Getting Started in Homebuilding, Part 2 of 11

Budd Davisson, EAA/Experimenter,, 1994

Which Material is Best For You?

Don't you just hate all the decisions that have to be made before you can actually start whittling out airplane parts? There's all that stuff about picking the right design and whether you can fly it or not. There's the worry about whether the design will fit in the back bedroom with the wings on or not and the horrible decision about the proper name to paint on the cowling: Spouse's name (politically sound move), kid's name (safe, but doesn't build spousal support) or some unpronounceable Japanese word which originally meant "Star of the morning gliding across azure skies", but because of the misspelling now means "Bagel with lox, 147 yen."

One of the most basic of decisions is which type of building material is best for you? Okay, so the definition of "best" is open to some definition and interpretation but here we are going to say that "best" is the material that:
· You feel the most comfortable with
· Has a skill demand that you are sure you can match
· Is applicable to the aircraft you feel best fits your personal
needs and wants
· Will work within the restrictions presented by your workshop environment
· Requires an equipment investment you can live with
· Fits into your financial scheme of things

In reality, there are only four basic different types of material; wood, steel, aluminum and composite. In the case of almost all airplanes, they will see some combination of steel, aluminum and composite, while a wooden airplane may see all four materials used.

Incidentally, the term "composite" actually means the airplane uses a combination of materials, so a rag and tube airplane is technically a composite airplane because it uses wooden wings, steel fuselage and fabric everything else. Be grudgingly, we'll bow to modern convention and use the term "composites" in its most recent sense, meaning some form of high tech fabric held rigid in an epoxy or polyester resin matrix.

A Builder's Comfort Zone
The question of what you are the most comfortable with can probably be answered by asking what kind of materials you've worked with the most. The more experience, the more comfort.

Wood is the only aircraft material that most of us civilians have any serious experience with, because that's what everything around us is made of. If we were living in sheet metal houses, we'd probably feel the same way about sheet metal because the association with housing type materials breeds a familiarity with it whether we want it to or not.

Steel is probably the second most common material in the real world because it is so commonly used in industrial applications that we run up against it in the work place. Composites follow steel in terms of familiarity outside of aviation courtesy of the boating industry. Yes, you can run a boat for your entire life and not know a thing about fiberglass work. However, the type of person who is thinking about building an airplane is also the type of person who probably tinkered with his boat and did a little bit of fiberglass work.

Aluminum is absolutely the last material with which civilians are likely to have any familiarity. When was the last time you made a birdhouse for your daughter out of 2024 or put an aluminum deck on the house? There's not much call for aluminum fabrication around the average house.

The most important thing to know about being comfortable with a material is that it's not important you be comfortable with the material to build an airplane, although it will make the initial stages a little easier. As you begin to work with the material, you'll gradually become comfortable with it. You'll come to grips with composites and wood the fastest, with steel right behind and aluminum dead last by a fairly good margin.

A note concerning builder comfort and composites: Some builders have an existing allergy to some of the resins used, while others can build up a reaction to them through use, if not properly protected. If allergies are one of your curses, it might pay to have yourself checked out for any kind of allergenic reaction or skin sensitivity to any of the resins used.

Skill Demands
Here again, don't worry about the skills required by any material in any airplane. We're building flying machines, not restructuring DNA or doing brain surgery. Regardless of the material and builder background, every skill can be learned by every builder. Period.

Yes, the skills can be learned, but some materials not only lend themselves to faster learning, but are also much more tolerant of skill shortages. In some materials this tolerance takes the form of letting the builder be a little short of skill while still letting him/her produce a strong, although not necessarily beautiful, structure. Other materials are much more picky about how they are handled and how the work is done and they demand a certain level of competence just to produce a safe structure over the long haul.

Structurally, composites are probably the most tolerant of less than professional skill levels. In most composite designs there is so much redundancy built in, the builder has only to understand some of the basics and he'll quickly build enough skill to produce a sound structure. Also, almost all of the kits have been designed for the weakest link in the builder chain. They have engineered out all the hard stuff, making it easy to not only learn the material, but also to rapidly progress to the level that the workmanship is going to produce a safe airplane.

Oddly enough, steel sits in the number two spot structurally. This may sound unusual because there is a general misconception that learning to weld is difficult and it is hard to produce a safe weld. That's baloney! Because of the way aircraft structures are designed, a weld has to be unbelievably awful to be structurally dangerous and, in that case, it will look just as awful, so there is no secret about its inadequacies.

Wood could most likely be even with steel. The modern glues have made it much easier to produce a sound structure, but there are still some basic techniques that must be followed and other's to be avoided.

Aluminum is structurally intolerant of sloppy workmanship. Scratches, dinged edges, holes with burrs all contribute to a structure that could see some fatigue problems in the long run. An aluminum structure with those kinds of workmanship problems will fly safely for years, but at some point could start developing cracks, etc.

If a kit isn't being used when building in aluminum, chances are there will be some fairly extensive shearing and forming to be done and some of those operations do strain the skill envelope. Fortunately, however, most aluminum kit manufactures have carefully engineered the need for forming, bending or trimming skills out of their kits. Then all the builder has to realize is there can't be any scratches, burrs, etc. He also has to master the skill of riveting, which doesn't appear to be a big deal, but it can be because aluminum doesn't let the builder easily cover a mistake, as with so many other materials.

Nobody wants an unsafe airplane but no one wants an ugly one either. We are like a bunch of kids, when it comes to the finish on our airplanes, and we delight in pointing out the flaws in the next guys. Kids can be cruel. The type of materials used can make a definite difference in how much innate skill is needed to produce a finish we can be proud off.

Although the techniques are grossly different between fabric and composites, the actual skill required for a product with a good finish is about the same and is composed primarily of elbow grease, attention to detail and patience. Aluminum can be a different story.

Aluminum doesn't have the luxury of an additional level of finish over the basic structure, as do the rest of the materials. There is no layer of fabric to be doped and finished as with rag/tube or wood and there is no ability to easily fill a ding and feather edge it smooth, as with composites.

Aluminum, once the basic structure is finished, is ready to be flown because the skin actually is the skin. However, that means the surface to be finish is largely dependent on the craftsmanship that went into building the structure in the first place. In aluminum many of the structural details show through to the surface, so everything inside has to be up to snuff for the final structure to have a surface that takes a good finish. This includes things like clean, straight sheet edges and, most important, smooth, even riveting. Although riveting isn't difficult, it takes a while to develop the skill necessary to produce a surface that would be smooth and clean enough to go without paint.

Every aluminum builder lays awake nights worrying about coming right up to the last few rivets and then having a rivet set skip across the skin, leaving a trail of tell tale half-moon marks...half-moon marks that can't be removed and are practically impossible to cover up. If looking for a flawless, unpainted surface in aluminum, the builder has to be willing to make the part over again, if it doesn't turn out right the first time because blemishes can't be corrected with the finishing process.

Does the Design you Want Use the Material You Want
In a lot of cases a builder doesn't have any choice concerning the materials he wants to work with because he has already locked in on a design and that drives the material question. It is doubtful, for instance, if Van would look kindly upon an RV-6 built of rag and tube, or composites.

If there is going to be any concern about the materials, then they become a sizable part of the design selection. That being the case, as you read through this treatise, remember that being comfortable with the material and developing the requisite skills shouldn't enter the design decision. As we've said, you can learn any skill and those you can't, you can buy.

The areas where materials become part of the design selection process should be based on cost or allergies, or other factors which preclude the use of that material and therefore, eliminate that design.

Workshop Environment
The workshop isn't a free standing, self contained personal space that is unaffected by the environment around it. Quite the opposite: Workshops are part of a pre-existing environment and only part of it can be controlled or changed.

One of the workshop questions has to do with how it interfaces with the rest of the house and the rest of the family who lives in that house. Some materials aren't family-friendly because they produce what airplane builders see as lovely aromas, but other family members see as smelling awful. Composites always have a dull undercurrent of resin odor, sort of like an auto body shop. Anything with fabric will, during the finishing process, generate eau de butyrate aromas and most structures will, at some time in the process, have the smell of enamel floating around while small parts or entire fuselages are being painted.

Some of us see those smells as something exotic and wonderful. For instance, we wish we could put that smell which develops while using rubbing compound on dope in a can and sell it. It is a warm combination of dope, compound, a little nitrate and other things which say "airplane." Kids and spouses think it says "stink."

A powerful, explosion-proof exhaust fan is a good investment for family peace.

A very powerful environmental factor driving material choice is temperature. Composites and fabric work demand a reasonable temperature to work properly while aluminum and steel can be worked in about any temperature the builder finds personally comfortable. If keeping the workshop temperature up is a problem, then either the material chosen should be immune to temperature effects or the builder will have to schedule temperature-sensitive processes during the summer.

Steel airplanes are not temperature sensitive until it comes time to cover them. Aluminum airplanes can pretty much be built in any temperature, so long as it isn't so extreme that expansion and contraction could cause problems. If the builder can stand there and work without gloves, chances are the temperature is okay. However, the builder should be aware that letting a workshop drop to ridiculously low temperatures over night and then coming out to work in the morning means he is also going to have to give some thought to warming the structures he is working on. It's hard to know, for instance, the exact effect of starting a weld on a fuselage that has been standing in zero degree temperatures overnight, but it can't be good.

Heating workshops is a never-ending project for parts of the country, but not even a factor worth discussion for the rest.

Equipment Investment
The tools needed to build an airplane depend very much on the material and even more so on whether the airplane is kit-built or not. This is especially true of aluminum airplanes. An aluminum airplane built strictly from plans, with no kit parts, is one animal, while the same airplane built from a kit is an entirely different animal.

Most airplanes can be built with the most basic of hand tools: A hacksaw, electric drill, etc. Steel airplanes introduce the need for a welding rig ($300-$400), but until getting into the aluminum airplanes, a builder could, theoretically, get along with nothing else. And many have.

Regardless of the material, however, there are some basic tools that will make life a lot easier and move the airplane along much faster. Fortunately, although it's not doing the national balance of trade any good, many of the Taiwan imports will work just fine for our purposes. A $35 bench grinder and $65 3/8" drill press will make many things happen faster and better. Steel builders will fall in love with a 6 x 48 stationary belt sander and a power-hack saw ($175 each), especially once they see how fast they can crank out fittings and how much truer they are.

A wooden airplane might go together a little easier and truer if a small joiner (4") was purchased and that would run around $150, but this isn't a necessity since a sharp hand plane will do essentially the same work.

Most materials will eventually need the ability to apply finish. Steel needs it to finish the fuselage, etc. and then needs it again to finish the fabric. In reality, a lot of airplanes have been finished right up to the final coats with a brush. Some of the new epoxies brush onto tubing and flow-out so well it is really hard to tell they weren't sprayed. Fabric work can be done by brush and only the final coats sprayed on. Wood structures are actually best done with a brush and don't need the ability to blow paint until the final coats are applied. So, most building materials don't really need the air compressor until the final stages and then, it can be rented.

The foregoing is not true with aluminum. Although some aluminum designs, like those from Zenair, depend on hand pulled, blind rivets which don't need a compressor or rivet guns, most aluminum airplanes demand the ability to drive AN rivets. As soon as it is decided rivets have to be driven, it is automatically said an investment must be made in the following:
· Reasonably powerful air compressor - $300-$400
· Rivet gun and sets
· Bucking Bars
· hoses
· In-line filters
· Clecos
It has been estimated that over and above the air compressor itself, getting completely set up to do riveting would add $400-$550 to the cost of equipment. Therefore, the decision to build in aluminum carries an automatic investment of around $1,000 attached to it.

Cost of Materials
The cost materials becomes a major factor only if the airplane isn't being built from a kit. Most of the kits actually offer the material at a cost below that which a consumer would pay at retail. They buy the material in bulk and get low prices, mark them up and they still come in less expensive than retail. The kit cost is driven more by the value added in working the material and in paying back the developmental cost.

The cost of materials is worth considering primarily if an individual is building from plans or using a minimum of purchased components.

Aluminum is going to come out the cheapest material, no matter how it is figured. There are so many uses for the material outside of aviation that we can tag along and reap some of the benefits associated with large scale production of a material. A similar thing could be said for composites, although some of the foams and resins are climbing.

it is impossible to separate steel and wood, since a rag and tube airplane uses plenty of both. Unfortunately, 4130 prices have taken it right on the nose until its per-foot cost makes it look as if we are buying prime filet mignon. That is because there is very little of it being used in non-aviation situations and the production is low. Fortunately, even the larger airplanes, like the Skybolt don't need that much.

The price of aircraft quality spruce is a classic case of supply and demand driving the price. There is darned little of it available so it is priced accordingly. That is why many designers have a Douglas fir alternative that is slightly heavier, but much more available and less expensive. A lot of us theorize there are huge stands of old growth Sitka spruce trees in Siberia where they are probably using it for firewood. Whether we'll ever see any of that wood is any body's guess, but we can keep hoping.

In Summary
In case you didn't notice, we never did develop an argument for the "best" aircraft material for you. That's because there is no such thing as the best material and only you can decide what works best for you and your own peculiar combination of decision factors.

If we've made any points here, we hope the one that comes through loud and clear is that any one can develop the skills to work any material. There is no magic involved. Just a little dedication and the desire to learn.

So have at it. Pick the material that makes you happy and start whittling.