|BlueSky Business Aviation News|
That is, we humans tend to conflate the new with the old if they seem similar enough to us. It’s a kind of stereotyping, I think. We are all subject to it at some level.
Let us discuss wood or cellulose-based composites as a substitute for aluminum. The best of the aluminum alloys has a higher specific strength and a higher specific stiffness than the various timber products. The difference isn’t huge, but it’s there. However, none of the data I can access shows the characteristics of the best bonded wood plies, much less any data on the yet to exist cellulose fiber composites. So, the currently available data are incomplete.
Is there hope for wood or cellulose composites? Yes, certainly. Aluminum is generally characterized as isotropic. That is, for discussion purposes, we say that the strength of aluminum doesn’t vary (much) along different axes. This is not strictly true. It would be more fair to say that aluminum is weakly anisotropic, which is to say that there really are differences in the strength of the metal depending on grain direction, generally, somewhere between five and fifteen percent.
Composites are ALL anisotropic
All aerospace composites are strongly anisotropic. That means the strength varies quite a lot depending on the orientation of the fibers. This is not the problem you might think, at first glance. Designers are able to orient the fibers in the composite lay-ups to correspond with the direction of the loads. So, really, composites can be a far more structurally efficient material than metals. This is why composite aircraft usually have a weight advantage over metallic aircraft.
A cellulose fiber based composite would act exactly the same as a carbon fiber based composite or a fiberglass based composite; the only differences being the tensile strength and stiffness. The fibers would be embedded in a compatible matrix, oriented along the expected load paths with no strength wasted in directions from which no loads can come.
Wood is like tape, wait a moment
Interestingly, laid up plies of natural wood are little different from tape plies of fibers. Let me back up a moment. Composites are made up of a load carrying fiber surrounded by a matrix that connects the various fibers together into a single load-carrying entity. The matrix is what distributes the load among the various fibers so efficiently. Without a matrix, the individual fibers are strong, but they can easily be torn apart from their neighbors. The matrix connects each fiber with the fibers on all sides, making it much more difficult to pull apart. The matrix itself is not terribly strong, nor should it be, its role is to enable the fibers to act as a group, and those fibers are very strong along their longitudinal axes, which brings us to the interesting question of shear.
OK, let me take another step back. Tension and compression are longitudinal stresses; they act along the long axis of the part. You can only have one or the other. If the tension and compression stresses going into a part are exactly the same, the resulting load on the part is zero. Shear, on the other hand, is in some ways the opposite. It acts in two directions at once.
To handle tension loadings, designers use a form of composite known as “tape.” In a tape, all of the fibers are going in the same direction; we say they are unidirectional. This allows for the maximum usage of the fibers’ greatest strength, their ability to handle tension loads. For two different parts made of different materials, we can get equivalent strengths by making one part thicker than the other. We utilize this fairly often in repairs to aircraft structure when we run into problems with the thickness of the stack-up, sometimes resorting to a steel or titanium layer in an aluminum stack-up to bring the dimensions down to something we can tolerate structurally. We also use that same principle when we switch between different aluminum alloys for various reasons.
To handle shear, designers use a different form, called a cloth or a fabric. That’s exactly what it looks like, a fabric woven more or less loosely of the fiber. Generally speaking, those fibers are not twisted like we find in ordinary textiles, but are laid as straight as we can to maximize the stiffness of the material. The more the fibers are twisted, the more elastic they become, which is not what we want. This makes weaving the fibers into a cloth or fabric more difficult. And, of course, the weave has to be a bit loose in order to accommodate the matrix, which must permeate every part of the cloth. The designer will usually vary the orientation of the layers in the same way we do with plywood—and for the same reason: to achieve greater resistance to shear loads. Here again, if we have two parts made of materials with different strengths, we simply make one thicker than the other. Interestingly, there may be no significant difference in weight as the material with less strength is often less dense. In fact, it is the differences in density that makes both aluminum and titanium superior to steel in aircraft design, not the differences in strength; steel is actually stronger than either.
To handle compression loads, sometimes the most challenging of all, composite structures designers tend to use layups built into more or less complex shapes that are intended to provide resistance to buckling by translating the axial compressive load into shear and bending loads where we can once again use the fibers’ natural strength in tension. Buckling behavior is described by some fairly sophisticated mathematics, but suffice to say that buckling is sensitive to thickness. The thicker the material, the less likely it is to buckle, all other things being equal. So, if I have two parts made of different materials, the less dense material will probably be easier to design for compression loading as it will require a thicker part. In this application, the lower density materials rule. Steel is awful for compression loads because it is so strong that it becomes very thin and buckles shamelessly. For decades, wood was used in aircraft structure for this very reason, resistance to buckling. Between WWI and WWII, the US government spent millions and millions of dollars helping metal structural designers figure out how to make metallic structures the equivalent of wooden ones. In contrast, the US government invested very little money in improving wood structure, and that tiny bit of funding was cut off shortly after the end of WWI.
Characteristic stresses in different parts of the airplane
Certain parts of the airplane have characteristic loadings. In the crown of the fuselage, aft of the main gear, we tend to have mostly tension. The wing skins see both compression and tension, depending on whether the airplane is generating lift, though the lower skin tends to see a lot higher tension loading than compression loading and is designed with that in mind (vice versa for the wing upper skin). The fuselage between the nose gear and the main landing gear sees loads going in both directions. The belly skin, for example, sees tension loads while on the ground, and compression loads while in the air; the crown skin above it sees the reverse. So, designing for one part of the airplane or another isn’t quite as simple as one might hope. It’s a rare part that sees only one kind of load. Fortunately, the designer knows what kinds of loads come from where and with a composite material, we can tailor the strength of the part to exactly match the design loads without waste.
Wood really is like tape
Picking back up again, wood fibers look a lot like a fiber tape, with their generally uniaxial orientation. However, there is a certain amount of cross-linking between the cellulose fibers in a natural wood that we don’t get in a fiber tape. This cross-linking gives the wood ply strength in what we might label as the transverse (long or short) direction. This means that a layup made of several wood plies all oriented in the same direction will still have some shear strength—more than we would ever see in a part built up of several layers of tape made from discrete fibers—and all without compromising the tension capabilities of the fibers. This is pretty cool.
The Ultimate Matrix
Moreover, the lignin acts as a perfect natural matrix. Wait, let me back up again. Wood is a twopart composite material, just like fiberglass or carbon composites. I.e., it has both a fiber for the load carrying capability—cellulose, in this case—and a matrix to efficiently distribute the loads to the various fibers and keep them together working as a unit, the lignin. A conventional resin-fiber composite for cellulose fibers is simply an effort to use an artificial resin matrix to replicate the natural lignin in wood. A much more interesting technology would be for someone to figure out how to re-activate the lignin in the various wood plies and re-knit those links, fully replicating a naturally grown piece of wood, eliminating any need for adhesives between plies. This would be awesome for several reasons, not least because of the potential for extremely low amounts of embodied energy in aircraft structure. The resulting structure would also be completely non-toxic and could be gently restored to the biosphere by composting it, at the end of its economic life. It would be really hard to find something “greener,” even in theory.
This issue of embodied energy is important. There seem to be a lot of folks currently running around expressing their corrosive unhappiness with with those they classify as “environmentalists” (and worse), but without those environmentalists, at least here in the US, we would still have rivers that catch fire in the summer lightning storms, water that would be unsafe to drink, life-threatening levels of smog in our cities, and no old growth forests whatsoever. Taking care of what for all intents and purposes we can analyze as a closed system--the earth--makes perfect sense. We don’t take off from Wichita in our Lear 45 expecting to fly to Mumbai non-stop, right? We don’t fly around expecting that in some miraculous fashion fuel will somehow magically appear in the tanks when we’ve run out, do we? Why would we then expect some sort of miraculous replacement for paleo-carbon-based fuels? Haven’t all of those kinds of people died off by now? Surely, we in aviation would be the last group of people to advocate magical thinking. The world is perfectly understandable for those who are willing to do the work.
The cost of energy will continue to increase
Resources here on earth are limited by the very nature of the system. Energy is expensive now and can only be expected to get more expensive as the population continues to grow and the global economy will continue to grow even faster than the global population, making energy an ever more expensive commodity as time goes by. So, weaning ourselves off energy-expensive materials is a very smart long-term strategy. Since it takes decades, quite literally, to introduce a new material into broad industry usage—keep in mind how long it has taken for carbon fiber composites to be to become popular—it only makes sense to start now, or last year, if possible.
My hobby horse
Let me make another point in my by now traditional flogging of the expired equine. We in business aviation will get little or no help from government. We have no NACA, no BACA, and the various space agencies like NASA and the ESA have no time for our concerns. Moreover, much as I admire the work done by GAMA and the NBAA, they do not perform any sort of basic research functions. The best we can hope for is that someone at a university somewhere thinks that research on this kind of composite might be a worthwhile path to a doctorate, and THAT does us in the industry zero good since it will neither adequately characterizes the materials nor certifies them, much less build up the necessary production base. Yes, we need our own think tank. If we in business aviation believe our own advertising, we are an enormous multiplier of executive effectiveness. If we truly are that valuable, we should be willing to invest in our own future. No one else will.
Terry Drinkard is a Contract Structural Engineer based in Jacksonville, Florida whose interests and desire are being involved in cool developments around airplanes and in the aviation industry. He has held senior positions with Boeing and Gulfstream Aerospace and has years of experience at MROs designing structural repairs. Terry’s areas of specialty are aircraft design, development, manufacturing, maintenance, and modification; lean manufacturing; Six-sigma; worker-directed teams; project management; organization development and start-ups.
Terry welcomes your comments, questions or feedback. You may contact him via email@example.com
Other recent articles by Terry Drinkard: