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Note that for a wing rigid in bending we have — 0, and thus zD — which is the exact solution for pure torsional divergence. Also, for a torsionally rigid wing we have xD — 0, and thus Pd — —19/3, which is very close to —6.3297, the exact solution for bending divergence. For the cases in between the error is quite small. It is very important to note that the sign of r is driven by the sign of e, whereas the sign of p is driven by the sign of A. The approximate solution in Eq. (3.86) is plotted along with some of the exact solution branches in Fig. 3.19. Note the excellent agreement between the straight line approximation and the exact solution near the origin. Also note that the intersections of the solution with the xD axis (where fii> — 0) coincide with the squares of the roots previously obtained in Section 3.2.2, Eq. (3.62), as (2n - 1)2tt2/4 for n = 1, 2, ..., oo (i.e., tt2/4, 9tt2/4, ...).

A somewhat more convenient way of depicting the behavior of the divergence dynamic pressure is to plot xD versus a parameter that depends only on the configuration. This can be accomplished by introducing the dimensionless parameter r, given by

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