I am solving a wing structure for normal modes using SOL103 in NASTRAN. Since I am beginner, I started with just the wingskin (lower and upper wingskins) but that results were far from what was expected. Only the local modes of the wingskin was observed. I modelled the spars and ribs then the 1st mode of bending was observed, rest all were the local modes of wingskin. Increasing the number of ribs caused the 2nd mode of bending to appear. Introducing stringers stiffened the structure where the 1st twisting mode and bending in x-y plane (my model has chord along x-axis and span along y-axis) were obtained. However still the local bending modes of the wingskin are seen where seen. So what is the appropriate spar-rib-stringer spacing and their thicknesses and the wingksin thickness so as to obtain the expected normal modes of a wing?
My wing is a sweptback wing with 30deg sweep angle, taper ratio of 0.6, span of 2m, and made of NACA0012 airfoil. I intend to simulate the cruise condition.