Composite Construction By Ron Alexander

Within the sport aviation world, the term “composite aircraft” is synonymous with sleekness of design and speed. These airplanes, composed largely of fiberglass, are becoming more and more popular. Actually, composite aircraft construction is not a new idea. Gliders have been constructed using fiberglass for many years. Throughout aviation history, advances in design have been made. Beginning with wooden structures that were covered with fabric, technology then advanced to welded steel frameworks and on to aluminum. As each type of construction was introduced, design improvements were made in strength and aircraft performance. Composite construction is yet another advancement for the aircraft industry. Fiberglass construction has been and continues to be used in manufacturing a number of parts found on most airplanes. Of course we now see many airplanes that are constructed almost exclusively out of composite material. Composite technology has certainly changed the entire aviation industry and in particular sport aviation.

History of Composite Aircraft

Amateur-built composite airplanes were actually introduced during the 1970’s when Ken Rand introduced the KR-1. Burt Rutan also introduced the VariViggen that featured some composite construction. Rutan introduced the VariEze in 1976. This airplane design included a more comprehensive type of composite construction using moldless techniques. The term moldless will be defined later. The VariEze was very successful inspiring Rutan to develop the Long-Eze. During the 1980’s, several other designs were introduced to sport aviation enthusiasts as popularity of this type of construction heightened. It was during this period of time that aircraft “kits” were first introduced. Supply companies began offering material kits to builders to simplify the building process. Plans for composite airplanes could be purchased and then materials for each phase of construction could be obtained on an as needed basis. The amount of time needed for completion is a factor in building an airplane from a set of plans. With this in mind, several companies began introducing their own airplane design in a kit form. The objective was to allow the builder to spend less time actually constructing the airplane. A large number of parts and pieces were manufactured by the company and sold to individuals. This concept introduced the pre-fabricated kit airplane that is popular today in all types of construction.

During the late 1980’s through today we have seen many composite aircraft kits offered to prospective airplane builders. Higher performance airplanes with many varying appearances are being offered by a large number of kit manufacturers and also by designers who offer plans.

Defining Composite Construction

Before beginning our discussion of aircraft composite construction let’s define the word “composite.” The dictionary defines a composite as “a complex material such as wood or fiberglass, in which two or more distinct, structurally complementary substances combine to produce structural or functional properties not present in any individual component.” In simple terms, a composite structure has more strength than the individual components that make up the structure itself. For our purposes, the component parts comprising a composite structure consist of a core material, a reinforcing material, and a resin binder. Each of these substances alone has very little strength but combined properly they become a composite structure that is very strong.

To further explain the structure, the core material keeps the reinforcement fibers separated so that they can be kept in maximum tensile (tension or stretching) strength. The reinforcement fibers carry the load. They must be properly oriented to achieve their maximum potential. The resin keeps the fibers in place so they can maintain straightness and deliver their maximum strength. The resin also binds the fibers to the core. Therefore a composite structure is really a mixture of critical components. When loads are applied to a wing, as an example, the majority of the stress occurs at the outer surfaces. To take advantage of this principle, a sandwich panel is designed with two working skins on the outside that are separated by a lightweight core. This type of design concentrates the strength in the area of high stress (outer surfaces) while reducing the weight in the area of low stress (inside the wing).

Moldless and molded type construction are terms used in composite construction. To define these words as they apply to us is relatively simple. Moldless construction, as the name infers, does not use a mold. This technique allows the builder to construct a part by forming a core material to a desired shape and then laminating the reinforcement material to the shaped piece to make up the final part. The core structure, usually a foam like material, allows the builder to employ virtually any shape desired. Original designs such as the Vari-Eze used moldless type construction. Many airplane designs continue to use this type of fabrication. Moldless techniques allow the builder to produce a safe, superior airplane without the requirement of expensive equipment or extensive experience.

In contrast, molded fabrication uses a mold to build the part. A master mold or “plug” must first be built in the same manner as you would build a moldless part. You then construct a working mold from the master and then finally make the actual part from the working mold. Within the homebuilt industry molded composite construction is very popular. A large majority of kit manufacturers use this type of fabrication. Molds are made by the kit manufacturer who then fabricates the parts from the mold. The manufacturer then supplies you, the builder, with the parts. As an example, a wing kit might consist of two wing halves, built from a mold, along with the necessary ribs. You would then assemble the wing by bonding the ribs to the wing halves and, of course, bond the halves themselves together. Compare this with moldless construction where you actually form the wing, following a set of plans, out of a foam material. You then place several layers of fiberglass on the foam using resin to bind the two. The end result would be very similar. One type of construction (moldless) has a core material you have shaped that is solid whereas molded usually has thin cores that are sandwiched between skins and you actually assemble the supplied parts. Building a molded type composite kit is very similar to assembling a plastic model airplane. The building of most amateur-built composite airplanes will often require use of both types of construction.

To summarize our general discussion, composite structures that combine the best qualities of diverse materials have opened a new world to the airplane builder. Modern composite construction offers several advantages over conventional techniques. While safety tolerances for metal structures are often designed at 1.5 to 1, lightweight reinforced composites allow “overdesign” by factors of several times, increasing both safety and performance. These designs also achieve better aerodynamics by eliminating joints and rivets in addition to reducing problems of corrosion. Composite design allows an easy way to achieve a low drag airfoil. Composite airplanes are usually faster for a given horsepower than their counterparts because of airfoil shape and smoothness. One common misconception that does exist is that composite airplanes always weigh less than metal airplanes. This is often not the case. Fiberglass is heavy. If we were to construct an airplane wing out of solid fiberglass we would have a very heavy airplane. Remember though, instead of doing this we insert a piece of core material between layers of fiberglass to reduce the weight. Kit airplanes use ribs and more contemporary types of construction to achieve the high strength with a lower weight.

Steps of Construction

Building a composite airplane entails five stages of construction. These five stages are (1) planning, (2) basic building and assembly, (3) systems installation, (4) filling and finishing, and (5) inspection, certification, and test flight. Certainly there exists overlap within these stages. A brief overview of each step follows.

Planning

This phase of construction is critical to the successful completion of an amateur-built airplane. You cannot spend too much time planning. A large part of the planning process is technical knowledge. Composite construction, like all types of construction, requires a certain amount of basic knowledge. The EAA offers a 2-day workshop (EAA SportAir Workshops program) explaining the techniques of composite construction with time spent actually building airfoil sections utilizing this method of construction. I would highly recommend that you attend one of these courses prior to starting to work on your project.

Study the assembly manual and/or plans early on. Start laying out your workshop space and tools (more on this later). In other words, have an overall plan before you actually start building.

Basic Building and Assembly

Start the building process with a small control surface. The majority of kit manufacturers and designers will make this recommendation. This stage of construction is obviously the most time consuming. You will build all of the component parts of the airplane and then begin to assemble them.

Systems Installation

During this step the builder will install the electrical system, hydraulics (if present), engine, propeller, avionics, etc. Many of these items will be started during the basic building stage and will be completed during this time.

Filling and Finishing

This can be a challenging stage of composite construction. Filling and finishing can often take a large amount of time. It depends upon the plans/kit and how the parts have been finished from the factory. If you are building a plans-built airplane, this step will be time consuming. There is usually a lot of sanding and filling to be done to get that perfect finish.

Inspection, Certification, and Test Flight

After you have completed the building process, the FAA Inspector must make a final inspection on the airplane and deem it airworthy. After finding it safe for flight they will issue an airworthiness certificate. You are then ready to test fly the airplane.