Divergence

Divergence is a static aeroelastic instability, most frequently found in the torsion response of a wing or airfoil. When divergence occurs, the steady aerodynamic forces of a static configuration overwhelm the structural restoration forces, resulting in a structural failure.

As a fixed-wing airplane flys, the steady aerodynamic loads cause its wings to twist. In normal conditions, this twist may be very small or imperceptible. Now consider the case where the twist causes the angle of attack to increase, thus causing more lift which therefore causes even more twist. This process can be stable, neutrally stable, or unstable (divergent). In the case of a stable deflection, the wing will twist to some final steady airloads and deflections. This is the normal condition of the plane. However, in the unstable case, the increase of lift and twist tends towards infinity and the wing is ripped off the fuselage.

There exists a point between stability and instability where the wing may possess arbitrary deflection. This is a point of “neutral stability” which marks the edge of the divergence zone. The dynamic pressure at this point is referred to as “divergence dynamic pressure”, and is used to restrict the plane’s flight envelope for safe operation.

It is possible to construct a vehicle which has either infinite or negative divergence dynamic pressure. In both cases, such a vehicle cannot ever encounter divergence — a very useful feature.

Analysis of divergence can be done by solving for deflections (e.g. beam torsion with torsion boundary conditions) as a function of aerodynamic load. If the expression is expressed as a single fraction, then the denominator tending towards zero is a condition for divergence (since then the deflection will tend towards infinity).