2 nd International Conference on Engineering Optimization September 6 - 9, 2010, Lisbon, Portugal Multi Spar Composite Vertical Empennage Structure Optimization Thiago A. M. Guimarães, José Hernandes , Saullo G. Castro ITA, SP São José dos Campos, São José dos Campos, Brasil, hernandes@ita.br Abstract In the present work a multi spar vertical empennage optimization fully constructed by laminate composite materials had been optimized to find not only the number of plies for each laminate, but also the stacking sequence of the plies. The buckling load factor is the design constraint, objective function is minimizing structure weight, and analysis is done considering the critical maneuver load case for left and right side. Two steps are proposed. First the finite element model was constructed using four adjacent elements forming a square-like group with the same composite property. The purpose is to find the regions with similar thickness in the same component (spar or skim) in this step no stacking sequence is evaluate, the design variables is only the thickness. After this step the finite element model was reorganized by attributing properties to the regions founded. In this second step angle and thickness discrete variables were used to evaluate the full optimization. The structural optimization software GENESIS ® is used in both steps. Result comparison is done evaluating and not evaluating the first step Keywords: Composite, multi spar, stacking sequence, vertical empennage. 1. Introduction Optimization is constantly related with the best project desire. Every time engineers are developing new products adjusting and evaluating constrained parameters to minimize or maximize results always looking for higher performance. In accord of Vanderplaats [1] no mater what kind of problem; we are always solving an optimization problem. Otherwise this traditional approach to analyze and revise normally involves only changing a few variables at a time and does not account very well for the interaction among the variables. Numerical optimization allows the change in large number of design parameters simultaneous improving designs while realizes design constraints checks. Composite materials have been applying to aircraft construction to provide flexibility to develop lightweight primary and secondary structures. It has been more attractive since last decade due to recent development of new features to perform design optimization. An important feature of laminated composites is their layered construction that guarantees stiffness and mechanical behavior of structure increase. However, efficient and simple optimization strategies are necessary to find such properties. Therefore, the purpose of this paper is to explore methodology developed for Barker et all [2] that propose to organize the zones in a composite structure merging adjacent elements with similarity thickness, without consider different percentages for each ply, combining with a simple methodology to evaluates stacking sequence optimization developed for Guimarães et al [3] to optimize for buckling failure a multi spar vertical empennage concept. To evaluates computational optimization is used the commercial optimization software GENESIS ® in both steps. Different approaches were developed to perform optimization in composite laminate structures. Yamazaki[4] proposed to maximize the buckling and frequency performance of a composite plate through gradient-based optimization using design variables to approximate near the optimum discrete design . A two-step approach was proposed by Todoroki and Haftka [5] to maximize buckling load of a composite plate . Leiva [6] developed a new approach in stacking sequence optimization of composite laminates using GENESIS ® structural optimization software. The structural optimization software GENESIS ® uses the strategy to optimize structures modeled with finite elements using non linear programming methodology (MPNL), consisted of generalities that reduces necessity of many design cycles approximating objective and constraints functions by math programming methods. The first step similar to Barker [2] methodology with some modifications (do not consider ply percentages) will be called topometry. The objective of this step is reorganizing zones into a new finite element model to ensure that composite layered structure will be lighter and simple to manufacturing. In order hand the second step is a simple strategy developed to realize lay-up optimization based on the use of PCOMP bulk card fields where specific spaces are changed by integer design variables. 2. Optimization Methodology 2.1 Topometry Optimization Following it is described the methodology used to select the best organizing of zones for a multi-spar composite 1