Building a Composite Aircraft
Sport Aviation - October, 1997
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.
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. This decade (1990’s), has seen a tremendous growth in the popularity of amateur-built composite airplanes. Higher performance airplanes with many varying appearances are being offered by a large number of kit manufacturers and also by designers who offer plans. This is truly an exciting time for our industry.
Before beginning our discussion of 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). I will further expand on the specific types of materials used later in the article.
To further complicate the issue, you will hear the words moldless and molded 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 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 our 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 in which you actually form the wing, complying with 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 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 IN BUILDING A COMPOSITE AIRPLANE
DECISION AND PLANNING
MATERIALS USED IN COMPOSITE CONSTRUCTION
Choosing the proper core material is critical to the overall composite’s performance. Note the illustration in figure 1. The first item is one piece of material with its respective weight and strength being shown as 1.0. When we insert a core material doubling the thickness of the composite notice that the strength increases to 3.5 the stiffness to 7.0 but the weight only increases by 3%. Further strength is noted by increasing the thickness 4 times. Observe even in this case the weight only increases by 6%.
Lightweight core materials include wood, foam, and honeycomb. Wood has obviously been around for a long time. It serves as a good core material for many composite designs. It is stiff, strong, and has high shear properties. However, its variations in density and physical properties along with the difficulty in fabricating limit its application.
Foam is usually the choice of material for the custom aircraft builder. Foams are easy to shape and reasonable in cost. Three types of foam are generally used within our industry. Polystyrene foam is the first. It is blue in color and is supplied in large billets. Polystyrene foam is often used to construct boat docks. This type of foam can be easily shaped using a "hot-wire" technique described later in this article. Polystyrene foam is the type used in several popular composite airplanes in the wings and control surfaces. It does have the disadvantage of being softened by exposure to gasoline and several other solvents. This type of foam cannot be used with polyester or vinyl ester resins both of which will be discussed later.
Polyurethane foam is basically a low-density insulating type foam also used for the construction of surf boards. Polyurethane foams are often used within a fuselage structure or for parts requiring detailed shaping. This type of foam is impervious to most solvents. Its color is usually tan or green. Polyurethane foam has certain hazards. It emits a poisonous gas when burned. DO NOT USE A HOT WIRE DEVICE TO CUT POLYURETHANE FOAMS. You also do not want to burn any scraps of this type foam. Carving and cutting should be accomplished using a knife, saw, or other cutting tools.
Polyvinyl chloride foams (PVC) are based on the same chemistry used in common PVC water pipe material. Divinycell and Klegecell are tradenames for this type of foam. Both of these are suited for structural cores. This material is resistant to most solvents and it can withstand a high temperature.
The last type of core material is honeycomb. This material has an appearance much as the honeycomb found in a beehive. The sheet material used to form honeycomb can be woven fabric, metal, or paper. Honeycomb cores are used very extensively in the aerospace industry. Varying thicknesses are available along with a wide variety of materials. Honeycomb is usually supplied in 4 feet by eight feet sheets. Honeycomb materials offer exceptional strength to weight ratios but reliable bonding to outer skins is more difficult to achieve.
Glass fiber or fiberglass is the most widely used reinforcing material. Fiberglass is manufactured with varying physical characteristics and cost. One of the most widely used is termed E-glass. This type of glass fiber has the best physical characteristics at the lowest price. One other type with limited use in our area is S-glass that is about 30% stronger than E-glass but the cost is often 2-3 times higher. Fiberglass is also offered in various weaves. The terms unidirectional and bi-directional are used. Unidirectional simply means all of the glass fibers are running in one direction-lengthwise. They are held together with threads running parallel to the glass fibers. Bi-directional fabric means the same number of fibers go across the material as found lengthwise. The type of weave is then defined. Several weaves are available such as plain, basket, satin, twill, etc. Fiberglass also is available in varying weights from less than 1 ounce per square yard to over 10 ounces per square yard.
Carbon fiber or graphite is a very strong reinforcement material. It is used on sail boat masts, golf clubs, etc. Carbon fibers combine low weight, high strength, and high stiffness. In the custom aircraft area, carbon is used in critical areas such as spars, etc. Working with carbon fiber is somewhat difficult and when it fails it will snap like a carrot snapping in two. Of course, the failure point where this occurs is extremely high.
Kevlar is a product of the DuPont Corporation. It is a very tough material with a high strength. It is used in making bulletproof vests. Kevlar is very effective in applications requiring resistance to abrasion and puncture. However, its use in primary structures is often limited by the relatively low compression strength and difficulty in handling.
Polyesters are most widely used for industrial applications and within the boat industry. They are cheap and they set up fast. A typical polyester is Bondo. Polyesters are easy to mix with the amount of hardener added only affecting the time needed to develop full strength. Polyesters are not suitable for applications requiring high strength. They also will shrink over a period of time. You may have noticed an automobile fender repair where the paint cracked over a period of time. Chances are Bondo was used as a filler and since it is a polyester it cracked under the paint. In a few words, polyesters are the least capable resin for structural aircraft use.
Vinyl esters are used extensively throughout our industry. Vinyl esters are a crossbreed between polyesters and epoxies. They are much more capable than polyesters in strength and bonding. Vinyl esters are low in viscosity making them easy to use. The cure time can also be easily affected by adding more hardener thus speeding up the cure time. Despite the cure time, hardened vinyl ester usually exhibits consistent properties of strength and flexibility. Vinyl esters are not subject to moisture problems during application and are also lower in price than epoxies. One of the disadvantages found in using vinyl esters is in the mixing of the chemicals. Vinyl ester resin is usually "awakened" from its dormant state with cobalt napthenate (CONAP) prior to use. Just before using the system dimethyl aniline (DMA) is added as an accelerator that determines how quickly the mix will cure and, in addition, methyl ethyl ketone peroxide (MEKP) is added as the hardener that actually starts the curing process. Mixing of these chemicals can be somewhat complicated in addition to being hazardous. MEKP mixed directly with DMA or CONAP , apart from the base resin, can be explosive. Overall, vinyl esters provide an easy to use, inexpensive resin system. Proper care certainly must be taken during the mixing process.
Epoxies have come to dominate the aerospace industry and are the basic resins used in most amateur-built aircraft. Epoxies differ from polyesters and vinyl esters in that they harden through a process termed "crosslinking." Epoxies are essentially long chains of molecules that intertwine when hardened to form a strong matrix of crosslinked chains. This provides an inner structural strength to the resin. When combined with the proper reinforcement material, composite structures using epoxies are unmatched in strength and lightness. Epoxies are packaged in two parts: a resin and a hardener. Unlike polyesters and vinyl esters, the resin to hardener mixture must be strictly followed. Adding more hardener will not accelerate the cure time, in fact, it may seriously impede the drying and strength of the cured resin. Epoxies are offered with different characteristics including strength, curing time, etc.. Care must be taken to follow the manufacturer’s recommendation regarding the type to use. Most epoxy cures at room temperature. Once this is complete additional strength is obtainable by raising the temperature of the epoxy through a process called "post curing." Usually this involves raising the temperature above 140 degrees Fahrenheit for a period of time. If this has not been properly accomplished the heat from a ramp on a hot day can "post cure" the epoxy on an airplane. Working time with epoxies can be much longer than polyester and vinyl-ester because you can use specific hardeners that have custom working times some as short as 4 minutes others over 24 hours at 70 degrees. This makes removing excess resin that may accumulate much less of a problem. Proper skin protection is a must with epoxies due to skin dermatitis that can be caused by the chemicals.
TOOLS REQUIRED FOR COMPOSITE CONSTRUCTION
In addition, you will need mixing cups, tongue depressors for stirring, and a large supply of latex gloves.
Building a composite airplane can be a very rewarding experience. The basics of composites have been presented in this article. Next month I will expand on the actual building techniques used with this type of construction. I will discuss safety issues, cutting and shaping foam, mixing resins, applying layers of fiberglass cloth, post curing, vacuum bagging, bonding, and many other composite building procedures.
All content, logos, pictures, and videos are the property of EAA
Copyright © 2011 Experimental Aircraft Association, Inc.