Thanks for the responses, everyone. Really great information. -Terry
> On Jul 29, 2022, at 8:38 PM, kiwi faulkner via KRnet <krnet@list.krnet.org> > wrote: > > > "Any idea if a list of these mods are available anywhere? " > > From PFA commentary paper dated 13 Feb 98: > > MW1 Wing main spar joint plates > MW2 Wing spar > MW3 Fin and tailplane > MW4 and MW5 Details of load test (optional) > MW6 Lap strap attachments > MW7 Longeron tripler and Shoulder Harness Attachment > > "Were any aircraft tested with physical weights or were these design > analysis?" > > As stated, the proposed modifications were the result of a stress analysis. > I do not know what load testing took place on UK examples. > > "...the wing attach fittings, long considered the weakest part of the wing." > > I don't know why - although it's a pretty straightforward hand analysis to > get in the ballpark and the fittings as designed seem to be proven in > service. Mike did express concern over load peaking at the end bolts due to > the strap plates being wasted by cutting progressively larger holes between > the rows of bolts, but then in the analysis itself he noted that it was > "difficult to stress with confidence." Hence he took a conservative approach > in his modification to ensure load on the fittings was transferred gradually > with equal load on each fastener. > > "...the weakest part of the aircraft is the top skin between the spars of the > stub wing." > > This needs to be taken in context with reference to the analysis. The UK PFA > at the time correctly noted that the KR construction method (foam/glass) > whilst having demonstrated adequacy in service "does not lend itself to any > known analytical methods". Mike Whittaker notes that while his conservative > analysis with a composite safety factor of 2 looks satisfactory, " experience > has shown a large strength scatter is not uncommon with this type of > construction". > > In the analysis Mike stressed the spars conventionally, then considered > critical stress in the skins neglecting the spars. Being a professional > engineer Mike had access to datasheets which I do not, so one has to take a > leap of faith following the analysis. He also had data from a 'computer > analysis' on the standard KR2 which I am not privy to, and deduced a factor > to be applied to all loads. > > In any case in his first iteration with a simplified method all the bending > moment was taken by the main spar which was clearly not satisfactory so the > stiffening effect of the skins had to be taken into account to relieve rear > spar bending load. The torsional stiffness of the wing was analysed by > considering the front D-cell forward of the main spar and integrating twist > in 20" sections of the semi-span using shears at the mid-section of each > cell. The net result bypassing a few steps was to establish main and rear > spar stresses which he referred to as being "believable". > > The bottom line ... > > There are plenty of KR's out there that have been flying successfully for > many years. If you are able to stick to the plans and keep as close as > possible to the design weights then in all likelihood you will end up with a > good flying machine. If you are going to change engines, dimensions, MAUW, > structure and so forth, in absence of an analysis and subsequent testing, > don't be the first!!! Copy someone else who has thoroughly proven the > modification in service. Whilst a stress analysis is a bit of a chore, > establishing realistic loads to be expected in service is not and will allow > a series of static load tests which should engender some confidence that your > new machine will stay in one piece whilst airborne. Failing that, you are > truly an experimenter and test pilot ... > > Nga mihi > > Kiwi > > > -- > KRnet mailing list > KRnet@list.krnet.org <mailto:KRnet@list.krnet.org> > https://list.krnet.org/mailman/listinfo/krnet > <https://list.krnet.org/mailman/listinfo/krnet>
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