This paper investigates the aerodynamic characteristics of a winglet design for a Fair-child Merlin III 8-seat, twin turboprop aircraft. The winglet was designed by the aircraft owner to improve the aesthetics of the aeroplane. To ensure that the winglet will not have any negative effect on the aircraft, a series of comparative CFD simulations was carried out. Confidence in the results was gained by comparison with accepted computational and empirical methods. Good agreement was achieved. The solutions were found to be mesh dependent due to hardware restrictions, but converging with decreasing mesh size. The results for the winglet showed a maximum 1.3% increase in L/D of the wing. A strong leading edge vortex created by the high sweep of the winglet severely limited the performance gains. Based on the findings for the original winglet, a new, improved design was tested. It featured less sweep and twice the surface area, with a NACA 6-series aerofoil. This new design improved the cruise L/D of the wing by 5%, which is a threefold improvement over the original winglet. At the same time, the skin friction drag was found to be similar, despite the large increase in area.