The Houston Hawk Project
Selection of Wood
The following is a transcript of an e-mail message form Jack Womack to a Houston Hawks club member. Jack was questioned regarding how to select the best balsa and spruce for the Houston Hawk project. This message was copied to all members of the club on our e-mail system.
Steve asked me for my take on wood selection. I didn’t consider the fact some of you are not familiar with the selection of good balsa and spruce.
The first thing that you need to know is that balsa comes in three grades. The light stuff comes in about 5 to 8 pounds per cubic foot. Medium balsa is generally
9 to 12 pounds per cubic foot, and hard balsa is that which comes in over the 12 pounds per cubic foot. I have never used the light material, as it’s far too spongy. For balsa spar stock, look for the heavy, long grain hard balsa. For sheeting, look for sheets that have consistent color with no soft spots, dark spots or imperfections in the grain. Flex the sheet along the grain like it would sit when glued down to a set of ribs. See if it’s stiff or too soft. Look at the grain from the side and from the top. It won’t be easy to see the grain on the side but look at it very closely. With some concentration you can see if the grain is straight or crosses the edge of the sheet at an angle. If you find a sheet with too much angle, then look at the face of the sheet in that area. The grain will be very short and will be rough in that area. More importantly, it will be weak because it has too much grain diversion. For my models I use aircraft selection parameters. The rule states; for 1” of width in any area, the grain must be visible for 12” before crossing from one side of the 1” boundary to the other side of the boundary. For a 1/16” sheet, looking at the edge grain (not the end grain), the grain has to pass at least ¾” along the edge before leaving the sheet. Look down the whole sheet if the grain doesn’t match those rules then don’t buy it.
Watch for sheets that change grade. It may be hard on one edge and soft on the other. Again, don’t buy it. Stick with balsa that is on the light side of medium for the most durability. Use the lightest balsa in places of low stress where weight causes a premium, such as some of the tailpieces.
When the word “CONTEST” appears on balsa, it usually means it’s in the lightest grade. As you gain experience in the grading of woods, you will be able to buy the best wood and leave the rest for someone else to struggle with.
There is also a “C” grain” balsa that is used for spars and structural parts, but never for sheeting. This material has an average vertical end grain that forms a tighter and tighter “C” as it gets closer and closer to the center of the tree from which it was cut.
This material does not make good sheeting material because it splits very easily. This material is used for ribs and spars as it the strongest across its grain.
Spruce is another type of animal but not totally different. Look for the same criteria as listed for balsa, but add one parameter. Count the number of grains per inch. There need to be at grain least 12 per inch. If you are looking at 1/4 “ stock, there should be at least three grain lines in that ¼” cross section and more is better. The same grain diversion note in balsa is even more important when selecting spruce. You want the grain lines to run parallel to the length of the part not across the part. The only reason spruce is used in model building is for it’s very high strength to weight ratio. Spruce is second only to a wood called Ramin, which is commercially almost impossible to find. Spruce is used in high stress areas on models such as wing spars, some leading edges and fuselage longerons. I sometime use a piece of 1/16” x ¼” spruce on the wing trailing edge. This piece of spruce can then be sharpened down to a fine durable edge. When gluing spruce I never use CA glue if I can avoid it. If you do have to use CA on spruce, then sponge down a paper towel with water than has baking soda dissolved into it. There are oils in the spruce that are not compatible with CA glues. These oils somehow seem to retard the setup of the CA glue. This baking soda and water mixture seems to do way with the oils on the surface, at least.
The best spruce is becoming somewhat difficult to find. Some of the best balsa suppliers will try and substitute basswood for spruce. You need to be aware that for strength per cross section bass comes up short every time. A piece of 1/8” x ‘1/2” basswood is not much stronger than the same size piece of balsa. If you do substitute basswood for spruce, add thickness by a factor of .5 – in other words, if the plans call for 1/8” x 1/2” spruce you must use a 3/16” x 1/2” basswood for the same strength. If you do use basswood use the same rules to judge the material. One nice thing about basswood is that it is compatible with CA glue.
I can’t think of anything that I haven’t covered on this subject, but e-mail questions, or different opinions, if you have them. I’ am not too old to learn.
<Back to the Houston Hawk>