Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

24 Dossier | Quantum Tron endurance figures yield a theoretical linear range of 160 km including take-off and landing in VTOL mode. The Tron’s overall proportions are those of a glider, with its 3.5 m wing span being nearly twice its overall length of 1.8 m. For transport and storage, the Tron is dismantled into sub-assemblies comprising the fuselage, wing, V-tail, engine nacelles and batteries, all of which fit into a ruggedised box measuring 180 x 40 x 35 cm. While providing an aircraft with VTOL capability always adds complexity, in some ways it can simplify life for aerodynamicists tasked with developing hybrids, as Seibel points out. “Optimising a VTOL for fixed-wing flight is fairly easy, as you only have to focus on one thing – cruise flight,” he says. “The aerofoil doesn’t have to be a compromise that also has to produce high lift at low airspeeds during take-off and landing. We were able to reduce aerofoil drag by about 10-15% compared with an aerofoil that has to provide both good cruise flight characteristics and high lift.” The aerofoil sections and planform that together determine the lift and its distribution are Quantum’s own work, and were designed and tested with the aid of the wind tunnel at the University of Stuttgart, achieving an overall lift-to-drag ratio better than 20:1, Seibel says. This cruise-optimised, ‘slippery’ wing goes some way towards offsetting the inevitable loss of range that comes with VTOL capability courtesy of the weight of the extra motors. Again though, Seibel points out, there is a weight saving available from not needing the extra structural strength required to withstand catapult launches and hard landings. He emphasises that launch and recovery are much smoother all round, which is of great benefit to some of the more sensitive payloads. The fuselage is tadpole-like, the bulbous body tapering back to a thin boom terminating in a V-tail and forward more abruptly to an approximately ogival nose cone. The long wings have almost straight trailing edges, with the leading edges curving back to meet them at the tips, making for an elliptical shape to give the lowest possible induced drag, Seibel says. For greater aerodynamic efficiency there is only one small aileron on each wing, and because there is no need to accommodate high-lift devices such as flaps or slats, the wings can also be made lighter as well as structurally and mechanically simpler. Each aileron is controlled by a brushless dc servo motor. The servos are Hitec’s highest grade units, although customers can opt for more expensive Volz items with a higher specification. Each surface of the V-tail has its own servo-controlled ‘ruddervator’ (a combination of rudder and elevator). The four electric motors, each with its own propeller, are housed in pairs in wing-mounted nacelles, the forward and aft portions of which tilt under the influence of electric servo actuators. As they must provide sufficient thrust to lift the aircraft vertically off the ground, the motors are significantly overpowered for cruise flight, so the rear pair are routinely switched off after transition and the propeller blades folded back to minimise drag. Two motors good Even with two motors switched off, the remaining two are still larger than would be optimum for cruising flight – as are the propellers, which are also sized for VTOL operation. However, Quantum argues that this is a better compromise than carrying four extra motors in cruise flight, the dead weight of which would be exacerbated by the drag of the inactive vertical propellers in hybrid VTOL UAVs with separate lift and cruise modes. A quad tiltrotor design also needs one motor fewer than more conventional hybrids, because it does not need a dedicated motor for cruising flight. The company has found a way to sidestep one of the main drawbacks of fitting overpowered motors, by switching them on and off and exploiting the Tron’s gliding abilities, something that would be risky with an internal combustion engine but presents no problems with electric propulsion. At a constant power setting in cruise mode, they would not be operating within their most efficient range, but if they are used in short bursts at a high power setting the aircraft can follow a ‘sawtooth’ climb and glide profile that lets the motors work within that range. For mission profiles June/July 2017 | Unmanned Systems Technology Testing the aerodynamics in the wind tunnel at the University of Stuttgart