Thanks to everyone who has chipped in so far with advice and assistance. This is a bit long, for which my apologies, ......
Bolt to bolt hole clearance and the effect at the wing tip As far as I can see, if parallel bolts and bolt holes are used, there has to be some clearance in order to fit the bolts. Given that the two WAfs on the main spar (on my a/c at least) are 145 mm apart and the tip is 2.38 metres from the bolt holes (again, my aircraft), 0.1 mm clearance (4 thou approx) at each main spar bolt will result in about 3mm vertical movement at the tip and .8 mm (31 thou) will give close to the 25 mm (1 inch) I am seeing. Standard average location fit tolerances For comparison, an average location fit for this size bolt will have a maximum clearance of 1.5 thou and is unlikely to be achieved in practice using 3/8 drills and aircraft bolts (an AN6 3/8 inch bolt has a diameter tolerance of -1 to -3 thou and then there is the drilled hole tolerance to add in as well.) Likely wear mechanism and applied loads True that KRs have been flying for 30 years now with little evidence of WAF failure in normal service, but it's also true that everyone speaks of WAF bolt hole wear as a "need to carefully inspect" if buying a used one. I suspect that the wear mechanism is that, with the WAF bolts tightened up, the friction between the WAFs of the stub wing and its outer panel makes the fit appear to be tight when a "casual" wing waggle test is done by hand, and no tip movement is detected. However, when G loads are applied, I believe that the WAFs will move one to the other due to the necessary assembly clearance and the G force overcoming the friction, thus causing wear on the bolt and fitting. Note that a 2 G load on a 1000 lb aeroplane will be applying about 3,800 lbs across each WAF fitting on each wing (being 500 lbs vertical lift plus about 3800lbs spanwise reacting the wing bending moment.) And then there are the significant and high frequency inertial loads imposed by taxiing over uneven surfaces .... especially if you have wing tanks. Correcting the problem Fixing the wear on the standard design is not that straightforward ... you need to jig the wings into the correct position and incidence and then drill / ream out the WAF fitting to the next size up, a procedure that seems to need some cutting out and subsequent repair of the wing leading edge. And you are still left with a joint that will wear. With this in mind, unless anyone knows of an existing design, I have determined to design a taper pin and spacer system that will allow for easy assembly of the wing to the aircraft and that will accommodate any wear that does occur. As a bonus, With this system, wear is likely to be much less because the inner and outer wing WAFs will not be moving relative to each other as both tapered pins will be firmly located into their tapered holes with all clearance taken up. Recommended annual inspection One last thought ...... I would recommend that every KR owner should loosen the WAFs annually and check / measure any wing tip movement. This will indicate the presence of wear and enable the owner to monitor its progress. Your thoughts and comments would be appreciated ... Cheers Martin Martin Pearce - KR2 with Subaru EA81 + KR2 S with GMH Saturn - Both in my garage, neither registered or flying in Au ---yet! rocketdri...@optusnet.com.au -----Original Message----- From: krnet-boun...@mylist.net [mailto:krnet-boun...@mylist.net] On Behalf Of krnet-requ...@mylist.net Sent: Friday, 20 May 2011 6:34 AM To: kr...@mylist.net Subject: KRnet Digest, Vol 353, Issue 138 Send KRnet mailing list submissions to kr...@mylist.net To subscribe or unsubscribe via the World Wide Web, visit http://mylist.net/listinfo/krnet or, via email, send a message with subject or body 'help' to krnet-requ...@mylist.net You can reach the person managing the list at krnet-ow...@mylist.net When replying, please edit your Subject line so it is more specific than "Re: Contents of KRnet digest..."
