1. An aircraft wing and a winglet on the tip thereof, the winglet comprising:an inner end and an outer end, and
the winglet having a varying radius of curvature (R) extending from the inner end to the outer end, and wherein the radius of curvature of the winglet:
(i) decreases along the winglet over a first distance d1 from the inner end to a first location along the winglet; wherein over the first distance d1 the radius of curvature varies according to an equation R=k1/dn, where k1 is a constant, d is a distance along the winglet measured in an outward direction, and n is greater than zero;
(ii) remains constant over a second distance d2 from the first location to a second location along the winglet; and
(iii) increases along the winglet over a third distance d3 from the second location to a third location along the winglet, wherein the sum of the first and third distances (d1+d3) is greater than the second distance (d2).