Larry writes:
"The simplest way to determine correct empty CG location would be to use a bit of reverse engineering. Build the airplane complete to the firewall and then determine the desired location of the engine that will give you the most aft CG location when considering arm and weight of all expected loading. Make sure you stay in range with the extreme opposite loading also. " Alas not so simple Larry. It is indeed possible to move the engine and relocate the empty CG such that the CG of the loaded aeroplane is more or less wherever you desire. But that does not address the issue of the where exactly the CG limits lie (forward and aft) that were initially designed into the aeroplane on paper by Mssrs Rand and Robinson. Said CG limits would have then been verified and/or refined by subsequent flight testing of their specific example (the prototype). And as I mentioned, any modification of the forward fuselage for an alternative powerplant or moving the engine forward are destabilizing and you cannot know their effect on the static margin without analysis and testing . "The plans, at least my set from 1990, list only the CG range with no regard to weight." The assumption therefore is that there is no critical loading situation below MAUW for the prototype. They state the "structural design" is rated at 7 G's at 800 pounds". I stand corrected - thank you. The only information that I have for the KR-2 is a little yellow pamphlet put out by RR in the '70's/'80's which specifies gross weight of the KR-2 as 900lb and a stress analysis done in the UK by Mike Whittaker (who is an aeronautical engineer by profession) which indicates the same. "The 900 pound gross, to my knowledge, is a limit imposed by the authorities in the U.K. or somewhere in Europe." Again, I bow to your expertise as I do not have a copy of the KR-2 plans. Mike's 1994 stress analysis covered the major structural components of the KR2S. I visited Mike at his home and he was kind enough to provide me with a copy, given that I had been asked to carry out a flight evaluation of the type (which never transpired). In the introduction to the document Mike notes that the KR2 aircraft was modified by increasing wing span, lengthening the fuselage, increased empennage area and a MAUW increase from 900 to 1100 pounds. Mike's 'hand' stress analysis is extremely thorough and very well documented with all of his calculations. In the conclusion Mike writes that "The strength of the aircraft was found to be marginal in many areas." Now before all of you KR2(S) owners burst into a frenzy of email recriminations please recall that Mike's analysis was just that - a stress analysis carried out on paper by hand calculation. Mike did not carry out any static testing (although he specified a loading regime by which this could be achieved). Furthermore, the composite construction technique of a fibreglass sheet skin over one side of a foam core (as opposed to wing skins) does not lend itself to simple structural analysis. So yes, I am sure builders have carried a couple of big blokes, lots of fuel, luggage and a Jersey cow in their KR2 aircraft and happily flown from A to B behind a 1835cc VW engine. "95% of my flights are made at 1150 pounds with the CG at the center of the range. " Is this the CG range specified in the original plans or have you established the CG range for your particular aeroplane? Perhaps you could share the method by which you established sensible forward and aft limits so that other KR-2 builders/flyers can apply a similar methodology? To conclude, I reiterate the point about it being a good investment in time researching the matter of safely and sensibly investigating the operating CG limits of any given version of the aeroplane and then religiously adhering to them. Nga mihi Kiwi
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