Look at all the bracing in this photo (hi-res version) of a classic Stearman biplane. Without this structure, the torque produced by the wing flaps could produce a dangerous twist in the wooden wings. For more on biplane design, see the Wikipedia entry Biplane and "Structures--or Why Things Don't Fall Down," by J. E. Gordon (in print only), pp. 259-271. To see what might have been the first biplane, visit Earliest Known Flying Dinosaurs.