Abstract:

ESDU 74025 gives a method for estimating the natural frequency of a box structure that is rectangular in section and has a number of supporting shear spars and webs to create an array of rectangular boxes. It is initially unstressed. The analysis assumes coupled vibration of the skin and internal supporting structure; the validity of the assumption is discussed and an alternative method suggested for when it is no longer acceptable. Curves give the lowest natural frequency for a structure made from an array of identical boxes with identical cover plate thicknesses; they cover a range of box geometries and spar/rib/cover plate thickness ratios. A computer program allows the method to be applied for higher modes and to structures in which the boxes are not identical and the cover plates have different thicknesses. The program is provided as ESDUpac A7425. The predictions are compared with 40 experimental results drawn from the literature.