CARBON ROD in Aircraft Wing Design

Whether your spars are constructed of fiberglass or carbon roving you must take a serious look at this new miracle structural material.  It's more than six times stronger than 2024-T3 aluminum, twice as stiff and nearly half its weight. Compare this remarkable carbon rod with wet lay-up carbon roving; it's nearly 3-1/2 times stronger in tension and 5-1/2 times as strong in compression.

It's the most exciting material to hit the aerospace market since the introduction of fiberglass.

Commonly known as micro-carbon, this material is cost effective and consistent in properties. You can build wing spars faster with much greater reliability. Fabricated in one operation, in a female mold, it cuts assembly time in half. Another benefit of a female mold is that the outside dimensions are always consistent.

The current problem with wet-hand layup carbon roving is that you cannot lay all the filaments down straight and/or achieve a consistent resin content. In addition, air bubbles invariably develop. Trying to push them out ends up disturbing the filament alignment. As a result, your laminate strength has a lot of localized strength deficiencies.

Here is how this problem was solved. A new, highly modified pultrusion process forms a round or rectangular carbon rod in a machine which lies in all filaments straight, parallel and under equal tension. Resin content is closely controlled to +/- one percent. Maximum performance is obtained in every fiber resulting in tensile strengths exceeding 300,000 psi and 200,000 psi compressive strengths, far above the nearest contender. Coupon testing has shown consistent strength values with very small scatter. Modulus of elasticity is 21 million.

The stickler here is deflection. Because so little carbon rod is required and the material is so strong the beam just keeps bending as the load is increased. Don’t let the high modulus fool you. The modulus of elasticity of Micro Carbon rod (21 mil) is twice that of aluminum (10.5 mil). If you are building a short thick wing, the deflection is probably not a problem. However, some wings are long and thin like a sailplane’s.  By doubling the number of rods in the cap, the deflection is cut in half equaling that of aluminum. Despite the added cap weight, you are still way ahead in total weight and cost savings.

Some builders want to mix the rod into an existing wood spar. Yes, you can do this. It's not as weight efficient as an all-carbon spar but the little weight added is far offset by the strength gained. You need not scuff the surface of the rod prior to bonding. In fact, no surface preparation is required. Most pultruded rod has oil added to the resin to assure smooth flow through the heated dies. The manufacturing process of Micro carbon rod does not require any foreign matter to be mixed into the resin or injected into the die during the manufacturing cycle. The matrix, or binder, of the carbon fibers is "BIS F" Epoxy. Keep in mind that epoxy likes to bond itself to epoxy.

The micro-carbon rods are rolled off a spool, cut to length and laid into a female wing spar mold and embedded in an unfilled epoxy resin matrix. The rod ends are cut off square with a Dremel cut off wheel or a cable cutter. The rods do not require cleaning or sanding to improve bond strength. Rather than weaving glass fabric through the rod pack we let the epoxy matrix carry all the shear loads. This is similar to steel reinforcing rod set in concrete for structural beams or roadways. 

Over 90 pages of detailed, technical information, instruction and workshop techniques. Based on Jim Marske's extensive experience working with composites, this manual is invaluable for aircraft designers and builders working with composites including carbon micro-fiber rod.

Click here for details on the Composite Design Manual