Lateral stability analysis of frames for civilianaircraft structures

Abstract: The fuselage of a civilian airplane is a quite complex structure build from slendermembers and panels. Slender members are essential to minimize the weight of theairplane. These slender structures are at risk of buckling due to high compressiveloads. The scope of this thesis is to evaluate the lateral stability of one of thesestructures, a curved beam later referred to as the frame. Three main factorsthat affect the lateral stability of the frame will be discussed: The effect of thesurrounding geometry, the load conditions and the initial curvature of the frame.A literature study was carried out to find current information and methodologiesregarding such structures. The most promising techniques were later implementedto evaluate the lateral stability of the frame.The results from the analysis indicate that the structures surrounding the frameare sufficiently stiff so they won’t deflect and rotate together with the frame whenit buckles laterally. The whole cross section of the frame was assumed to be a rigidbody in this thesis but the experience from the present work indicates that thismight be wrong. The stress distribution in the frame was seen as a combination oftwo different load cases, plane bending and pure compression. By analyzing theseload cases individually we found that for the pure compression case the criticalbuckling stress was lower than for the maximum stress level of the plane bendingcase. The effect of the initial curvature of the frame was evaluated for the planebending load case. The initial curvature increased the resistance to the lateralbuckling modes but had also local effects on the stability of the frame. The crosssection of the frame is of the type of an open channel and if one were to take theinitial curvature into consideration one would also have to make sure that the webdoesn’t buckle locally due to increased radial compressive stress in the web.It was found that the surrounding structures could be disregarded if one appliesfixed boundary conditions to the fastener line of the frame where it is connected tothe skin panel. Further evaluation of how the initial curvature affects the bucklingmode is needed but the results look promising.v