This work proposes a novel reduced-order modelling approach in time domain for the coupled flight dynamics and aeroelastic response of manoeuvring very flexible aircraft. The
starting point is the coupling of a displacement-based, geometrically-nonlinear flexible-body
dynamics formulation with a 3-D unsteady vortex-lattice method. This is followed
by a linearisation of the structural degrees of freedom, which are assumed to be small in
a body- fixed reference frame. The translations and rotations of that reference frame and
their time derivatives, which describe the vehicle flight dynamics, can be arbitrarily large.
As a result, all couplings between the rigid and elastic motions are introduced without
the a priori assumptions of the mean axes approximation, traditionally used to decouple
the equations in flexible-aircraft dynamics. The resulting system can be projected onto
a few vibration modes of the unconstrained aircraft with geometrically-nonlinear static deflections at a trim condition. Equally, the unsteady aerodynamics are approximated
on a fixed lattice defined by the deformed static geometry. The resulting high-order
aerodynamic system, which defines the mapping between the small number of generalised
coordinates and unsteady aerodynamic loads, is then reduced through balanced truncation.
This unified description of the flexible aircraft dynamics provides a hierarchy of aeroelastic
model fidelities, which will be illustrated on a representative high-altitude, long-endurance
aircraft to identify the importance of geometrically-nonlinear wing deformations on the
vehicle dynamics. Application of the reduced-order modelling approach further shows a
very substantial reduction in model size that leads to model orders (and computational
cost) similar to those in conventional frequency-based methods but with higher modelling fidelity to compute manoeuvre loads. Closed-loop results for the Goland wing finally demonstrate the application of this approach in the synthesis of a robust flutter suppression
controller.