Introduction to Aircraft Aeroelasticity and Loads

F

Aircraft Antisymmetric Flexible Modes

In Chapter 15 a simple dynamic roll manoeuvre is introduced for rigid and flexible aircraft and so some definition of simple antisymmetric free–free whole aircraft modes is required.

F.1 AIRCRAFT MODEL

Because of the complexity of the lateral manoeuvres, the effect of the fuselage, fin and tailplane flexibility is ignored in this analysis, so only the wing will behave elastically in twisting or bending. The aircraft once again has unswept and untapered uniform wings which are permitted to bend or twist. Fuselage and empennage roll motion is present for the antisymmetric bending mode.

F.2 ANTISYMMETRIC FREE–FREE FLEXIBLE MODES

F.2.1 Antisymmetric Wing Torsion Mode

In this case, the flexible mode shape involves a (nose up) twist of the starboard wing given by the function γ_e(y); the port wing will deform antisymmetrically (nose down), as illustrated in Figure F.1(a). The flexible mode is assumed to involve wing twist with no bending and so essentially the wing mass and elastic (or flexural) axes coincide to avoid bending/torsion coupling. The fuselage would not roll or pitch in the mode shape, since the torsional inertia moments in the two wings are balanced by antisymmetry.

For simplicity, it may be assumed that the wing torsion mode shape is given for y ≥ 0 (starboard wing) by the linear function (refer to Chapter 3 and Appendix C)

where ...