V. I. Biryuk - Ph.D. (Eng.), Associate Professor, Lecturer, Department of Aircraft Strength, Moscow Institute of Physics and Technology (State University), Chief Research Scientist, Central Aerohydrodynamic Institute. E-mail: v_biryuk@yahoo.com
H. Ph. Nguyen - Post-graduate Student, Department of Aeromechanics and Flight Engineering, Moscow Institute of Physics and Technology (State University). E-mail: loicuagio100884@yahoo.com

The object of this research is the UAV like "Predator", which has, like other UAVs, straight wing with a high aspect ratio. The study was conducted using the finite element software complex MSC.Patran/Nastran. The objectives of this work were: to define the optimal orientation distribution of the fibers stacking sequence in the composite package, in which the wing twisted to highest degrees in direction of angle of attack reduction; to define the minimal weight in condition that all the layers of composite package are not destroyed (using three composite failure criteria); and to evaluate what percentage of design bending moment and structural weight can be reduced respectively. The results show that for straight wing of UAV with high aspect ratio, the use of anisotropy properties of composite material in wing skins allows to reduce angles of attack of each wing cross-sections. This is an additional factor for weight reduction. For the considered wing box, when the staking sequence was -20/-45/-20/45/-20/90/-20/45/-20/-45/-20, the wing twisted to highest degrees in direction of angle of attack reduction (6,9 degrees) that led to a significant reduction of the bending moments (50%), and the weight of the structure also decreased by 11% respectively. The application of the three composite failure criteria led to slight differences in optimized weight. But, despite the same trend of twisting the wing towards the reduction of angles of attack, the differences in the distribution of torsion angles and their values give evidence for detailed experimental research on constructive samples in order to study this issue thoroughly.