Weight Reduction Through Optimal Arrangement of Force-Carrying Components Within Wing Box Structures

Abstract

Weight is always playing a dominant role in aircraft design, because it affects the aircraft performance significantly. In general, more weight means more drag, which dictated an engine with more power, which meant even more weight. The structural weight is part of the takeoff gross weight. A historical statistical data showed that an amplified impact of any weight- component increase on the gross weight is about 4.525, that is, 1 lb saved in any manner - payload reduction, reduced structural weight, reduced fuel weight, etc. - results in a 4.525 lb reduction in overall gross weight. Therefore, weight reduction is always a very sensitive issue in aircraft design. Optimization can be a powerful tool in the conceptual and preliminary phases of aircraft design. The focus of this paper is to reduce the structural weight through the optimal arrangement of the force-carrying components within wing-box structures under a specified air-loading condition, that is, to find the “best” locations of the longitudinal wing spars and transversal wing ribs. The “best” means that the shear flows in spar webs and wing skins, and the axial stresses in the stringers of the wing spars will be the smallest under a specified air- loading condition, thus, the corresponding weight will be the smallest. The essence of the optimization here is actually to find a “best” distribution of the internal forces among the force- carrying structural components such that each of the components tries to share its biggest effort to support the external loading based on its own capacity. A redundant weight on all structural components is eliminated. The computer program searched for 110,400 times, and found the optimal locations of the two wing spars and the four ribs as shown in the demonstrated example. The weight reduction is demonstrated by comparing the structural weight corresponding to the optimal arrangement with that corresponding to a “bad” arrangement. The effect is salient - a 21% reduction in wing structural weight can be achieved.