As a fundamental study for the flutter analysis of swept-back wings, the vibration characteristics of a cantilever parallelogram box beam are analytically and experimentally studied. The fundamental equations are the same as those in the author's previous paper on the static deformation. Under the condition of large aspect ratio, the first approximation leads to the beam theory and the St. VENANT torsion theory for the natural frequencies and yields coupling between bending and torsion for the normal modes of vibration. For the second approximation of the natural frequencies, the effect of the shear deformation of the spar webs is treated in the bending vibration and the effect of the constraining of the warping at the root is treated in the torsional vibration. An experiment has been carried out on a specimen of the aspect ratio 7.5 and the swept-back angleof 30 degrees. Good agreement is found between the analytical values and the experimental results for both the natural frequencies and the normal modes.
