tried about 200 different designs of rotors with dual, triple, quadruple and octuple blades. In the end, we came back to two- blade rotors inboard and outboard.” “Efficiency was our real Achilles heel at the beginning: we were looking at 100-130 W/lb of thrust,” he says. “We just couldn’t put enough power in the system – even with batteries added – to achieve a reasonable payload and endurance.” After asking several aerospace engineering firms to design the rotors, without success, the company went back to KDE Direct and asked it to design a propeller to match its motors and ESCs, which it achieved within six months. “We found their efficiency was roughly 30-40 W/lb of thrust – KDE almost tripled the efficiency.” The Hummingbird XRP now uses KDE’s 8218xf-120 motors and UAS125UVC ESCs. The outboard rotors use the same company’s CF275-DP blades, which are 27.5 in in diameter and have an 8.9 pitch. Their twist ensures a consistent pressure along the length of each blade, says Bishop, while August/September 2019 | Unmanned Systems Technology Like the external rotors, the one in the central duct is electrically driven, but it is controlled by the power management system rather than the flight control system (Courtesy of Reference Technologies) We tried about 200 different rotor designs but in the end we came back to two-blade rotors on the inboard and outboard