In the last few years, the interest regarding solar powered aircraft has increased significantly and several research programs are carried on all over the world. In particular, the interest is focused on Unmanned Aerial Vehicles (UAV) able to perform High Altitude and Long Endurance (HALE) missions for telecommunication, remote sensing, surveillance and control, both in civil and military field. Starting from a previous work in which different aerodynamic configurations have been compared, this paper presents a design procedure for a non-conventional biplane aircraft, with vertical link-wings. The first part of the paper deals with the definition of the geometrical parameters which both minimize the aircraft weight and fulfil the daily energy balance; a vortex-lattice aerodynamic code and a solar energy model have been implemented. Once the optimised configuration is found, an iterative process for a “fine tuning” is activated up to weight convergence. In order to define the range of application of a given aircraft, in terms of day and latitude, the “Operating Domain” (OD) chart is introduced. When aircraft configuration and mission are given, this chart shows those combinations of day and latitude that satisfy the energy balance. In this paper a simplified method for OD-chart tracing is shown. By means of the present procedure, optimised aircraft at winter and summer solstices are shown and the related OD-charts discussed.