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The requirement for an aircraft to operate in the thin atmosphere at high altitudes for long periods provides a particular challenge to all areas of aeronautical engineering. This is reflected in a range of difficulties encountered when attempting to design aircraft for this type of operation. This paper first considers the reasons for adopting an unmanned solution for high altitude long endurance (Hale) aircraft. It then indicates how the demands of Hale operations lead to problems in applying the design approach taken for more common aircraft types. Some of the work performed at the College of Aeronautics (CoA), in attempting to address the problem areas for the design process and produce initial designs for a range of Hale unmanned aircraft (UMA) types, is then reviewed. The lack of data/methods to allow the prediction of Hale UMA structure mass, engine performance at high altitudes and aerodynamic parameters for low Reynolds number operation and high aspect ratio configurations are identified as particular problems. However, initial design, based on standard approaches, has been shown to be possible, provided caution is exercised.

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