- Mon Nov 07, 2011 4:32 pm
There is SO much that can be said about this topic, and so little time and space for it. :-) First of all, thanks Steve for forwarding my note about the possibility of the damage being initiated by landing loads. I was at a marching band parade my daughter was in when I read the initial note, and had to reply with my phone, then yesterday was spent flying (woo hoo!), but now I have a keyboard to respond myself.
Like Steve said, I really did do even worse damage to my Xtralite keel, which had to be removed, cut apart and repaired in the Telluride LZ, following a landing after an aero round. I suppose that event could have been seen as an aero failure had it been a dingle dangle instead of a kingpost hang system, but when the kingpost gets torn off it's easy to know just when it happens. The Xtralites always had an Xtra-long keel too, so a really solid flare always ended up with the keel banging the ground first, and since my harness kept me from reaching my feet to the ground with the glider stood up vertically on its keel, I'd end up losing grip on the downtubes sometimes, or breaking the keel if I held on to the downtubes too firmly when the keel struck the ground. Most sites were nice enough to have soft ground for the keel to bury itself into, enough for my feet to reach the ground. Eventually, I cut 8 inches off the length of the keel and that solved the problem completely.
But seeing that photo of the damage to the keel from the dingle dangle bending back just didn't jive with what I would expect from an aero induced load. I always try to look at things as numbers, if I can. I have a couple of G meters that I have used now and then to get an idea of what loads I pull in loops. For me, 4 G's is pretty standard for most of my loops. I have done some nasty hard pullups by accident, way back when, and saw as high as 10.5 G's (no s***), complete with face bashing on the basetube and the complete inability to hold the basetube anywhere normal, it was yanked out to full arm extension. Long time ago, but that set the upper extreme for any loops I have done. I've seen as high as 6 in fairly hard, unusually hard pull ups, but still manageable loops. So I figure it's not unreasonable to assume people that do lousy loops, with too much or too strong of a pull up would see 6-7 G's. If you use that number, multiply the pilot weight by that, say if the pilot is hooking in at 200 pounds, you'd be seeing 1,400 pounds on the hang strap.
Then, consider the angle that that load would be pulling on the hang point, whether a kingpost or dingle dangle. On a nice, normal loop, the peak load happens with the bar further back, without "push-out", so like maybe just in form trim position, mid chest. If you blow one bad, do one of the scary "positive pitch divergency" style pull ups, that load would probably be peaking with the bar around face level or a bit out from that. So considering a normal peak load and an extreme peak load, and looking at the angle the hang point would be at the keel when pushed out to those degrees gives you an idea of load compared with angle of that load. I'll post some photos showing angle on my own glider, and anyone else can do he same, so we would have an idea whether that angle is sufficient to cause an in air failure like this one. I suspect not, but that's just me.
A reasonably easy onset of load in looping is only going to peak at whatever it peaks at, from 4 -6 or 7 G's usually. A landing where the pilot sticks the keel will peak a G load far in excess of 10 G's as a sudden, sharp, shock load. Plus, a nose high flare will put the hang strap at close to a parallel angle to the keel, the most advantageous angle possible for both creating a strong enough load and a right enough angle to rip the dingle dangle backwards.
Aaron Flare.JPG (385.22 KiB) Viewed 3510 times