This paper is devoted to lightweight rotary wing UAVs for applications at extreme high altitudes of up to 9000m, such as search and rescue as well as different types of environmental monitoring. Up to now, such rotary wing UAVs have not been available. Consequently, experience in design and operation of rotary wing UAVs in such environments is lacking. This situation motivates our team from DLR and TUM to work on an all-electric prototype of this kind of rotary wing UAV. The first experimental missions should be performed at altitudes of 5000 to 9000m above sea level. This paper shows the project's current status and preliminary results. We present procedures for preliminary rotor designs and electrical power supplies as well as a tool developed for exploration of the influence of critical design parameters from a mission based point of view. Using the described procedures and the tool, we present the preliminary design of the UAV prototype, which is a synchropter with two counter-rotating and intermeshing rotors. Some of preliminary calculations were verified with flight experiments (hover performance) at different altitudes of up to 5200m above sea level. The results presented, illustrate the feasibility of developing rotary wing UAVs for selected applications at extremely high altitudes and will be the cornerstone for a detailed design of the first prototype.
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