Joe is exactly right on this count. The plywood on the spar is called "Sheer Webbing" as it's loading is all in sheer with the top spar cap attempting to move one direction and the bottom spar cap in another direction. In the case of the center spar, the spar caps want to bow in flight and the sheer webbing keeps them from bowing. There are some good pictures of sheer webbing failure in a KR in the middle of the page at <http://www.krnet.org/mvn2008/>. This was a not so well built, weathered KR carcass that was bent to failure of the spar. It took an incredible amount of force to cause it to fail, but the pictures of the webbing bucking are IMHO, very valuable as a learning tool.
I have holes through the sheer webbing in the spar in my KR to accommodate routing of fuel lines and electrical components. At 1200# gross and 900 hours flight time there are no signs of distress in the spar. Round holes through the sheer webbing are best as they don't leave a stress riser at the corners. -Jeff Scott Los Alamos, NM ----- Original Message ----- From: joe.kr2s.buil...@juno.com Sent: 02/09/12 08:54 AM To: kr...@mylist.net Subject: Fw: KR> Drilling through the spar phill,I am not an engineer but have some real life experience with this idea. (construction & structural steel) Typically a small hole in the centerline of the web is not an issue. The futher the hole needs to be up or down from center determines if it would need reinforcement. Also the distance from the end of the span is a factor. I think that I have a picture somewere out there of what you describe. I have a 3/8" fuel line run through the main spar on centerline and very near the point that the spar is supported at the fuselage side. I also have (2) 1/4" extension tubes for fuel shut off's through the spar so that the fuel shut off's are right between my legs on the front side of the spar. But i stopped there with holes and ran wiring around or over the spar I am sure there is math that can be done to show the maximum size that could be put through and the reinforcing would most likely be an additional layer on each face fully bonded. The holes must be neat and without tearout or splintering of the web material. Edges need to be sealed. Personally I concider a 3/8" hole in the center line to be minor. A hole for something the size of a torque tube i think is major. The box spars as shown for a KR2S (fully enclosed) are incredably strong.Hope this helps,Joe Horton . ---------- Forwarded Message ----------
