All (non-glider) aircraft require powerful, lightweight, efficient propulsion. For medium and small unmanned air vehicles (UAVs), this almost always means some form of propeller-based system. As it is difficult to engineer variable-pitch propellers on the scales required, fixed pitch systems are generally used. Combined with the high costs of developing bespoke prime movers, this means that the designer of small and medium UAVs is generally faced with rather difficult compromises in selecting the appropriate propulsion system from components already available on the market. We have found no concrete solutions: for long-endurance aircraft, we tend toward gasoline-fueled internal combustion (IC) engines with multiple cylinders, via multicylinder engines, multiple engines, or both. For short-endurance systems, especially if low vibration is important, battery-powered electric motors are often preferable, and as battery technology improves, electric propulsion will become more useful for longer endurance systems. Here we consider both these forms of propulsion in various configurations and also hybrid systems where both IC engines and electric motors are fitted to a single airframe.
In sizing and assessing propulsion systems, we rely on various sets of results we have obtained from a dedicated engine/propeller test facility we have built and also the first-rate data produced at the University of Illinois at Urbana-Champaign (UIUC) by Brandt and Selig . It has been our experience that it is wisest to actually measure static and dynamic thrust, fuel, and battery consumption, and so on, for ourselves whenever possible. Dedicated test cells also provide an ideal place for setting up and running in new engines in controlled conditions, see Figures 5.1 and 5.2. We deal with the range of engines and motors we have direct experience with in the following sections. See also Section 11.6 for various expressions we use in estimating likely propeller and engine sizes.