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Builders guide to safe aircraft materials
Properties of plywoods
Rev. 5 — page content was last changed 6 January 2010
The plies are generally laid so that the grain directions alternate between lengthwise and crosswise, with the grain direction of the two surface veneers parallel to the longer dimension of rectangular sheets. Three-ply is stronger along the surface grain axis, But as the number of plies increases, the lengthwise/crosswise strengths and stiffness of a plywood sheet will become more equal. Three-ply bends easier along the grain direction of the surface plies because only the core ply will have cross-wise grain. To ensure the strength and stiffness characteristics of three-ply in the face grain direction, it is required that the thickness of each surface veneer is between 25% and 33% of the total sheet thickness; i.e. the two surface sheets comprise 50% to 66% of the total sheet thickness.
Three-ply manufactured from very thin veneers is predominantly utilised as aircraft fuselage and wing panels (stressed skins), for example. Finnish birch plywood sheet thicknesses start as low as 0.4 mm (1/64"). Such panels are supported/restrained on all sides. Thicker three-ply has many uses — spar webs for example, where just two opposite edges are supported or restrained. Five or more plies are usually available in sheet thicknesses of 5 mm or greater and such plywood designs should be balanced; that is, the total thickness of the odd-numbered plies (number 1 being the face ply) should be about the same total thickness as the even-numbered plies. The centre ply is known as the 'core', the outer plies as the 'faces' (or the 'face' and 'back') and the intermediate plies are the 'crossbands'. The density of plywood is usually much the same as the parent wood, slightly higher if it's softwood.
(When softwood plywood has to be bent to form a curve perpendicular to the surface grain, some checking is likely to occur. To protect the surface it may be necessary to apply a light fabric coating to the skin.)
The stiffness of plywood can be increased without an increase in weight by laying the two outer plies so that their grain direction is at 45° to the long axis of the sheet. Forty-five degree plywood is slightly stronger than the normal 90° sheets but it is much more expensive and difficult to acquire. Humidity effects may be less apparent in the 45° plywood. ANO 108.23 'Specification: marine plywood for aircraft use', refers to the AS/NZS 2272 – 1979 'Plywood – Marine', which has been superseded.
Australian-produced plywood used for aircraft construction should be chosen from that manufactured to the standard AS/NZS 2272 – 1996 Plywood – Marine — and no other, even if stated to be an equivalent standard. Marine plywood manufactured to the British standard BS 1088 should not be substituted for AS/NZS 2272 marine plywood, nor should exterior plywood manufactured to AS/NZS 2271.
AS/NZS 2272 – 1996 provides a purpose-designed structural plywood intended for use in marine applications and manufactured from timber species selected for density, bending strength, impact resistance and surface finishing characteristics. The standard also specifies veneer qualities, bond quality, joints, veneer construction, moisture content, dimensions and tolerances, finishing, and branding. A series of performance requirements relating to species, and a procedure for evaluating and admitting new species are included. The density of marine plywood should be between 450 and 720 kg/m³, and MR should not be less than 75 MPa.
In the USA plywood sold as 'aircraft grade' should meet the specifications in the MIL-P-6070 standard and could be African mahogany or American birch hardwood veneers that are hot press laminated to a hardwood core with waterproof glue. 'Marine grade' plywood, if softwood, should meet products standard PS 1-74, which allows face and back veneers A-A, A-B or B-B while 'marine grade' for tropical hardwoods must meet British Standard specification BS 1088. Engineered Wood Products Association of Australasia.
There are five veneer qualities specified for plywood in the Australian and New Zealand grading system — A, S, B, C and D. Only A-grade surface veneers are suitable for aircraft construction
Other plywoods available from merchants are:
The next module in this group is 'Wood joints and adhesives'
Builders guide to safe 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 |
ANO 108.23 Specification: marine plywood for aircraft use. This is an out-of-date document that is still of interest (pdf document 17 kb).
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