A study of propeller for high altitude unmanned airplane

Over the last 20 years, we can observe growing interest in the field of unmanned high altitude aerial vehicles. Especially the light aircrafts with electric motors become popular. The use of electricity to drive enables application of non-conventional energy sources like solar energy. The propeller remains the best propulsion system for planes powered by electricity. High altitude planes powered by propellers are able to fly over the wide range of altitude: from 0 to 25 kilometres. Such wide range of altitude is connected with variability of propeller work conditions (for example the density of the air is changing 10 times) which remarkably complicate the process of matching propeller to motor. The paper presents optimization model of propeller destined to unmanned high altitude airplane. Computational procedure deals with searching optimal distribution of lift coefficient line along the blade span of the propeller to obtain maximal efficiency for selected altitudes of flight. Genetic algorithm was used during the searching procedure. The database of optimal solutions is created as a result of computation and can be later exploited for choosing the best solution able to meet the requirements. Because the same propeller is used for take-off, climb and high altitude flight, then the airfoils of the propeller blades must be capable of operating over an extremely different flow condition caused by large change in air density. Propeller blades airfoils are required to operate within a low Reynolds number (below 105) and high subsonic Mach number (up to 0.6) flow field during high altitude flight. At low Reynolds number, the airfoils generate lesser lift and higher drags. The performance of the airfoils and consequently of the whole propeller can decline significantly. The objective is to find airfoils with good performance in all condition, which can appear during the flight, and to find the best shape of lift coefficient curve along the blade span of the propeller. That is necessary in order to obtain acceptable efficiency of the propeller for all flight conditions.