Canopy causes cockpit fire

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

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

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

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.

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

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.

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

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