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Builders guide to safe aircraft materials
Selecting aircraft timber
Rev. 2 — page content was last changed 8 January 2010
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.
The 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.
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.
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
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.
• 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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