Preliminary study of an airplane for electric propulsion testing at high altitudes

Technology of photovoltaic cells and lithium batteries is being developed rapidly. As a result, attempts to build solar High Altitude Long Endurance (HALE) airplanes are more and more frequent. In the future, such airplanes may appear very useful for the economy because they may replace geostationary satellites in several applications. Unfortunately, data on altitude effect on photovoltaic cells and batteries performance are not easily available. Moreover, acquisition cost of cells is very high. Therefore, a tool for inexpensive testing of cells is needed. This article shows a study of very light unmanned airplane that could be used as a testbed for this purpose. Weight assumptions are presented together with concept of geometry and aerodynamic characteristics. Propulsion system is proposed, so also airplane performance is estimated. Finally, results are discussed leading to the conclusion. It appears that unmanned airplane with maximum take-off weight of 1.3 kg can climb to the altitude of 10 km within 4 hours during sunny summer day about the noon. However, only 30% of such days can be used because of strong winds blowing at high altitudes, quite small optimal airspeed of the airplane and constraints due to Air Traffic Management. Moreover, application of variable pitch propeller is recommended as well as some kind of take-off assist. For example, towing or take-off from the hill is desirable to avoid threats resulting from small climb rate.