Effect of rain on gliders in flight

Hi all,

Beginner student here again.

The weather during my last training session was overcast with ceiling
at 2,500 ft. Right after our last tow for the day to 2,000.....after
the release and a turn or two....it started to rain, perhaps for just
5-10 seconds. I could see the rain drops on the canopy and see them
pushed back by the relative wind.

My instructor made no mention of it and I forgot to ask about it post
landing.

What are the implications about getting rained on in flight?
Obviously, if the ceiling is 2500 and overcast, this is primarily a
"training day." How should a glider pilot react to rain? How does it
affect the control of the aircraft?

How does one fly safely in the event or threat of rain?

--Michael

Michael,

First, welcome to soaring. All of us were beginners once, and it's
great that you are asking questions. The things that come to mind with
rain:

Is there a possibility of lightning? If so, stay clear. Composite
ships are especially prone to lightning damage and I seem to remember
hearing of damage to the metal control linkages in one glider.

High performance gliders will suffer more loss of L/D in rain due to
their laminar flow airfoils -- which rain makes non-laminar flow, with
higher drag.

Will visibility stay VFR? Aside from seeing other traffic, is there
any danger of loss of spatial orientation?

A good general rule: If it feels dangerous, it probably is. At a
minimum, err on the side of believing that until you confirm
otherwise.

Martin

Rain on the wings will almost certainly increase the stall speed - by
how much depends on the particular airfoil. As a rule of thumb, older
wood & fabric or metal gliders suffer only a little degradation, early
glass (like my Open Cirrus) suffers quite a lot, newer glass suffers
something in between.

This means that if you are landing with wet wings you need a higher
approach speed to retain the same safety margin over the stall. Your
instructor will advise you (but maybe later in your training - now
you've got too much to think about just following the tow plane).

*Never* winch launch with wet wings, as this could lead to a stall and
spin off the wire. I wouldn't aerotow with anything more than mildly
damp wings either.

There is also an illusion caused by rain on the canopy that makes the
glider appear higher than it is - causing the pilot to flare higher
than normal. When you have rain on the windscreen, you should gently
fly it on; it is a surprise when you find that the height you expect
to touch the ground is not correct. Also, because you think you're
higher, you tend to fly a lower approach, so add a few feet for
obstacle clearance in rain.

There will also be a tendancy for the inside of the canopy to mist up
more, so open the vents.

There is also an illusion caused by rain on the canopy that makes the
glider appear higher than it is - causing the pilot to flare higher
than normal. When you have rain on the windscreen, you should gently
fly it on; it is a surprise when you find that the height you expect
to touch the ground is not correct. Also, because you think you're
higher, you tend to fly a lower approach, so add a few feet for
obstacle clearance in rain.

There will also be a tendancy for the inside of the canopy to mist up
more, so open the vents.


Hey !

I have never heard of this wet canopy illusion, Dan, can you expand for me?
Why does this happen? Is it just from looking through the water like when looking into a pond?

thanks
bagger

On Apr 27, 9:41*pm, bagmaker
wrote:
-There is also an illusion caused by rain on the canopy that makes the
glider appear higher than it is - causing the pilot to flare higher
than normal.

I have never heard of this wet canopy illusion...

Not one of the standard texbook visual illusions... which doesn't mean
it's not real.

The things I noticed when the dark cloud overhead opened up when I was
on downwind one day was that visibility was terrible AND the yaw
string stuck to the canopy AND there was terrific sink. I flew a very
small pattern...

DJ

Bagger, everything looks normal - until the ground isn't there when
you expect it to be; seems like you flare about 1-2 feet higher.
There is an abrupt arrival once your fully-held-off approach leaves
you will the flying qualities of a brick and too much altitude below
you. With the addition of a contaminated airfoil making stall speed
higher. carrying a couple of knots/mph/kph/furlongs per fortnight and
flying it on seems prudent.

Details mentioned at:
http://www.nappf.com/nappf_aero_medi...%20in%20Flight

From the Canadian Tansport Canada Aeronautical Information Manual
(AIM), Airmanship, Section 2.5:

"An error in vision can occur when flying in rain. The presence of
rain on the windscreen, in addition to causing poor visibility,
introduces a refraction error. This error is because of two things:
firstly, the reduced transparency of the rain-covered windscreen
causes the eye to see a horizon below the true one (because of the eye
response to the relative brightness of the upper bright part and the
lower dark part); and secondly, the shape and pattern of the ripples
formed on the windscreen, particularly on sloping ones, which cause
objects to appear lower. The error may be present as a result of one
or other of the two causes, or of both, in which case it is cumulative
and is of the order of about 5° in angle. Therefore, a hilltop or peak
1/2 NM ahead of an aircraft could appear to be approximately 260 ft
lower, (230 ft lower at 1/2 SM) than it actually is.

Pilots should remember this additional hazard when flying in
conditions of low visibility in rain and should maintain sufficient
altitude and take other precautions, as necessary, to allow for the
presence of this error. Also, pilots should ensure proper terrain
clearance during enroute flight and on final approach to landing. "

The AIM is now online, and a good reference for things like this.

Dan

On Mon, 27 Apr 2009 11:58:30 +0100, Chris Reed
wrote:


*Never* winch launch with wet wings, as this could lead to a stall and
spin off the wire. I wouldn't aerotow with anything more than mildly
damp wings either.

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. I've also aerotowed on many occasions with wet wings -
"operations normal" at 60-65 kts airspeed.

In cruising flight there is a tendency to fly fast to "blow" the wings
free of water - this tends to be counter productive and I've settled
on 60-65 kts as the optimum for my PIK 20B. I estimate about a 20%
reduction in L/D with wet wings. Simply allow for a nominal 40:1
dropping to 32:1!

Geoff Vincent
Melbourne, Australia

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!

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.

Whatever the performance issues I would agree with the rule that a winch
launch should not be undertaken with wet wings, doing so could result in
you being served by undertakers.

At 15:11 29 April 2009, Chris Reed wrote:
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!