Unmanned Systems Technology 008 | Alti Transition UAS | Ground control systems | Xponential 2016 report | Insitu Orbital N20 | UAVs | Solar power | Oceanology International 2016 report

90 June/July 2016 | Unmanned Systems Technology PS | The Robotic Hummingbird G iven the current requirement for bird-sized unmanned air vehicles for surveillance and similar applications, coupled with our understanding of aerodynamics and the latest advancements in microelectronics and power sources, designers are now able to miniaturise flying craft down to a size that is feasible for flapping wing flight (writes Stewart Mitchell). David Colman, of the Department of Aerospace Engineering at Texas A&M University, has been researching this area, and he says “Our recent investigations have proved that biological flapping wing flight offers superior manoeuvrability with excellent gust tolerance and disturbance rejection capabilities compared with that of conventional craft. “Also, flapping wings use several unsteady aerodynamic phenomena, such as leading-edge vortices, at low Reynolds numbers [air boundary layer flow separation caused by turbulence], that significantly enhance lift production over what is seen in steady flow conditions such as those on conventional aircraft.” After research into the potential of a flapping wing micro air vehicle (MAV) in early 2015, Colman and his colleagues at the university designed, manufactured and tested a biologically inspired two- winged, hover-capable autonomous MAV which they call the Robotic Hummingbird. It has a wingspan of 12 in and flaps at 22 Hz to manoeuvre its 62 g weight. Wing control is taken care of by a unique modified five-bar crank-rocker system, which turns the rotary motion of the motor into a linear arc (flapping) motion. The system is similar in principle to a four-bar crank-rocker linkage but with an extra shaft that enhances the flapping amplitude and modulates the right- and left-wing flapping speed and amplitude independently. It also means the MAV can provide more lift than any other flapping wing aircraft of its size, and has much more intricate aerial control. Each wing is made from a lightweight plastic material, and weighs only 0.85 g. The wings also have flexible spars that aid lift by using aeroelasticity, flexing so as to increase their lift area. A modular mechanism controls each wing’s flapping frequency and amplitude independently to keep it true to its orientation and direction along a specified course or to hover. The MAV’s control system incorporates an autopilot, which auto-stabilises the craft from almost any orientation using the response from a series of high feedback- rate gyros and sensors on board. “The biggest challenge was to develop a design with the strength throughout its components to withstand the large forces imposed during flapping,” says Colman. To that end, the main body of the vehicle, which functions as an anchor point for all the mechanical subsystems – including a 29 KV brushless dc motor, flapping mechanism, actuators and wing kinematic modulation device – is rapid prototyped from ABS plastic, giving it a weight of only 5.3 g. A 2.75 mm diameter carbon fibre rod protrudes vertically down from the body to support the base of the craft on landing. Results from flight tests on the Hummingbird in a small enclosure show that it has better hovering and turning capabilities than other contemporary MAVs, which could mean that in the future we may see MAVs with the same agility as nature’s own hummingbird. Now, here’s a thing “ ” The biggest challenge was to develop a design with the strength to withstand the large forces during flapping