SITKA SPRUCE ALTERNATIVES
Du Quoin, Illinois
How much... $16 per board-foot... Are you nuts?!
I like the idea of a wood airplane... simple construction using techniques that I already know. But the designs I have been looking at call for Sitka Spruce, which, as I'm told, is the ideal wood for aircraft. Because of this common understanding and its unavailability, aircraft quality Sitka Spruce is marketed at a premium price. The first thought that came into my mind was "what wood can I use as an alternative?"
I set up some criteria. First, it had to be a similar wood in weight and strength. I wanted this so that existing designs would not have to be too greatly modified. Because of this, I didn't consider the heavier but stronger hardwoods. Second, it had to commonly available. I wanted to be able to shop at any of the discount home supercenters. Third, it had to be inexpensive.
To familiarize myself with some of the alternatives, I lookedthrough the Timber Construction Manual written by the American Institute of Timber Construction. This primary search was based on wood having the same strength to weight ratio. Granted, there are several species that are much stronger than Sitka Spruce but they are also heavier. Listed under tables of the unit weights and under the design values of specific species, several possibilities were noted, particularly a few of the pines, Yellow Poplar, and Douglas Fir. What was also illustrated from the table of design values was that the strengths vary greatly with the grade of the wood. It is therefore important to not only select the right species but to also assure that the appropriate grade can be acquired.
In an effort to see if anyone else had done studies on alternative wood selection, I searched the Internet. The only site that turned up fruitful was the NACA server (http://naca.larc.nasa.gov). The National Advisory Committee for Aeronautics was the predecessor for NASA and existed between 1917 and 1958. This site contains most all the reports that the NACA wrote while that committee was active. The report numbered Number 354 and titled Aircraft Woods: Their Properties, Selection, and Characteristics (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091423_1993091423.pdf) was exactly what I was looking for. My suggestion, READ THIS REPORT!
The biggest thing that Report 354 did for me was to verify that Sitka Spruce is by far not the only species that can successfully used in an aircraft. This report compares almost all species against Sitka Spruce and individually lists conclusions about each species. This is very helpful as suggestions are made to the use for specific parts. Additionally, one can gain an insight into the selection process at that time from the individual conclusions if you read between the lines.
MY TAKE ON THE SITUATION (i.e. CONJECTURE / ASSUMPTION / OPINION)
I pieced together an opinion about the almost exclusive use of Sitka Spruce in wooden aircraft. It is a great wood to use, and, at one time, was the best choice. I think it may not be the best choice now.
Airplane design where wood was the primary structural component really only happened at the beginning of last century. At the time, Sitka Spruce was the most appropriate choice. It was light and strong, 30'-50' of straight lumber was not uncommon, and its level of availability was like any other lumber species. As lumberjacks mowed through old growth forests, they harvested whatever was growing and therefore many species were available. By the time World War II rolled around, metal was becoming the primary material for aircraft design. No new designs based on wood structures were being generated and consequently, no new progress with wood was made. The few aircraft designs that did use wood after that time period were based on older designs. There are several plans today that are simply copies of designs from the 1930's.
The use of Sitka Spruce for all components in the aircraft is most likely a result of its demand in the wing spar. Construction becomes easier if only one type of wood is required and the smaller components can be made from the scrap. That's why Sitka is used for turtle-decks, formers, and rib stock in these designs.
Now, lumber production is standardized and mainstream. Since balloon framing is a thing of the past, production of lumber longer than 14' is a special order. Lumberjacks, sawmills, and the distribution systems are optimized for 8', 10', 12', and 14'. No longer are old growth forests available for primary lumber production, lumber is planted and harvested like a crop. With this, only a few species are widely available, Southern Yellow Pine and Douglas Fir as examples. Therefore, the list of cost effective species is much shorter than the choices available 100 years ago.
When making the species choice using strength and weight, Sitka Spruce is at the top of the list. Actually, if this were the case, all designs would use only carbon fiber. Set the criteria to strength, weight, and cost, Sitka becomes much less attractive.
A FEW CHOICES
I went to both Lowes and Home Depot to see what choices I had. The list of possibilities was Southern Yellow Pine, Douglas Fir, Aspen, Oak, White Pine, and Yellow Poplar. I ruled out Southern Yellow Pine immediately, it's too wild when cut. It contains all sorts of internal stress and is therefore unstable. Due to the report noted above, I also skipped Aspen, Oak, and Douglas Fir. I purchased a piece of Yellow Poplar at about $4.00 a board foot and a piece of White Pine at about $3.00 a board foot. Care was taken to find samples that were straight-grained and without flaws. I wanted to compare these side by side with Sitka Spruce.
A quick note about costs per board foot, a "board foot" is a 12" by 12" square one inch thick. It is the thickness that becomes the rub in the equation. Lumber is cut at the sawmill in increments of 1/4". Generally, 4/4 and 8/4 are produced which gives a nominal 1" and 2" lumber. Traditionally, sawmills "sell you the kerf" which means your lumber is thinner by the kerf or thickness of the blade. This is why a 2 x 4 is not actually 2" by 4". To add to this, the nominal thickness remains the same even after the board is dried, planned, and sanded. So to compare apples to apples with cost, 4/4 rough-cut lumber and a 3/4" finished board is considered a nominal 1-inch.
I took the samples of White Pine and Yellow Poplar, a piece of Sitka Spruce, and also found a piece of Ponderosa Pine for my own strength tests. Three samples of each species were cut into 3/8" x 3/8" sticks 13" long. I then used a bucket full of weight to fail the samples spanned across a 12" gap.
Samples 1,2,3 are Sitka Spruce
Samples 4,5,6 are Yellow Poplar
Samples 7,8,9 are White Pine
Samples 10,11,12 are Ponderosa Pine
In this picture, the grain structure can be seen. All samples were oriented so that the tests would be performed with the grain of the cross-section vertical or normal to the force.
GRAIN STRAIGHTNESS - The Sitka Spruce had the straightest of the grains along the source lumber. The Ponderosa Pine was a close second. The Yellow Poplar had some wave in the grain along the source lumber while it was impossible to find White Pine with straight grain.
GRAIN DISTANCE - The space between individual grains was the smallest in the Ponderosa Pine at 1/16". Spruce was second with 1/8" and the Yellow Poplar was 3/16". The White Pine was random ranging from 1/16" to 3/16".
MACHINING - In processing the samples, it was noted that Sitka Spruce and White Pine cut with moderate force, the Yellow Poplar required somewhat more force to cut, and the Ponderosa Pine was very easy to cut.
STRAIGHTNESS - All samples remained straight after cutting except for the Yellow Poplar. Both the samples and the source changed in straightness indicative of existing internal stress.
The test setup is shown here.
Using a test sample, an initial failure point was determined to be greater than 25 pounds. With a sample in place, the bucket was preloaded with that amount and weight (cat litter) was added until failure. The weight was poured as quickly as possible without being abrupt as time is a major consideration with the failure of wood. The bucket was then weighed.
The samples after failure.
Note the failure modes of the group of samples. The Sitka Spruce failed without separating. The Yellow Poplar broke the cleanest with almost no grain failure. The sample of White Pine that did break had a major grain failure that extended about 3". The Ponderosa Pine had a clean break with a small amount of grain failure.
The least strong given the same size was the Sitka Spruce followed closely by the Ponderosa Pine. The Yellow Poplar was notably stronger and the White Pine was stronger still. Weights were not taken so no strength to weight ratios is available.
In my opinion, the alternatives tested would all be good choices for rib stock in truss ribs, formers in the turtle-deck and airframe, and, with the exception of Ponderosa Pine, wing spars. Granted, the total weight of the aircraft may be heavier if the alternatives are substituted using the same dimensions. Although, reduction of the size of the components proportional to their increased strength may result in acceptable total weight.
One afterthought on Douglas Fir, I initially excluded because it splinters when cut. In discussions with others, it is actually a good substitute and I regret not including it in these tests.
Regardless, using an acceptable alternative will provide substantial savings in material costs over Sitka Spruce.