Why do we not see more main spars made with wood as the sheer web and CF capstrips? I am assuming the length and area of the epoxy bondline would be the ultimate determinate of failure or would it go to the cap strips to call it quits?
The short answer is that WEIGHT IS THE ENEMY. There are a whole bunch of reasons, all of them related to making the spars lighter at the required strengths.
Webs must simultaneously do a couple things:
- Carry the shear load between the caps, and;
- Extend or contract with the cap motion under bending deformation;
This drives a need for the the connection between caps and web(s) to be pretty darned sturdy. A successful and reasonably efficient built up wooden spar then usually has webs attached by one of these methods:
- Bonded to the sides of the caps;
- Bonded into a substantial kerf in the caps;
- Attached with blocking.
This takes the strains and stresses in the epoxy into the durable range. If you simply tried to edge bond the plywood webs to the caps, the epoxy is unlikely to carry it durably unless you add the bonding area given by each of the above methods.
In composite webs, we usually laminate the web with the caps and carry the cloth onto the caps to get interlaminar shear stress significantly below epoxy strength and strains in the application.
Strength of the epoxy is higher than the wood, but much lower than the glass-epoxy, which is again substantially lower than graphite-epoxy;
The ideal lay of plywood for shear webs is with grain at +45/-45 degrees, and gets expensive that way. Composite webs are also ideally laid up at +/-45, but that costs no more than any other orientation in composites;
Plywood shear webs are not easily tailored for thickness - thicknesses are 1/16, 3/32, 1/8, so the steps for tailoring are kind of big, areas are large making laminating and clamping cumbersome unless you vacuum bag. The result is many wooden webs go untailored. Composite webs are easily tailored in 0.006 - 0.018" thick steps, and so are readily tailored to thickness where needed, thinning as you go outboard. They can be open laid up, wet vacuum bagged, or even vacuum infused;
The webs not only have to carry shear loads between the caps, but they must resist buckling under the loads discussed. This frequently drives more wooden web thickness than if buckling were not an issue or requires design and application of a bunch of web stiffeners. In composites, we just build the web with a foam core, which generally pushes buckling way out and allows tailoring the web plies based upon shear and bending inputs.
Foam cores generally run from 2 to 8 pounds per cubic foot, while birch plywood is around 40 pcf. That combined with a nicely tailored fiberglass or carbon fiber web is usually substantially lighter that wooden webs.
There are more reasons all related to wood being heavier. If anyone is certain that they can reinforce wood with modern composites and come out lighter and/or cheaper, I recommend Mechanics of Materials book of their choosing (I like Timoshenko), focusing on the beam theory sections, then bone up on matrix algebra, then get into either Tsai and Hahn or Jones for composite calculation methods.
Wooden webs work OK with wooden spars when done correctly. Glass webs work OK with glass or carbon fiber caps. Carbon fiber caps and webs work together OK too. Wood with carbon in spars has been done, but is just not as good an idea as you might think - yes, it can be done strong enough, but that usually means more weight than by doing an all composite spar.
Yes, looking at my spars, you will find some laminated wooden cores. They are located where shear plates are bolted through the web, and are meant to carry the crush loads from those bolts. Wood and phenolic are applied local to these loads only, with foam cores elsewhere. I have 388" of main spar system, and wooden cores are in 24" of that length. The other 94% of the length of these spars is either 1" or 1/2" polyurethane foam at 2 pcf.
Billski
