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The punch-test gives an approximation of the tensile strength of the
fabric. It has nothing to do with how long the fabric will last.
New Grade 'A' cotton ('standard' weight; about 6oz per square yard)
must have a minimum tensile strength of 80 pounds, equal for both warp
& weft. When applied to a wing or control surface, once the strength
degrades by 30% the fabric should be replaced. (The test is normally
done to the upper surface; that which has the most exposure to
sunlight.)
For comparison, new 2.7oz/yd polyester (ie, Dacron) has a tensile
strength of about 85 pounds, 3.7 oz about 130, although those numbers
may vary slightly from one manufacturer to another. When warp & weft
are not equal you must use the lower strength for your calculations.
Other than fiberglas, all fabrics used for aircraft covering are
subject to degradation. And even WITH fibergals (ie, 'Razorback') the
airframe UNDER the fabric requires periodic maintenance, making claims
of 'Lifetime' coverings a bit iffy :-)
The fact a 20 year old covering measures high-green on a strength test
is no guarantee the thing won't fall apart next week. There could be
corrosion, dry-rot or rampant rusting going on under that slick,
always-hangared fabric skin.
-R.S.Hoover
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Re: Punch test?
