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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. --- This email has been checked for viruses by AVG. https://www.avg.com |
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Thanks for the good and detailed information.Â* I think I'll migrate over to the homebuiltairplanes.com forums, which are quite active and have interest-specific areas like "Aircraft Design", "Composites", etc.Â* I don't know if you're there, but I see at least one of the other soaring luminaries you mentioned (BoKu, a.k.a. Bob K.) posting regularly in those forums..
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