M.Ing. / Sustained or long endurance solar powered flight is defined as an aircraft capable of main- taining flight through multiple day-night flight cycles, using only solar power and rechargable energy stores. The project is focused on developing solar powered flight theory and real-world unmanned aerial vehicle implementations. The important aspects of system design are es- tablished and studied at a fundamental theoretical level. A preliminary design is conducted with endurance optimisation as the main aim. The optimisation process aims to establish a theoretical basis for sustained solar powered flight. The project is started with a feasibility and relevance study. A literature study was used to gather the required theoretical information. A novel theoretical preliminary design basis is conducted. The study is aimed at answering many questions in the field. The study is the first to show how previously varied aircraft from 3 m to 80 m are valid solutions to the long endurance flight requirement. The optimisation results correlates well with the current state-of-the-art. The theoretical models were then characterised through the development of two unmanned aerial vehicles. The development required a multidisciplinary integration of various fields. The development process was characterised and discussed. Flight automation was successfully integrated into the system. Multiple test flights were conducted. An interpretation of multi- faceted results are given. This project has contributed to international theory regarding solar powered and sustained endurance aircraft. Many specific contributions were made to the field. The project has achieved multiple unofficial records from the flight tests in the Southern Hemisphere and African continent.