Hey all.. I'm new to the list, though I've been building for about a year. You may have stopped by my site already, and I've pestered quite a few of you on occasion, so some know who I am :-)
I had some thoughts on the G-limit questions, and it sounds like the discussion is heading more towards wing theory pretty soon. Constant span-wise lift distribution is not a very efficient design (though it is cheap and easy; maybe more like cheap because it's easy... i dunno). Wing structure has to be made stronger all the way out, because as you guys pointed out in other emails, the wing spar is you basic cantilevered beam problem. (Yes, not exactly that simple, but for the purpose of discussion we'll treat it as such) Supposedly, an elliptical distribution is the most efficient insofar as strength-to-weight goes. Tapered wings closely approximate this distribution, especially when washout (geometrical twist of the wing) is considered. If you look at the stock spars in a KR1, 2 or any tapered wing aircraft design, you'll note that they look pretty wimpy on the ends. Elliptical wings such as those on the P-47 Thunderbolt and the Spitfires of WWII era were pretty damned good too :-) Elliptical planform wings however are expensive and difficult to make (production-line wise) which is I think one reason we don't see them much on GA aircraft. Aerodynamically speaking, you get better roll-rates with most of your lift being generated near the root, but generally less stability (hand in hand kinda thing, but remember, the weight is distributed is the same way, but opposite sense that the lift is distributed, in effect changing your moments of inertia). It's tough to compare apples to apples with wings because so many variables go into making one. You can't just say take this with this and you get that. Any variable has a portion to do with another variable... Sounds like dependency problem to me... :-) That's what it's an optimization PROCESS instead of a recipe. I could go on and on about the different stuff, but there are many books by many great minds. Daniel P. Raymer comes to mind... Anyways back to the question at hand. G-Limits.... I have of course changed my spars, spans and wing attach locations slightly, along with making the spars deeper to accommodate a different airfoil, but I ran some FEA's which agreed with the simple calcs I did by hand. Loading for my particular setup was +/- 10G's, and that was the WAF's breaking. My through bolts would break at 11G's (which means they're about the right size... ) and the spars are going to pieces around there too... Now, that's not to say I'm gonna go test it to that... Hah... I'll make the limit about 6+, 4- to give a safety factor that I'm comfortable with. By the way, the wing isn't the only thing catastrophic that can depart the aircraft that you need to figure out the effects of G-forces on! Sigh... Too long.. Sorry guys. Matt Elder ---------------------------------------------- melder "at" infinigral.com http://kr1.infinigral.com
