SOME CONSTRUCTION TECHNIQUES FOR BUILDING HOMEBUILT WOODEN AIRCRAFT

by Fred F. McCallum
Light Miniature Aircraft

Many aviation enthusiast today do not realize that aviation was conceived in the womb of the home work shop. Indeed, you may go back two centuries , before man actually flew, to when he could only imagine flight through the success of his models, and find him gluing and sanding away. And up to around 1926, wood was the primary building material for most airplanes, including most of the WWI fighters and bombers. There were even all wood planes built for WWII, notably the British Halifax bomber and the Mosquito fighter.

The most amazing thing is , when aviation began to grow and evolved into a huge industry, the home builder stayed in his shop experimenting with models and building thousands of experimental planes. The racing planes of the thirties required more strength to carry the bigger engines, so many of them had aluminum and steel airframes. But these planes were the ultimate in homebuilt (or very small company) planes. The average man stayed with the parasols and hundreds of other home style designs that were either unsuccessful or perhaps one of a kind. But the cost of flying also soared so the average man continued to build homebuilt aircraft to satisfy his desire to fly. And the homebuilt dream burns on ever stronger in the hearts of many men (and women) today. You won't find much proof of it now, but the Experimental Aviation Association, founded in 1952, was originally dedicated to the promotion of the experimental homebuilder.

This article is not a only a dissertation on the methods of constructing new wooden aircraft, but rather, is written to dispell old myths and out dated methods that may inhibit the uninformed novice from ever getting beyond the "dream" stage of creating his own plane. Introducing you to new techniques and methods of fabrication may be one way of doing it. The mysteries of flight have long been dispelled by scientific study, experience and experimentation, but because of the nature of aviation this means the limits have merely been extended. In the beginning , aviation advanced mostly by experimentation; some things worked, while many didn't. And, as many young aviators found out, the only thing harder than inexperience is the ground. Perhaps because man has always held flight in awe, a mystique pervades aviation to this day. Imagined or real, seasoned airmen seem to revel in stories that promote the mystery. Going from the imagined to the real is only one little step. The only problem is, that step must be made in the right direction. So, eyes right , about face, and to the rear march! Go back. You didn't say, "May I?"

Many people think about designing and building their own plane for several different reasons. They may have had some flying experience or have been a model builder . Or they might have a designers eye and believe they can design something different with special lines and capabilities. These are all noble aspirations, but not for the first time builder or designer of a full size plane. So where does a person begin? I think this question may stop more projects than any other. Just how does a person get started?

In my opinion, I believe the best way to become involved is to find a project in progress in your town or city. It may be someone who already has completed a plane. Most builders will encourage you. If the person is in a wheel chair, go on to someone else! Another place for inspiration is at your local airport.

Believe it or not, within reason, cost is not a primary concern. The first thing to consider is what the utility of your plane is going to be; single place or two place, sport plane or cross contry flyer, range, flight characteristics, portability, storage and any other requirement you feel necessary. I don't believe anyone has ever sat down with his design parameters and found them all satisfied in one plane. If you limit your selection based on price, you may find yourself waiting and waiting for a time which may never come. Very soon you discover a whole new meaning for the word "compromise". But that is what you must try to do by researching the industry for planes that come closest to filling your requirements. Write for literature, go to fly-ins and visit local EAA Chapters. If you are truly looking, sooner or later, you will find a plane in kit form and is as close as you can find to what you can afford and what you want. You may be surprised to find that it does not look very much like the plane you originally envisioned.

At any given time, the "kit" industry only represents a small fraction of the number of planes available to you. Your alternative is to build from plans without the benefit of a kit. This may be a good option for your second plane. However, "scratch building", as it is called, makes your effort that much more complicated and time consuming. Even though you may save some money over the cost of a kit, the actual cost in materials alone (plus mailing or shipping costs) may not be as attractive as you think. In addition, finding and using your own parts and materials, in effect, makes you the designer! This is a good point to remember when you are tempted to alter the plans of the plane you decide on.

Now, if you really have done your homework, you will know if and how the plane is kitted and you will have some idea of how long the company has been around. Can you buy just plans? Is there a construction manual or is it a 4 or 5 page pamphlet that says somthing like, "build the fuselage first, then build the landing gear, then build the tail, then build the wing. Make sure everything is straight and glue them together." I'm being facetious, of course, but not very much. I have seen plans I think would be about as much help as a "Ouija" board. Also, are the plans full size working drawings, or do you have to layout your own from 8 1/2" X 11" sheets? Unless you are a draftsman, more mistakes can be made in this area than you can believe. Try to avoid it or get help to make your own full size construction drawings.

Needless to say, during the above process you have been keeping your eye on the price tags. Just because a kit may cost more does not mean it is worth more. And sometimes a kit may cost more just because it is in demand. Unlike most markets, it seems the more demand there is for a certain type plane, the higher the price. However, even though you may not be a skilled builder, you can save money by purchasing your own materials such as lumber, plywood, aluminum tubing, strap steel, wheels, tires, lexan, rivets, fabric, paint and many other things. Or partial kits such as L.G. kits, wing rib kits, or tail kits may be available from the manufacturer along with fiberglass cowls and tanks and other specialty items. If you don't ask, no one will tell you. If a certain complete airframe kit costs seven or eight thousand dollars, I believe almost any (second time) builder can scratch build it for 20 to 25% less.

THE DAY HAS FINALLY ARRIVED! While you were doing your research and sorting out the facts and rejecting almost everything for some reason or another, you have had a garage sale to clear it out, and have sold your wife's car to finance your work table (4 ft. x 12 or 14 ft. long x 3 ft. high, with a 5/8" particle board top) and a couple of power tools, such as a small drill press and 12" bandsaw (and a table saw if you are scratch building). You are very fortunate if you have a wife who appreciates the new rollerblades you bought for her even though she can only carry one bag of groceries home at a time. What a gal! You lucky S.O.G.!

To keep your initial investment as low as possible, you have purchased a set of plans and a construction manual for a plane that comes as close to what you want as you could find. You already have the color scheme in mind and know you can dress her up with the proper trim. The manual is already getting a little dog eared from you thumbing through it, and there are a few things that just don't make sense, but you figure by the time you get to them you will know more about what you are doing. This a good attitude. Good instructions will usually prevent you from getting ahead of yourself , if you just follow them, and will explain things when you get there..

THE HOUR HAS FINALLY ARRIVED!
Please bear in mind that this article is not written for any specific type of construction, and some of the methods of fabrication and/or assembly may not be applicable to your project. The four types of wooden construction that come to mind are what I call Traditional, Geodesic, Formed and Modern.

If you are working from a kit or partial kit and have not already examined the contents, you should do so now. There should be a shipping list in there somewhere. Find it and use it as a check list for every item you have received in each box.. If you have a shortage or damage, now is the time to call the mfg. and let him know about it. Hopefully, you have received everything that will allow you to get started.

Most manufacturers will not furnish you with full size plans. If you have not received any, go back a couple of paragraphs and see what you have to do. On the other hand, if you have received a set, then you may lay out the fuselage drawing on your long table. If your table is too short, let the front of the drawing hang over the edge or add onto your table.

A drawing can be very complicated looking if the designer has decided to show you all the details such as seats, controls, doors, etc. However, if he has done that, then he should have heavy lined the longerons, uprights and diagonal struts to indicate those parts are to be assembled on the table. Also, his instructions should be telling you some of these same things.

To hold the various pieces of wood to the table, make about 60 blocks from 1/4" plywood about 1 1/2" X 3" to use on either side of your pieces to be assembled on the table. One 3/4" drywall screw through the center of each block can be driven with a hole gun very quickly. DON'T EVER NAIL, SCREW OR DRILL ANY HOLES IN ANY PART OF THE AIRFRAME UNLESS YOU ARE SPECIFICALLY ASKED TO DO IT.

Next, take a role of waxed paper and cut it on your bandsaw into three equal rolls. Leave one of them on your table so you can tear off squares to place under your work wherever you are going to use epoxy. This is so simple to do, and it will keep your structure from sticking to the plans and you won't have to waste time trying to clean up your work later.

It is important to follow the lines on the plans. If pieces of wood cross each other, be sure you break the right one. For instance, you may see a vertical member going from the bottom of the fuselage to the top in the cabin area. these members usually connect with the center section wing spar (if there is one) and carry the vertical load down to the landing gear. If they are there, they are more vital then than any horizontal member they may cross because they carry more load!

A BIG MOMENT HAS FINALLY ARRIVED!
Always start by laying out your LONG PIECES FIRST to make sure you don't cut them up. Let's assume you are starting with the bottom longeron. Somewhere near its location on the plans there will be a notation as to what size that piece is. It may be in a little circle with an arrow pointing to the piece. Or there may be a circle with a code number in it, like RS-4. This is a Raw Stock number which (hopefully) is leading you to an index in the manual for its dimension.

Usually, the lines will be outside of the piece you are fitting because this is how the original drawing was made. (The pieces of wood were laid out on vellum paper, anchored in place and then outlined with a good pen. The wood is then removed and the drawing is completed.) The plan you receive is a copy. Reversing the process, you now start to lay pieces of wood onto your plan. Looking straight down, lay a straight edge across one end of the piece and draw a pencil line at the angle of the intersection on the drawing. Use a band saw to cut the marked end off and check the angle by putting the piece back in place. If it is wrong, mark it again and re-cut it. when it is right, carefully go to the other end and mark and cut that end in the same way. Then lay this piece on top of another piece of the same wood, mark and cut it. This second piece is to make the second side of the fuselage. Label it with masking tape so you will know its position and direction. Both ends may not be identically beveled.

Screw plywood hold -down blocks on either side of one set of longerons. Use just enough to hold them steady. The first side is (usually) the right side. When all the first side longerons are in place, you may start to lay in the vertical struts between them as shown on the plan. Do not force them into place. They also need hold-down blocks. Don't forget to make and mark two of everything. Proceed with the diagonals in the same way. Verticals may not all have the same cross section. Work steadily and accurately. If you make a mistake, re-do the part. Never force a piece into a close fit., it may distort the frame later on. When using expoxy, a loose fit is acceptable up to 1/32". The epoxy glue will fill the gap and actually be stronger than a tight joint that has pressed most of the epoxy out of it.

As I pointed out earlier, all systems are not the same. Older plans will show small triangular blocks at all junctions. If the plan calls for them, then you must put them all in place before you start to glue the first side and they usually require pressure depending on the glue. Geodesic plans will have deviated before now, and will have different assembly methods, while more modern plans will call for long 1/8" corner gussetts the length of the longeron indicated with a broken line.

If you have arrived at this point late in the evening, it is a good time to stop for the night. Don't start using epoxy unless you have a couple hours to complete that which you are working on.When you do start to use Epoxy Cement, there are a few tips that will save you time, waste and labor. First of all, you should use an epoxy that uses a one to one mix and does not require to be weighed 50-50 or two-to-one for ease of use. Also, find an epoxy that has a good temperature range. Guys in the north sometimes have a problem getting the garage up to temperature in the winter.

Make a 10" square pallet out of 1/8" plywood to mix your epoxy on. In the beginning it is hard to judge how much epoxy you may need, so get in the habit of mixing small quantities at a time. You can always mix more and there isen't much you can do with left overs. Even if you have a large job ahead of you, mix small. For a good mix, make a small pile of epoxy about the size of a half dollar on your pallet. Then, using a different clean stick, make a similar pile of hardner right next to it. It is easy if the piles are side by side and not on top of each other. With a third stick mix the two pile together. two or three minutes of mixing will insure a good set. Different epoxies have different set up times and different half strength times. The one I use has a pot life of about 1 hour and 20 minutes at about 80 degrees.

Most instructions say you should "butter" both pieces of wood where they are going to be in contact. I have always found that to be a real good way to make a big mess, and I can't see any reason to do it this way if you make sure there are no voids in the epoxy. So I have always used enough epoxy to make a good joint, and then wiped off the excess immediately. That is probably why it is so important for me to use waxed paper under each joint. You may have a better method. If so, use it.

If your plans call for small blocks to be epoxied into all the joints, you may do this when you please. If you have already made them, it is easier to set them in while your other epoxy is still soft. Sometimes a second pair of hands come in very handy at times like this. If you wait for the first epoxy to set up, then it will be harder to put the blocks in place.

With the more modern method, just make sure the top of the joint is wiped. Then the long side gussets can be put on later, but before you lift the side off the table. By the way, don't be too much in a hurry to lift the side. Overnight drying never leads to any problems unless you made it wrong. There are few things more discouraging than to find a structure coming apart because you did not let it set up long enough.

To remove the side easily, just un-screw your hold-down blocks and save them for building the wing.
This clears the table for you to turn the first side over and BUILD THE SECOND SIDE ON THE BACK OF THE FIRST. If you have made all second side parts as instructed, then it becomes a fast job to lay up the second side.

Mix the epoxy the same way and don't forget to separate each junction with a square of waxed paper. Steel clamps with at least a 1" opening of their jaws is a very fast way to clamp the second side to the first. If you are wondering why the second side cannot be made in the same jig as the first side was, it is because the gusset is on the other side of the second side. Also, using the first side as a jig to make the second side, assures you of having identical sides which is absolutely necessary in order to have a straight fuselage.

By now you should have become adroit at cutting angles and mixing epoxy. When you have done everything to your side panels, shown on the full size drawings, and if the epoxy is dry , it is time to prepare for the joining of the sides. Note that the longeron gussets go to the outside.

The tail post is the last vertical member at the tail end of the fuselage. When the two side panels are joined , the two tailposts come together. However, their combined width should be the same as the width of the vertical fin spar. Since this dimention may vary between planes, you must refer to the tail parts list to find out what it is for this plane. It should be about 1/4" too wide. If that should be the case, then the inside of each tail post must be sanded down by 1/8. I usually accomplish this by laying the sides inside up on the edge of a table and using a small belt sander to bevel the tail post on the inside. If you want to get an idea of how long the bevel should be, stand the sides up, spread the front ends apart so you have the proper width at the station nearest the trailing edge of the wing, and putting the tail posts together. The angles you make do not have to fit perfectly together at the end. Note: If you over sand, you can always fit a triangler piece of wood between the top and bottom spars later.

If the front end of the fuselage side breaks at the instrument panel, it is easier to join the front side panels after the rear panels have been joined. If your cabin sides are parallel from front to back, you may join the sides by laying one on the table, inside up, and standing 6 pre-cut top and bottom fuselage cross members vertically in that area. The pieces should be merely clamped in place using auxiliary 4'X4" right angle 1/8" gussets. (Some builders find it easier to join the sides while they (the sides) are standing up.)

Find a carboard box of approximately the same dimension as the cabin width, and place it 2/3 of the way back towards the tail post. Then (two people) place the second side on top of it and clamp the front cabin cross members to it. Carefully (two people) pick up the fuselage from the front and rear (on the box) and rotate it 90 degrees to sit on its bottom. The cross members at the top and bottom front corners of the cabin may be epoxied in at this time. Use waxed paper under the auxiliary gussets. One or two pipe clamps come in very handy about now.

Gingerly, remove the cardboard box from between the sides and pull the two tail posts together. When they come together, the fuselage should become perfectly square. Sight down the fuselage from the rear to make sure it is straight. Drill a 3/16" hole through the tailpost to hold them together with bolt. Later, before completing all of the cross members, you can open the tail post a little to put some epoxy between them.

If your fuselage sides are tapered in the cabin area, then you may have to stand them up initially, prop them up and clamp them together using the pre-cut cross members as calculated from the front and rear cabin widths. In some planes, the sides of the fuselage may lean in or out slightly, which complicates the precedure.

On the bright side, joining the fuselage sides is the most trying of any other structure in the plane. The wise builder checks alignment before and after every piece he adds to assure good final alignment. This will tell you immediately where some stress may be, and you can take care of it immediately. Later, after all members are in place, if there is a misalignment, it is almost impossible to find the culprit. DO NOT GO ON FROM THIS POINT UNTIL YOU (AND A COUPLE OF OBSERVERS) ARE CONVINCED THAT THE STRUCTURE IS AS TRUE AS YOU CAN POSSIBLY MAKE IT. Almost everything you do from this point on will depend on this accuracy, including your flight characteristics!

Before going too far into the paneling you should consider coating the structure with Epoxy Wood Sealer. It is far easier to treat the struts and diagonals of the sides and the inside bottom panaling before adding too much side paneling. The epoxy and wood sealer I use is compatible, so it is not necessary to mask off un-cemented surfaces.

If called for, the bottom panels should be done first to hold the shape of the fuselage. Then add paneling to one side of the structure, only. until most of the interior and controls have been installed. If there is going to be a door, work the opposite side first. It is much, much easier to work on a half open fuselage than a closed one. After every major installation, pause to apply Epoxy Wood Sealer. It makes the work so much more enjoyable, it stops the wood from picking up moisture and it makes your work look very good.

On most planes, the landing gear merely bolts onto the bottom of the fuselage. Most are fabricated from aluminum and steel. It is nice to get the wheels on it so you can move the fuselage around more easily. Also, it makes varrroooom, varrroooooom, a lot more fun. I can't recall sitting in any of my planes until both sides were paneled or sheeted, but it is not a good idea not to.

MID ARTICLE STRETCH!!
At a certain point, you may want to set your fuselage aside and start on another section. The empennage is a good choice and there are some interesting little innovations to be considered. First of all let's take inventory on "lightness". The tail moment on most planes is usually three or more times longer than the nose moment. Therefore, it is much better to build a nose heavy plane than a tail heavy one. Why? Because it takes three times as much weight added to the nose of a tail heavy plane than to balance a nose heavy one. Also, a tail heavy plane is much less flyable than a nose heavy one. CONCLUSION! Build the tail as light as possible without compromising strength.

Tail structures may be just a little bit more dificult to build than you think at first glance. First of all the material will most likely be a little lighter. Also, you may run into some laminating work on the parimeters. Just because you might never have done any laminating, don't let it frighten you. Actually, if you realize that laminating is done in place with very thin peices of wood, the job takes on different dimension. If you have no laminated parts, then skip the next paragraph.

If the tail spars are fabricated, that is the place to start. Cut the proper materials to length, then fasten them to your work table to have caps installed. When dry, mount them back to back for hinge drill holes. If you displace them lengthwise by about 1/8", your hinges will not butt one another. If you forget to do this, you can always trim the longer spar by an 1/8 " or so. A careless approach to any deviation from the plans is to think , " from 2,000 feet no one will ever notice it!"

I like to build the hoizontal stabilizer and elevator in one piece. There are several reasons for this. First of all, they are more apt to stay flat thoughout. They have to fit oneanother when completed, and the curve of the tips will be contiuous and well shaped when the two are cut apart later. But to attain this perfection, I fabricate the two spars and actually put the hinges in place as indicated on the plan. Eye-bolts and clevis pins make good hinges. Once in place on your plans , you have the heart of your horizontal tail plane done, so it is pretty hard to go wrong . Fabricate the ribs and epoxy them to the spars. The vertical stabilizer and rudder are built in the same manner. It is always prudent to measure the seat of these parts on the fuselage to make sure they are going to fit later. Small adjustments either way are acceptable.

When actually laying up the perimeters of the tail pieces, I use narrower hold down blocks about 1" X 2" behind the curve on the inside line to give the first lamination more support in the curve. If laminating is necessary Spruce should be used. 1/8" thick laminations work very well. You can form spruce into a hoop with a 16" radius or smaller just from the heat of your hands if you rub it and work the curve in slowly. Don't worry too much about the degree or shape of the curve. If your shop is too cool, you can make steam in a small pan and just pass the strip of wood over it as you are working it. Naturally, laminations are applied one at a time. Make the first one follow the line of the drawing as closely as you can, cementing it to the ribs or the end of the spar and let it dry thoroughly. The next lanination can be glued to the first and held in place with small clamps or clothes pins. By using pre-formed 1/8" half round as your final laminiation, you can avoid having to sandpaper a radius on the perimeters. Also, you can get a much more uniform curve. This procdure turns a dread into a very enjoyable part of building. When I built my first plane, laminating was one of the things I dreaded most. As it turned out, I was sorry when I had completed the tail, but looked forward to building the wing tip bows.

In my opinion, the wing spars are one of the most important parts of your plane. Although I design light wings, I do not compromise the weight or strength of the spars. Spar caps must be made of spruce or fir. The wood must be straight and tight grained, clear with no knots or splits. It should also be carefully examined for checks and other damage. Webbing must be marine birch plywood or better with the grain set VERTICALLY between the caps. Never make a drill or screw hole in the spar caps unless it is specifically called for in the plans! However, holes may be made in the webbing for the attachment of the wing struts, compression members etc.

There are almost as many ways to make wing ribs as there are planes. The traditional way is to form them from 1/4" X 1/4" sticks laid on a jig and held together with small gussets, glue and even tiny nails. Some are cut from light plywood stacked up and cut on a bandsaw. Whatever system is used, the ribs must be uniform, light and relatively stiff. If you make plywood ribs, the fabrication is much quicker and more uniform. Make a stack of 12 rib blanks from 1/8" luan door skins, with the airfoil pattern drawn on the top rib, hold the stack in alignment by using two 3" wood screws from the bottom, and cut the stack on a bandsaw. After cutting, make four or five 4" lightening holes between the spars.

Due to it's size, a full wing plan may not be provided. If that is the case, the designer should provide you with a small layout drawing and dimensions, which will allow you to draw lines on your table to represent each rib location. This is all you need to layout a wing panel.

Your table should be made such that one edge is perfectly straight and flat. Use it as guide to layout the bottom side of your trailing edge. If flaps are going to be built, you must provide for them at this time by drawing them on the table with the proper clearence at their ends. Naturally, the aileron outlines must also be drawn. Some wings require the wing ribs to be slipped onto the spar (s). In other cases, the ribs are built into the wing as it is assembled. Don't change the layout of the ribs because a drag-anti-drag system usually has to pass through them at 45 degrees, so their openings may be critical. Any place where epoxy may come in contact with the surface of the table, waxed paper should be used. Some wing tips are square and some are rounded. A square wing tip usually is formed by merely putting a tip plate on the last rib. However, a round tip is more complicated and usually requires laminating. If you laminated the tail, you know how to do it. If not, read that section of this paper again.

It is unlikely that you will encounter a sheeted wing. However, if you should, provision must be made to build in a few degrees of wash-out. This involves twisting a wing slightly and applying surface skins while it is held in that position. Generally speaking, though, wings will be fabric covered and the washout can be set by the lift struts even after fabric has been applied.

Wings in particular must be protected against moisture and rotting by the application of epoxy wood sealer. Pay particular attention to the inside of the trailing edge and the roots ribs and second ribs.

Plan ahead. If wing tanks are going to be used, the drag-anti-drag system will have to be moved out one or two bays. Don't fully complete any section of the plane until you have had the opportunity to set it up with the wings and tail on. It is so much easier to work on it before it is complete. For instance, if you put the leading edge skin on the leading edge of the wing panels, you can't attach the upper end of the lift struts. The "D" section is closed.

These instructions have purposely been kept general and in support of the basic airframe. My philosophy has always been to help the new builder form up a major part of the structure as quickly as possible to preserve the wonderful excitement that comes when you actually start to build your own plane. It is an enthusiasm that can keep you on track and holds your interest throughout the project. The later details and custom work are more easily accomplished when you have the whole plane to work on. Also, it is the part of the building which allows you to express some of your own ideas concerning interior design, instruments, color skemes and many other features which make the plane "your" design.

When you are getting close to completion, I hope you will agree with me that building a plane is merely performing hundreds of simple tasks such a cutting, drilling, gluing (and repairing) wood. It is when to do all these things, not how, that is the problem. And I sincerely hope I have been more help than hinderance. You may not realize how much you have learned from the experience of building your first plane, but you are much closer to the time when you might want to design your own. Just a thought!!

Founded in 1982, Light Miniature Aircraft was one of the first companies to develop replicas of classic American designs as ultralights, two place ultralight trainers and experimentals. Designer, builder and "test pilot" Fred McCallum, during the next ten years, developed ten different models in Bonded Aluminum and All Wood up to and including a full size Super Cub he affectionately named "Golden Girl".