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Firestar project main spar attachment

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jindoguy(at)gmail.com
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Posted: Wed Jan 30, 2008 6:25 pm Post subject: Firestar project main spar attachment

William, I pulled out my Firestar's assembly manual and checked the Final wing rigging and lift struts section (pages 32 and 33 on my manual, dated 1985, yours could be different).

There is no mention of three holes, just a single 5/16" hole. There is an admonition to "maintain a healthy edge distance" when drilling this hole. Perhaps this was a homegrown modification that gained traction later on, I don't know. 

In general empty holes are considered a place for cracks to start in aircraft structures. Do you have any precision measuring capability? It would be good to know just how much distance there is between the edges of holes and the distance from each hole to the edge of the fitting. This area is in compression during normal flight, but in a negative G situation there is a possibility of a shear tear out when this area is loaded in tension. This would be subject to just how much material is there.

 I do not have 5 rib wing drawings, just the 7 rib. It shows the pin center to edge as 5/16" or a .156" (5/32") edge margin. 

By contrast Boeing specifies a default .35 edge margin unless otherwise shown on the drawing.

  This is one you might want to give the factory a call on and get their blessing.



Rick



P.S. The pic is of the right wing attachment on mine. - Show quoted text -

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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Thu Jan 31, 2008 7:53 am Post subject: Firestar project main spar attachment

FWIW, standard design practice is to have a MINIMUM edge distance,  

measured from the center of the hole to the nearest edge, greater than  

or equal to 1.5 x hole diameter. For a 5/16 diameter hole the closest  

edge distance from the center of the hole, according to standard  

practice, would be 15/32" (.47"). Also important is that there be no  

sharp edge, nicks etc, where stresses concentrate. Will it fail if  

slightly less than .47"? Probably not due to design safety factors  

built into "standard practice".



Thom in Buffalo

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Thom Riddle
Buffalo, NY (9G0)

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