Date: Mon, 6 Nov 1995 06:26:30 -1000 From: dlw@odi.com (Dan Weinreb) Message-Id: Organization: Object Design Inc., Burlington, MA Subject: Breaking threshold of spar material? I've seen some useful information about the rigidity/stiffness of various spar materials. There are useful tables in Eden's "Kiteworks" and in Servaas and Velthuizen's "Stunt Kites to Make & Fly." What they are measuring is how much a spar is deflected if you anchor one and and apply a force at right-angles at the other end. However, I'd also like to know something about how much force various spar materials can endure before the spar breaks. I am ignorant when it comes to materials science and mechanical engineering, so I have no idea to what degree this is related to the rigidity/stiffness mentioned above. But common-sense tells me it's not the same thing. My immediate problem is a corner kite whose spars tend to snap in relatively high winds. One of the external bracing spars is being pushed on at both ends, i.e. =====> ----------------- <===== push poor helpless push to spar caught to right in the middle left The spar bows out, and eventually during some strong gust or something, it snaps. I had been using 1/4 inch ramin dowels, and they had been snapping. So I rebuilt the kite to use 5/16 inch dowels, which are suppposed to be 2.5 times more "rigid/stiff". Nevertheless, it snapped. I know that graphite tubes are a lot more "rigid/stiff" than ramin dowels. But before I shell out ten bucks or so to replace the four dowels, I'd like to know if they are also a lot stronger in the sense of being less likely to snap under this kind of pressure. By the way, despite all the disappointing broken wood, I'm having a great time building and trying out kites! Thanks. = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Wed, 8 Nov 1995 20:24:54 -1000 From: andrew@tug.com (Andrew Beattie) Message-Id: Organization: /usr/lib/news/organisation Subject: Re: Breaking threshold of spar material? dlw@odi.com writes: > =====> ----------------- <===== > push poor helpless push > to spar caught to > right in the middle left > >The spar bows out, and eventually during some strong gust or >something, it snaps. I don't have the answer for you, but you're looking at a *difficult* question. I guess that you're diagram is probably incomplete too - I bet there is a stand-off half way. It's only small, but it has a great mechanical advantage. I figure that as soon as the spar starts to bend, the geometry of the thing will make it easier (require less force on the ends) to bend further, and buckling is almost inevitable. Unfortunatley, the research that has been done (See Simo's or Dave Lord's Web pages or look it up at www.kfs.org/kites) covers forces at 90 degrees to the one that you've indicated, but it might still be useful. One thing that might help is to reduce the pressure on the standoff. Andrew -- New to rec.kites? START HERE! | To: www@kfs.org send an email message like this-> | Subject: service /-\ () >< () |\/| () >< () /-\ | http://www.kfs.org/kites/welcome Never *ever* cross-post from rec.kites to alt.folklore = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Fri, 10 Nov 1995 06:17:02 -1000 From: dlw@odi.com (Dan Weinreb) Message-Id: Organization: Object Design Inc., Burlington, MA Subject: Re: Breaking threshold of spar material? In article andrew@tug.com (Andrew Beattie) writes: I don't have the answer for you, but you're looking at a *difficult* question. I guess that you're diagram is probably incomplete too - I bet there is a stand-off half way. No, there isn't. This isn't a spreader on a delta stunt kite. It'a an external spar on a single-line corner/facet kite. It's a wooden dowel, pushed into a little lengths of vinyl tubing at each end, with nothing touching the middle. One thing I've since realized is that the more interesting facet kites, which have additional, smaller sails, would suffer from this problem less (or not at all), because the small sails attach along the length of the spar and would help prevent buckling. I'm in the middle of making one of these now, so I'll be trying it out soon. Thanks! = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =