A winglet 3 has an inner end 5 and an outer 7. The winglet 3 has a varying radius of curvature (R) which:(i) decreases along the winglet over a first distanced1; (ii) remains constant over a second distanced2; and (iii) increases along the winglet 3 over a third distance d3. The sum of the first and third distances (d1+d3) is greater than the second distance (d2). The radius of curvature may vary according to the equation R=k1/dn. The parameter n may be equal to 1, such that the curvature follows an Euler spiral.