The effects of h/s (tip gap height-to-span ratio) on heat/mass transfer on the plane (flat) tip surface of a full coverage (FC) winglet have been investigated by employing the naphthalene sublimation technique along with oil film flow visualizations. For a winglet width-to-pitch ratio of w/p = 10.55%, the tip gap is widely changed to be h/s = 0.68%, 1.02%, 1.36%, 1.70% and 2.04%. The result shows that regardless of h/s, local heat/mass transfer rate on the tip surface is very high on the protruding winglet top surface at the leading edge and on the pressure side. On the other hand, there is relatively low local heat/mass transfer rate on the protruding winglet top surface on the suction side as well as in an area behind the leading edge separation bubble. The tip surface inside the baseline blade profile with the FC winglet is exposed to much less severe thermal load than the baseline plane tip surface with no winglet, whereas total thermal load on the protruding FC winglet is more severe than thermal load on the baseline plane tip surface with no winglet. (C) 2015 Elsevier Ltd. All rights reserved.