Foils with serrations

A foil having a leading edge and a trailing edge, of which at least a portion of one or both of the leading edge and trailing edge has a serrated profile comprising a plurality of adjoining teeth, each tooth having a tip point that represents a local maximum chord-wise extent of the tooth, a root point, on each side span-wise of the tip point, that represents a local minimum chord-wise extent of the tooth and at which the tooth adjoins a respective adjacent tooth so that the root point is shared by the tooth and the respective adjacent tooth, and a tooth edge, on each side span-wise of the tip point, between the tip point and an adjacent root point,

• wherein the profile of the tooth edge between the tip point and an adjacent root point is subject to a limitation defined by relative chord-wise positions of the tooth edge at first, second, third and fourth intermediate positions located respectively at 5%, 35%, 65% and 95% of a span-wise distance from the tip point to the root point, a chord-wise distance between the second and third intermediate positions being less than the chord-wise distance between the first and second intermediate positions and the chord-wise distance between the second and third intermediate positions being less than the chord-wise distance between the third and fourth intermediate positions, and
• wherein the profile of the tooth edge between the tip point and the adjacent root point is subject to a further limitation in that a tangent to the tooth edge is not perpendicular to a chord-wise direction at any location on the tooth edge between the first intermediate position and the fourth intermediate position.