A split winglet configured for attachment to a wing of an airplane. The split winglet may include an upper winglet smoothly extending from the wing tip above a chord plane of the wing and a ventral fin projecting below the chord plane from a lower surface of the upper winglet. The upper winglet may include a transition section which curves upward from the wing tip into a substantially planar section. Upper and lower surfaces of the upper winglet may be bounded by leading and trailing edges which are swept toward an airstream direction, parallel with the chord plane, and curve toward the airstream direction before terminating at a point distal of the wing tip. Similarly, upper and lower surfaces of the ventral fin may be bounded by leading and trailing edges which curve toward the airstream direction and terminate at a point distal of the wing tip.