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Builders guide to safe aircraft materials

Selecting aircraft timber

Rev. 2 — page content was last changed 8 January 2010
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7.1 Defects in timber
The primary natural defects that reduce the structural strength of boards are those which cause the grain slope to deviate significantly from the longitudinal axis of the board (knots and irregular grain) and those which produce reaction wood. Knots, common in softwoods, are the sections of branches that have been engulfed within the trunk as it expanded producing quite irregular grain surrounding them. Spiral grain occurs when cells forming in the outer layer of the trunk grow at an angle to the vertical axis, with the growth continuing in this fashion for several years. Interlocked grain occurs when the direction of the spiral changes with new growth; i.e. several years growth spirals in one direction, then the next several years spiral in the opposite direction. Sap veins, bark and other growth anomalies or damage may be included within the trunk as it expands.

Reaction wood: when a tree experiences continuing forces that cause it to lean (prevailing winds, growth on sloping ground, interference from other trees), the lateral growth of the trunk, or portions of it, will be eccentric. Hardwoods will produce additional softer growth (tension wood) on the higher side, while softwoods will add weaker and possibly brittle growth (compression wood) on the low side. The tension or compression wood may be layered because the forces that cause it may not be constantly applied. The strength of reaction wood is unpredictable and thus should not be used in aircraft structures. Also tension wood is difficult to machine, and produces fibrous or woolly surfaces.

Stresses during growth cause fibre separation within the trunk, resulting in shakes (usually internal longitudinal cracking between annual rings) some possibly resin-filled (gum or pitch). Such defects are readily discerned in a board. However, buckling damage to internal fibres — caused by storm stress or as happens in logging operations when a tree falls across an obstruction on the ground — may not be readily noticeable in boards subsequently cut from the tree. The visible wrinkles in the fibres are known as compression shakes.

Compression shakeThe image on the left shows a compression shake caused by excessive in-flight loading and extending across the width of the main spar of a Citabria aerobatic aircraft. Note the small wave anomaly in the grain at the compression failure, which probably acted to concentrate the stresses.

Image and comments courtesy of the Bellanca-Champion Club

Warping and curving defects

Milling and seasoning defects: there are often residual stresses in a log delivered to the mill causing some deformation in the sawn board, which may be emphasised during the drying process and show up as:

curvature – bowing and spring over the board length

warping – cupping in board cross-section or twisting over the board length

splits or longitudinal cracks that go right through the thickness of the board.

seasoning checks due to uneven shrinkage and seen as small fibre separations on the surface of a board — or may show as a longitudinal crack perpendicular to the growth rings. Such checks could also appear in seasoned wood that is exposed to large humidity changes even with a water vapour inhibiting barrier; plywood, however, is much more resistant to checking of that nature.

Seasoned boards are subject to decay caused by wood-rotting fungi or bacteria and to insect attack— borers for example.

See the notes regarding defects permitted/not permitted following table 1-1 in AC 43.13-1B   FAA advisory circular chapter 1-1; wood structure — materials and practices.
7.2 Stock dimensions
Timber merchants usually refer to fully seasoned boards in dimensions that indicate the board thickness and width as sawn; for example 50 × 100 mm (or 2×4 inches) even though the board might have been 'dressed' or 'planed all round' to smooth sides and edges. Planing will remove 2 to 3 mm of material from each surface, thus the actual dimensions of a dressed board will be about 5 mm less than the nominal size; a 50 × 100 mm dressed board will actually be about 45 × 95 mm. Timber classified as 'clear grade' or 'first grade clears' (or similar) has been inspected and found to have both faces and both edges free from most defects or blemishes.

Plantation-grown hoop pine is produced in Australia by Hyne Timber as No.1 clear sawn in the following sizes:
       15 mm × 100, 150 mm
       21 mm × 100 mm
       25 mm × 100, 125, 150, 200 mm
       33 mm × 75, 100, 125, 150, 200 mm
       38 mm × 100, 150, 200 mm
       50 mm × 100, 150, 200 mm

7.3 Selection of boards
When selecting individual boards for airframe construction, the primary requirement is the general grain slope, which must be better than 1:16 — or 1:20 if required as spar material. The second priority is rate of growth. The amount of natural and seasoning defects that could be acceptable depends on the intended use of a particular board.

Of course the timber species and the board dimensions and lengths needed must be known before suitable commercial boards can be selected at a timberyard.

7.4 Ordering plywood
When ordering plywood, the following information should be supplied:

      •   number of panels x length (mm) x width (mm) x thickness (mm) and construction

      •   plywood type and Standard

      •   face and back grades, and the glue bond type

      •   EWPAA quality control and product certification stamp.

For example, an order for marine plywood might be stated as:

5 sheets of 1200 x 1200 x 2.5 mm 3-ply
Marine plywood to AS/NZS 2272
AA - A bond
EWPAA product certified: PAA TESTED MARINE

The next module in this group is 'Basic strength and elastic properties of wood'

Builders guide to aircraft materials – wood, plywood and adhesives modules

| Guide contents | Properties of wood | Properties of plywoods |

| Wood joints and adhesives | Wood beams in aircraft | [Selecting aircraft timber] |

| Basic strength and elastic properties of wood |

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