Aircraft Products
Laminated Wing Spars
We are applying modern vacuum-clamping processes to the manufacturing of laminated spars and spar blanks from aircraft-quality Sitka Spruce. Extensive experimentation and an instrumented destructive testing program have demonstrated several advantages over a traditional solid spar:

• Laminated material is at least 25% stronger than natural (solid) wood. ** See note below
• Laminated spar sections remain dead straight to provide for a more accurate structure, free of warps or bows.
• Using a laminated spar incurs no weight penalty over a solid spar.
• Laminated spars are easy to work with – it’s no different than solid spars.
• Laminated spars use thinner sections of wood, which gives a greatly reduced chance of hidden defects (such as pitch pockets.)
• If stressed to the point of failure, a laminated spar will fail incrementally by layers, not all at once… this may provide a chance to land an overstressed, damaged aircraft.
• Using laminated spars is well-proven and FAA-accepted… this was routine practice in the 1930’s and 1940’s.
• Since we can selectively remove a defect within otherwise good wood, the wood quality can be greater throughout a laminated spar.

** NOTE ON “STRENGTH”: Steen does NOT recommend considering our laminated spars as a means to increase the ultimate STRENGTH of any aircraft design, including our own. Laminated spars will, however, increase the SAFETY MARGIN available before failure, as compared to a solid spar of the same size and material. The FAA considers a properly laminated wooden beam to be equivalent in strength to the corresponding solid piece, though all of our testing has demonstrated a significant increase in strength over solid sections (test samples withstood at least 25% more force before the onset of failure than the solid samples.) The epoxy joints between layers are at least as strong as the original wood — the wood will fail before the glue joints will!

We have installed a bandsaw with power feed for the express purpose of planking down from large ‘cants’ of spar-grade wood. We are able to buy high-quality spar wood and finish-saw it ourselves, and are able to be very competitive on spar wood pricing both for solid and laminated materials.

Laminated spar technology is not a new phenomenon; it was routine in the Golden Age of wooden aircraft design in the 1930’s and 40’s. The process requires more labor than solid spars, but since thinner sections are used, any defects such as knots or pitch pockets are much easier to find. In addition, significant defects just can’t hide within a typical lamination (usually about 1/4-inch thick) as easily as they can disappear within the heart of a 1-inch thick (or more) solid spar section. The overall quality of the laminated spar is uniformly excellent, as we can “cherry-pick” the very best wood sections available.

Solid spars are still perfectly acceptable and safe, of course, and we will continue to provide finished solid spars and spar blanks to those who want them. In our experience, it nearly always makes sense to go with laminated spars… their many advantages greatly outweigh the minor additional cost.

You may view many more laminated spar test photos here in the Skybolt Construction Gallery.

To confirm the strength of the laminated spar design, it was tested to failure using a computerized measuring and recording system. A large number of identically sized solid and laminated samples were tested.		This graph is typical of a laminated spar failure… a series of incremental failures as each lamination fails, rather than a single catastrophic explosion typical of a solid sample.
A calibrated press applies many thousands of pounds of force to the samples.		Eventually, the samples break, demonstrating the failure mode.
The results of the testing were dramatic.