The recent series of posts concerning Powder Coating for Wing Attach Fittings
(WAF) have assumed that the 3/16 bolts have to fit snug in the steel WAF and
the Spruce spars since the bolts are stressed in sheer. I read an explanation
some years ago in the KR newsletters that the WAF are clamped to the spars and
generate their holding action by friction. The 3/16 attach bolts are stressed
primarily in tension not in sheer.
Another newsletter article reported on KR-2 main spar testing. The ultimate
breaking force on the spar was 21g. (I don't know how hard folks fly their
KRs, but 6g would be much more than enough to break parts of me.) One surprise
was: The point of failure was about 12 inches inboard of the WAF and not at the
WAF. After the test the WAFs were dismantled and examined for sheer damage.
There was no evidence of any sheer stress on the eight WAF nor the AN3 and AN6
bolts. The report did not say there was not any sheer force, just no evidence.
I guess they did not find any dents in the bolts.
Bear with me for a little real world analysis. I assume a 4.2g load.
The tensile strength for AN bolts is 125,000 psi.
AN3 bolts are (3/16 x ½) x (3/16 x ½) x 3.14 = 0.0276 sq in
125,000 x 0.0276 = 3450 pound yield. The usual recommended bolt tension is 1/3
the yield rating.
We don't want to crush the lumber, so only snug up to a fifth of that or 690
pounds.
There are eight AN3 bolts per WAF or 8 x 690 = 5520 pounds pressure on each
side of the spar.
Assume the coefficient of friction to be 0.8 and the force needed to move the
WAF along the spar would be:
0.8 x 5520 = 4416 pounds or 8832 pounds for the WAF pair.
For the 4.2g loading on a 900 pound gross weight KR-2 the WAF forces would be:
The total wing load would be 4.2 x 900 = 3780 pounds. The center section
contributes 1/3 the lift.
The outer wing panels share the remainder or 1260 pounds each.
Assume that load is concentrated 36 inches outboard of the WAF AN6 bolts.
For 6.5 inches vertical separation between WAF AN6 bolt pairs, the moment would
generate:
36/6.5 x 1260 = 6978 pounds directed inboard along the top set of WAFs.
My conclusion: There would be minimal sheer load on the AN bolts for KR WAFs.
Sid Wood
Tri-gear KR-2 N6242
Mechanicsville, MD USA
sidney.w...@l-3com.com
