High altitude high endurance solar powered UAVcan be a solution for many missions. The designcomplexity is due to the very high altitudes expected and the low available energy to supply theengines. During time of daylight, the solar energyis converted by photovoltaic cells and then usedto supply electrical motors and payload. Theenergy remaining part is stored in a regenerativefuel cell and in potential energy with a climb maneuver. At night, fuel cell provides energy necessary to motors and payload. Optimisation carriedout here consists in maximising the payload for afifixed total mass. It requires mass model for eachconstitutive part of the aircraft. In particular, themass of the wing is minimised by the use of composite materials and by tolerate a large flflexibility.An new analytical mass model is proposed herevery useful for this particular application. Optimisation shows the existence of the UAV in a cruisespeed versus lift coeffificient diagram. This onerevealed an optimal solution having a payload ofabout 4 % of the total mass of 817 kg for a 69 mwing span.