ILLUSTRATION 4 fabric tester was made by the Langley Corporation, San Diego, California. It was calibrated for Grade A cotton fabric only and was not intended to be used as a universal measuring device for other fabrics or finishes. It probably would not be used as a field-test instrument for determining fabric strength because it was replaced by the Maule tester. The Maule fabric tester shown in illustration 4 was designed to test in-service fabric on structures causing as little damage to the surface as possible. When testing fabric the tester was placed on the fabric surface and pressure slowly applied to the tester while reading the numbers on the scale. I used to push until I read above 46 pounds for intermediate fabric and above 56 pounds for heavier weight fabric. It would work on both Grade A and synthetic fabric, and unless the fabric was below minimum requirement, would not punch a hole in the surface. Unlike the Seyboth tester, the Maule unit reads in pounds per inch along the shaft, beginning with 5 pounds and ending with 80 pounds per inch. The most accurate method to determine aircraft fabric strength is by a pull test under controlled laboratory conditions. Left is a very old pull test on my Aeronca 7AC done
by the Twining Lab in Fresno, California. Note how accurate the readings are. The minimum deteriorated fabric strength for a 7AC is 46 pounds per inch warp and fill. When testing fabric always test on the top surface in the darkest color because that is where deterioration will be the greatest. Recall Ray Stits raw fabric test; deterioration was the greatest on the top surface and less on the bottom surface. These three samples indicate the fabric is still airworthy. I recall sending Twining three samples of new raw Dacron cloth for tensile testing. The first sample was Ceconite 101 (3.6 ounce per square yard), and it pull tested more than 150 pounds per inch and a notation was made that the fabric tester read 150 pounds maximum and it failed above that amount. The second sample was Ceconite 103 (2.6 ounces per square yard and a suitable replacement for the old intermediate Grade A cotton fabric), and it pull tested 97 pounds per inch. The third sample was advertised as Dacron cloth for experimental aircraft only, and it pull tested around 75 pounds per inch as I recall. However, Twining noted that the fibers pulled apart rather than breaking. I just removed Ceconite 101 fabric covering from my Aeronca 7AC that had been in place since 1971. I intend to have a couple samples pull tested just to see what the value is after more than 38 years of service. I expect to find it still good because it had eight cross coats of silver dope applied over six coats of clear nitrate dope when I covered it back in 1971. Illustration 5 details the results of a fabric pull test under laboratory conditions on my Aeronca Champ when it was first done in 1964. When aircraft woven fabric cloth is pull tested the sample must measure 1 inch wide and 6 inches long , and the pull must be directly along the fibers, not across them. This test necessitated cutting substantial openings in the fabric’s upper surfaces in the darkest colors. These openings would generally measure 1-1/2 inches by 6-1/2 inches, giving the laboratory some excess for trimming. The samples were sent in with coatings intact. Repairing those holes took a fair amount of labor; a pigmented doped surface was the easiest, and an enamel or polyurethane surface much more difficult depending on the synthetic fabric process. Inspection of fabric surfaces consists of an examination of coatings to make sure there are no cracks that expose woven cloth fibers. Look for wrinkles, particularly at the trailing edge of wood wings that would indicate rib dam-
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