"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
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