TopCat wrote:Bit late to the thread, but I'd say it was clearly the right decision. The temperature was dropping rapidly enough for the runway to start icing up and droplets of water on the wing to freeze, and as has been said, with the temperature approaching the dewpoint, goodness only knows what the visibility might have been like an hour later. Icy aircraft and runway, potentially poor vis, dark soon... I'm really not fancying it.
On another note, a question.
If frozen drops of water are bad for lift, why are liquid drops not also bad for lift if they don't run off? Because they don't always, and often if they partially do, they leave trails of smaller droplets behind that stick even more to the wing.
I realise that for takeoff, it's premature departure of the airflow over the leading edge that is most critical, but I've always felt a little uneasy when the wing is wet. I've never read anything to suggest different takeoff performance with a wet wing, though, only an icy one.
What's the obvious thing I'm missing?
Wet wings are bad - it's just that iced-up wings are even worse. It shows up most (as @PA28 says) on modern laminar flow glider wings. A wet wing increases stall speed and can dramatically increase drag - an iced up wing even more. To give an example I once got a glider iced up in a cloud climb, when I emerged at 4000ft AGL I could'nt even make it to a nearby gliding club 7 miles away whereas with dry wings in that glider I would expect to be able to glide 20 miles and still have a 1000ft left for a circuit.
Some older (Wortman) wing sections are particularly bad - more modern sections have a degree of tolerance. On the worst types (eg DG200 or PIK20) you may need to to add 20% to your approach speed with wet wings and your achieved glide angle will be around half what it should be (I used to own a DG200 and a light shower put me into a field on several occasions).
We normally would not even try and launch a modern glider with wet wings & definitely not with iced up ones. By contrast older (wooden winged) machines are pretty tolerant of wet wings - their wing sections are non-laminar or limited laminar.
So you need to understand the wing section of what you are flying, highly laminar flow section + contaminated surface can produce all sorts effects at low speed, whereas a non-laminar wing might be quite benign. So you are right to be cautious taking off or landing with wet wings