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About Building Wood Fuselages
By Tony Bingelis (originally published in EAA Sport Aviation, May 1996)
It is a fact. Most wood aircraft can be built cheaper than metal or composite kit aircraft . . . and with relative ease, too, for a number of reasons:
1. Almost any aspiring first-time builder is quite likely to be familiar with wood and would be comfortable working with it, especially since only a few common workshop tools are all that are needed.
2. Most wooden aircraft are built from raw materials, therefore, there are no added costs for kit fabrication, mold and jig development and, of course, amortization.
3. Often overlooked, too, are the very expensive crating and shipping costs for the bulky prefabricated kit components. These savings are very real and provide the opportunity to start construction almost immediately on your dream aircraft.
Quite the opposite seems to be true for a number of the newer composite designs for which deposits may be required simply to get your name on a long waiting list with a rather vague delivery date.
Unfortunately for the builder who is looking for so-called "quick-build" kits, there are none for wood aircraft designs. For that matter, complete construction kits are rarely available for wood aircraft . . . one notable exception is the sophisticated and expensive high performance Falco.
The amateur building movement, however, is changing very rapidly as more and more eager builders arrive on the scene. As a result, recently a number of light and ultralight wood aircraft designs are now also being offered in kit form.
Included in this group are designs like the Loehle Corporation designs - 5151 Mustang, P-40 and Sport Parasol.
Another prominent producer of wood kits is TEAM, Inc. Its fleet of wood kit designs is becoming very popular with the cost conscious builders eager to get airborne. Their simple economical kit designs include the miniMAX, Hi-MAX and a number of others. Most of these lightplane kit designs have been selling for less than $4,000.
But, unfortunately, if you want to build a more powerful, higher performance wood aircraft, you will probably have to build much of it from raw materials. That is, you will have to expend more time for shopping and for purchasing your own lumber, plywood, glue and other construction materials. In other words, you will be trading more of your personal time for money you will not have to spend on expensive kit components.
Nevertheless, in some instances partial wood and materials kits for a number of popular designs are available from: Wicks Aircraft Supply (Catalog orders 1-800/221-9425) and Aircraft Spruce & Specialty (Catalog orders 1-800/824-1930).
These are partial material kits for wood designs like the Barracuda, Celerity, Corby CJ-1 Starlet, GP-4, and Osprey amphibian - these may be available from either company. These packaged material sources for the listed designs can be very helpful in getting a good jump-start on your project.
In addition, difficult-to-fabricate parts such as engine mounts, landing gear components and other welded and machined parts can usually be purchased from specialized sources listed in the plans.
After you order some wood and glue, then what? Just where do you start?
Although the classic approach to building an airplane is to begin with the smaller less expensive tail surfaces, a surprising number of builders of wood aircraft prefer to start construction with the fuselage first. Not a bad idea at all. After all, cutting and fitting pieces of wood for the fuselage is no different than cutting and fitting pieces of wood for the tail surfaces.
At least starting with the fuselage will give you something that looks like an airplane sooner. Besides, it offers you the opportunity to sit in it and do a little advanced planning . . . and maybe a bit of daydreaming.
Types of Fueslages
From a construction point of view, wood fuselages may be classified in two broad categories:
1. The relatively easy-to-build four longeron slab sided type. This type can be streamlined by fairing it with formers and stringers over which a covering of fabric, fiberglass and/or plywood skins may be applied.
2. The second is the more difficult to build semi-monocoque, round or oval, type fuselage where the outer plywood skins carry a major part of the stresses. Very few homebuilt designs fall into this category. In the U.S. the Falco and a one-of-a-kind Lockheed Vega replica come to mind.
Of the two construction types, the four longeron slab sided fuselage designs are by far the most numerous, most popular, most economical and the easiest to build.
Their top longerons are generally level with the thrust line making it a simple matter to erect the two simple flat fuselage sides by jigging them upside down on a large flat plywood work table. It is then a simple matter to cut, fit and gusset the various cross members and diagonals.
A semi-monocoque fuselage, on the other hand, has to be built around numerous labor-intensive round or elliptical bulkheads cut out of solid plywood sheet or laminated of 1/8" wood strips. Each of these bulkheads must then be positioned, aligned accurately and clamped to a separate assembly jig.
Needless to say, a careful leveling and alignment of this type of fuselage is more demanding than the simple centerline alignment required of a slab sided fuselage.
In either case, it does indeed matter very much that your fuselage be accurately assembled around a well marked centerline on your work table or in your assembly jig so you can maintain the proper symmetrical shape and alignment of the fuselage during construction.
Unless a specific type of jig or work bench is described in the plans, most builders find that a long work bench (28" to 30" tall) surfaced with two 4x8x3/4" plywood sheets provides the ideal work surface.
Such a bench will probably be much longer and wider than absolutely necessary to accommodate the fuselage layout but the extra space all around is a tremendous asset.
It will give you plenty of workspace for cutting your material and allow you to lay tools, parts and stuff where it can be handily reached no matter what end of the fuselage you happen to be working on at the time.
Before You Cut It
The experienced builder will always assure himself that he doesn’t ruin an expensive sheet of plywood by cutting off part of it for some small parts . . . or for that matter, inadvertently cutting short a long strip of wood that was intended to be used as a longeron. Therefore, keep the following in mind:
1. Plywood sheets. Always cut out, label and set aside your roughly cut large fuselage skins/bulkheads before you start cutting away smaller pieces from those expensive plywood sheets. You will still have plenty of material left over for gussets, plates and the like.
2. Select your best wood strips for longerons. Tape them together and set them aside until needed. This will help assure that you don’t accidentally cut and use the long pieces for other parts.
Your Working Tools
You can get started on your project using the ordinary hand tools you undoubtedly already have on hand.
Begin your preconstruction preparations by cleaning, sharpening and adjusting your cutting tools (knives, chisels, plane blades, etc.). Your block plane, if you have one, should be kept razor sharp.
Also, prepare several sanding blocks. These are useful for leveling dried glued joints. One sanding block should be long enough to span across the maximum width between the upper and lower longerons of the assembled fuselage sides. A typical sanding block can be made by gluing #80 grit sandpaper to one side of a straight 3/4" board about 3" wide, using contact cement or rubber cement.
A couple of shorter sanding blocks would also be handy to have around. For example, one of the shorter sanding blocks could be fitted with a coarse grade of sandpaper (#35 or #40 grit) for rough leveling of glue joints and similar clean-up work.
Often a large (10" or 12") bastard-cut file is more effective in leveling dried glued joints than sandpaper. Sandpaper tends to ride over the bumps and sand away the surrounding areas. A file will not do this. You will find that your clean-up and leveling of dried glue drops and lumps may be quicker and easier using a file instead of sandpaper.
In order of importance and frequency of use, I find the following tools and equipment to be very useful:
1. A 10" bench saw. Building a wood fuselage from raw materials makes the availability of a bench saw essential for dimensioning your lumber to size for the longerons, uprights, diagonals and various filler blocks. Equip your saw with a 10" satin cut blade (combination or trim type) and you should be able to get nice smooth cuts that will not have to be planed or sanded before gluing. I prefer a 10" bench saw . . . not a radial arm saw, because I find that ripping long strips (longerons, etc.) with a radial arm saw is a bit more difficult.
2. A saber saw for rough cutting of parts from large plywood sheets.
3. A band saw with a metal cutting blade. You can cut wood, plywood and aluminum on it without any speed reduction.
4. A large 9-12" disc sander equipped with a self-adhesive backed #80 grit sanding disk is ideal. However, some builders might prefer a belt/bench sander instead. It all depends on what you have and what you are used to using.
5. A drill press.
6. A good, 3/8" capacity, electric drill.
7. You should also have an adjustable height roller stand to support long lengths of wood you may have to rip on your bench saw. This is an essential aid when working alone.
If your plans have a "Bill of Materials," consider yourself fortunate as it will give you a fairly good idea of how much plywood and wood you will need . . . and in what sizes. Nevertheless, most materials lists are seldom complete and often do not allow for waste or for miscellaneous extra needs.
Your materials list will, therefore, generally be "short" a few odd sizes and pieces. This is where a bench saw becomes valuable for custom cutting extra various odd sized pieces, as needed, instead of having to order more dimensioned stock for your project.
Preparing A Slab Side Fuselage Jig
Lay out fuselage side dimensions full size directly onto the plywood work surface using a #2 soft pencil, or a fine point Sharpie pen. Since you will have to build each of the two sides in the same jig to ensure exact duplication, your outline blocks should be securely nailed or screwed to the table.
Use short wood blocks to accurately position every upright and diagonal piece in the side frames. The blocks of wood should have a slightly smaller thickness than the longerons and uprights. This will ensure that any squeezed out glue will not fuse the aircraft gussets or plywood skins and frames to the jig blocks making the removal of the fuselage frame from the jig extremely difficult.
Incidentally, it is better not to locate the jig blocks exactly at the joint centers, as instinct might tell you to do. Instead, locate the small blocks about an inch away from each intersection to simplify sliding wax paper under the joint and, later, to allow better access for prying the fuselage frame out of the jig. This is sometimes difficult to do after the gussets or plywood skin have been glued in place.
When building each side separately, you must be alert to avoid making two "left sides" or two "right sides." Be sure you apply the plywood skin or gussets to the correct side on each of the frames. With all the blocking in place, load the jig by slipping in the longerons and begin cutting and fitting the various uprights and diagonals. Since you will have to make two duplicate side frames, why not cut and label two identical uprights or diagonals at the same time for each location to ensure that both sides will be exact duplicates
A small dovetail saw is ideal to use for smoothly cutting and fitting the spruce longerons, uprights and diagonals.
Coat the ends of the uprights and diagonals with glue as you slip them in permanently. Although an unreinforced end grain joint is a rather weak glued joint, the glue film does help seal the wood and keeps moisture out. The full strength of a wood joint is typically developed by adding gussets or a plywood skin overlay.
Use the type of glue recommended in your instructions.
Although rescorcinol glue is assembled to be the superior glue, it is not. That is, it is not unless the joint is almost perfect and properly clamped. Furthermore, it is temperature sensitive and the wood and surrounding air must be at least 70° F to yield an effective joint.
For these reasons, quite a few homebuilders prefer to use an epoxy adhesive such as T-88. It tolerates much lower working temperatures, doesn’t require high clamping pressures nor, thanks to its gap filling qualities, perfect joints.
If a hardwood like ash, birch or oak is specified for a particular blocking or member, by all means comply with the instruction. Obviously, additional strength must be needed in that particular area to take engine loads, landing gear loads, etc.
On the other hand, do not "reinforce" areas you think may need it. You could be wrong . . . unless you can back it up with engineering know-how and experience. All you may be doing is adding useless weight - and weight does accumulate fast if careful control is not maintained over the sizes and types of materials specified in the plans and instructions.
Compare the structures illustrated in the accompanying photos and you will see that, although designs vary, the wood construction methods used are quite simple, similar and well proven.