EP 413987 A UPAB: 19930928 The axial-flow fan with bladed impeller is housed in a duct in an enclosing wall (1). One or more apertures (20) are formed in the latter at the blade tip (15) region, each with a cover (21) with ports (22) at the side facing the duct. The covers pref. have fine ports. They are typically of textile fabric, or gauze. There can also be a coarse cover with a large number of holes. ADVANTAGE - No turbulent discharge flow, and stable characteristic over the entire output range. 1/6