By Budd Davisson, EAA 22483
This article first appeared in the March 2019 issue of EAA Sport Aviation.
When the vast majority of general aviation pilots (as opposed to purely recreational pilots) think about buying a more-or-less modern airplane, the question of what material that airplane uses in its construction isn’t a deciding factor. This is because modern general aviation aircraft basically use only two materials — aluminum and composites, and GA pilots don’t seem to prefer one over the other. The design is what makes the decision.
There is a small subcategory of modern aircraft — which includes all of the American Champion, Aviat Husky, CubCrafters, and Maule lines — that still builds rag and tube aircraft. However, almost without exception, those aircraft fall outside of the interest range of most general aviation pilots and edge into the primarily recreational category of interests.
Beginning before World War II and continuing up through the classics to the classic-contemporary aircraft of the ’60s and ’70s, three of the four primary building materials were used. Amateur-built aircraft started out with wood and rag/tube and then added aluminum and composites in recent years, bringing the total up to four.
When viewing those four different materials, which are often mixed together in the same airframe, the issues can get fairly complicated. Further, when we think certified aircraft versus experimental, it gets even more complicated. So, we’ll break the discussion into two parts, certified/factory built and experimental, and within each part discuss all four materials. We’ll address their design and construction characteristics as well as ownership issues.
Certified Aircraft
Prior to WWII, the majority (but not all) of certified, single-engine aircraft available were tube fuselages with wood-framed wings and wood fuselage formers covered in fabric. By the late ’30s, the occasional all-aluminum airplane would be seen, such as Luscombes and Ercoupes. WWII pushed the balance of materials toward aluminum, and little by little, all major manufacturers (the first Bonanzas had steel tube center sections) went to aluminum with the exception of Piper. However, even Piper, which doggedly held to rag and tube, eliminated structural wood and went to aluminum spars right after the war and introduced monocoque aluminum with its Apache, but even it had a steel tube center section. Piper got out of the rag and tube world with its Cherokee in 1960. However, even then the Super Cub would pop up from time to time to remind the world that rag and tube still has its place in aviation.
The Fatigue Differences Between Steel and Aluminum
A little-known fact that differentiates steel from aluminum is contained in what engineers refer to as the S-N curve. Every material has such a curve that shows the way in which alternating loads affect the strength and eventual life of that material. The S-N curve for steel says that if the loads are kept below a certain limit, which is relatively easy to calculate, all things being equal, it has an effectively infinite fatigue life. Bend it as often as you want within that range and it won’t fail. Aluminum is a different story.
The S-N curves for aluminum say that no matter how thick it is, or how light the loads are, if you flex aluminum enough times, it’ll develop fatigue cracks, period. It may not fail in our lifetimes or those of our grandchildren, but keep flexing it and it’ll fail. So, yes, flight hours do matter. However, aircraft designers do their best to use conservative numbers so the fail point is pushed so far out into the future, that it won’t be a factor. However, exactly where in the future it’ll fail is difficult to predict, so they just over-design it. This is why some foreign countries place time limits on aircraft.
Corrosion Differences Between Steel and Aluminum
Most aluminum used in modern times is alclad, meaning that it’ll have a very thin coating of pure aluminum applied. Pure aluminum oxidizes quickly and looks dull when exposed to most common environmental conditions. Once a coat of oxide is developed, for a time, that coat protects the patent metal from further oxidation. Plus, aluminum is pretty resistant to corrosion caused by pure water. That’s why bass boats and such used in fresh water are often built out of aluminum, but manufacturers avoid using aluminum in boats meant for saltwater use. The same, obviously, cannot be said about steel.
Steel, especially 4130 chromoly, which is the most common alloy used in aircraft today, rusts like crazy. Leave it unprotected by oil or paint in many parts of the country and it’ll rust overnight. If moisture and air are present, even in the desert, eventually it’ll rust unless it is continually protected by a coating that has no faults in it. Scratch the paint/coating to the surface of steel and eventually you can be guaranteed rust will form. If it’s actually pitted, that’s a deal breaker, and the hacksaw and torch are called for.
What About Wood?
Wood is nature’s original composite. Its many internal fibers cling together in a matrix, making it react like fiberglass cloth held ridged in epoxy. Obviously, it doesn’t rust or corrode. In fact, if kept in minimal humidity and out of the sun, wood can be considered to have a life as long as any other material. In terms of fatigue, it can last longer.
Wood is not without its downsides. For one thing, because it is organic it has to be protected from moisture or things can grow on it. Actually, rot isn’t something growing on wood. It is a living “something” (fungus or bacteria) that is eating it. Being organic, wood is part of the food chain, so we use finishes to keep the moisture content of the wood too low to sponsor growth. Some of the finishes are purposely toxic because the fungus and bacteria are living things that would love to eat our dead-but-strong wood, and the finish kills them.
Aircraft and marine plywoods use special glues and finishes to protect the layers of glue and keep them from delaminating, which, if the conditions are wrong, generally takes a long time to occur. However, when a wing skin starts delaminating, be prepared for a long job replacing that skin and repairing the damage that’s under it. If there’s enough moisture to delaminate a plywood skin, the internal structures are likely to be suffering as well.
Woods like spruce have good strength to weight ratios, so it’s possible to design and build really light, strong structures. However, because it is a purely “linear” material, it takes a lot of different-shaped pieces glued together to make the structure work. In a vintage, all-wood structure, like a Bellanca’s wing, what this means is lots and lots of pieces, large and small, glued together to make sure the forces are transitioning smoothly from one part to another. This, in turn, means that the bazillions of glue joints have to be intact so the structure can work as advertised. By “intact” we mean they haven’t been compromised by moisture or anything attacking the glue. However, if the wood is allowed to get too dry, as in the desert, there is not enough moisture to make the wood appetizing to fungi or bacteria, but it then has to worry about shrinkage compromising the glue joints or splitting the wood itself.
Wood airplanes require some understanding and care to keep them from becoming dinner for bad things or self-destructing. This care begins with a hangar or spending an inordinate amount of time making sure every surface is totally watertight and shaded, if possible.
Certified Composites
One of the truly important airframe changes in the last few decades is the development and acceptance of composite structures, which are light, strong, and can be shaped to any imagined form.
Basically, composites used in modern airplanes are impervious to just about everything except sunlight and condensation. The fabric (glass or carbon fiber) is little affected, but the ultraviolet rays/water combination can attack the matrix (the resin, etc.), which weakens the bond between layers. It’s a lot more complicated than that, but this explains why composite airplanes are usually, but not always, white. Recently, the development of really good UV blockers and stronger matrix materials has let darker colors enter the composite world.
In terms of caring for composites, it’s the same as wood and fabric-covering: keep it dry and out of the sun. Not always easy.
The Big Four Materials and Experimental Aircraft
During World War I, Anthony Fokker pioneered welded steel fuselages, and it didn’t take long for stick-built wooden fuselages to disappear. They lasted a little longer in experimentals because welding wasn’t (and still isn’t) a common skill, where woodworking is. So, Pietenpols and such had both tubing and stick-build fuselage options. From WWI to the 1960s, the steel tube fuselage, generally combined with fabric or wood-covered wooden structure wings (the Tailwind) was the EAA norm. This changed during EAA’s 1963 design contest that featured the T-18 designed by John Thorp. With its revolutionary “matched hole tooling,” good looks, and high performance, the aluminum bullet took the homebuilt community by storm and spawned a lot of semi-clones.
Aluminum as a Building Material: Pros and Cons
When you’re looking at acres and acres of RVs at EAA AirVenture Oshkosh, it’s easy to see that aluminum is the choice of many homebuilders. That wasn’t always the case. Before the advent of kits, scratchbuilding in aluminum was fairly laborious.
Kits solved most of that. For that reason, when talking about homebuilding an aluminum airplane, it has to be done in two entirely different categories: kits and scratchbuilds.
Aluminum Kits
Aluminum and the kit concept are made for one another. The concept received a real shot in the arm when CNC machining entered the kit-building world. Through the magic of computers, a kit manufacturer can push a button and a sheet of aluminum is turned into a flat sheet component for a kit that is the right size with all the holes already punched. The real magic is that the holes will match every hole in every adjoining piece. In a quick-build kit, the pieces are folded and rolled using other machines, and major components (wings, fuselage shells, etc.) are partially assembled by factory personnel in record time. What comes out of the box on Christmas day are components that actually look like an airplane and can be jury-rigged together so the new owner can sit in it in the driveway making airplane noises with their mouth.
Other than cost, there are no downsides to building an aluminum kit airplane. There are, however, some caveats — and these apply to aluminum construction no matter what form it is in.
Aluminum, more than any other building material, is dependent on clean craftsmanship. That means there are some small details that have to be paid attention to or they can cause long-term threats to the builder. Those are:
- Square corners, such as sheet edges, edges of fittings, or edges of holes, must be “broken” or chamfered, deburred, whatever you want to call it, to eliminate stress risers.
- Scratches of any kind are stress risers and can promote local fatigue cracks. Every effort must be made to protect the aluminum from developing scratched surfaces.
- Rivet “tails” must be consistent and not “clinched” or bent over.
- Steel to aluminum contact must be buffered with some sort of primer. Epoxy works well.
All of the other building materials are much more tolerant — within certain limitations.
Aluminum Scratchbuilding
Some aluminum airplane kits provide plans so it can also be built from scratch. However, most provide “flat kits” in which the sheets are punched but not bent. Pure plansbuilding, or scratchbuilding, means that every part must be cut and then bent with most holes drilled on assembly. This can be a very laborious process.
In many aluminum-building situations, you build the airplane twice: once in wood for the form blocks and again in the metal pieces you form around the blocks. Then you have to build some jigs to properly position the parts so you can locate and drill a million holes. This sounds harder than it is, and it has been done tens of thousands of times throughout EAA history. It’s not at all impossible, but it does require an obscene amount of patience and determination. We’re not saying kits are easy. They’re not. Plansbuilding is just less easy.
Another factor about scratchbuilding in aluminum is that you spend a lot of time and all you have are bunches of parts that only you can identify as an airplane. Visual progress, which is essential to keeping building energy up, is very slow. Then you start Clecoing the pieces together. In a matter of days, you have what looks like an airplane. Actually, because of the hundreds of Clecos, it looks like a flying porcupine.
Aluminum is also the only medium that, when you have the airframe finished and all the systems are functional, you can fly it and totally skip the always drawn-out process of painting it. A completed aluminum component doesn’t need anything cosmetic to protect it. Other aircraft using any of the other materials have to go that extra cosmetic step to protect the airframe. Not so for aluminum.
Rag and Tube Airframes
More than all of the other airframe designs, rag and tube airplanes use all of the other materials in one way or the other. So, inasmuch as the word “composite” literally means “composed of many different things,” we’d have to class a Hatz biplane, for instance, as a composite airplane. The same would hold for any rag and tube airplane. Also, these airplanes use different skills than any other type of construction. At the same time, every one of those skills is pretty basic and easy to learn. That’s why that kind of construction was the backbone of the early EAA in the ’50s and ’60s.
Of all the types of materials, there’s more visual progress with a tubing fuselage at the beginning, so it gives the illusion of rapid progress. It takes a month to have a fuselage rough-tacked together and another month to have it on its gear. The builder feels elated. Then he spends far more time putting all the tabs and bushings on the fuselage, and it looks as if it’ll take forever.
Then come the fabric and paint. This is where rag and tube really slows down, especially if the builder wants a super slick finish and fancy paint scheme. This can easily increase the build time by 30 percent. This is true of every kind of finish on every material, but doing fabric and paint just takes more time to get it up to the fancy level if that’s what you want. Also, you’ll have a totally assembled, functional rag and tube airplane waiting for fabric and paint while the guy building an RV or Thorp is out flying his hours off in bare aluminum.
Wood Airframes
Basically, an all-wood airplane is a big model airplane with everything, fuselage and all, covered in fabric or fiberglass, even if the fuselage is a box. The wood all has to be protected, and a layer of fabric is the most common way to do it. One of the downsides to building an all-wood airplane, like a Falco or even a Pietenpol, is that there are a million small pieces. The parts count is higher than any other building medium. However, every single one of those pieces is relatively simple and easy to build. Plus, working wood is in just about everyone’s workshop comfort zone and requires less equipment than some of the other materials.
One downside to wood, when used in a stick-built fuselage like one of the Pietenpol options, is that it offers less crash protection than steel tube. A lot less. Wood’s yield and ultimate strength points are very close together. It’ll bend just a little and then suddenly snap. This is not a concern in flying, only in crashing. So, don’t crash!
Composites
The first composite homebuilts, Rutan’s VariEze and Long-EZ, were called “moldless” composites. A foam core was cut to the needed shape, usually by a hot wire setup, then it was covered in several layers of fiberglass. There were different weaves of cloth with the fibers oriented in specific ways to supply the strength needed in specific areas. It didn’t vary at all from traditional foam surfboard construction. It was cheap, it was easy, and it didn’t require super precision or craftsmanship. However, it was labor intensive. If a glass-like finish was wanted, a lot of filling and sanding was involved. The resulting aircraft were amazingly fast for their horsepower and were terrific cross-country machines.
Today, most composite homebuilts are built from kits, like the Cozy or Lancair. These are far more sophisticated than the earlier airplanes in that they are built from a series of completely shaped “shells” that are glass-foam-glass sandwiches laid up in molds. The latest of these are so precise that the different sections of the fuselage and wing actually have joggled edges so they snap together like plastic model airplanes. Better than that, the as-delivered surface finishes are exact and require a fraction of the surface prep for painting that the earlier aircraft needed.
At the beginning of the composite period, say the early ’70s, the big question about the unknown and totally unorthodox construction method was how it would handle fatigue. We are now 40 years into the process, and there are virtually no reports of problems of any kind with the designs. And the process has been applied to a wide range of aircraft types.
The Ezes were pavement airplanes, and most composite aircraft are the same way. This isn’t because they are composite but because they were built for speed, not the bush, so the tires are small for low drag. Tiny wheels and grass are not a good combination. However, Rutan, and then a few others, built some other-worldly-looking bush birds with fat tires and canards and went out to challenge sandbars and such.
There Is No Best
By now it should be obvious that there is no “best” aircraft construction method. Each has its strong and weak points. It’s up to the buyer/builder to use or buy whatever best fits their mission.
Budd Davisson is an aeronautical engineer, has flown more than 300 different types, and has published four books and more than 4,000 articles. He is editor-in-chief of Flight Journal magazine and a flight instructor primarily in Pitts/tailwheel aircraft. Visit him on www.AirBum.com. For more from Budd, read his Shop Talk column every month in EAA Sport Aviation.