An experimental study of aerodynamic characteristics of an optimum horizontal axis windmill is described. The windmill, rated at 20KW at 8m/s with a two bladed-rotor of 14m diameter, is designed so as to vary the geometry of the blade in such a way that the aerodynamic efficiency becomes maximum. The combined blade element-momentum theory is used as an analytical tool. To check the design method and get some useful aerodynamic data, a windtunnel test of a 1/7th scale model (2m diameter) is performed in NAL low-speed tunnel, whose test section is 5.5m by 6.5m. Two models, whose blades have the same optimum chord distribution but have different planforms, are tested. Measurements are made of the efficiency, torque, axial drag force and initial torque for various combinations of the pitch angle and the tip speed ratio. The yaw characteristics of the windmill are also measured. The test shows that the model windmills operate in almost the same manner as the design method predicts, at least, for the design conditions. 資料番号: NALTR0698000 レポート番号: NAL TR-698