Heliograph

[RVDan]

Originally Posted by DanH

Just to clarify, the canopy is believed to focus the sun's energy (a) like a parabolic reflector (sunshine reflects off the inside of the canopy to a focal point), or (b) by refraction (sunshine passes through the plexiglass at an oblique angle, probably twice, to a focal point, much like a magnifying glass)?

"B". Not believed, but empirical data as it has been observed many times as the sun is refracted by the canopy.

[RKellogg]

Originally Posted by DanH

Just to clarify, the canopy is believed to focus the sun's energy (a) like a parabolic reflector (sunshine reflects off the inside of the canopy to a focal point), or (b) by refraction (sunshine passes through the plexiglass at an oblique angle, probably twice, to a focal point, much like a magnifying glass)?

Based on personal experience with a Sport Criuser, the interior surface of the open canopy reflects and focuses the sun's rays in a concentrated spot that melts/burns the top of the glareshield. Only occurs with tip-up canopy OPEN and tail toward the sun. Sun has to be at relatively high angle, IIRC, at least in the Sport Cruiser geometry. Happens in the blink of an eye. Pick your combustables carefully...

With regard to the question of reflection vs. Refraction... the relatively low angles of incidence leads me to believe that the phenominon is based on reflection rather than refraction. Refraction at shallow angles dissipates so much energy within the plexi. I have not done any ray tracing to verify this.

Roger

[DavidBurton]

I was sitting in my truck having lunch one day and noticed smoke rising from the console. A partially filled plastic bottle of water refracting the sun's rays to a point was the cause...

[flyboy1963]

.....so my starboard nav light lens catches the sunlight just right, & melted the wingtip plexi light cover.
.....can't really park differently, so just replaced the cover, and put some green tape over the nav lite, at least until I need it for night flying! ( not happening this century!)

darn parabolic geometry eh?

[Steve Barnes]

I found two parallel deep burnt gouges in the upholstery of one of the seats in my RV6. One was about 4" long and the other a little shorter. Had to reupholster the seat.

Steve

[airguy]

I always said I wasn't going to be one of those guys that flies my airplane somewhere, parks it, and then covers it up with a cover - but I just ordered a cover.