A method for frequency-limited balancing of the unsteady vortex-lattice equations is introduced that results in compact models suitable for computational-intensive applications
in load analysis, aeroelastic optimisation, and control synthesis. The balancing algorithm
relies on a frequency-domain solution of the vortex-lattice equations that effectively eliminates the cost associated to the wake states. It is obtained from a Z-transform of the
underlying discrete-time equations, and requires no additional geometrical or kinematic
assumptions for the lifting surfaces. Parametric reduced-order modelling is demonstrated
through interpolation over (a) projection matrices, (b) state-space realisations and (c)
transfer functions, which trade accuracy, robustness and cost. Methods are finally exemplified in the dynamic stability of a T-tail configuration with varying incidence. Numerical
studies show that a very small number of balanced realisations is sufficient to accurately
capture the unconventional aeroelastic response of this system, which includes in-plane
kinematics and steady loads, over a wide range of operation conditions.