Many gliders as well as having positive flap settings for slow speed flight and landing can also put their flaps negative to -5 or -10 degrees settings to reduce drag and improve the L/D at higher speeds - but in doing so increase the frontal area... It's more a case of having an appropriate camber for the speed at which the aircraft is being flown for small flap settings either plus or minus. Larger positive settings do add drag disproportionately to deliberately worsen the L/D.