In Defense of the Composite Homebuilder

Brian Martinez

Originally published October 1992

This is just a little note about a disturbing trend which I have noticed recently in the aviation press concerning the perceived weaknesses of composite aircraft. Recent articles have implied the following:

  1. Composite aircraft are no lighter than aircraft of aluminum construction. Learfan and the Beechcraft Starship are obvious proof of the inferiority of all composite aircraft.
  2. Only aviation gas can be used in foam/fiberglass airplanes which use Epolite resins as the primary structural adhesive.
  3. All composite aircraft must be painted white and stored in a hangar.
  4. Composite surface impacts will cause delamination and reduce the strength of the part.

As a background for a lot of this controversy, let me say that in the past a lot of inflated claims have been made for the benefits of composite construction. Many of these claims grew out of the great furor which accompanied the advent of this type of aircraft on the homebuilding scene. As with most things there are realities and there are fantasies. Additionally, I should further define the types of homebuilt composite aircraft which most of us know. These types include:

  1. First generation composites in which most or all of the structural components were built up using room-temperature cure resins, E-glass, and hand shaped foam core.
  2. Second generation composite kits containing some major structural elements which are prefabricated in molds.
  3. Third generation composite kits having most or all structural elements fabricated in molds.
  4. Fourth generation composite kits having all molded structures utilizing high-temperature curing resins, S-glass and carbon fiber reinforcements, and autoclaved/vacuum bagged parts.

In general, first generation composite aircraft can probably be built as light or lighter out of aluminum. The great benefit of the first generation of composite aircraft is the ease of fabrication with primarily woodworking tools and the simplicity of forming complex shapes. The various "Ezes" are all fabricated using E-glass (the less costly and lower performing variety of fiberglass) and room-temperature curing resins. The result of the "easy to use" compromise is that under heat and load the finished components may creep and deform at fairly low temperatures (180 degrees F). The foam cores upon which the laminates are bonded are also sensitive to heat and solvent contamination. As long as these types of structures are sealed, temperature protected, and ultraviolet light shielded, they will work fine. Constant inspection will assure safe operation of the structure. Where these first generation composites still show performance advantages is more likely in the area of configuration. They are typically smaller in size than comparable production aircraft using the same power, and as a consequence have a lower weight, wetted area, and drag. Modern laminar flow airfoils further enhance the performance of first generation composite airplanes.

Learfan is the perfect example of the wrong way to design a composite aircraft. Instead of building an aircraft to capitalize upon the strengths of high performance composites, the Learfan team chose to apply composites in the same way they would metal in conventional aircraft with the associated fasteners and metallic load carrying structure. Beechcraft side-stepped a novel approach to reducing weight and production man-hours. There was an early attempt to filament wind the fuselage which met with some early success until conservative management decided upon tape layups in order to reduce risk. The Learfan and the Beechcraft Starship in many ways were either ill-conceived or poorly executed. Metal construction is fine. Composite construction is also fine. Each must be applied to a design according to its strengths and with a carefully thought out set of tradeoffs in mind.

One of the biggest problems in homebuilding is "Bone-Headed Engineering". In other words, there are too many folks out there in the homebuilt movement who think one-dimensionally concerning building materials and applications. There is also too much religious belief in one design guru or another. Also remember that kit manufacturers and designers frequently have different motivations from those of the builders. Sales and liability may dictate that alternate construction methods and competing designs be trashed in the popular press. Aircraft are compromises and neither wood, metal, nor plastics will present a perfect solution to the design problem. If you want to be safe and don't want to strain your brain, just build per plan. If you want to get a little more performance out of your project just do the research and make reasonable changes. Also, be sure and check out your work by doing tests. For example, if you want to significantly improve the chemical resistance of your composite fuel tanks, line them with DOW Derakane resin. The chemical resistance charts are fantastic for this stuff and the 470 formulation of Derakane has a demonstrated ten year service temperature of 350 to 450 degrees F. It is not compatible with blue foam but it is easily applicable to fuel tank construction and sealing of composite tanks in first generation kits. Auto gas is no problem. I tested it.

If your airplane is a first, second, or third generation composite, paint it white and try to store it in a hangar. If you want a high quality part which will far exceed the performance of aluminum use high temperature toughened resins, AS4 carbon fiber or better, and compatible core materials. Properly designed, bagged, and cured, your part will "slamma-jamma" traditional metal construction. And by the way, once you finish your truly advanced composite part, go ahead and paint it B-2 Carbon Gray or just plain F-18 Dark. After all, your uncle does.

Composite surface impacts will cause damage to the structure, but in most cases the structure is redundant enough to allow for it. Carefully inspect all surfaces on all composite parts. Tap test regularly, N-ray (that's neutron-ray) if you must. One hint which I got from some folks in the Structural Mechanics Code at China Lake was to use a thin layer of Kevlar on surfaces subject to routine impact damage. This method has the potential of keeping up to 90 percent of the strength of a part after impact and had been demonstrated on a number of carbon fiber missile motor cases.

Summarizing, don't believe everything you read in the popular aviation press. Like it or not, designers and manufacturers are out to make money and stay in business. Competitors will slam each other at will. Remember that sources of independent test data do exist and if you do your homework you can access it. The most important thing is to get started and finish your project. If there is a problem, just fix it.

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Revised -- 22 February 1997