Melting of door fabrics

I'm curious to know if anyone else has had the same issue. I installed fabric on the doors, and live in Connecticut. Last summer a mysterious burn mark appeared on the fabric near the rear of the left door approximately 8 inches from the bottom. I never did figure it out. Last week, during a stop in North Carolina, I left the doors open on a hot sunny day. When we returned to the plane, there was a large burn mark in the right door in almost the same spot. Our only guess is that the door is somehow focusing the sunlight to a point that gets hot enough to melt fabric. Has anyone else seen this?

Is the plane painted????

Yes, mine did the same thing! I never considered the sun was the culprit, but since you mentioned it, my friend's RV-9 did this last summer. He had the tip up canopy opened and I noticed smoke coming from his cockpit. The canopy was acting as a magnifying glass and burning a hole in the canopy seal!

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Steve Sullivan
RV-10, N781S

RV10 SS said:

Yes, mine did the same thing! I never considered the sun was the culprit, but since you mentioned it, my friend's RV-9 did this last summer. He had the tip up canopy opened and I noticed smoke coming from his cockpit. The canopy was acting as a magnifying glass and burning a hole in the canopy seal!

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We see this a lot in sailplanes. Usually, what happens is a product of reflection, not refraction.

The most common victim is the ASW19 or the Pegasus C101, which are essentially the same glider. They both have front-hinged canopies with gas springs that hold them open. With the canopy open and the sun coming in over the tail of the glider, the inner surface of the canopy forms a concave mirror that concentrates the rays of the sun. It usually results in what looks like cigarette burns on the glareshield, but it has also resulted in burns on parachutes.

A few years ago an ASH25 two-seater fell victim to this effect. As I recall, the concentrated sunlight burned through a tube to an oxygen regulator, and a fierce fire destroyed the cockpit and most of the forward fuselage. The glider was a total loss and was paid out to the tune of around $120,000. It was later rebuilt by a resourceful repair shop that made a new forward fuselage shell in molds pulled from a similar glider.

Anyhow, based on my experience, look more to reflection than refraction to prevent this. Be especially careful when your tip-up is open and the sun is coming in over the tail.

Thanks, Bob K.

Someone, (maybe Dave Saylor?) had to replace one of his rear windows a couple of years ago. The airplane was unpainted and the sun reflected from the wing onto the rear window and distorted it pretty badly.

The plane isn't painted yet, so I suppose its possible the light reflected off a metal surface. It seems with doors open and the temp around 75F it would be hard to melt a hole in the fabric from reflection. I was thinking it was focusing of the light through a window. Either way I now need to come up with some placards to hide the marks.

Aaron

Instead of placards, why not get some of the matching leather/leatherette material and make a patch pocket to cover it? You could use them to hold loose change for any toll booths you pass (g).
John

AaronG said:

The plane isn't painted yet, so I suppose its possible the light reflected off a metal surface. It seems with doors open and the temp around 75F it would be hard to melt a hole in the fabric from reflection. I was thinking it was focusing of the light through a window. Either way I now need to come up with some placards to hide the marks.

Aaron

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With the flaps in the reflex position, the aft part of the wing and flap basically form a concave shape. Could this concentrate the sunlight on the fuselage? Anyone seen this?

-Rob

AaronG said:

...It seems with doors open and the temp around 75F it would be hard to melt a hole in the fabric from reflection.

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Actually, it's pretty easy, given the size of the reflector. Consider that you can fairly easily light a piece of newspaper on fire using sunlight concentrated through a 4" magnifying glass with pi*r^2= 12.6 square inches of area. If you have a much larger reflector, say an 15"x15" window with about 225 square inches of collection area, the focus doesn't need to be nearly as precise to achieve ignition temperatures.

...I was thinking it was focusing of the light through a window. Either way I now need to come up with some placards to hide the marks.

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Constant-thickness transparencies such as airplane windows make pretty poor refractors, unless the rays are pretty close to tangential. And then you lose a ton of area, and lose a ton of energy to reflection. One way to look at it is that if it were a better lens, it would be a worse window, with much more optical distortion as you looked through it. That's why I remain convinced that this is a reflection issue, not a refraction issue.

Sailplane pilots have gotten savvy about this problem, and are careful about where the glider is pointed while the canopy is open. They avoid orienting the glider so that sunlight can reach the concave inner surface of the canopy. Anytime that happens, there is potential for sunlight concentration to ignition temperatures.

As another poster pointed out, there is also potential for light to reflect off of the wings, onto the concave surface of the window, and then get focused to the upholstery on the door. That's an added complication that calls for a bit of vigilance when the doors are open.

Thanks again, Bob K.