Your dead in a 20 G crash any way so what dose it matter? I fabricated a bulkhead behind the main spar and anchored a cable from the middle of the bulkhead to the tailwheel reinforcement area. The bulkhead serves many purposes for my KR including a rear hinge reinforcement. I will be dead if that breaks away.
KRron ----- Original Message ----- From: "Mark Langford" <n5...@hiwaay.net> To: "KRnet" <kr...@mylist.net> Sent: Wednesday, June 23, 2004 8:51 PM Subject: KR> seat belts and spar strength (dead horse alert!) > NetHeads, > > There has been a lot of talk about seat belt brackets, bolt diameters, etc > lately. I just ordered my seat belts today from Hooker Harness, and have > been forced to think about attachment in order to come up with the proper > belt lengths. The thing that concerns me most is the strength of the aft > spar itself. If you think about laying the thing out flat between two > sawhorses with a 32" space between them, I'll bet a lot of you would have > second thoughts about merely STANDING on the middle of it, much less jumping > up and down on it. I think the standard for seat belts is something like > 20g's, so if you weigh 200 pounds, and have a 200 pound passenger with you > (half of both would be 200 at the center), imagine 4000 pounds out there in > the middle of that spar (laid flat). Do you think it would hold it? I'll > bet you big money it won't. > > I stopped by my stress guy's office today, and talked him into working out > the details for me (I'm more than just a little rusty). We assumed a 1" x > 1" piece of spruce with 4000 pounds (pilot and passenger) of force out in > the middle of that 32" span, neglecting the upper cap, and the vertical > members and plywood connecting them to the "subject" lower cap (somewhat > conservative). The number we calculated was 192,000 psi applied to that > spar cap's spruce material in a 20g crash. The modulus of rupture in static > bending is 9600 psi for aircraft grade spruce. So my apprehension of > walking on that spar would be justified. Theoretically, 200 pounds would > break that 1x1, and that passes my "common sense" test. > > Larry's idea of spanning the two caps with a bracket is a good one, since it > calls both caps into play. Using that bracket to span the caps, and if > you're the optimistic type and assume that the shoulder belt and lap belt > will play equal parts bearing the load, and you have no passenger, then > there's only 25 pounds (200/8) acting out in the middle of each cap, so the > spar would handle something closer to an 8g crash. But I seriously doubt > the shoulder belts do as much work as the lap belts do, so we're probably > back to 6g's again, and that's with no passenger. > > My point is that although bolt diameter and bracket material are important > factors, you also need to make sure the spar itself can handle it. > Obviously the load of the seat belts needs to be shared with something other > than the aft spar if you are preparing for a 20g crash. The two best ways I > can think of are a compression member connecting the main spar to the aft > spar (connected to both caps of each), or a cable connected to something > like the tailwheel block. I know the cable thing will stir up the usual > arguments, but I think that's exactly what I'm going to do...run cables > from both center shoulder and center lap belt attachment points back to the > something substantial in the tail (like the tailwheel block). > > I know we've all heard of KR's torn to pieces in a crash, and the guy walks > away with the rear spar belted to his butt, but those are not the sort of > full frontal crash that this 20g standard is based on. You say you're not > going to worry about 20g frontal impacts because the chance of one is > unlikely? I don't blame you. They are. But then why worry about bolt > diameter or brace strength for a 20g crash if the spar's not going to take > it anyway? And I'm not saying the cable strap is a 20g solution either, but > that's what it'll take to make me comfortable, with minimal weight gain. > > Just thought I'd throw that out there. There are lots of ways to work this > problem, and you're welcome to work it the way you want to. I need to get > back to work if I'm going to fly to the Gathering. I just wanted to bring > it up... > > Mark Langford, Huntsville, Alabama > N56ML "at" hiwaay.net > see KR2S project at http://home.hiwaay.net/~langford > > > > > > _______________________________________ > to UNsubscribe from KRnet, send a message to krnet-le...@mylist.net > please see other KRnet info at http://www.krnet.org/info.html > >
