Multi-objective optimization of the composite wing box of solar powered HALE UAV

Nowadays there is a growing request for Very Long Endurance Solar-Powered Autonomous Aircraft (VESPAA) flying at stratospheric altitudes of 17-25 km because they can act as artificial satellites - with the advantage of being much cheaper, closer to the ground, and being able to perform missions that offer greater flexibility. A long experience has been acquired by the Polytechnic of Turin, Department of Mechanical and Aerospace Eng. (POLITO/DIMEAS, Scientific Responsible Prof. G. Romeo) in Design of Solar powered UAV as High Altitude Very long Endurance Platform positioned in the stratosphere and with an endurance of more than six months. Two different configurations are under investigation in order to understand the best solution that completely fulfill the a priori imposed constraints. The main goal is to obtain a structure as light as possible that ensures the capability to resist at loads evaluated from the flight envelope without catastrophic failures. A Genetic Algorithm has been developed to optimize composite laminates subjected to mechanical, thermal and hygroscopic loads. A set of penalty function has been defined in order to guarantee deformations less than the allowable limit and to avoid local or global buckling. A Progressive Ply Failure Analysis code has been developed in order to evaluate the Last Ply Failure Loads of composite laminates. Since, in agreement with international regulations, the structure must be able to resist at ultimate loads without failures for at least three seconds, Genetic Algorithm and PPFA have been coupled in order to optimize the lay-up of each panel that ensure the minimum wing box weight, a proper safety margin from FPF to LPF and respects of the penalty function. In particular, no failure are allowed at limit load while, at ultimate load, each panel must stay under LPF level.