Why is White Sacred?
by Bob Whelan
Editor's Note: This page is the result of a discussion on rec.aviation.homebuilt. Bob Whelan posted a summary that contained the essence of Jack's Soaring article "Why is White so Sacred?" in order to validate the credibility of the color/temperature chart. The Soaring article contains details relating to test box construction and thermometer placement, etc. I have heard that Jack's information is included in the Burt Rutan's Veri-Eze manual.
On pages 22 & 23 of the Sep, 1975 issue of "Soaring" is an article by (the late, I believe) John P. [Jack] Greene then living in New Jersey. Article title is "Why is White so Sacred?". Subtitle, "Energy absorption & color in fiberglass".
Per the article (I don't have a scanner or I'd shoot you a copy; instead I'll roughly paraphrase):
He gathered his data first-hand over the time span 1972 to 1974, collecting thousands of data points, with dozens of colored samples. Samples were prepared from polystyrene boxes measuring 7"x7"x5" (outside dimensions), with 1' thick sides/bottom. The top face of the box was removed and replaced with a colored fiberglass panel. A mercury thermometer was inserted through the side of each box with the sensing bulb attached/encapsulated to the underside of the fiberglass panel with epoxy resin.
Test samples were mounted in a simple frame with the colored surfaces aimed squarely at the sun. The frame was continuously tilted & turned to follow the sun. Ambient air/surface-temps-of-all-samples were continuously recorded until a peak was reached for existing conditions. Said procedure was repeated as often as possible to assure a statistically valid set of recorded values. "Ambient air" refers to dry bulb temps (deg F) measured in the shade. Test requirements called for a very clear sky without/o the slightest cloud formation or haze. The minutest development of high haze, hardly discernible to the eye, would immediately cause sample temps to drop and bring testing to a conclusion. Also the slightest breeze introduced an appreciable cooling factor and tests were conducted only in very calm air. The ultimate goal was to determine the highest skin surface temp a colored sailplane might experience when parked under a blazing sun with no cloud cover, no shade, not the slightest breeze, and a very high ambient.
"After two years of testing, significant data was sorted out & plotted on the curve sheet to develop temp rise curves for each color and for black and white. Referring to these curves, note that the baseline represents ambient air and the vertical scale represents maximum sample temperature. As might be expected, the curves are bounded on the top by black and on the bottom by white. These finished curves are simply a graphical presentation of the highest temps recorded for each color sheet on a broad ambient temp range from 30 degrees to 110 degrees."
"Now what does all this mean?...The curves clearly indicate a black sailplane could achieve a surface temp of 115 to 120 degrees above ambient. for example, on a day with temps of 90-95 in the shade, it's conceivable the skin surface of a black sailplane could reach the temp of boiling water..."
"The curve sheet also indicates an all-white sailplane could attain a peak temp of 45 to 50 degrees above ambient, about 70 degrees lower than the corresponding figure for black."
Brown isn't too different from black.
Colors like red & green should be avoided if moderately high surface temps are objectionable.
Orange and tan are near the middle, and orange has the property of being very visible.
The coolest colors are pink, yellow and light blue, along with all the pastel shades. Note the position of the aluminum sample.
To get an approximation of peak temp possibly expected for a specific color on a glass sailplane: select the color & determine the max ambient expected (for that area). On the curve baseline, find the ambient and move upward vertically to the appropriate color line, then move horizontally to read peak (surface) temp on the vertical scale.
End of paraphrasing/excerpting...
As I recall from decades-old gleanings from "Soaring" magazine, Jack Greene:
a) was an engineer in real life (don't remember what field);
b) was a homebuilder (HP-18 and others???); and,
c) had an engineer's grasp of structures and V-n diagrams, etc.
My opinion is the data plotted on the Color Curve Sheet was sensibly and meticulously gathered, and is usable engineering data. For what it is worth, my own observations of the man are through the eyes of a non-practicing aerospace engineer (1972), and glider pilot/nut.