Getting Started in Homebuilding, Part 8 of 11


Budd Davisson, EAA/Experimenter, 1994

Details - It's the Little Things That Count

What's the first thing you notice, when you walk up to a homebuilt airplane? That probably depends on the person. If he/she is an enthusiast who is head over heels in love with things that fly, but isn't a builder, they may not home-in on any one thing. The whole airplane may make the entire statement.

If an airplane is approached by a builder, he may be initially attracted by the overall statement made by the machine, but that quickly fades. As he gets closer, he/she begins visually taking the airplane apart.

Maybe the first thing seen is how straight the light reflects back off the wings. No waves. No ripples. Then the crack around the canopy may be noticed because it is particularly tight and even. Then the finished edge of the combing at the bottom of the windshield may call for some attention. The edge is carefully radiused and meets the windshield evenly with no gaps or filler in evidence.

The list goes on and on, detail after detail until it is all added up and becomes an airplane. If the details are judged to be above average the airplane is judged above average. If the details are super, the builder doing the inspecting may be talking about the airplane for the rest of the fly-in. At the very least, he will approach his own project with a little clearer idea in his/her mind as to what his own details should look like.

An airplane, like any other object, is the sum of its details. Get the details right and the airplane is right.

Certain details stick in an observer's mind for a long time. The polished aluminum cheek cowling on Jim Butler's Midget Mustang is still being talked about 20 years after it won grand champion. The little fairing at the intersection of the vertical and horizontal stab on Ed Lesher's Teal is a lesson in stretching metal. Jim Younkin's Pacer fairings all show aluminum actually is a viable alternative for fiberglass. Not the other way around. The inside of Hale Wallace's Skybolt wings make a builder wish they didn't have to be hidden under cover, they are such works of art.

Like we said...the sum of the details.

In reality details can sometimes to broken down into two categories; cosmetic and structural. The cosmetic details do a lot for a builder's ego. However, apply the same detail oriented attitude to the structure inside the airplane and the rewards are safety and longevity. These are tangible rewards that will be with the pilot long after the shine has gone off the paint and airframe nicks and bruises accumulate.

In building safety and longevity into an airframe there are an endless list of details that must be right. Some of these are craftsmanship oriented while others are simply paying attention to the plans and doing exactly what the plans call for.

Too often there is a tendency to look at the dimensions on the plans when comparing them to the dimensions on the airplane and say, "...that's close enough..." In some cases, that may actually be okay, but, unless the builder is either an engineer or an expert on that particular airplane, he can't be sure where he can fudge and where he can't.

The best assumption is that there are no places in the airplane where "good enough" is good enough. Every dimension should be held as closely as possible.

The most common problem is when there are minor deviations or misalignments in fittings. Each part in the airplane is designed to make use of materials and fasteners in a specific way and, when the parts aren't lined up right, the loads calculated are no longer valid. For instance, if two holes in a fitting, like a wing fitting, are perpindicular to the fitting material but not exactly in line with one another, the stresses at the edge of the holes are much higher than calculated. When the bolt is forced in to place, it contacts the material at the edges of the holes, so the load is spread over a much smaller area. The result is a small area of concentrated stress that will eventually lead to a localized failure of the material in bearing or the bolt may eventually shear.

Another type of problem exists when a fitting is moved slightly out of line with another one, so the load paths are no longer concentric and don't all end up in the same place. When that happens, the area between the two load paths is subjected to a bending moment for which it wasn't designed. The same thing happens, when tubing in a cluster isn't fitted exactly right and the centerline of one tube doesn't coincide with the rest.

Often a builder will look at a plan and decide the designer didn't know what he was doing, when, for instance, he specified an AN-3 bolt, so he substitutes an AN-4, drilling the hole in the fitting out to match. When he does that, he is removing metal from a fitting or lug that was sized for a smaller hole. In going for the bigger bolt, he could very well have reduced the strength, not increased it.

The same thing holds true, when increasing a sheet thickness or going up a size in tubing. It is always possible to change the load path so a concentration occurs elsewhere in the airframe.

Part of getting the details right is making sure they match the plans.

Another part of detailing an airframe is making certain scratches just don't exist. This is true on fittings, skin, tubing, anywhere.

Scratches are difficult to define, but in general, anything that can catch a fingernail is around .003" deep and is something to worry about. Scratches are a worry for several reasons. One is they reduce cross sectional area and the other is they act as stress risers by concentrating stress in a small local area.

Aluminum structures are particularly susceptible to scratch damage. First the material itself is easily scratched and second, it is usually much thinner so a scratch causes much more severe damage.

If, for instance, a .003" scratch runs across a apiece of .025 aluminum, that means it reduces the cross section thickness by 12%. That is a sizeable reduction. Much worse than just the reduction is what happens at the sharp apex of the scratch. The local stress is increased by anywhere from three to 15 times!

Besides reducing the amount of load that can be carried, the scratch also reduces the fatigue life. Unfortunately, there is no way of knowing exactly how much it reduces the fatigue life. Depending on the part, the types of loads and fatigue cycles it sees, it might fly forever or fall apart tomorrow afternoon sitting in the hangar. It is impossible to guess.

For that reason, aluminum has to be treated like very soft glass. When it is laid on anything, like a workbench, everything should be cleaned out from beneath it and something like carpet scraps used to protect it. The most common injury to sheet metal skins come from dragging them across a work bench that has drill shavings laying on it.

Scratches are more critical on aluminum than steel because aluminum fatigues much faster than steel, still, scratches on steel should be avoided, as well. The fatigue aspect of scratch damage can be fixed by gently sanding the scratch out and polishing the surface. However, the cross-section reduction damage will still exist. There will be less material left to resist the load.

if there is one detail in an airframe that is overlooked more than others it would be the proper finishing of holes. All holes. Whether they are bolt holes or rivet holes or simpe lightening holes, they should all receive the same amount of detailing.

In the first place, regardless of the type of fastener to be used, the roundness of a hole is important. If the hole isn't round, the fastener going into it will contact it in two or three small spots, so naturally, those spots will have to take up the entire load by themselves. With a smaller amount of material resisting the load, the stress is higher and local failure can occur.

As it happens, a regular AN rivet will flow and reshape itself to a certain extent and can make up for some irregularity in the shape of the hole. A bolt or pulled rivet generally will not.

Making a hole round is a little more difficult than it sounds. In fact, the thinner the material, the more difficult it becomes. A regular drill bit will almost always screw itself through a thin sheet producing a hole with three flat spots. The hole will be slightly triangular. Just about the only way to avoid that is by using a brad point sheet metal bit or a punch. Also, the slower the bit is fed through the metal, the closer the hole will be to being round.

In drilling thicker material, getting a round hole is much easier. Then it becomes a judgement call as to whether the holes should be reamed for fit and finish or not. If it is an important fitting, yes, they should be reamed. This will produce a round, well finished hole and eliminate any galling on the inside of the hole that may have been caused by the drill bit.

Once the hole is drilled, it is time to finish detailing it. This is one of the more important details that is often overlooked. The edges of the hole, because they are nice and square, are stress risers in themselves. Regardless of where the hole is, whether in a fitting or skin sheets, the edges of the hole have to be 'broken'. The edges need to be slightly champfered or rounded. This will remove any burrs and put a slight radius on the edge that will greatly reduce stress concentrations.

The finishing of fittings is another detailed operation that differs from other parts of the airframe only in that fittings carry and transfer more loads, so it is important their integrety not be in question. A little detailing will go a long way towards increasing their life span.

As designed, there are three basic parameters to a fitting or lug that don't want to be altered. They are the thickness of the fitting and the distance from the edge to the hole in two places' on both sides of the hole and on the end of the fitting in the direction of the load.

Here again, what is actually being talked about is cross sectional area. As either dimension is altered, i.e. the thickness or the edge distance, the amount of material available to carry the load is changed and that is unacceptable.

When finishing fittings it is possible to get entirely too enthusiastic about sanding and polishing the edges in an effort to remove scratches and nicks. When this is done, too much material is removed and the edge distances are reduced. This lowers the load carrying ability of the fitting drastically.

Still, there can be no scratches or nicks like those left by a hacksaw on the edges of the fittings. Besides the fact that the edge distance and, therefore, the cross section area available is reduced by the depth of the nick, a distinct stress riser would exist in a critical area.

Another problem in making fittings is getting the hole patterns to match. A way to do that is to cut the blanks for the fittings oversize and weld or bolt them all together in a stack so all the holes can be drilled at the same time. Then the blanks are cut to the right outline. If the outlines don't match exactly, that is okay, since the bolt patterns will be the same.

The surfase of fittings should be treated the same as aluminum marks, no scratches. It's not unusual to see a fitting that has marks on it from being held in the vice during bending. Sometimes the marks are the diamond shape pattern left by the jaws and other times they are from the sharp edges of the jaws. Both can be eliminated by making sheet metal jaw covers.

There should be no marks on the flats of the fittings or in the radii. If there are, they should be gently sanded and polished out. The operative word there is "gently." We don't want to remove any more material than absolutely necessary.

An entire book could be written on the details that need attention in building an airplane, but the most important facet of detailed building is the attitude of the builder. If, at any time, he looks at a part and asks himself whether that's the best he can do or not, it probably isn't. If he has any doubts whether he can get things aligned a little better, or polish an edge a little cleaner, or get a little more friction out of a system, then there is still room for improvement and he should take the time to do it.

An airplane is more than the sum of its details. It is a machine that takes us into a totally hostile environment in which we are totally dependent on every one of those details to bring us back safe. There is no such a thing as a detail that doesn't count.