This paper describes measurements of the root bending moment on a three-bladed 500 W horizontal-axis wind turbine, which were obtained using a purpose-built rotating data acquisition system. The main aim was to investigate the moment when the turbine was yawing in response to wind direction changes, particularly when the yaw rate was large. This situation usually causes the largest loads on operating small turbines. At sufficiently high yaw rates, the moment receives a significant contribution from the cyclic Coriolis acceleration of the rotating and yawing blades. It is shown that maximum of the Coriolis moment provides an accurate bound to the measured cyclic moment and that the remaining "average" moment is largely due to the thrust on the blade. However, further investigation is necessary to quantify the contributions to the average moment. Some of the consequences of the dominance of the Coriolis term for the assessment of blade safety are discussed.
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