Effect of rain on gliders in flight

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!

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

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

Bob,

This is WAY off subject; however, your old Ka-6E
(http://www.soaridaho.com/photogallery/valley/Ka-6E.jpg) is sitting in its'
trailer in a hangar here in Idaho. I sold it to the current owner a little
over 10 years ago. He has never flown it and seems to have no desire to
sale it.

Wayne
HP-14 "6F"
http://www.soaridaho.com/

"Bob Gibbons" wrote in message
...
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

On May 1, 9:29*am, "Wayne Paul" wrote:
Bob,

This is WAY off subject; however, your old Ka-6E
(http://www.soaridaho.com/photogallery/valley/Ka-6E.jpg) is sitting in its'
trailer in a hangar here in Idaho. *I sold it to the current owner a little
over 10 years ago. *He has never flown it and seems to have no desire to
sale it.

Wayne
HP-14 "6F"http://www.soaridaho.com/

"Bob Gibbons" wrote in message

...



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- Hide quoted text -

- Show quoted text -

That is really sad. A Ka-6 would make some beginning glider pilot very
happy. I sure do love mine....

Pete