Aerodynamic and structural optimization a male class unmanned aerial vehicle wing with genetic algorithm

Abstract

In this thesis, a genetic algorithm based airfoil, planform and wing structure design is utilized for a 1500-2000 kg class fixed wing reconnaissance MALE (Medium Altitude Long Endurance) type UAV (Unmanned Aerial Vehicle). Due to their mission descriptions, these UAV are generally designed to have long range and high loiter time. For these reasons, an aircraft to be designed for observation purposes must have high aerodynamic efficiency and high fuel load ratios (low empty mass) to maximize the range and loiter time. To achieve high aerodynamic efficiency, these aircraft have high wing span ratios, and their wings contain specially designed airfoils. These airfoils are generally designed to have high lift-to-drag ratios. On the other hand, due to long wing structures of high-span aircraft, the wings of such aircraft have high bending forces especially during maneuvers. Therefore, design of airfoils of such aircraft have a compromise between fitting the ideal lightweight inner skeleton for the wing and providing ideal aerodynamics. For these reasons, the wing profile located at the root of the wing is generally chosen to be thicker than the profile used at the tips of the same wing. To simplify the design process, each airfoil are constructed from two Bezier curves that create centreline and thickness distribution of airfoils. Control points of the Bezier curves are the most of the input parameters of the genetic algorithm program. From sets of control points, the airfoils are created. Then, the airfoils are analysed in XFoil by the interface of the function made in MATLAB. After analysis of airfoils, a planform that uses these root-tip airfoils is tested for having sufficiently high lift and low drag for the cruise altitude, cruise speed and cruise power. Then the airfoilplanform combination that pass the basic requirements are sorted by their maximum lift-to-drag ratios. The airfoil-planform combinations with higher maximum lift-todrag ratios are selected for creating the next generation, and the cycle continues. When the maximum number of generations are achieved, the best airfoil-planform combination of the last generation is selected as the best candidate. The fitness criterion of this first phase is the lift-to-drag ratio of the airfoil-planform combination. After the winner airfoil-planform combination is created, inner structure optimization process for the wing begins. Inner structure of the planform consists of four ribs and a twin-box spar structure made of 7068 aluminium alloy, the strongest commercial aluminium alloy available. The lift force and torsion moment of each wing segment is transferred to the spar by the ribs of the wing. The cross section of the spar consists of two closed cells with the support of four stiffeners and eight flanges. Vertical walls have thickness of 2.5 mm, while upper and lower walls have thickness of 1.5 mm. The flanges have cross section value of 400 mm2 , and are set to the upper and lower ends of vertical walls, filling the corners of each cells. The stiffeners have cross section value of 200 mm2 , and are set to the middle of the upper and lower walls, in between the vertical walls. While the stiffeners and flanges carry the tensile and compressive loads, the walls primarily carry the shear loads. To simplify the structural analysis, several assumptions are made. The main spar is assumed as a serial combination of smaller spar sections with constant cross sections. The stresses on cross sections are analysed with structural idealization method. The cross section is assumed as collections of idealized shear force carrying panels and normal force carrying area members called booms. At first, the effective areas at the positions of booms are found by adding effective boom area of wall sections to the actual stiffener or flange areas. From the effective areas and positions of each boom, area moments of inertia and bending moment centre are found. From area moments of inertia of the section and applied bending moment, the compressive and tensile forces of each boom are calculated. After the calculation of tensile and compressive forces, shear forces on the walls are calculated from the area and wall thicknesses of each cell, torsion moment inflicted on section, and compressive and tensile forces of each boom. After the stress calculations are made for each section, a selection process is carried out to control the stresses on the cross sections on each wing rib. If the stresses on the wing at any point is larger than safe limits, the spar is considered as infeasible specimen. Otherwise the fitness value for the spar is compared to the other successful specimens and the fittest specimen is chosen for each generation to create new specimens. The weight of the spar is the fitness value for the second phase. At the end of the process, the ideal cross section is obtained and the program finishes working.