Dear Matthias,
In BModes, the cross-sectional flapwise mass inertia, flp_iner, should be computed as the integral of the mass density times the square of the flapwise distance from the blade section mass center to the differential area element over the cross-sectional area of the section. That is, flp_iner = ∫∫ ρ(x, y)x^2dxdy, where ρ(x, y) is the mass density in kg/m^3 and x and y are the flapwise and edgewise distances in meters from the blade section mass center to the differential area element, respectively. Likewise, edge_iner should be computed using the edgewise distance: edge_iner = ∫∫ ρ(x, y)y^2dxdy.
For a solid cylindrical cross section of uniform density ρ and radius r, flp_iner = edge_iner = ρπr^4/4.
Best regards,