Abstract: Duct propeller can increase the propulsive efficiency. Comparing to the metal ducts, the composite duct can effectively reduce the weight of duct propeller, which is critical for the use of duct propeller. Through the designing of ring longitudinal and ring ribs, the disadvantage of composite duct's stiffness could be overcome. Based on the ANSYS APDL optimization design theory, two best duct structures have been given with the objective functions of dry and wet modal respectively. Accordingly, the best structure has been present. Meanwhile, the similarities and differences of dry and wet modal features, including the nature frequency and mode shape. The results show that the first nature frequency of wet modal is about one quarter of the dry's value, while the mode shapes are almost the same. The designing of composite duct and the optimization method have certain directive meaning to the engineering design.

Key words: optimization design, composite duct, wet mode