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Australian timber for aircraft use

Thomas Philippa

Building an ultralight aircraft
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Quick link page index R1 – draft page published April 24, 2005

There is no requirement that timber used in the construction of amateur built aircraft be certified in any way. So just follow the simple guide below to satisfy your own timber requirements. Pay maybe $500 and select your own timber and use all of it, if not in the aircraft then for jigs and fittings — for which you would still have to buy the timber separately if you were able to purchase certified aircraft timber.



Australian timbers suitable for aircraft construction
Most aircraft components are built with softwood. Hardwood is usually only used in places such as undercarriage blocks, engine mount blocks and other high stress areas.

Stressed parts: hoop pine, bollywood, coachwood, silver quandong, silver ash.

Plywood: hoop pine, bollywood, coachwood, silver quandong, alpine ash.

Spars: hoop pine, brown alder, coachwood, rose gum, yellowwood, silver quandong, alpine ash.

Main frame: hoop pine, brown alder, coachwood, rose gum, yellowwood, rose maple, silver quandong, alpine ash.

Wing ribs: hoop pine, coachwood, rose maple, silver quandong.

Botanical names
Alpine ash/mountain ash: Eucalyptus regnans/(Eucalyptus delegatensis 480_ 839
Bollywood: Litsea reticulata 480 to 530 kg/m3 at 12% moisture content
Brown alder: Ackama paniculata 480 _ 719
Coachwood: Ceratopetalum apetalum 625
Hoop pine: Araucaria cunninghamii 480 530
Rose gum/flooded gum: Eucalyptus grandis Can vary with maturity of the wood, with an average of about 800 kg/m3 at 12 %
Rose maple: Cryptocarya erythoxylon
Silver ash: Flindersia schottiana 640-675 kg/m3 at 12%
Silver quandong: Elaeocarpus grandis 495 kg/m3 at 12%
Yellowwood: Flindersia xanthoxyla medium

How strength properties of timber are evaluated
Modulus of rupture: this figure is used to assess the strength of timber beams.

Modulus of elasticity: the measure of beam stiffness or rigidity, its resistance to deflection.

Tensile strength perpendicular to the grain: the resistance of forces on wood that tend to split the timber.

Compression strength perpendicular to the grain: a measure of the maximum stress across the grain.

Shear strength parallel to the grain: a measure of the ability of timber to resist slipping along the grain.



Timber classifications at 12% moisture content
Name Basic density
kg/m³
[ lb/cu. foot]
Class Modulus of rupture
kg/cm²
[lb/sq. inch]
Compression strength
kg/cm²
[lb/sq. inch]
Alpine ash480 [30]C or S41120 [16 000]560 [8000]
Silver ash560 [35]C or S41120 [16 000]560 [8000]
Bollywood415 [26]D or S6 840 [12 000]420 [6000]
Brown alder610 [38]C or S41120 [16 000]560 [8000]
Coachwood480 [30]C or S41120 [16 000]560 [8000]
Rose gum560 [35]C or S31120 [16 000]560 [8000]
Hoop pine450 [28]D or S6 840 [12 000]420 [6000]
Silver quandong450 [28]D or S6 840 [12 000]420 [6000]
Rose maple560 [35]C or S31120 [16 000]560 [8000]
Yellowwood580 [36]C or S41120 [16 000]560 [8000]


Purchasing timber

Do not buy thick pieces of timber, there is too much chance there may be hidden defects. It is best to look for quality timber of 150 mm or 200 mm width and 25 mm thickness, and it will be back sawn, however that is not a problem, as you will see later on. For most small two-seat designs you need about 12 of the above size planks x 1.8 meters length. Be aware that you will need longer lengths for the spars and longerons, the above measurement of 1.8 m is only a guide.



What timber to look for

Look for nice clean straight timber and pick out the best for closer inspection. Wood is a living thing and whatever piece of timber you select it will have some defects in it. If the timber looks overall clean and straight and at one end or in the middle there is a big defect, do not reject it. Most of your aircraft timber is in small sections and you can always cut around the defects. In aircraft construction the main wing spars are the largest single piece of timber, make them laminated because this enables the builder to select a wide plank of back sawn timber and cut it into strips to glue them on top of each other so that the grain shows it is quarter sawn in the deepest section. Even in a spar the timber can be spliced at a 12:1 ratio provided there are no other splices near it above or below. After that 90% of the rest of the timber is so small in dimension that to specify quarter sawn timber is not much of an issue. quarter sawn timber is almost impossible to get so go with what the timber yard has to offer. Always remember that a laminated timber spar is better and stronger than a single piece of timber that may have many hidden defects inside it.



Types of defects

selecting timber

  •   Look along the edge of the plank to see it is straight. This means no obvious warping, twisting, cupping or rounding.

  •   Look at the end of the plank and the growth rings. In your mind form a circle from the section of growth ring — it should be a nice big circle of about 300 mm diameter or more, so that it shows it is not to close to the centre of the tree but rather further out in the trunk.

  •   Looking at the end of the plank see that there are no less than 8 growth rings per 25 mm.

  •   Look along the edge of the plank to see that the grain lines are reasonably even and straight. Please do not try to put a straight edge on it; after all it is a living thing and a little meandering of the grain or grain slope is OK. Remember that predominantly it should be reasonably straight.

  •   The top surface of the plank should have no large knots in it. If it does and the rest of the plank is good then do not discard it but see how many nice pieces of timber you could cut from the plank when cutting around and away from the knots. There is a point at which you must say it is OK or too much waste. Small knots of the size of a fingernail are usually no problem. If you do not like it, cut around it.

  •   Look on the side edge of the plank and see if the grain slope is at an angle to the edge and if so it should not be an obviously steep slope. If you do have a small bit in a large plank this is usually caused by a knot see how much you need to cut around it without too much waste. Otherwise stay away from timber with sloping grain in any direction. Very slight sloping (1:20) is very good.

  •   If you see a very fine hairline crack across the timber this is called a “felling shake” and it is caused by the way in which the tree fell to the ground when it was cut down. It means that the timber is broken across the grain. The rest of the timber is OK but this piece cannot be used with the crack in it. Where ever you come across a crack like this break the timber through at the crack and use the shorter pieces from either end.

  •   Other defects such a gum veins etc are obvious defects and you will see them. Look to see that you get the best part of the plank free from them, if not select another plank.



Moisture content

It is important that timber is properly dried after it is cut in the timber mill and that the moisture content is controlled and evenly distributed through the cut piece. If the timber was not properly dried after cutting, the effects will be obvious by the time you go to select you pieces. It is the kind of stuff you would automatically reject anyway.

However after you purchase the timber and take it home, the environment around your home may be different in moisture content and you cannot stop the timber from adopting the moisture content in the district where it is being used. What is important is that it is kept away from exposure to the sun. Keep it in a cool dry shaded place without a lot of air circulation. On the average in most places the timber moisture content will come to about 12%. It can fluctuate between 8% and 15%.



Thomas Philippa       tomp@mydesk.net.au




Other information about timber

Properties of timber   A short introduction to the properties of timber [html document]

Why not wood?   An article from Flight Safety Australia [html document]

Aircraft wood   An article about North American woods [external html document]

AC 43.13-1B   FAA advisory circular chapter 1-1; wood structure – materials and practices [pdf document 99 kb]

ANO 108-23   Specification: marine plywood for aircraft use [pdf document 17 kb]

ANO 108-29   Specification: timber for use in aircraft propellers [pdf document 21 kb]

ANO 108-22   Specification: klinki pine for aircraft use [pdf document 24 kb]