Dear David,
As discussed in the following forum topic: How to find the paramter of the first bending modal mass, damping, and stiffnees coefficients? - #2 by Jason.Jonkman, FAST v8 returns the linearized model in first-order form. So, while you can’t get the second-order matrices M, D, K, and F, you can get the same terms normalized by the mass matrix i.e. I, [ M^-1 ]*D, [ M^-1 ]*K, and [ M^-1 ]*F through the FAST v8 linearization analysis. This first-order form should also be amendable for controls design.
See the OutListParameters.xlsx file provided in the FAST archive for the complete list of ElastoDyn output parameters. TTDspFA is the tower-top fore-aft deflection. As discussed in the following forum topic, while you can get the tower-tip deflections and accelerations, the tower-top translational velocities are not currently output parameters available from ElastoDyn: Output parameters of Tower-Top,Yaw-bearing Motions - #2 by Jason.Jonkman.
Best regards,