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Old June 28th 19, 02:48 AM posted to rec.aviation.homebuilt
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Default Sailplane wing joint

On 6/27/2019 10:50 AM, Sky Surfer wrote:
Bob, did you have problems with the Zuni's spar end pins fatiguing or
cracking because of cyclic loads? That's a critique of spar end pins,
according to Marske's manual.

No problems of which I was aware in 2,000+ hours of flight in the ship. I know
the late Stan Hall - who actually DID use his AE degree to do structural
design on airplanes/sailplanes his entire working life - wasn't a fan of
spar-end pins for the reason you note, as well as the fact it introduced point
mounting loading issues into the spar end...bearing loads induced by spar
bending loads where the buried bits of the pin mount in the spar end
structure. Stan was seriously conservative in his structural-design thinking
IMO...which is not a criticism!
- - - - - -

The particular HP-14 I have in mind has the bottom of the two long knuckle
pins passing through an anchor tab which is just forward of the bottom
knuckles. That tab is welded to to the top tube of a steel-aluminum
bulkhead which makes up the front wall of the landing gear box.

Ah. My ship didn't have such a tab.
- - - - - -

The knuckle pins have cross-drilled holes at their ends. After the pins
are pushed rear-to-front through the knuckles and rotated so the small
holes line up vertically, a long 1/16" wire is pushed down through both
holes (just forward of the bottom anchor tab) to retain the knuckle pins.

After I wrote Dick Schreder about the absence of any safety pins on the main
pins of my ship (asking if the builder had merely ignored construction advice)
was when he replied with his thinking on the absence fore/aft forces requiring
them, though he suggested if I felt differently I could simply drill holes in
the ends of each and "do the safety-pin thing." Being both curious and lazy, I
opted before each flight for marking with a laundry marker at the interface
between each pin and the forward-outer face of top/bottom knuckles (to detect
fine movement) and a wrap of vinyl tape about 1/4" from the knuckle (to get a
handle on gross movement should my laundry mark disappear); never noticed any
movement in ~200 flight hours.

Bob W.

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