Oh, now this is a real bummer. I did a few minutes of searching through the archives and turned up this post from Don Reid, who says it doesn't matter how the grain is oriented in the main spar caps!
Don Reid is the resident expert on all things structural on this list. He's not only a mechanical engineer, but also holds a PE license. But that doesn't necessarily mean anything. What matters is that he is consumed by engineering and structural design, and now is working on either his masters or Phd in aeronautical engineering as well. He is, in short, an expert on the subject. A quick search at the KRnet search engine using "wood grain" as the keyword turned up the following post that Don made in 1999 on this subject. Whatever Don says, I believe, and his information comes straight out of the gospels of ANC... ------------------------------------------------------------------- Subject: Wood Grain and Bolted Joints (Very Long) From: Donald Reid <donr...@erols.com> List-Post: krnet@list.krnet.org Date: Wed, 11 Aug 1999 12:13:12 -0400 X-Message-Number: 11 There has been some incorrect information floating around the net the for a while concerning wood grain and bolted joints. If anyone is interested, here is the real story. References used are ANC-18, Design of Wood Aircraft Structures, and ANC-19, Wood Aircraft Inspection and Fabrication. There are tables of mechanical properties available which are derived from standard engineering tests. This tabulated data represents the best estimate of the strength of the wood specie under various loading configurations. There are two grain orientations, the load applied parallel to the grain, and the load applied perpendicular to the grain. The wood grain should not be confused with the growth rings. The grain direction is defined by the direction of the wood fibers, the longitudinal axis of the tree. The growth rings are a radial and tangential component of the wood structure. Quoting from ANC-18, "Tests indicate that the stress at proportional limit when the growth rings are placed horizontal does not differ greatly when the growth rings are placed vertical. For design purposes, therefore, the values of strength in compression perpendicular to grain as given in tables 2-6 and 2-7 may be used regardless of the ring placement." It further states, "Tests indicate that the plane of failure being tangential or radial makes little difference in strength in tension perpendicular to grain. Results from both types of tests have been combined to give the values shown..." The loading pattern mentioned above is one in which the wood grains are being pulled apart. In a wing attachment fitting, the load application will tend to crush the wood fibers, since the load path is in line with the grains. Again quoting, "The strength of wood in bearing parallel to the grain against solid steel aircraft bolts disposed along the member in single or double lines with the load divided equally between the two ends of the bolt ... is expressed in terms of the maximum crushing strength (of the wood)." The crushing strength of the wood is not dependent upon the orientation of growth rings. The loading pattern described in the quote is a wing attachment fitting, just like in a KR. Checks, Splits, and Shakes: (Paraphasing from ANC-19) "A check is a longitudinal crack in wood, generally in the radial direction and are usually due to uneven shrinkage in seasoning. A split is a longitudinal crack in the wood. It is caused by rough handling or other artificially induced stress. Typically, it extends through the thickness of a piece from side to side reguradless of whether the piece is edge-grained or flat-grained. A shake is a longitudinal crack in wood between two annual rings, they originate in green wood. It is obvious that relatively large checks, splits, and shakes may seriously weaken wooden members in resitance to longitudinal shear, and finished parts containing them should be rejected." So, we need to protect mainly against checking, which is caused by seasonal changes in the wood due to changes in humidity. There are several ways to prevent checking. Covering the endgrain of a piece of wood with paint or epoxy or varnish to minimize humidity changes is one way, but that mearly slows down the process of the change. Water will still pass through the chemical barrier of the coating, it will just do it slower. Yes, even the best epoxy will still allow moisture to migrate through the wood. The other method to prevent checking is to cross-band the wood. The plywood shear web on the spars is an example of cross-banding. The plywood is very resistant to checking due to the many thin layers of wood. The plywood in turn will stabilize the solid substrate under it (the spar caps). That just leaves the ends of the spar caps exposed. If they are laminated from two or more pieces of wood, the likelyhood of checking is further reduced. If the end of the spar caps are cross-banded in some way, this will virtually eliminate the chance of checking. I chose to cross-band my spars with the plywood of the wing ribs. Other methods are available. Now for the conclusions: A wooden spar may be fabricated with the growth rings oriented either horizontally or vertically and there is NO difference in the overall strength of the spar. It is my opinion that any lamination in the spar cap should be either oriented perpendicular to the bolts, or arranged so that the bolts do not fall on a glue line. End grain of any sort should be protected from checks and splits by cross-banding or a barrier coat (epoxy or varnish) A 3/16" bolt, passing through a 2 - 3/8" thick piece of spruce, which is reinforced with plywood on the outer surfaces (just like a KR main spar) will be able to carry a load of approximately 1,150 pounds. Since there are eight bolts through the spar cap in each wing attachment fitting, each fitting will carry approximately 9000 pounds in either tension or compression and that is all. -- Don Reid ---------------------------------------------------------------- Hmmmm. Maybe that was a perfectly good KR after all... Mark, the future RV Aviator
