WARNING - Long Post Oscar wrote - >You can do whatever you want to the verticals in the wing spars, you >can extend the stub spars or the outer spars, you can beef up the wing >attach fittings, change the wings and wingtips- but I believe prior >analysis has shown that the weak point in the whole thing is the >connection right at the spar and the fuselage and that is where the >maximum moment will occur on a cantilevered wing. Increase the load >that the spar has to carry and you need to increase its ability to >transfer that moment to or from the fuselage.
Kinda right, kinda not. The fuselage is not supposed to, nor is it designed to carry bending loads from the wing spar. That's why the center of the spar "carries through" the fuselage. Do a simple thought experiment. Metally cut the center spar, and just glue what's on the outside of the fuselage to the fuselage side... Would it stay? No. Possibly not even under it's own weight, let alone a flight load. When analyzing a wing spar, an engineer doesn't model that spar as a cantilever beam with a fixed end at the fuselage side. Fixed implies an infinitely stiff material, which the fuse sides definitely are not. The glue joint between the fuse sides and the spar are intended to carry the lifting loads only through shear. Max bending moment still occurs in the center of the carry-through section, though there is a slight increase in stresses in the local area where the fuselage meets the wing spar due compression and a bit of added bending stresses. The fuselage sides actually move quite a bit through this whole ordeal, though not enough to be visible while you are in the cockpit pulling 5 g's. Most low or mid wing aircraft that I've seen and/or worked on have the main spar attached to the fuselage with just two bolts. Again, this allows the spar to flex and carry the bending loads on its own while the bolts carry the lifting loads (via shear). Randomly beefing up areas on a wing spar is unwise and dangerous unless you know a little bit about structures and the lift distribution. Yea, a lot of people get away with it, and no, you don't have to be an "engineer" if you have some intuition on the matter and some luck. We aren't building spacecraft or anything, but rest assured a broken spar will cause you to have a bad day. There are lots of places that a seeming innocuous change to the structure, like adding an extra layer of ply to the front or rear, can cause a SIGNIFICANT increase in local stresses to the point something may break. Another scary thing I've seen one person do is to just use a flat plate for the wing attach fitting instead of putting the various sized holes along it's length. They thought they were making it stronger, when in fact they were really compromising the spar where the attach fitting bolts on. Not good. There are a lot more things to look at too like torsion, shear, etc... that I won't get into that you have to look at when designing or modifying a spar. Sorry to make such a long post, and I don't want to sound like I'm on a soap box. I'm all about making things better and experimenting (that's what we are doing this for anyway!)... I just want to make it clear for those people that sometimes forget, that there are consequences for every design decision or alteration that is made, good or bad. Be responsible and if you really don't know what it will do, ask someone that's qualified to answer the question. There are probably a couple of them on this list that are willing to help. "Good" engineers are indeed hard to find, but it's worth the extra energy to seek one out. All that said, I think the 1 pc spar is a wonderful idea, and I wish I had built mine that way back 7 years ago. Also, my analysis shows the KR-1 wing at the gross weight I'll be flying at to be 11g's... a bit overbuilt. I'm sure the -2 wing is somewhere close to that, but I haven't run the numbers. I'll go back to lurking now -- Matt Elder Orangeburg, SC KR-1 under construction...
