Working With Aircraft Timber:
By Thomas Philippa Dip Teach I A Design & Wood Technology (Retired)
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The Holy Grail: At least that is how some people treat their aircraft timber. There is nothing to fear. Just because a stick of timber is not perfectly cut or has not been sanded smooth, or has a minor dent or blemish does not mean it is to be rejected. There is a point between insanity and fanaticism where reality lies. It is that happy medium one must find otherwise it will take a lifetime to build the aircraft at triple the cost. The best part of a wooden aircraft is the fact that you can make mistakes and it can be fixed cheaply and without much fuss.
Wooden aircraft are over designed in strength and if you have to laminate a section together or put a splice in some part not indicated on the drawings does not mean the aircraft is less safe. Even if a part has been glued wrong it can always be cut apart and a new bit glued in, or repaired by laminating a bit of wood where the saw cut was made. The aircraft repair manual AC 43.13 – 1B/2A has a good section in the front for timber aircraft construction. It is especially useful when trying to make repairs to a damaged section of a timber aircraft. The book can be purchased from the RAAus.
STORING THE TIMBER: Store your timber lying flat in an area with low ventilation away from the heat and wet. Don’t wrap it up in plastic, the oils leach out of the plastic and cause problems when gluing. It is better to first wrap it up in unbleached calico and then in plastic.
MACHINING TIMBER: Machining your own timber is much cheaper then having it done for you. My first aircraft was done with a power handsaw that I fitted under a table and a hand plane. I ripped the lengths of timber on my homemade saw bench and planed it to thickness by hand. Not as big a job as it seems so just purchase a bit of timber and work at it one bit at a time.
CUTTING SEQUENCE: Because an aircraft consists of a lot of small sticks of timber it is very economical and wise to begin by cutting the biggest sections first. The off cuts can be machined down to the smaller parts and if this method is applied to the whole aircraft there will be very little waste and consequently the cost is kept down.
This is also the reason why a board of timber can have defects in it when you buy your timber because you cut around the defects. Pieces of timber with defects are then used for jigs and support blocks during construction once again saving money by not having to purchase more timber just for that purpose.
It is wise to select the best piece of timber first and use it to cut the spar material. Once the spar material is cut the plans should be scrutinized for the next largest piece of timber to be used in the aircraft. This way good material is used for the most important parts of the aircraft and the smaller sections can be machined out of timber that may have imperfections in it. If at all possible try to machine the timber to quarter sawn sections. Sometimes this cannot be done and the builder has to think where he could use the material without wasting it. Often it can be machined into smaller useful sections. When I talk of smaller sections I refer to the width and not to the length of the timber.
TIMBER SPLICING: Longerons are usually the longest section in the aircraft apart from the wing spar flanges. If the material is not long enough the longerons may be spliced to 1:12 in the tail of the aircraft. (Remember the aircraft is over designed in strength and the splice in the tail of the aircraft will bring the frame to about to 90% original strength and this is very safe. Similarly the main and auxiliary spars can be spliced if you do not have a sufficient length of good timber and the AC 43.13 manual will tell you how you can do it.)
When splicing timber the grain direction is very important. The timber must appear to be the same on both sides of the splice joint. Face grain must be to face grain and flower face must be to flower face. Grain face clearly shows the lines of the growth rings, which is not very attractive and the flower face clearly shows the wavy pattern that is so much admired in polished wood.
The grain in the joint must appear to continue from one piece into the other. This means no sloping grain in the joint. For all intentional purposes the timber joint must give the impression that it is a single piece of timber.
FINISHING TIMBER SURFACES: It is best not to sand the timber but to leave it clean cut by the saw or planer. If you have to sand it make sure you vacuum clean the surface before gluing otherwise the timber pores will be clogged with saw dust and the glue joint will be very weak because you are gluing more saw dust then timber. The timber surface does not have to be perfectly smooth. With modern gap filling epoxy glues minor gaps will not lessen the strength of the glue joint. Besides when using the modern glues you cannot clamp the timber tight because you will squeeze out the glue and the joint will be weakened. There is more about gluing in a different section on gluing.
STEAMING THE TIMBER: Use a lawn mower fuel can with water and let it boil. The steam coming out of the spout is directed to the timber through a 50 mm plastic drainpipe that is suspended over the can. When the timber is well saturated and hot put it in the jig and allow it to dry into shape. Do not glue until it is dry otherwise the glue will not hold.