Aircraft Construction Materials

The debate continues, which is the best construction material for aircraft. For those contemplating a homebuilt project, I offer my humble observations form my metal RV project and from my friend's experiences with their projects.

Metal

Aluminum alloy aircraft structures have been with us for over 75 years. More aircraft have been built from aluminum than from all other materials combined. This is not by chance. Aluminum has many attractive qualities. It has a high strength to weight ratio, good corrosion resistance, it is easy to work with, has good dimensional stability and low toxicity. The material also has good conductivity for lightning protection, is homogeneous and has predictable bonding strength between panels.

As with any material, working techniques need to be learned. In this case, cutting, bending, forming and riveting are the main ones. Metal is probably the most difficult and time consuming material to form into complex shapes but it can be done. For standard aircraft fuselage and wing assemblies, the material is fairly easily formed. Large amounts of time are consumed marking, drilling, deburring and riveting. Cutting and shaping most parts is quite fast, although many specialized tools are required.

Aluminum is the least toxic of the 3 main materials to work with until you reach the corrosion protection stage. Etching, alodining and priming stages all involve toxic chemicals. These processes are a pain and you may need a spray booth of some kind to do the work properly.

Wood

Wood and fabric covered structures were the first used in aircraft construction but their popularity has rapidly declined in the last 60 years. Wood has a good strength to weight ratio and is very easy and quick to form into complex shapes. Its main disadvantages are its dynamic dimensional nature and suseptibility to moisture, heat and rotting. It is not the best choice for an aircraft which will be stored outside. Properly protected and cared for, some wooden aircraft last many decades and there is something special about the way that this material can come together into flying art. The DeHavilland Mosquito was one of the most pleasing and effective high speed, wood designs ever built.

Bond strengths between parts can be somewhat unpredictable and much time is spent clamping, gluing and varnishing. You don't need a lot of expensive tools to shape wood for aircraft and the skills are perhaps easier to acquire than those for metal working. Fabric covering presents its own problems once the basic structure is completed. A course offered by one of the covering supply companies or a friend with previous experience would be a recommended asset here.

Wood is fairly non-toxic to work with although new studies point to possible respiratory system damage with long exposure to certain types of wood dust. While the material itself is quite benign, the adhesives, protection varnishes and dopes used to finish the fabric are not. Again, proper protection should be used when working with these substances.

Composites

In the last 15 years, composite materials have come into much wider use, especially in homebuilt aircraft. The smooth, compound curves possible with these materials are undeniably efficient and beautiful at the same time. Properly designed composite structures using Kevlar, carbon, graphite and glass fibers and epoxy or polyester resins have the highest strength to weight ratios of any construction material for aircraft. Once the proper skills are developed, the materials are easy to shape and form with a minimal number of expensive tools. Corrosion resistance is excellent with no extra preparation required as with wood and metal. UV protection in the priming and painting stages is about the only step needed. Minor skin damage is easy and quick to repair with composites. The Glasair and Lancair designs are popular examples of composite homebuilts offering speed and beauty.

There are many different layup techniques used in composites. Some use foam sandwich cores. Others use a more conventional semi-monocoque type of construction. The big bugaboo with composites is bond strength between assemblies. This is somewhat unpredictable. A a result, composite structures need a larger fudge factor to ensure that even with an imperfect bond, adequate strength is there to prevent failure. This increased fudge factor over metal aircraft structures usually negates any real world weight advantage that composites materials might offer. Smooth flowing curves and smooth surface finish are quite possible to achieve although lots of sanding and prep work is required. The extended outdoor longevity of certain resins has not been well documented as there is simply insufficient history here.

Composites have several serious drawbacks for some people. The fibers and resins can cause allergic reactions in some people. For those not allergic, the materials are undeniably smelly, messy and the fumes and sanding dust are toxic. Many people have had to sell their composite project when they discovered that the fumes were too much to deal with or that they has an adverse reaction to them. You will be doing a LOT of sanding as well, so be prepared for the dust and be sure to use proper personal protection. On top of these problems, resins require stable and warm temperatures to cure properly and composites are not something you can leave in the middle of a layup for dinner or to answer the phone.

Before starting your own project, I would advise visiting several similar projects to the one that you are contemplating to see exactly what is involved. This may sway you towards a different design.