Motivation: The last decade has seen a dramatic diffusion of battery-powered rotary-wing micro Unmanned Aerial Vehicles (UAVs, a.k.a. MAVs). Their low price, maneuverability and small launch-and-landing footprint has made them extremely popular, but their aerodynamics have disadvantages, such as the relatively poor energy efficiency, that divides multi-rotor UAVs from conventional helicopters and fixed-wing aircraft. The poor capacity of existing lithium-ion polymer (LiPo) batteries is another issue, typically limiting the flight endurance to 15 to 30 minutes.