Timber for Aircraft Use
By Thomas Philippa Dip Teach I A Design & Wood Technology (Retired)
I can be contacted by email at
CERTIFIED AIRCRAFT TIMBER: This process is basically not done anymore. Amateur built aircraft do not require an official release note for aircraft timber. So just follow the simple guide below to satisfy your own timber requirements. Pay about $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 purchase Certified Aircraft Timber.
AUSTRALIAN TIMBERS SUITABLE FOR AIRCRAFT CONSTRUCTION:
Stressed parts: Hoop Pine, Coachwood, Bollywood, Silver Quandong, Silver Ash
Plywood: Bollywood, Hoop Pine, Coachwood, Silver Quandong, Alpine Ash
Spars: Brown Elder, Coachwood, Rose Gum, Yellowwood, Silver Quandong, Alpine Ash
Main frame: Brown Elder, Coachwood, Rose Gum, Yellowwood, Hoop Pine, Rose Maple, Silver Quandong, Alpine Ash
Wing Ribs: Coachwood, Rose Maple, Silver Quandong
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 the beams 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: Is 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 CLASIFICATIONS AT 12% MOISTURE CONTENT
Name Basic densityClassModulus of ruptureCompression strength
Ash Alpine30 lb/cu. ft.C or S416000 lb/sq. in8000 lb/sq in
Ash Silver 35 lb/cu. ft C or S416000 lb/sq. in8000 lb/sq in
Bollywood26 lb/cu. ft.D or S612000 lb/sq. in6000 lb/sq in
Brown Elder38 lb/cu. ftC or S416000 lb/sq. in8000 lb/sq in
Coachwood30 lb/cu. ftC or S416000 lb/sq. in8000 lb/sq in
Rose Gum35 lb/cu. ftC or S316000 lb/sq. in8000 lb/sq in
Hoop Pine28 lb/cu. ft.D or S612000 lb/sq. in6000 lb/sq in
Quandong Silver 28 lb/cu. ft.D or S612000 lb/sq. in6000 lb/sq in
Rose Maple35 lb/cu. ft.C or S316000 lb/sq. in8000 lb/sq in
Yellowwood36 lb/cu. ftC or S416000 lb/sq. in8000 lb/sq in
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 1.8m 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 cut 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: 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 look at the growth rings. In your mind form a circle from the growth ring and it should be a nice big circle of about 12” diameter or more so that it shows it is not to close to the center 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 to the inch or 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. 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 a 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. Other wise stay away from timber with sloping gain 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%
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