Via Boing Boing:
From an engineering standpoint, we find two major limitations on achieving human flight modeled after birds. These are the ability to translate energy into thrust using these flapping wings and then the structural limitations. The purpose of the flapping wing on a bird, insect, or ornithopter is to create thrust. The body, after being thrusted forward, can sustain flight by simple aerodynamic manipulation through wing shape, the way it has been done for decades. The flapping motion moves air, imparting momentum on the body creating this thrusting force. This means that we would have to design wings that could flap at speeds that would be able to sustain the thrust force to keep the aircraft airborne while being structurally sound and as light as possible, a task easier said than done. Then comes the issue of hovering, which would require the wings to create a significant upward force equaling the weight of the aircraft. Following that, the structure of these wings must be considered. Aeronautical engineers have nearly perfected the design of fixed wings structure, but with a flapping wing, the wing is no longer fixed and is in a constant state of stress. There are generally two categories of stress, compression and tension, the pulling and pushing of material. With a flapping wing, the material will experience both these stresses on top of the other stresses associated with flowing air. This state of constant, high magnitude stress, leads to a very short fatigue life for the structure.
I always found the idea of the ‘thopter in the Dune novels needlessly unique, because it didn’t seem possible and the physics of the world the people inhabited in the novels were not different from ours. In the de Laurentis movie, they’re just boxy, luxe spacecraft. But a reader of the novel learns of a different machine altogether, which flaps and whomps and makes its own brand of noise.
Nevertheless, humans have in fact built flapping flying contraptions.