No one has mentioned the 5/8" vertical spruce blocks that the plans call for when building KR spars. The spars are a variant of an I-beam. The function of the web in any I beam or box beam is to keep the two caps from coming together. No matter what loading is put on the beam - plus or minus g's, or twisting, and any combination of these forces - the caps will tend to come closer together. Beam failure will be either crushing the cap that is under compression, breaking of the cap under tension, or crush of the web followed immediately by buckling or crush of the compression cap. Metal tends to buckle; wood tends to crush. The theory and practice is to always have the caps either in compression or tension, never in bending. The lumber is much stronger in tension or compression and poor in bending. The 3/32 plywood, used for a web, will always be subject to compression and is strongest along the length of the grain (as Don Ried cites). Plywood has an odd number of plies with outside plies in the same grain orientation. That is the strongest dimension orientation. The KR box beam construction is probably way over-built at 21 g failure. So, you could put the plywood on in any random orientation and probably still have a 6-g airplane. For the exact same weight would you prefer a 21-g wing or something less? Ken Rand and Stu Robinson got it right. Sid Wood, Tri-gear KR-2 N6242 Mechanicsville, MD USA sidney.w...@titan.com
---------------------------------------------------------------------------------------------------------------------------- And having said that, here's one from Don Reid where he advocates running the grain horizontal, rather than vertical. I'd trust just about anything Don says as gospel. ---------------------------------------------- List-Post: krnet@list.krnet.org Date: Jul 20, 1999 8:27 AM From: Donald Reid <donr...@erols.com> Subject: Re: Grain direction.....who cares it's plywood...my turn at a 'STUPID' Question Tim wrote: > Like Aircraft Plywood is either 90 or 45 degrees, I assume this is how > the ply's (3-7) are layered. So grain direction of the top sheet is of > interest, but I wouldn't think the orintation is as critical in dealing > with the Spar web as perhaps Aluminium ..... OK, here are some numbers. Anyone who is interested can make up their own mind. All data are for birch plywood and taken from ANC-18, Design of Wooden Aircraft Structures. (The thick pieces are included just to show the effect with more plys) thickness # plys parallel perpendicular 0.125" 3 15.17 5.544 0.160" 5 21.46 11.47 0.410" 7 131.1 80.91 All plys are equal thickness. The numbers are moment for fiber stress at the proportional limit in units of inch-pounds per inch of width. As to why the KR plans specify a vertical orientation, it is because Ken Rand and Stu Robinson got it wrong. ----------------------------------- Mark Langford, Huntsville, AL N56ML at hiwaay.net see KR2S project N56ML at http://home.hiwaay.net/~langford
