We recall the "typical section" aerofoil model with oscillating pitching and plunging motions to represent the torsional and bending behaviour of a wing structure with large aspect ratio, low sweep angle, and smoothly varying aerofoil shape. Further, we extend the original model as shown in fig. 8. In addition to an external system independent force, we introduce aerodynamic lift L (positive upwards) and pitching moment M (positive nose-up). The aerodynamic forces and moments act at the aerodynamic centre (ac), which for thin aerofoils is close to the quarter chord point. The aerodynamic centre is located a distance xec from the elastic centre. Using the chord length c rather than b is an arbitrary choice.