Canopy causes cockpit fire

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?

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 !

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 !

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

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

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.

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

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

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

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