Effect of rain on gliders in flight

Geoff Vincent wrote:

My experience with wet wings (DG-300 and PIK 20B) when winch launching
is that the droplets disappear from the wing surface long before you
enter rotation and to all intents and purposes the launch is normal.
Both above-mentioned gliders had a well polished surface finish
though.

The British Gliding Association advice is never to winch launch with wet
wings. From memory of Steve Longland's book (effectively part II of the
BGA Instructor's Manual), your minimum winch speed is calculated as 1.5
x Vs (free flight), to allow a safe margin over the increased stall
speed on the winch caused by the weight of cable + pull of winch. If wet
wings increase the stall speed by, say, 15% (for my Open Cirrus it's
15-20%), the safe winch launch window is much smaller.

For my aircraft the normal launch window is 90-110 kph (50-59 kt). With
wet wings, the free flight stall speed rises from 60kph to 69kph (say 70
for ease of calculation), and 1.5 x this is 105kph. Max winch speed
remains the same, so I now have a safe window of only 5 kph (3 kt)!

It might be that the rain in the Southern hemisphere runs off wings
quicker, but I'm not taking chances on that if I get the opportunity to
fly there!