Design Tradeoffs for Fiber Composite Fuselages Under Dynamic Loads Using Structural Optimization

*† ‡ In this paper a fuselage shell, made of fibers composi te material, of a passenger aircraft is optimized to get a minimum weight. Two dif ferent structural configurations of foam-filled sandwich and stiffened shells are analyze d under the dynamic loads due to the gust. A knowledge based engineering (KBE) approach is formulized to automate the multidisciplinary optimization problem in three diff erent layers. The first layer takes care of the optimization of full fuselage under the cons traints of real and negative eigenvalues to get an asymptotically stable closed-loop syst em. The second layer and third layers, take care of the optimization of the several sections along the length of the fuselage and the optimization of a particular panel of a s ection, respectively. The constraints in the second and third layers include the critical buckling and wrinkling stresses of the panels and the sections, under bending and torsion loads. The design variables are the stacking sequence of fibers orientati ons, sizes and positions of the stiffeners in a panel, section length along the longitu dinal axis of the fuselage, and the skin thicknesses of the panels. Finally both of the op timized designs are compared with each other in terms of minimum weight, stability and moreover in terms of manufacturing. I. Introduction The use of fiber composite material is getting common in the aircraft industry. Previously their use is quite common only in the light weight aircrafts and in the true se nse most of the time the home builders or hobbyists were the forerunners in using these types of materials . The main reason behind this is the ease of manufacturin g and its lower cost. We also see that in that period the ma nufacturers of large airplanes are always reluctant to use the fiber composite material and restricted most of the t ime to the parts that were less critical to loads. Contr ary to that we see a change in late 80s and early 90s that f ew of these manufacturers start using the fiber composite materials in tail-section which was quite encouraging to th e world of aircraft engineering and especially to the fiber composite industry. The start of 21 st century gives another break through in the sense that, apart fr om tail section, two of the major manufacturers come up with fibe r composite fuselages i.e. the fuselages of A380 and B787. The manufacturers of executive jets have already started producing the complete fiber composites airframes and we expect that those days are not far awa y when the wings of large airplanes will also be made of fiber composites. The use of fiber composites has several advantages over t he metals which include the better service life in terms of fatigue and moreover the recycling if made of a special kind of fiber composites known as thermoplastics. The knowledge of designing a metal airfra me is almost one hundred years old. This knowledge is transferred to us from generations to generations which i nclude the tons of record in the form of experiments and theories with the cases of successes and failures. Presently an aircraft structural designer must be better equipped to deal with the structural analysis of a metal airf rame than to a fiber composite one. One of the examples is to find the flexural or torsional stiffness of a fuselage or wing section. The stiffness plays an important role in achieving the stability of the load beari ng components of the aircraft, especially when these are exposed to the external disturbances during the atmospheric turbulences and takeoff/landing. In the case of stiffness of a structure made of metal alloys, one has t o find the cross-sectional areas and mechanical properties of the material, which for the Aluminum alloys are well known and recorded. In case of fiber composites one has to analytically find the stiffness for each and eve ry section/panel, which on the other hand changes if one of the fiber layers is oriented differently to that of th e adjacent section/panel. The lack of vast experience amon g the aircraft structure design engineers in dealing with th e effects of fibers orientations on the stiffness of a structure can be the one of the several reasons behind t he slow pace in adapting these materials in the designs of