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
