Issue 40 Unmanned Systems Technology October/November 2021 ANYbotics ANYmal D l AI systems focus l Aquatic Drones Phoenix 5 l Space vehicles insight l Sky Eye Rapier X-25 l FlyingBasket FB3 l GCS focus l AUVSI Xponential 2021

98 PS | Distributed propulsion in UAVs D istributed electric propulsion (DEP) is now common in UAVs, thanks largely to the wild success of small quadcopters, and the investment is now pouring into eVTOL vehicles (writes Peter Donaldson). Fundamentally, distributed propulsion (DP) is more than just multiple engines or propulsion units fitted to an aircraft in different places. While there isn’t a formal definition yet, NASA for example says the capabilities of a DP system should serve to improve the system-level efficiency, capabilities or performance of an airborne vehicle. Such synergistic effects generally involve tight aero-propulsive coupling in which the propulsors – whether they are propellers, rotors or ducted fans and so on – contribute to lift, thrust, control and even drag reduction. While the current focus is on electric power transmission, DP has been used with various means of sending power from one or more engines to propulsors, including chains and shafts, and engine exhaust gas. Electric power distribution is fundamentally more flexible though, as generators, batteries and modern motors offer much greater torque and power densities than their predecessors. As with IC engines, large electric motors are generally more efficient than small ones, but they are much less sensitive to size effects than, for example, gas turbines, so the efficiency penalty for using multiple small motors for a given total power rather than one large one is nothing like as severe. Concepts for larger vehicles that need to combine high power with long range and endurance tend to be hybrids with gas turbine-driven generators. Without a mechanical connection between them, the engines and propulsors can be operated in their most efficient speed ranges for more of the time, although using mechanical transmissions between the electric motor and propulsor can still bring weight and efficiency benefits. Also, the use of power inverters between generators and motors allows the speed ratio to be changed during flight, giving the effect of a variable-ratio gearbox, NASA points out. That opens up a lot of design scope for innovative vehicle and propulsion system architectures, whether pure electric or hybrid, manned or unmanned. It allows greater fuel efficiency, safety through redundancy, and the combination of VTOL/STOL and high speed. For example, NASA’s Single-Aisle Turboelectric Commercial Transport with Fuselage Boundary Layer Ingestion aircraft features two wing-mounted turbofans with generators that power an aft propulsor, which consists of an inlet and fan that ingests some of the airflow’s boundary layer and uses it to generate thrust. The electric propulsor reduces the load on the engines, and the drag caused by the boundary layer is reduced. While the concept is for an environmentally friendly airliner, the potential for longer range and endurance for high-speed UAVs is clear. In another example, Aurora Flight Sciences’ XV-24A LightningStrike UAV concept features a single gas turbine driving three generators to power a total of 24 electric variable-pitch ducted fans built into the tilting mainplane and canard foreplane. Aurora’s aim was to combine a 15% increase in hover efficiency over an equivalent helicopter with sustained speeds of 300-400 knots. DEP is still in its infancy but it is already showing great potential for unmanned aviation. Now, here’s a thing “ ” October/November 2021 | Unmanned Systems Technology The use of power inverters opens up a lot of design scope for innovative vehicle and propulsion system architectures, allowing greater fuel efficiency and safety