Comment on 'General formulation of the aeroelastic divergence of composite swept-forward wing structures'

Abstract

O can only wonder at the reasons for the incessant refinements to an academic formulation of an aeroelastic problem that does not exist in flight. Reference 1 and many of its references have addressed the problem of divergence of a cantilevered composite forward-swept wing (FSW), whereas a FSW airplane will flutter in flight rather than undergo a quasistatic divergence. A number of papers' have shown that the flutter speed of a FSW aircraft is lower than the divergence speed of its cantilevered wing. A limited study' of a simple two-dimensional wing/fuselage with only a rigid-body degree of freedom in plunge also showed that the instability is always oscillatory for a large range of fuselage to wing mass ratio. (An earlier attempt to study the divergence characteristics of an unrestrained FSW configuration by a quasistatic stability analysis was unsuccessful, but indicated the necessity of a dynamic stability analysis.) A number of other papers show that an oblique-wing aircraft does not diverge either, but has a flutter speed higher than the divergence speed of its cantilevered FSW component. Weisshaar has compared the oblique wing and FSW situations: An important conclusion of these (oblique wing) studies was that the inclusion of the freedom of the oblique aircraft to roll as a rigid body during aeroelastic oscillation enhances the stability of the system. In particular, the aeroelastic divergence mode of instability associated with cantilevered forward-swept wings disappears and is replaced by a low-frequency, oscillatory instability that occurs at a higher flight speed than does divergence. (This is unlike the freely flying FSW, which experiences body freedom flutter at an airspeed lower than its divergence speed.) The inclusion of pitch and plunge rigid body freedoms further modifies the low-frequency instability speed, but not to the extent seen with the inclusion of rigid body roll freedom. The main purpose for this Comment, however, is to criticize the use of aerodynamic strip theory. The assumption of strip theory is, of course, a sine qua non for a closed-form solution. The assumption has no compensating errors in a static aeroelastic problem, in contrast to the flutter problem where it sometimes enjoys some compensation (An even number of mistakes), viz., a conservative aerodynamic stiffness may offset an unconservative aerodynamic damping, depending on