An analysis is given for determining the lift fluctuation on an aerofoil due to a gust parallel to the undisturbed flow. By combining the new analysis with an established theory for “transverse” gusts perpendicular to the undisturbed flow, it is possible to obtain the fluctuating lift on a rotating fan or compressor blade moving through a flow disturbance. It is shown that simple design rules may be derived for fans and compressors if the fluctuating lift forces, and possibly therefore the associated noise, are to be reduced.