Significant variation occurs in the calculated critical buckling load with variation of stringer representation and model mesh density. The stringer representation was left as it was in the static analysis studies, i.e. the stringer bar elements were offset by the neutral axis distance and the bending inertias of the bars were those corresponding to the stringer neutral axis location. The results of these buckling solutions are shown by the following two plots. An appropriate stiffness representation of the stringer elements without using bar offsets is being investigated in a generic fuselage model. Insight gained from study of this model will be incorporated into the 747SP fuselage studies.
Early analyses show the fuselage buckling value increasing with removal of the bar offsets.
A refined mesh, generic fuselage model has been used to study the effect of mesh density upon fuselage buckling. Numerous lower value buckling modes are displayed by this model. Further studies are planned. Because of stringer representation and mesh density concerns, the results of the 747SP fuselage buckling analyses are felt to be more qualitative than quantitative. The global buckling load values may vary significantly from those presently predicted, but the global buckling performance of the modified fuselage versus the unmodified can be reasonably gauged.