Composite applications


Written by Lucas Weakley
New Technology
Column
As seen in the Fall 2018 issue of
Park Pilot.


>> The modern aerospace industry is being flooded with potential applications of cutting-edge materials that are stronger, lighter, and more easily manipulatable than other previously released materials. Companies such as Boeing and Airbus are starting to make most of their airplanes from these materials, as are many other industry leaders. Although these materials are used in multibillion-dollar programs, park pilots and RC modelers can also benefit from their use.

What I’m referring to are composites. In this column, I’ll discuss some composite applications for RC model making, such as the use of pultruded carbon-fiber rods and tubes, building structures from cured composite sheet material, and fabricating aircraft components with raw fabric and epoxy resin.

What exactly are composites? The term is incredibly vague. The definition of a composite is simply the combination of two or more materials to create a new material with better properties than the individual components.

Wood, bone, reinforced concrete, taped foamboard, and many other materials we interact with daily are all technically composites. However, what I refer to in this column are two composite materials: glass fibers suspended in epoxy resin and carbon fibers suspended in epoxy resin.

Both composites consist of two components: the fibers (glass or carbon) and the matrix (epoxy resin). Fibers are very strong when tension is applied (pulling on both ends) but cannot support themselves in any other direction (think of a wet noodle).




This is a close-up of 6-ounce plain-weave carbon fiber. It’s best used in structural components because the fabric is heavy for an RC airplane.


A matrix material is incredibly strong and hard in all directions and is especially good at taking compressive forces. Because the matrix is so hard, however, it is also brittle and breaks easily under tensile and shear forces (offset opposing forces; think of rubbing your hands together). Combining the two materials stabilizes the flexible fibers and pulls together the brittle matrix, making an extremely hard, strong, and tough composite material.

In the RC world, we usually turn to composites to strengthen foam aircraft. Gluing a round, solid, carbon-fiber rod into a sheet of foam can bring a lot of rigidity to a flat wing or control surface. I’m willing to bet that most of you have used this building technique, but are you aware of the variety of these composite rods? There are rods and tubes in various shapes (trapezoidal, square, round, triangular, etc.) and a variety of diameters and wall thicknesses. There are also flat ribbon rods in all thicknesses and widths, which are incredibly useful for monodirectional stiffening.

These rods and tubes are made using a manufacturing technique called pultrusion. Fibers are pulled through a die that infuses heated epoxy. This produces a high fiber-to-matrix ratio composite with the fibers running in the same direction. This unidirectionality is useful in resisting bending forces, but the tubes are susceptible to collapsing because there are no fibers to resist splitting forces.

I’ve built many airplanes and multirotors using these rods and tubes. My favorites are the carbon-fiber ribbons because they are easier to use than round rods. When I want to keep a wing from bending, I can cut a slit using my X-Acto knife. I slide a carbon-fiber ribbon that reaches to both sides of the foam board into the slit.




The far left shows how a plain-weave fabric’s fibers are oriented at +30°. The middle shows -30° to the centerline. The far right shows the two pieces of fabric stacked. Notice how the fibers go in several directions.


With a little epoxy to keep it in place, the wing won’t even deflect when I bend it. You can visualize why this works so well by trying to break a Popsicle stick while only pushing on its edges instead of its wider faces. These rods are available online and usually cost a few dollars for a meter-long segment.

Another form of composite that is frequently used in RC is precured sheet composites. Like the pultruded rods, the sheets are formed under a lot of heat and pressure, creating a homogeneous material that is incredibly strong and dense. You can find these composites in multirotors, circuit boards, motor mounts, and any other application that needs a material harder than metal and lighter than most wood. These sheets are easily drilled and sanded and are often CNC cut on a router table.

I keep a 3 mm sheet around for hand fabrication of motor mounts and battery trays for my airplanes. Carbon-fiber sheets are expensive and offer greater performance, but G10 fiberglass sheets are widely available and are affordable for these applications.

Finally, there are wet layup composites. These involve laying dry fabric onto an object and wetting the fiber out with resin. Several layers of fabric are usually added to create a hard outer shell on a foam airplane. The same method could be used to mold a set of spring landing gear.

Wet composite layup techniques could be a column of their own, but I wanted to mention them here because the processes are usually foreign to people.

What weight of fabric should I use? What’s the best epoxy resin? Why did my layup come out all floppy? All of these were questions I first had when experimenting with composites, so let’s take them one at a time.




Here is a selection of pultruded carbon-fiber rods that Lucas had lying around. He uses the tubes to make hinges or spars that he can lock together with a second inner tube.


First, for strengthening RC models, fiberglass fabric is the best option. Carbon-fiber composites in the sizes that we need are too expensive for a simple park flyer. Fiberglass comes in a variety of weights and weaves, but for our purposes, plain-weave fabrics between 1 and 3 ounces will be plenty to add a rigid skin to a foam aircraft.

For epoxies, West System’s 105 Epoxy Resin products (westsystem.com) offer an affordable and versatile set of hardeners and fillers to meet almost every wet layup need, and they won’t melt foam. The epoxy is also great for joint bonding, and the products are even available at some hardware and marine supply stores, such as West Marine (westmarine.com). The company has a helpful selection guide on its website.

Fiber orientation is usually where most people run into problems and why some composites end up weak. The basic idea is that you want to ensure that there are fibers running in the direction of the load that will be acting on the structure. For general airplane structures, we want fibers to run in all directions along the exterior of the airplane.

I typically cut my fabric to be laid up in two sheets of -30° and +30° of the structure’s centerline. With plain-weave fabric, this two-layer composite makes a strong, stacked hexagonal pattern. I could elaborate on this, but wet layup is a skill that is best learned through trial, error, and YouTube videos.

Composites can make simple park flyers into very rigid and streamlined aircraft, making them handle better, be more crash resistant, and have longer lives. Look for more projects from me involving composites. I have several in the works!

-Lucas Weakley
lucas.weakley@gmail.com






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