Abstract: In an axial flow fan impeller, the natural frequency of vibration of the individual blades is increased by mounting each blade to the annular support rim or hub for swinging movement relative to the ring about an axis, which lies generally perpendicular to a radius passing through the mounting. For a given blade, the mounting enables the fan to be rotated at higher speeds with a corresponding increase in performance. The mounting also permits the use of thinner, lighter-weight blades for a given operating speed and this permits a reduction in the cross sectional area of the rim. If the impeller includes a mechanism for controlling the pitch of all the blades during operation, the use of lighter-weight blades reduces the stresses on the mechanism so that the design and strength of the latter does not tend to be a limiting factor on the maximum permissible speed for the impeller.