This paper is concerned with the case of a propeller working inside a symmetrical duct of finite length, zero thickness, zero camber and zero tip clearance. The propeller is modeled by the well-known Lerbs lifting-line theory while its radial circulation distribution is represented by a double Fourier series which allows for a nonzero circulation to occur at the blade tip. At the same time the duct circulation is considered to vary axially and circumferentially, which results in a system of longitudinal vortex filaments shed from each point on the duct surface. A comparison is made between the present method and the more sophisticated Tachmindji potential theory and the agreement is very good. Comparison is also made between the ducted and free-running (open) propeller.

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