Figure 3.24 Normalized divergence dynamic pressure for an elastically coupled, swept wing with GJ/EI = 0.2ande/e = 0.02 ;k = -0.4 (dots and dashes),/.- = 0 (solid lines), k = 0.4 (dashed lines).

Again, for positive <?, divergence is possible only if —90° < A < A^. Thus, because of the presence of k as an additional design parameter, the designer can at least partially compensate for the destabilizing effect of forward sweep by appropriately choosing k < 0, which for an increment of upward bending of the wing provides an increment of nose-down twisting. There is a limit to how much coupling can be achieved, however, as typically, \k\ < 0.6.

There are two main differences in isotropic wing design versus design with composite wings. First, it is possible to achieve a much wider range of values for GJ/EI. Second, and significantly more powerful, is the fact that composite wings can be designed with nonzero values of k. From Eq. (3.109), the value of A^ is decreased as k is decreased, which means that the range of A over which divergence occurs is decreased. To confirm this and our earlier statement about positive k being destabilizing, Fig. 3.24 shows results for k — —0.4, 0, and 0.4. It is clear that one can sweep a composite wing forward and still avoid divergence with a proper choice (i.e., a sufficiently large and negative value) of k. Since forward sweep has advantages for the design of highly maneuverable aircraft, this is a result of practical importance. The sweep angles at which divergence becomes impossible, Aco, are also somewhat sensitive to GJ/EI and e/t as shown in Figs. 3.25 and 3.26. Evidently, one may design divergence-free, forward-swept wings with larger sweep angles by decreasing torsional stiffness relative to bending stiffness and by decreasing e/t.

Figure 3.25 Sweep angle for which divergence dynamic pressure is infinite for a wing with GJ/EI = 0.5; solid line is for e/t = 0.01; dashed line is for e/t = 0.04.

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