Effect of rain on gliders in flight

On Thu, 30 Apr 2009 20:33:38 -0500, Bob Gibbons
wrote:

On 30 Apr 2009 09:30:03 GMT, Don Johnstone
wrote:

The performance of some gliders is badly effected by wet wings. PIK 20,
Kestrel 17 and 19, Grob 103 spring to mind but there are others. The
Kestrel and PIK have a Wortman Section and the Grob an Eppler. All these
gliders suffer from very high sink with wet wings. Other gliders are not
so badly effected, the ASW17 would climb in rain. It very much depends on
the wing section as the ASW17 has a different Wortman Section.

... text deleted

It has been said a number of times before in this group, but probably
needs repeating for those new to the sport as ships like the PIK come
on the used market.
The impact of rain on the wings of the PIK is significant and easily
recognized, but based on my 2000+ hrs in a PIK20B, including multiple
500K+ flights, the effect is not dramatic. You will hear this effect
described by those with no experience in the type as "falling out of
the sky". This is NOT the case. When encountering rain in the PIK, you
can feel the separation as a low frequency vibration usually before
you can actually see the rain accumulation on the wing. As rain builds
up on the wings, the sink impact is probably about 20%, significant,
but not severe. I have continued flight many times penetrating though
rain showers. and, if the lift is strong enough to compensate for the
slight sink increase (less severe at slower climb airspeeds), the PIK
will climb perfectly well in the rain. I never noticed any effect in
landing during rain showers, though I would add 5 knots or so to my
landing speed.
That said, there is no arguing with the fact that the PIK (and others
with this airfoil) were more strongly effected by rain when compared
with the earlier non-laminar airfoil ships, and the later thin airfoil
ships. But this concern should not be a significant factor in the
decision of a pilot to consider one of these older, and often very
affordable sailplanes.
I sold my PIK over 10 years ago, but still would have no safety
concerns about flying the ship through the rain.

Bob


Well spoken Bob - exactly my own experience, albeit with only 1600 hrs
in the PIK 20B. It is rarely acknowledged, but many other early glass
ships world-wide were equipped with the same Wortman FX 67-K- 170
(150) airfoil and suffered from performance degradation when the wings
were wet:

e.g. Kestrel 17/19, Glasflugel 604, Nimbus 2, Mini-Nimbus, Janus,
Caproni Calif, Open Jantar 1& 2, Mosquito, LS-2 (only one built, but
won a world championship - Reichmann, Australia 1974), LS-3, HP-18,
Zuni and several others.

It is such a pity that the PIK 20 seems to cop all the flak when the
subject of wet wings comes up! In almost all respects it's a great
aircraft.

Geoff V
Melbourne, Australia