TABLE OF CONTENTS Figure 1 shows the typical wing section used to derive the structural equations of motion. This model has been well established for two-dimensional aeroelastic analysis (Dowell et al., 1994; Fung, 1955; Glaser, 1987). The degrees of freedom associated with the aerofoil are shown in Figure 1. The pitching and plunging displacements are restrained by a pair of springs attached to the elastic axis (EA) with spring constants K ? and K h, respectively. A torsional spring is also attached at the hinge axis whose spring constant is K ?.
Djayapertapa (2001) and Scanlan and Rosenbaum (1951) describe the derivation of the two degrees of freedom aeroelastic equation of motion from Lagrange s equation, and the same principal can be applied to a three degree of freedom system. The resulting governing equations are given by:
| (1) |  |
| (2) |  |
| (3) |  |
where the symbol definitions are shown in Figure 1. S ? is the static moment of the aerofoil about the EA and is given by S ? = mX ? b. S ? is the static moment of the control surface about the hinge axis and is given by S ? = mX ? b. 
is the aerofoil moment of inertia about the EA, and 
is the control surface moment of inertia about the EA.
In order to obtain the full non-dimensional form of the equation, non-dimensional plunge ( ? = h/ b
