Canopy causes cockpit fire

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

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

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 :

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.

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

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

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