Aeroelasticity

Aeroelastic phenomena can classified to be either of dynamic or static nature, depending on whether they are time-dependent or not. In either case, the structural response to the aerodynamic loads can be stable or unstable [Megs16]. Dynamic aeroelastic phenomena, such as classical flutter, involve unsteady aerodynamic loads and structural dynamics. The unsteady loads continuously feed energy into a structure – a wing for instance – causing the mass distributed along the wing span to be accelerated in an oscillating motion. Hence, not only elastic properties but also the mass distribution influences the response of the wing.

Static deformations and divergence

Even in quasi-steady flight situations aircraft structures, only having a finite stiffness, will deform. In case of sufficiently stiff structures the deformation will converge and form a quasi-static equilibrium with the aerodynamic forces. This situation is usually found during steady level flight but also during quasi-steady flight manoeuvres such as steady climbs and descents, pull-up manoeuvres or steady turns in which the aero- dynamic loads can be considered to be steady [Megs16] [ToWi09]. In a quasi-static equilibrium, the aircraft will take on a so-called flight shape, the deformed shape during flight. As a result of the changed aircraft geometry, aerodynamic parameters, such as lift or drag, do not only depend on the incoming flow but also on the flight shape [Brau07] [RoWH14]. In fact, wings of large transport aircraft may be designed, optimised and built in such a way, that, at cruise, when deformed by aerodynamic and gravitational loads, they take on a minimum drag configuration [RoWH14] [Bouc03].

Aeroelastic divergence is a static instability that may occur if a lifting surface does not have the required stiffness to sustain the aerodynamic loads. At divergence the deformation of the wing is continuously amplified as the aerodynamic loading increases which contributes to even further structural deformation. When restoring elastic forces are overcome, a point of structural failure is reached. Main factors influencing this phenomenon are the dynamic pressure and wing torsional stiffness. Forward swept wings are particularly sensitive to divergence as the forward sweep introduces a disadvantageous coupling between bending and torsion deformations [RoWH14].

Control efficiency and reversal

When flight control surfaces such as ailerons are being deflected, they usually introduce forces into the wing at some distance away from its elastic axis. These torsional loads can cause the wing to twist which decreases the effectiveness of the control surface deflection. For instance, ailerons mounted on a real, elastic wing will produce smaller roll moments than ailerons mounted on an idealised, perfectly stiff structure. This adverse effect of fluid-structure interaction, also known as loss of control efficiency, typically becomes more pronounced with increasing airspeeds due to accompanying higher torsional loads. Insufficiently stiff structures can encounter an instability called control reversal, in which the lifting surfaces have deformed so much that control surface deflections produce reversed effects, i.e. moments opposite to their intended direction.

Note

This summary is based on/copied from [Dett19] with the authors permission.