Flap–torsion coupling in sandwich beams and filled box sections

Slender aerodynamic structures such as helicopter rotor blades are routinely idealised as 1D beams with associated cross-sectional stiffness properties, and often exhibit structural couplings. A strength of materials based analytical model is presented for determining the cross-sectional stiffness that couples flap bending and torsion (a coupling that is of particular interest in helicopter rotor blade design), by considering strains of individual substructures of the cross-section. The section may be square or rectangular in nature and be foam-filled or hollow. The top and bottom skins may exhibit anisotropic elastic behaviour through non-symmetry and both membrane and flexural anisotropy. This allows a more detailed parametric investigation of this cross-sectional property than is possible with existing analytical models. The analytical model is validated by comparison with a finite element model of the cross-section.