Canopy causes cockpit fire

[Bob C]

Yesterday afternoon Mark Mocho (MM Fabrication) opened
the canopy of his Pegasus to get his cell phone. By
the time he made a call to me, his soaring hat, which
was lying atop the glare shield, was smoking. The
canopy was acting like a big magnifying glass and focusing
the sun's rays onto a quarter-size spot just above
the instrument panel. Scary to think what would have
happened if he'd walked away for a few minutes.

Photos are at http://www.silentwingsairshows.com/canopy.html
Notice the burn marks on the instrument panel from
previous solar encounters. One shot is of Mark (no
that's NOT Jack Nicholson) duplicating the sun angle
with a flashlight.

I'd heard of this phenomenon before, but couldn't figure
out exactly how it happened. Seems it's not the light
coming through the canopy, but the light reflected
from the inner surface (which is roughly parabolic
in shape), that causes the problem. At a shallow grazing
angle, enough light is reflected to get smokin' hot.
Nice trick to know if you ever get ants in your cockpit
;o)

Bob

[Stephen Haley]

Just take a look at the backseat headrest of most newer K21s - They are all
pitted with burn marks from the magnifying affects of the canopy.
rgds
Stephen

"Bob C" > wrote in message
...
> Yesterday afternoon Mark Mocho (MM Fabrication) opened
> the canopy of his Pegasus to get his cell phone. By
> the time he made a call to me, his soaring hat, which
> was lying atop the glare shield, was smoking. The
> canopy was acting like a big magnifying glass and focusing
> the sun's rays onto a quarter-size spot just above
> the instrument panel. Scary to think what would have
> happened if he'd walked away for a few minutes.
>
> Photos are at http://www.silentwingsairshows.com/canopy.html
> Notice the burn marks on the instrument panel from
> previous solar encounters. One shot is of Mark (no
> that's NOT Jack Nicholson) duplicating the sun angle
> with a flashlight.
>
> I'd heard of this phenomenon before, but couldn't figure
> out exactly how it happened. Seems it's not the light
> coming through the canopy, but the light reflected
> from the inner surface (which is roughly parabolic
> in shape), that causes the problem. At a shallow grazing
> angle, enough light is reflected to get smokin' hot.
> Nice trick to know if you ever get ants in your cockpit
> ;o)
>
> Bob
>
>
>

[JJ Sinclair]

>
>I'd heard of this phenomenon before, but couldn't figure
>out exactly how it happened.

I believe it occurs when the sun angle is parralel with the top of the open
canopy and the magnification is caused by rays passing through the plexiglass
at almost a zero angle. The fire is started on any object that is close to the
focus point, ie. instrument panels, head rest on K-21 or DG-300. I know of an
ASH-25 that was *totaled* after a cockpit fire caused by this phenomenon.
JJ Sinclair

[Mark Navarre]

>> Photos are at http://www.silentwingsairshows.com/canopy.html

what is that, THREE moving map displays on the panel? And what are those wires
coiled around the hat band of the bucket hat-some sort of induction device that
is part of an experimental head-up display?
There is secret experimental nav testing going on here and the cat is out of
the bag....spill the details!!


ps: how does that Ball GC work for you? I sold one to a Russia pilot about five
years ago after I could not make it work with GPS input.

-


-
Mark Navarre
2/5 black ace
LoCal, USA
remove brain to reply
-

[Ray Lovinggood]

I've seen it happen on an ASW-24. The 24 was oriented
with the tail towards the southwest (northern hemisphere,
in North Carolina, USA). The canopy was fully open
and pointed towards the sun and we smelled plastic.
It was melting a portion of the trim around the glare
shield. We then moved the canopy up and down and watched
the bright white spot form on the glare shield when
the canopy was in the 'sweet spot.' It looked just
like it looks when using a magnifying glass out in
the sun when burning paper or ants.

Since my LS-1d has the removable canopy rather than
a hinged one, I guess I don't have to worry about it.
:-)

Ray Lovinggood
Carrboro, North Carolina, USA

At 13:54 04 May 2004, Jj Sinclair wrote:
>>
>>I'd heard of this phenomenon before, but couldn't figure
>>out exactly how it happened.
>
>I believe it occurs when the sun angle is parralel
>with the top of the open
>canopy and the magnification is caused by rays passing
>through the plexiglass
>at almost a zero angle. The fire is started on any
>object that is close to the
>focus point, ie. instrument panels, head rest on K-21
>or DG-300. I know of an
>ASH-25 that was *totaled* after a cockpit fire caused
>by this phenomenon.
>JJ Sinclair
>

[Kizuno]

And thus the checklist for my ASH-26E states "tail NOT pointed at sun".

Last season, the canopy reflected sun burned holes in my little PZL compass on
top of my instrument panel cover. It gets your attention real fast to whiff a
bit of that petroleum based liquid and think you have a fuel leak!

Kemp
9J

>I'd heard of this phenomenon before, but couldn't figure
>out exactly how it happened. Seems it's not the light
>coming through the canopy, but the light reflected
>from the inner surface (which is roughly parabolic
>in shape), that causes the problem. At a shallow grazing
>angle, enough light is reflected to get smokin' hot.
> Nice trick to know if you ever get ants in your cockpit
> ;o)
>
>Bob

[Mark Zivley]

I melted the aft facing edge of the GPS antenna that was mounted on the
shroud that covered the instrument panel on my LS-1F. I think it's not
the light passing through the canopy, but reflecting off the inside
surface when the open canopy is facing the sun. My glider was tail
pointed to the east and this was in the morning sun. Fortunately the
design of the LS-1 is such that it's unlikely that anything would find
it's way into the focal point of the beam in most cases.

Bob C wrote:
> Yesterday afternoon Mark Mocho (MM Fabrication) opened
> the canopy of his Pegasus to get his cell phone. By
> the time he made a call to me, his soaring hat, which
> was lying atop the glare shield, was smoking. The
> canopy was acting like a big magnifying glass and focusing
> the sun's rays onto a quarter-size spot just above
> the instrument panel. Scary to think what would have
> happened if he'd walked away for a few minutes.
>
> Photos are at http://www.silentwingsairshows.com/canopy.html
> Notice the burn marks on the instrument panel from
> previous solar encounters. One shot is of Mark (no
> that's NOT Jack Nicholson) duplicating the sun angle
> with a flashlight.
>
> I'd heard of this phenomenon before, but couldn't figure
> out exactly how it happened. Seems it's not the light
> coming through the canopy, but the light reflected
> from the inner surface (which is roughly parabolic
> in shape), that causes the problem. At a shallow grazing
> angle, enough light is reflected to get smokin' hot.
> Nice trick to know if you ever get ants in your cockpit
> ;o)
>
> Bob
>
>
>

[Greg Arnold]

I had the great idea of putting felt on the glare shield of my LS-3 to
hold down reflections. After burning a couple holes in it, I removed it.



Mark Zivley wrote:

> I melted the aft facing edge of the GPS antenna that was mounted on the
> shroud that covered the instrument panel on my LS-1F. I think it's not
> the light passing through the canopy, but reflecting off the inside
> surface when the open canopy is facing the sun. My glider was tail
> pointed to the east and this was in the morning sun. Fortunately the
> design of the LS-1 is such that it's unlikely that anything would find
> it's way into the focal point of the beam in most cases.
>
> Bob C wrote:
>
>> Yesterday afternoon Mark Mocho (MM Fabrication) opened
>> the canopy of his Pegasus to get his cell phone. By
>> the time he made a call to me, his soaring hat, which
>> was lying atop the glare shield, was smoking. The
>> canopy was acting like a big magnifying glass and focusing
>> the sun's rays onto a quarter-size spot just above
>> the instrument panel. Scary to think what would have
>> happened if he'd walked away for a few minutes.
>>
>> Photos are at http://www.silentwingsairshows.com/canopy.html
>> Notice the burn marks on the instrument panel from
>> previous solar encounters. One shot is of Mark (no
>> that's NOT Jack Nicholson) duplicating the sun angle
>> with a flashlight.
>>
>> I'd heard of this phenomenon before, but couldn't figure
>> out exactly how it happened. Seems it's not the light
>> coming through the canopy, but the light reflected
>> from the inner surface (which is roughly parabolic
>> in shape), that causes the problem. At a shallow grazing
>> angle, enough light is reflected to get smokin' hot.
>> Nice trick to know if you ever get ants in your cockpit
>> ;o)
>>
>> Bob
>>
>>
>>
>

[Bob Salvo]

>I believe it occurs when the sun angle is parralel with the top of the open
>canopy and the magnification is caused by rays passing through the plexiglass
>at almost a zero angle.

I've had it happen twice over the many years. I found that it happens when the
open canopy top is almost parallel to the sun rays, which are reflected off the
bottom surface and are focussed somewhere near the instrument panel. I don't
believe it happens if the sunrays pass through the canopy, since the reflected
rays are significantly lower in strength, having had two surfaces to pass
through. Maybe someone would like to experiment?

Bob

[Mike Borgelt]

On 04 May 2004 13:56:28 GMT, (Mark Navarre)
wrote:

>>> Photos are at http://www.silentwingsairshows.com/canopy.html
>
>what is that, THREE moving map displays on the panel? And what are those wires
>coiled around the hat band of the bucket hat-some sort of induction device that
>is part of an experimental head-up display?
>There is secret experimental nav testing going on here and the cat is out of
>the bag....spill the details!!


It is the experimental thought controlled instrument interface -
obviously.

Mike Borgelt

[Finbar]

I think I can see how this would happen with the canopy open, and the
sunlight hitting it from "inside," reflecting off the canopy surface
onto the focal point of the almost-parabolic canopy. I'm still
holding onto the idea, though, that this can't happen in flight
because the sun can't hit the canopy from the inside. The thought of
an in-flight cockpit fire is really not comforting. Has anyone had
solar/canopy - related charring happen in flight?

[Robert Ehrlich]

Finbar wrote:
>
> I think I can see how this would happen with the canopy open, and the
> sunlight hitting it from "inside," reflecting off the canopy surface
> onto the focal point of the almost-parabolic canopy. I'm still
> holding onto the idea, though, that this can't happen in flight
> because the sun can't hit the canopy from the inside. The thought of
> an in-flight cockpit fire is really not comforting. Has anyone had
> solar/canopy - related charring happen in flight?

I completely agree, focusing solar light can only occur by reflection,
by transmission, as the canopy has parallel inner and outer surfaces, it
cannot change the direction of light rays, only slightly offset them.
Hey, this is just why we can see through it as if it were not there !

[Bob C]

If you look at the photo link in my original post,
there is a photo of Mark duplicating the hot spot with
a flashlight.

http://www.silentwingsairshows.com/canopy.html

I tried it with the jet Silent this morning, and sure
as s**t when the canopy is open and the tail is pointed
toward a low sun, it gets smokin' hot just up near
the rudder pedals.

Bob C.

At 18:30 05 May 2004, Robert Ehrlich wrote:
>Finbar wrote:
>>
>> I think I can see how this would happen with the canopy
>>open, and the
>> sunlight hitting it from 'inside,' reflecting off
>>the canopy surface
>> onto the focal point of the almost-parabolic canopy.
>> I'm still
>> holding onto the idea, though, that this can't happen
>>in flight
>> because the sun can't hit the canopy from the inside.
>> The thought of
>> an in-flight cockpit fire is really not comforting.
>> Has anyone had
>> solar/canopy - related charring happen in flight?
>
>I completely agree, focusing solar light can only occur
>by reflection,
>by transmission, as the canopy has parallel inner and
>outer surfaces, it
>cannot change the direction of light rays, only slightly
>offset them.
>Hey, this is just why we can see through it as if it
>were not there !
>

[Eric Greenwell]

Willy VINKEN wrote:
ar/canopy - related charring happen in flight?
>>
>>I completely agree, focusing solar light can only occur by reflection,
>>by transmission, as the canopy has parallel inner and outer surfaces, it
>>cannot change the direction of light rays, only slightly offset them.
>>Hey, this is just why we can see through it as if it were not there !
>
>
> I don't agree.
> I've seen it happen at EBSH on the rear headrest of a Ka21 with open
> canopy. The sunrays incidence was almost tangential to the uprised
> dome and definitly crossed the plexiglass from the outside.
> Even if outer and inner surfaces are parallel (which has not been
> proved), near-tangential rays have to cross more material where they
> hit the vertex of the curved dome, which acts as a biconvex lens,
> with in this case, a bright focal point on top of the headrest.
> I don't believe in reflection-related damage. The sun would have to
> be too low on the horizon for this to be possible with open canopies
> and due to the see-through qualities of plexiglass, at best only a
> very small part of the energy would be reflected.

Believe it! Numerous people, including myself, have carried out the
experiment. Schleicher gliders, in particular, raise the canopy high
enough that it can easily happen during the soaring hours. I always
check the sun position before raising the canopy on my ASH 26 E (as I
also did on my ASW 20) to avoid any more burns on the black cloth I have
on the glare shield. Even the manual warns about it.

> The transmission-type damage I witnessed could not happen in flight
> either: the sun would also have to be too low -thus weak-, even the
> slightest movement of the glider would prevent the same spot to be
> focused continuously, and above all, the lens-effect of closed
> canopies would focus well above anything in the cockpit.

I am not aware of any transmission-related damage, and have never seen
any through-the-canopy focusing with the gliders I've had.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Bert Willing]

You should get an optics textbook. The surfaces ARE parallel due to the
manufacturing process, and as Robert stated, if there were optical
distortion, you wouldn't be able to look through properly.

On an untreated surface, you have about 25-30% of the incident light which
is reflected.

--
Bert Willing

ASW20 "TW"


"Willy VINKEN" > a écrit dans le message de
...
> On Wed, 05 May 2004 18:15:06 +0000, Robert Ehrlich
> > wrote:
>
> >Finbar wrote:
> >>
> >> I think I can see how this would happen with the canopy open, and the
> >> sunlight hitting it from "inside," reflecting off the canopy surface
> >> onto the focal point of the almost-parabolic canopy. I'm still
> >> holding onto the idea, though, that this can't happen in flight
> >> because the sun can't hit the canopy from the inside. The thought of
> >> an in-flight cockpit fire is really not comforting. Has anyone had
> >> solar/canopy - related charring happen in flight?
> >
> >I completely agree, focusing solar light can only occur by reflection,
> >by transmission, as the canopy has parallel inner and outer surfaces, it
> >cannot change the direction of light rays, only slightly offset them.
> >Hey, this is just why we can see through it as if it were not there !
>
> I don't agree.
> I've seen it happen at EBSH on the rear headrest of a Ka21 with open
> canopy. The sunrays incidence was almost tangential to the uprised
> dome and definitly crossed the plexiglass from the outside.
> Even if outer and inner surfaces are parallel (which has not been
> proved), near-tangential rays have to cross more material where they
> hit the vertex of the curved dome, which acts as a biconvex lens,
> with in this case, a bright focal point on top of the headrest.
> I don't believe in reflection-related damage. The sun would have to
> be too low on the horizon for this to be possible with open canopies
> and due to the see-through qualities of plexiglass, at best only a
> very small part of the energy would be reflected.
> The transmission-type damage I witnessed could not happen in flight
> either: the sun would also have to be too low -thus weak-, even the
> slightest movement of the glider would prevent the same spot to be
> focused continuously, and above all, the lens-effect of closed
> canopies would focus well above anything in the cockpit.
> Willy

[Robert Ehrlich]

Willy VINKEN wrote:
> ...
> I don't believe in reflection-related damage. The sun would have to
> be too low on the horizon for this to be possible with open canopies
> and due to the see-through qualities of plexiglass, at best only a
> very small part of the energy would be reflected.
> ...


I just tested it a few days ago with the ASH25 of a couple of friends.
It was waiting for launch in the tow line, the rear canopy was open and
the sun clearly was being reflected and focused on the head rest. I put
my hand at this place in order to feel how hot it was and had to withdraw
it after after a few seconds. And in this season the sun in not already
at its maximum bright.

[BllFs6]

Just an optical tidbit guys....


The amount of light reflected from such surfaces is usually only gonna be 5 to
10 percent...not much....and not enough to cause a problem

HOWEVER, if a light "ray" intercepts a surface at a VERY shallow angle, you can
get VERY high percentage rate reflections...80/90 percent or more....ie the
light ray is "grazing" the surface....focus THAT much light on a small area and
you can very definitly cause problems...as a matter of fact thats how space
telescopes focus xrays (think about it...how else would you focus xrays :)

Try it with a piece of glass or plastic or a canopy or a car windshield...

Put the old eyeball close to the surface , looking almost parallel the surface
and you will see stuff in the distance reflected VERY brightly.....almost as if
the surface was a mirror...

So, Im pretty sure that both the shape of the inside of the canopy and the fact
the sun is "grazing" some area of the inside surface is whats causing the
problem....

take care

Blll

[Greg Arnold]

BllFs6 wrote:

> Just an optical tidbit guys....
>
>
> The amount of light reflected from such surfaces is usually only gonna be 5 to
> 10 percent...not much....and not enough to cause a problem

5% of the sun's energy is a big problem


>
> HOWEVER, if a light "ray" intercepts a surface at a VERY shallow angle, you can
> get VERY high percentage rate reflections...80/90 percent or more....ie the
> light ray is "grazing" the surface....focus THAT much light on a small area and
> you can very definitly cause problems...as a matter of fact thats how space
> telescopes focus xrays (think about it...how else would you focus xrays :)
>
> Try it with a piece of glass or plastic or a canopy or a car windshield...
>
> Put the old eyeball close to the surface , looking almost parallel the surface
> and you will see stuff in the distance reflected VERY brightly.....almost as if
> the surface was a mirror...
>
> So, Im pretty sure that both the shape of the inside of the canopy and the fact
> the sun is "grazing" some area of the inside surface is whats causing the
> problem....
>
> take care
>
> Blll

[BllFs6]

>5% of the sun's energy is a big problem
>

Do the cals and get back to me....

And dont say the but the canopy is HUGE....because size in this case doesnt
matter

Within reason, the ONLY parameter that makes a difference in melting/burning
something using the sun and a lens or a mirror is the F ratio.....ie the focal
length of the lens/mirror divided by effective diameter....

And to get stuff hot enough that number needs to be around 2...give or take
....and that assumes a very high transmission/reflection number....drop that
number to 5 percent and no real problem...unless your target is a dark
chocolate bar....

Now bring that number back up to MOSTLY transmitting or reflecting and
poof.....hence worrying about grazing reflections....

Now, you may not believe me....but the "hotness" of the "burn spot" doesnt
really matter (to first order) whether I have a lens/mirror 2 inches across or
60 inches across....only the f ratio matters...

take care

Blll

[Greg Arnold]

BllFs6 wrote:

>>5% of the sun's energy is a big problem
>>
>
>
> Do the cals and get back to me....
>
> And dont say the but the canopy is HUGE....because size in this case doesnt
> matter

Hmm, I wonder why astronomers use those huge telescopes. Or why a huge
array of mirrors is used for attempts to harness the sun's energy.


>
> Within reason, the ONLY parameter that makes a difference in melting/burning
> something using the sun and a lens or a mirror is the F ratio.....ie the focal
> length of the lens/mirror divided by effective diameter....
>
> And to get stuff hot enough that number needs to be around 2...give or take
> ...and that assumes a very high transmission/reflection number....drop that
> number to 5 percent and no real problem...unless your target is a dark
> chocolate bar....
>
> Now bring that number back up to MOSTLY transmitting or reflecting and
> poof.....hence worrying about grazing reflections....
>
> Now, you may not believe me....but the "hotness" of the "burn spot" doesnt
> really matter (to first order) whether I have a lens/mirror 2 inches across or
> 60 inches across....only the f ratio matters...
>
> take care
>
> Blll

[Greg Arnold]

BllFs6 wrote:

>>5% of the sun's energy is a big problem
>>
>
>
> Do the cals and get back to me....
>
> And dont say the but the canopy is HUGE....because size in this case doesnt
> matter
>
> Within reason, the ONLY parameter that makes a difference in melting/burning
> something using the sun and a lens or a mirror is the F ratio.....ie the focal
> length of the lens/mirror divided by effective diameter....

The focal ratio of my canopy is about 1 when it is reflecting light onto
my glare shield


>
> And to get stuff hot enough that number needs to be around 2...give or take
> ...and that assumes a very high transmission/reflection number....drop that
> number to 5 percent and no real problem...unless your target is a dark
> chocolate bar....
>
> Now bring that number back up to MOSTLY transmitting or reflecting and
> poof.....hence worrying about grazing reflections....
>
> Now, you may not believe me....but the "hotness" of the "burn spot" doesnt
> really matter (to first order) whether I have a lens/mirror 2 inches across or
> 60 inches across....only the f ratio matters...
>
> take care
>
> Blll

[BllFs6]

>The focal ratio of my canopy is about 1 when it is reflecting light onto
>my glare shield

okay,

thats a number I can play with...

whats the focal length or effective diameter?

take care

Blll

[BllFs6]

>Hmm, I wonder why astronomers use those huge telescopes. Or why a huge
>array of mirrors is used for attempts to harness the sun's energy.

THATS about signal to noise ratios and image scale and total energy collected
and detecting FAINTER stuff....a totally seperate issue...trust me, ive been
playing with optics and astronomy for 20 years....

power density (how hot the hot spot gets) is pretty much a function the f
ratio, how bright the source is...in this case a constant....the sun...and the
target is black, white or in between....

here's the scoop....

two lenses.....same f ratio....one 3 times the size of the other...so larger
one has 3 times as long a focal length...

Both form an IMAGE of the sun (it isnt a point, its a disk....)

The longer/bigger lens has an image 3 times as large as the small one, so its
image has 3^2 or 9 times the area.....but the larger lens ALSO collects 3^2 or
nine times the total energy....so the power density (or how bright the focused
sun is) is the SAME for both lenses...

And as an aside....thats why photography lens are so concerned with F
ratios...because thats what determines what the exposure time will be.....the
focal length of the lens ONLY determines the image scale/effective
magnification....

If you half your f ratio..ie go from f2 to f1 youve increased the power density
a factor of 4 times...ie same amount of energy in an image half the size AND
1/4th the area....

So, if you want to compare 5 percent reflection to 80 percent say.....80/5 =
16.....sqrt 16 equals 4....so an F4 system at 80 percent equals an F1 system at
5 percent "burn/melt hazard wise"....

take care

Blll

[Robert Ehrlich]

Greg Arnold wrote:
> ...
> Hmm, I wonder why astronomers use those huge telescopes. Or why a huge
> array of mirrors is used for attempts to harness the sun's energy.
> ...

Astronomers use huge telescopes because they are primarily concerned
by resolution and resolution is better with big mirrors.

Attempts to harness the sun's energy are primarily concerned by the
amount of energy collected rather than by its concentration. The amount
of energy collected is proportional to the surface of the mirror(s).
The concentration, i.e. ratio of energy per surface unit on the target,
is proportional to the (square of the) focal ratio. This is because the
energy collected is proportional to the collecting surface, i.e. to
the square of the mirror's radius, and the surface on which it is
collected is the image of the sun, this surface is proportional to
the square of its radius, itself proportional to the focal length.

[BllFs6]

Okay!

The experimental results are in!

Drum roll please....... :)

I happened to have an F4 mirror handy.....4 inch diameter, 16 inch focal
length, reflectivity 90 percent or so....

Remember I said an F4 80 percenter was equal to an F1 5 percenter? And someone
here noted that their canopy was roughly an F1 system?

Well, the sun is out bright and clear today....

I had 4 targets, some thick black garden plastic (like a very heavy duty
garbage bag material), some black plastic about a millimeter thick, an old
black neoprene mouse pad between and 1/8 and a 1/4 inch thick, and a BONE dry
paper thin piece of wood loaded with sap....

The mirror had no trouble melting the 2 plastics...but a fire seemed
impossible/improbable, though there as plenty of smoke.....and note that
thicker plastics probably wouldnt even smoke....but would probably
melt/distort some....

with a little care I got the wood to ignite, but a similiar piece a 1/4 inch
thick was a no go no matter what I tried, but again a nice amount of smoke....

Now, to me the mousepad seems like the most realisitic substitute target...

When I focused the mirror on that it smoked quite nicely...but trying my best
with all kinds of variations an actual fire just didnt seem possible....

Now, if you increased the power density a factor of 16 I have little doubt it
woulda burst into flames...which is what would happen if an equivalent F1
system at 5 percent suddenly was to operate at a grazing angle and reflect 80
percent or so.....

So, I guess the big question would be HOW much above 5 percent reflection or
how much faster than F1 would be required to start a fire?

Note that this mirror was DESIGNED to focus properly, while I doubt a canopy is
anywhere near as accurate an optical surface (regardless of its percent
reflectivitiy), which would bring down the power density significantly...and to
just accidently get a canopy faster than F1 (ie F ratio less than 1)..that is
ALSO the right shape optically would be pretty improbable....

So, I'll retract my statement some....

If your worried about a FIRE (which was what I was thinking about when I first
posted).....AND you have DO NOT have grazing reflections...your probably
okay....

If you dont want stuff melting and smoking...your gonna have to be a bit more
careful!

Best bet? Be prudent and FACE away (perpendicular?) from the sun (or whatever
the owners manual says to do)....

Again, I guess my main point that got me into this discussion in the first
place was that the grazing angle and its resulting very high reflectivities
could be a MAJOR factor that some folks might NOT be aware of...and hence since
they never had problems at non grazing angles they might not realize the
dangers (fires OR just melting stuff) of what happens when they DO happen to be
in grazing angle conditions....

Anybody want to buy a slightly melted mousepad? :)

take care

Blll

[Robert Ehrlich]

Todd Pattist wrote:
>
> Robert Ehrlich > wrote:
>
> >Astronomers use huge telescopes because they are primarily concerned
> >by resolution and resolution is better with big mirrors.
>
> I used to work on mirrors larger than 50" looking at the
> satellites of a country whose satellites were looking at
> mine. Trust me (I hate it when someone writes that :-) -
> almost all the large mirrors astronomers use were built to
> gather light, not improve resolution. Over a period of
> about a second, atmospheric motion produces image wander
> that limits resolution to about the same resolution as a
> 6-12" diameter telescope. Any exposure longer than a few
> 100 milliseconds is blurred out to the resolution limit of
> the isoplanatic patch. All the rest of the diameter used to
> be to there to just gather light. The big scopes were
> called "light buckets" as they just served to gather more
> photons into the blurry patches set by the atmospheric
> resolution limit. The "light bucket" ability to gather lots
> of photons from dim objects is still a major reason for
> large diameter.
>
> In the last 20 years, active image correction has been
> developed that can eliminate much of the image wander and
> recover the lost resolution, even on the older scopes.
> Space telescopes don't suffer from this resolution loss and
> they get both the higher resolution and the higher light
> gathering power of a big scope mirror.
>
> Todd Pattist - "WH" Ventus C
> (Remove DONTSPAMME from address to email reply.)

Yes, I agree that the very huge mirrors used are primarily
designed for collecting more energy. What I was meaning is that
the resolution is the motivation for using diameters larger
that what you find in e.g. terrestrial binoculars.

Anyway resolution had also its motivation for bulding big
instruments in the previous century. IIRC I read in a book
from the famous astronomer Eddington, written in the mid thirties,
that Michelson built an interferometer (which is essentially a very big
mirror reduced to two pieces of its border) with which he was able to
directly determine the diameter of the star Betelgeuse. In the
same book it is mentionned that the companion of the star Sirius
was discovered by its gravitational perturbation on Sirius before
it could be optically observed, which became later possible with
instruments with better resolution.

[Pete Russell]

Well I am no rocket scientist like some of you guys. But this lowly
instrument maker knows that over the last 8 years or so I have seen and
replaced about 15-20 dial faces that had obvious burn marks from the Induced
Direct Infrared Objective Thermal System, (IDIOTS) for short. One was to the
point that the inside of the instrument was obscured from the smoke residue.

Pete
Sage Variometers

[Martin Gregorie]

On 06 May 2004 19:16:45 GMT, (BllFs6) wrote:

>Okay!
>
>The experimental results are in!
>
>Drum roll please....... :)
>
>I happened to have an F4 mirror handy.....4 inch diameter, 16 inch focal
>length, reflectivity 90 percent or so....
>
>Remember I said an F4 80 percenter was equal to an F1 5 percenter? And someone
>here noted that their canopy was roughly an F1 system?
>
>Well, the sun is out bright and clear today....
>
>I had 4 targets, some thick black garden plastic (like a very heavy duty
>garbage bag material), some black plastic about a millimeter thick, an old
>black neoprene mouse pad between and 1/8 and a 1/4 inch thick, and a BONE dry
>paper thin piece of wood loaded with sap....
>
>The mirror had no trouble melting the 2 plastics...but a fire seemed
>impossible/improbable, though there as plenty of smoke.....and note that
>thicker plastics probably wouldnt even smoke....but would probably
>melt/distort some....
>
>with a little care I got the wood to ignite, but a similiar piece a 1/4 inch
>thick was a no go no matter what I tried, but again a nice amount of smoke....
>
>Now, to me the mousepad seems like the most realisitic substitute target...
>
>When I focused the mirror on that it smoked quite nicely...but trying my best
>with all kinds of variations an actual fire just didnt seem possible....
>
>Now, if you increased the power density a factor of 16 I have little doubt it
>woulda burst into flames...which is what would happen if an equivalent F1
>system at 5 percent suddenly was to operate at a grazing angle and reflect 80
>percent or so.....
>
>So, I guess the big question would be HOW much above 5 percent reflection or
>how much faster than F1 would be required to start a fire?
>
>Note that this mirror was DESIGNED to focus properly, while I doubt a canopy is
>anywhere near as accurate an optical surface (regardless of its percent
>reflectivitiy), which would bring down the power density significantly...and to
>just accidently get a canopy faster than F1 (ie F ratio less than 1)..that is
>ALSO the right shape optically would be pretty improbable....
>
>So, I'll retract my statement some....
>
>If your worried about a FIRE (which was what I was thinking about when I first
>posted).....AND you have DO NOT have grazing reflections...your probably
>okay....
>
>If you dont want stuff melting and smoking...your gonna have to be a bit more
>careful!
>
>Best bet? Be prudent and FACE away (perpendicular?) from the sun (or whatever
>the owners manual says to do)....
>
>Again, I guess my main point that got me into this discussion in the first
>place was that the grazing angle and its resulting very high reflectivities
>could be a MAJOR factor that some folks might NOT be aware of...and hence since
>they never had problems at non grazing angles they might not realize the
>dangers (fires OR just melting stuff) of what happens when they DO happen to be
>in grazing angle conditions....
>
>Anybody want to buy a slightly melted mousepad? :)
>

An interesting set of experiments. Thanks for the report.

I'd like to add another thought: at my club we are taught to leave
canopies closed whenever the glider is unoccupied and unattended. If
you leave the glider for 5 minutes you still close the canopy. The
reasoning is to prevent canopy damage rather than to stop fires, but
from your observations I suspect that the closed canopy discipline
(and better - with the cover on if its sunny) will also prevent
solar-started burns and fires.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

[Robert Ehrlich]

Martin Gregorie wrote:
> ...
> I'd like to add another thought: at my club we are taught to leave
> canopies closed whenever the glider is unoccupied and unattended. If
> you leave the glider for 5 minutes you still close the canopy. The
> reasoning is to prevent canopy damage rather than to stop fires, but
> from your observations I suspect that the closed canopy discipline
> (and better - with the cover on if its sunny) will also prevent
> solar-started burns and fires.
> ...

The same rule is in effect in my club. Furthermore, each glider has
a canopy cover which stays in the glider when not used, so if you leave
the glider for some time, you close the canopy and cover it.

[JJ Sinclair]

>so if you leave
>the glider for some time, you close the canopy and cover it.

In the cockpit fire that I know about, the soft cotton canopy cover was placed
over the instrument panels to keep them cool and the canopies were left open.
Not a good idea to leave any combustable material on the instrument panel, not
even your hat. Close the canopy and put the cover on is the best rule.
JJ Sinclair

[Eric Greenwell]

JJ Sinclair wrote:

>>so if you leave
>>the glider for some time, you close the canopy and cover it.
>
>
> In the cockpit fire that I know about, the soft cotton canopy cover was placed
> over the instrument panels to keep them cool and the canopies were left open.
> Not a good idea to leave any combustable material on the instrument panel, not
> even your hat. Close the canopy and put the cover on is the best rule.
> JJ Sinclair

Was it the cover that ignited? I use my cover, hat or towel, all white,
and they all stay cool, even with the canopy focused on them. Not so for
the black glare shield or instrument face.

Some people hang the cover over the back end of the open canopy to block
the sun's rays from hitting the cover in the first place. Works well as
long as it isn't windy.
--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[JJ Sinclair]

>Was it the cover that ignited?

Yes, The open canopy struck a sharp focus on the instrument panel and ignited
the cotton canopy cover. By the time anyone knew it was happening, both
cockpits were on fire. The scene after the fire department was through was
nothing short of appalling. There were parachutes, instruments, seat cushons,
plexiglass, canopie rails, little sliding windows and stuff floating in
cockpits that were half full of water. It was hard to look at, even for a
veteran salvage bidder, like me. I figured the ASH-25 would be a steal at 20K,
about right for 30K and too much at 40K. It went for 38K.
BTW, the owner normally placed the canopy cover over the open canopies, but
decided to cover the instrument panel because it had blown off the day before.
Canopies closed and covered is the safest way to avoid this problem.
JJ Sinclair

[Eric Greenwell]

JJ Sinclair wrote:
>>Was it the cover that ignited?
>
>
> Yes, The open canopy struck a sharp focus on the instrument panel and ignited
> the cotton canopy cover. By the time anyone knew it was happening, both
> cockpits were on fire. The scene after the fire department was through was
> nothing short of appalling. There were parachutes, instruments, seat cushons,
> plexiglass, canopie rails, little sliding windows and stuff floating in
> cockpits that were half full of water. It was hard to look at, even for a
> veteran salvage bidder, like me. I figured the ASH-25 would be a steal at 20K,
> about right for 30K and too much at 40K. It went for 38K.
> BTW, the owner normally placed the canopy cover over the open canopies, but
> decided to cover the instrument panel because it had blown off the day before.
> Canopies closed and covered is the safest way to avoid this problem.
> JJ Sinclair

I guess I'll stop "protecting" the panel with the canopy cover! Covering
the canopy isn't neccessary, since there isn't any focusing through the
canopy, but it will keep the cockpit cooler.

I usually prefer to prop open the back of the canopy with a 2" foam
block to allow air circulation, but almost any prop will do. This is on
forward-hinged canopies that have quite a bit of closing force when
almost closed, not side-hinged canopies. I prefer it because my canopy
often isn't dust-free, so I'm concerned the cover will scratch the
canopy or the the dust will get into the cover; also, lots of times the
cover is back at the trailer.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Pete Reinhart]

Well,
Friday, at the end of the flight around % 0'clock on a sunny Central Texas
day I put my canopy on the ground and my Garmin GPS down beside it for not
more tha 15 minutes.
Now I've got black spot the size of a dime ont he screen.
Don't know what it's gonna cot to get fixed.
Be careful out there!
Cheers!, Pete (sun power believer now!)

"BllFs6" > wrote in message
...
> >The focal ratio of my canopy is about 1 when it is reflecting light onto
> >my glare shield
>
> okay,
>
> thats a number I can play with...
>
> whats the focal length or effective diameter?
>
> take care
>
> Blll