CRAFTSMAN'S CORNER

AIRCRAFT WOOD GRAIN SLOPE BY JOSEPH R. MARIDON, P.E. I have been involved in the construction of aircraft using wood spars since 1968. In the early years I did a lot of research in the selection of aircraft quality wood. Much of the information came from the old CAM 18 and the U.S. Forest Product Laboratory. This information was used to select the Douglas Fir spars used in my first homebuilt, a Starduster Too. The wood was selected from vertical grain stair tread stock. Over the years, talking to builders, technical advisors and others involved

in the construction of aircraft using wood components, I have asked the question, "How do you check the grain direction on the narrow face of a spar." Very few answer this question correctly and often admit they do not know. This is what has inspired me to write this article. The grain direction on the wide face of a vertical grain board is very obvious and easy to measure. Just measure the deviation of the annual growth rings from a line parallel to the center line or edge of the board.

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Now what about the narrow face? What we see on the narrow face of a vertical grain board is a pattern produced by the annual growth rings as they undulate in and/or out of this surface. Nothing in this pattern indicates the grain direction on the narrow face. There are two practical tests for grain slope on the narrow face. The direction or course taken by a narrow strip lifted by a knife point or the direction of free flowing ink applied to this surface. Touch a felt tip pen to this face and you can observe small streaks caused by capillary action spreading from the ink dot. This is the grain direction on the narrow face (see drawing). The ink dots can be left on the spar and varnished over providing a permanent verification of grain direction. AC 43-13.1A Acceptable Methods, Techniques, and Practices, Aircraft Inspection, Repair and Alteration, Chapter 1 Aircraft Wood Structures, states, "Spiral grain, diagonal grain or a combination of the two is acceptable providing the grain does not diverge from the longitudinal axis of the material more than specified in column 3 (i.e. 1:15). A check of all four faces of the board is necessary to determine the amount of divergence. The direction of free flowing ink will frequently assist in determining grain direction." Aircraft spruce sold today through reputable suppliers is advertised as meeting Military Specification; Spruce, Aircraft, MIL-S-6073. This

specification was written for the procurement of large quantities of wood and permits up to 30% of any lot of shipment to have a grain slope up to one in 12. This specification states

FLAT GRAINED FACE

INK DOT WITH CAPILLARY STREAKS SHOWING GRAIN DIRECTION EDGE GRAINED FACE

wood member rapidly decreases. Subject all wood for aircraft components to careful final inspection. • j ' '•'• (This article was provided by Joseph Maridon, EAA 45956, of 6429 Ouida Way, Las Vegas, NV89108.) '"-• Articles for Craftsman's Corner are obtained and edited by Ben Owen, Director of Information Services. ^

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that this wood will be subject to further inspection when used for highly stressed structural aircraft parts. MIL-S-6073 provides additional information on equivalent or total slope of the grain. When the edge grained (wide) face is straight grained the true slope of the grain is shown on the flat grained (narrow) face or vice versa. When there is a slope on both edge grained and flat grained faces the true combined slope is greater than the slope on either face. Combinations equivalent to one in 15 on the flat grained face and straight grain on the edge grained face or (vice versa) are:

1 in 16 and 1 in 43 1 in 17 and 1 in 32 1 in 18 and 1 in 27 1 in 19 and 1 in 24 1 in 20 and 1 in 23 1 in 21 and 1 in 21 Tf any of the above combinations are exceeded in checking all four faces of the board it does not meet the requirements of AC 43.13-1A. As the true or total grain slope exceeds one in 15 the ultimate strength in bending of a spar or other structural

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