Tips for Composites in Experimental Aircraft Design

Table of Contents

Design and construction of experimental aircraft has a unique set of challenges compared to other products. While reducing weight to increase performance is not a new concept, aircraft design takes it to the extreme. Lightweight, strong, rigid, durable and often low cost are all critical characteristics of components used in aircraft. That’s where composite materials can shine. 

Whether you’re developing a complete aircraft design, manufacturing production parts or just building individual parts, composite materials can be a useful addition to your materials library. 

In this article we’ll discuss some different types of composite materials, including some examples and give you a small taste in manufacturing them on your own. We’ll also cover where composites make sense and some reasons you may not want to use them. SendCutSend carries composite materials you can order, laser cut to your specifications and delivered to your door. 

Check out our article on composite materials for some more info on what we offer and help to choose between them.

What is a Composite Material?

Composite material is a general term used to describe two or more unique and individual materials put together. In the new composite, the individual materials remain, they aren’t mixed together like a metal alloy. The individual materials still have their unique physical properties, however the compositing of them together can create new, often superior, physical properties when compared to any of the individual components.

Because the term composite is so general, it means composite materials and their properties can vary quite a bit. The right combinations of materials and processes to bring them together can produce a multitude of desirable (or undesirable) properties. Improved strength to weight ratios, higher fatigue resistance, better corrosion resistance, stiffness, thermal properties, electrical conductivity, vibration damping characteristics and much more can all be tuned to excel in specific applications using composite materials. New composites and techniques are constantly being developed for use in all sorts of applications. Experimental aircraft are certainly not being left out. 

8 Examples of Composite Materials

1. FR4/G10

FR4/G10 – A combination of fiberglass cloth and epoxy resin, G10 is used a lot in electrical applications due to its electrical and thermal properties. For more information, we’ve written an entire article on FR4 and its benefits.

2. Carbon Fiber

Carbon Fiber has a nearly unmatched strength to weight ratio. It’s used in the highest performance applications from exotic supercar construction, motorsports and aerospace. Because SendCutSend carries carbon fiber, we’ve an article dedicated to it. Learn more about how to use carbon fiber for your next build.

3. Graphite Epoxy

Graphite Epoxy is not known for its strength, but rather for its ability to create a low friction surface coating. Graphite is used often by itself as a lubricant. As an additive to the right epoxy, it can create a low friction composite coating. It is used a lot in the marine and aerospace industry.

4. Aramid Fiber

Aramid Fiber — you may also know it by one of its common trade names, Kevlar and Nomex — are both made from Aramid fiber. Aramid fiber is similar to fiberglass and carbon fiber, but instead of glass or carbon, it uses aramid fibers. Often lighter than carbon fiber and known for its exceptional impact resistance and toughness rather than tensile strength like carbon fiber. It can also have fire resistant properties making it useful in protective equipment. Aramid fiber is commonly used in body armor and safety equipment.

5. Sandwich Composites

We’ve lumped together a number of different composites under a single category called Sandwich Composites. A common practice to create a new composite is to build a sandwich from different materials. Typically the outer skin material provides the durability, abrasion resistance, corrosion resistance, or whatever properties are desired. While the inner material can be less durable, lighter weight, thermally insulative, etc. This is beneficial because often the strength of a material comes from its ability to resist stress or carry loads along its outer skin, while the core contributes much less. 

This is the same reason structural tubing is nearly as strong as solid material of the same dimensions, but is considerably lighter. Some examples of sandwich composites are Foam Core with a paper or plastic skin over a lightweight foam interior. ACM or Aluminum Composite Material, which is aluminum skin surrounding a plastic core. SendCutSend carries ACM panels. Even plywood can be considered a sandwich composite.

6. CFRP

CFRP (Carbon Fiber Reinforced Polymer) — or  Glass filled nylon — is used in many commercial products like power tool frames. Swapping the fiberglass for carbon fiber just takes it up a notch. Carbon fiber nylon has become a popular material for 3d printing, often compared to aluminum, which is high praise for a part that can be made on a sub $500 3d printer. Adding chopped carbon fiber to polymer can increase strength, hardness and stiffness, improve fatigue and creep resistance and more. Intelligently routing continuous strands of carbon fiber within the polymer allows designers to specify exactly where the increased strength is needed such as along stress concentrations or around holes or even just at evenly spaced intervals.

7. Cement Based Composites

Cement Based Composites is a common composite that uses concrete known as Ferro-Cement. This is simply concrete poured over a metal (typically steel or iron – that’s where the “Ferro” comes from) skeleton or frame structure. You see this all the time in construction. Rebar is placed and concrete is poured over it. While concrete alone is very strong in compression, it has very little strength in tension. By embedding steel into it, it greatly increases the overall strength significantly reducing the brittle nature of the concrete structure. 

Lately, additives such as glass fibers and carbon fibers have been added to concrete mixes. For smaller structures, this can perform similarly to steel reinforced concrete. Finally, while not as popular likely due to cost, some concrete structures have been wrapped in carbon fiber cloth, which shrinks slightly as it cures and applies a compressive load to the concrete, helping to keep it from chipping and cracking on the surface over time. There are other cement based composites, but they tend to be less useful for developing experimental aircraft.

8. SiC Fibers

SiC (Silicon Carbide) Fibers and SiC Ceramic Matrix Composites is at the more exotic end of the spectrum for composites as it’s still being studied and developed. Meant to be used in extreme high-temperature structural applications with performance improvements over other materials including metals and other composites.

Considerations for Composite Design

There are several design considerations to be aware of when dealing with composite materials. Often testing samples of materials can help you determine which is the best for your application. This is certainly not an all inclusive list, but at least provides a starting point of aspects to consider when dealing with composite materials.

Anisotropic

Many composite materials are anisotropic. What this means is that the physical properties of the material, such as tensile strength, are not consistent in all directions. Carbon fiber for example can have a specific weave pattern to give it properties in one or two directions, but will have different properties in the third orientation. This is true of many composite materials that are laid out in planar or sheet form, but can also be true of materials with specific shapes such as the steel rebar in a concrete form. It’s important to understand the load conditions (all of them) of your parts before deciding which composite materials may be useful. If you’re building out the composite materials yourself you’ll have some control over this. When buying composite material, this information should be available.

Strengths

Composite materials get their strengths AND weaknesses from the constituent materials that make them up. While they may work together to give you superior properties in one area, that may come at a cost in another. For example, you may gain high temperature resistance to use a part in or around hot exhaust components, but if the trade off is an unacceptable loss in strength, then you should look into other materials. Engineering parts and systems for experimental aircraft involves more than just one consideration at a time. Strength at the cost of fatigue or vibration damping at the cost of corrosion resistance may not be worthwhile trade-offs.

Cost 

Building your own composite materials can be a good way to save costs in the long run, but depending on the process may require a higher initial investment. Some composites require special equipment like vacuum bags or heated chambers to correctly assemble. You may need to construct molds to get your composite material into the correct shape. Building the composite in-house also gives the ability to control how the materials go together whether that’s specific layer orientation, layer thickness, layer orientation and quantity. It may make sense to start (or prototype) building parts using out-sourced composite materials until you determine your exact needs.

Keep in mind SendCutSend carries a variety of composite materials like FR4, carbon fiber and ACM panels. We can also supply other materials and services to help build out tooling and molds.

How to Use Composite Materials in Experimental Aircraft

Composite materials can be effective on their own, but when building experimental aircraft it’s critical to be able to integrate them into the rest of the system. That may include adhesive bonding, fasteners, press fits or any other type of joint between similar composites, different composites or composites to metal components. 

Many composites aren’t ideal for tapping threads. In those cases you may want to mold a composite material around another metal component, effectively making an inseparable assembly. This is also useful to reinforce specific areas of a part, like panel hinges or to add embedded hardware like studs, nuts or parts with tapped holes. Note that if you need tapped holes, countersunk holes or PEM hardware installed, SendCutSend offers those as a service.

Advantages and Disadvantages of Composite Materials

The advantages and disadvantages of any composite will depend on both the application and the specific composite. Carbon fiber is going to have an advantage over mild steel  for many aircraft components because of its strength and low weight. Rebar reinforced concrete on the other hand, doesn’t share those same advantages in aircraft applications.

Since we’re focusing on composites for use in experimental aircraft, let’s take a closer look at the more traditional/common composites for that application, like carbon fiber and compare against traditional metals like steel and aluminum.

AdvantagesDisadvantages
High Strength-to-Weight
Carbon fiber can be stronger and stiffer than steel and many times lighter. Even compared to aluminum, CF still has the advantage.
Susceptibility to Impact Damage
Carbon fiber is very strong in the correct orientation, but impacts tend to happen in the weaker directions and can easily cause damage.
Corrosion Resistance
Unlike steel, carbon fiber won’t rust. It can be susceptible to UV, but that can be easily mitigated with the appropriate coatings.
Limited Temperature Resistance
Carbon fiber can have good temperature resistance, but at extreme temperatures it can be damaged or start to warp.
Design Flexibility
Sheet metal can be bent and welded to form complex shapes, but that requires a level of skill to get quality results. With carbon fiber (and similarly fiberglass), it can be molded around a form with little expertise or skill to get the shapes you need.
Complex Repair Procedures
Depending on the level of damage, carbon fiber can be patched, but to get a similar level of strength as the original, the patch may suffer cosmetically. It can also require a lot more skill to make a quality repair.
Fatigue Resistance
When compared to materials like aluminum which suffer from fatigue cracking, composites are better at resisting fatigue.
Material Degradation with TimeAs with many plastics or composites that contain epoxy, they will degrade over time. Protection from oxygen and UV light can help prolong that life.
Reduced Manufacturing CostsMaking parts in quantity takes little more than materials once initial equipment and molds are completed. There is some labor involved, but it doesn’t need to be highly skilled labor.High Initial Cost
Equipment and mold making can be a higher initial cost when doing carbon fiber layups. You may need to invest in vacuum bags or heated chambers. Making molds can require some skill to design.

Composite Manufacturing Techniques

We’ve touched on composite manufacturing techniques in previous sections of this article, but let’s take a more detailed look here at a couple examples.

Epoxy Resin Based Composites

Traditional manufacturing of composites like fiberglass and carbon fiber involve building a negative mold. This is done from scratch or by copying an existing part. The mold is used to layup alternating layers of resin cloth. Any embedded metal components can be placed into the desired layers. Once cured, the rough part is removed from the mold, cleaned up and finished.

While this method still works, it’s not the only method that can be used. 

Alternatively, vacuum methods can be used. The cloth is laid over the form and a vacuum is pulled to draw the resin into it. This method requires more equipment, but can produce higher quality parts. There are also methods involving heat and pressure curing using pre-impregnated (prepreg) cloth. 

Forged carbon fiber involves small pieces of chopped up carbon rather than sheets of cloth that gets compressed (along with resin) into a mold under pressure. It produces a unique aesthetic and can allow for more intricate shapes than cloth.

Molds can be made in any convenient way. They can be copied from existing components, whether that’s a body panel or machined prototype. They can be built up from raw materials like steel or plywood. They can even be 3D printed.

Sandwich Composites

The critical piece to manufacturing sandwich composites are proper bonding between the layers. This means the gaps between layers need to be closed and an appropriate adhesive needs to be used otherwise you risk the layers delaminating. The easiest shape for sandwich-style composites is flat sheets. Sometimes simple clamping pressure evenly across the surface is sufficient. Sometimes you’ll need to pull a vacuum or apply heat to properly cure everything. It just depends on the materials you’re using.

Composites and Experimental Aircraft

We hope we’ve given you enough information here to whet your appetite for composite materials. Composite materials really are a no-brainer for modern experimental aircraft, not as a complete replacement for metals, but as a supplement to take your aircraft to the next level. The varieties of composite materials mean you can pick and choose the best for the application and your manufacturing capabilities. From tooling, prototyping and testing to full-on production of composite parts for experimental aircraft, SendCutSend is here to provide the precision cut materials and services you need. Get free instant pricing today!

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Hole size in sheet (+0.003/-.0.000).168″
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When determining the distance between two or more fasteners, you can calculate the distance by the formula, C/L to edge + 1/2 the diameter of the second mounting hole..345″
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