Wood Wings

    Sport Aviation  1/96

    By Tony Bingelis

    Aviation magazines these days seem to be crammed with articles exuberantly extolling the virtues of one "quick-build" composite homebuilt after another, paying special homage to any newly launched extra-powerful and glamorous speedster. You get the impression that they are the "in thing" these days, and are being built by almost everybody. Not so.

    After all, how many of us can afford or are willing to spend $50,000 to $100,000 for a homemade airplane? If I had had to spend that kind of money to fly, I never would have gotten off the ground. Fortunately there have been, and still are, numerous exciting and less expensive options.

    Thousands of builders, worldwide, are building a mind-boggling variety of different aircraft designs, and they are not all composites or kit-built projects. Many are building their airplanes from plans and raw materials. Others are building using plans, partial kits, and a few prefabricated hard-to-make parts. They are building these airplanes of aluminum, tubing, wood, and, yes, even of composite and fiberglass materials fabricated at home.

    The least expensive way to build an airplane is to start building the small parts. Building the wing ribs first is a very popular procedure because you are, in great measure, substituting your own extra work and deferring the high dollar parts until you are really sure you want to build that airplane.

    But, assuming you are willing to do the extra work involved when building from plans, what choices would you have in selecting a design you like to build this way? Plenty.

    It all depends on what you want. Do you want something slow and stable? Nimble and aerobatic? Or perhaps something fast and sassy? The choice is yours to make. Yes, it can be a difficult choice, indeed, because at my last quick count I found well over 100 designs for which plans were being sold separately. In many instances partial and complete kits for them were available, too.

    Why Wood Construction?
    Most of the more than 500 aircraft designs available to amateur builders can be purchased in kit form. Of these, close to 200 of the kits utilize some wood components as essential structure. But, of this number, you still have more than 100 aircraft designs to pick from if you prefer to build from the plans and scrounge or buy your own materials.

    Wood is a readily available and durable construction material. It is easily worked with simple tools and, surprisingly, a properly constructed wood structure increases in strength with age.

    Most of us have some experience in working with wood . . . you know, things like making bird houses, boats, benches, cabinets and perhaps furniture. In short, wood is a good clean inexpensive construction medium for the first-time builder.

    To give you a better idea of what wood aircraft structures look like and how they are built, let’s examine the accompanying photos and study some of the structural details of wood wings.

    Incidentally, the strength and reliability of wood wings is well established because most aerobatic biplanes have them-and this includes the famous Pitts designs, the EAA Acro Sport, Christen Eagles, Skybolts, the One Design, and other sporty homebuilts quite familiar to most EAAers.

    The Wood In Wood Wings
    Two commonly used woods used in aircraft structures are Sitka Spruce and Douglas Fir. Other carefully selected limited wood species are also used, but be guided by the recommendations the designer specifies in his plans and instructions.

    A special manufactured wood product is most essential to the construction of wood wings. It is plywood. Not just any plywood, but "aircraft quality" plywood. Mahogany and birch plywoods are the most common of those used in aircraft. Both types are equally acceptable in most designs. Birch plywood is somewhat stronger (but heavier) than mahogany plywood and this should be taken into consideration when selecting the plywood sizes for a particular application.

    Aircraft plywood is free from knots, voids and other imperfections. It is made with waterproof glues and may be purchased in thicknesses as thin as 1/32" and 1/16"! It is hard to believe that wood veneers that thin can be glued together and manufactured as 3-ply plywood sheets.

    Thicker thicknesses of aircraft plywood are also available in 5 ply sheets. You can imagine how much stronger this plywood is compared to the variety commonly seen in builder supply outlets.

    Of course, it depends on the aircraft’s design but, for the most part, the thickness of plywood skins used in cantilever wings range from 1/8" in the wing inboard areas to 1/16" in the lighter loaded outer portions of the wing.

    Some light and ultralight aircraft designs call for limited use of thin plywood leading edge wing skins and more often rely on fabric covering as a means for reducing the need for extensive use of plywood. The reason being, of course, is that plywood is a much heavier material than ordinary wood members and fabric covering.

    Plywood gussets are used extensively in typical wood structures to reinforce built-up wood wing rib assemblies and fuselage joints. Since gussets are relatively small, you will save money if you first cut out the large wing skins and set them aside until needed. Then you can use the large sheet off-cut remnants for your gussets and other small parts. It is interesting to note that a completed plywood gussetted joint will be stronger than the individual wood pieces making up that assembly.

    Wood aircraft structures are held together with glues . . . call them adhesives if you want. Nails or staples, if used, are only incidental to making any assembly and may be removed after the adhesive has cured.

    If adhesives are good enough to withstand the cruel environment of outer space, they should certainly be suitable for use in homebuilts here on earth. The best of these adhesives are truly amazing. A glued joint, properly made, will be waterproof and virtually indestructible. It will not come apart. The wood will rupture first.

    The government (FAA) believes that rescorcinol glues are the longest lasting and most reliable of glues suitable for use in certificated aircraft. But, there are a couple of drawbacks to the use of rescorcinol. This glue does not have gap-filling characteristics and does require well-fitting joints and considerable pressure during assembly. Furthermore, the working temperature must be over 70 F or the joints may not cure properly and the results will be unreliable.

    On the other hand, there are the more forgiving-epoxies. They are truly the new miracle adhesives. Even though the FAA has not enthusiastically embraced their use as it has the rescorcinols, most inspectors will accept whatever glue you use provided you can furnish sample test pieces which can be destroyed to prove that the wood will fail and not the glued joints.

    Wing structures take two forms. They are either strut braced or cantilever.

    Strut Braced Wings
    A strut braced wing is typically lighter than a cantilever wing because its spars need not be as robust nor must the wings be skinned with plywood. Most strut braced wings are fabric covered although occasionally they might have lightweight aluminum skins. Less frequently a judicious use of plywood skins are used to cover critical areas.

    Wing spars for strut braced wings are normally cut from solid spruce or Douglas Fir blanks. Although the spars are cut from a single slab, they can be laminated of two or more pieces when perfect spar blanks of the proper length and width are difficult to obtain.

    Leading edges are covered with thin aluminum sheet or plywood . . . usually on the top only to help maintain the correct airfoil shape. This is necessary because the tightly shrunk fabric tends to sag somewhat between the ribs. This, in effect, changes the airfoil to one with less camber in those areas and it does affect performance.

    Strut-braced fabric covered wings require internal drag and anti-drag bracing to maintain the wing’s geometric planform alignment. The drag and anti-drag bracing is usually made from 1/8" or 3/16" diameter 4130 wire threaded at both ends. When drag and anti-drag braces are aligned and tightened, they will tend to pull the front spar and rear spar together. To prevent this from happening, it is necessary to install compression ribs, or struts, at each wire intersection.

    Fabric covered wings are usually built in two pieces although a single piece wing section is not uncommon. Quite a few biplanes utilize a three-piece top wing to simplify assembly and rigging. For that matter, a two-piece top wing, or a single-piece upper wing is not a rarity.

    A most useful characteristic of a strut-braced wing is that its tips can be rigged to have a deliberate warp in order to correct a wing heavy condition should that be necessary.

    After the initial test flights, this type of wing can be rigged with a slight twist to provide whatever wash-in or wash-out is necessary to achieve perfect lateral trim. You can’t do this with a cantilever wing.

    Cantilever Wood Wings
    Cantilevered wings are considerably heavier than the strut braced types because the internal structure must be more robust to handle all the loads without the help of wing braces.

    The lightest version of a cantilever wing will be one that is built as a single full span unit. Unfortunately, for builders this means that the workshop space must be large enough to accommodate the entire wing span and still allow a bit of workspace at each wing tip. For this reason, and to permit easier handling during construction, many cantilever designs offer a two-piece or three-piece option. This, of course, means the wing must be assembled and held together with strong metal wing straps and bolts . . . resulting in a 50 pound, or greater, weight penalty for the added structure.

    Cantilever wood wings are assembled around a stout box spar. The wings are then skinned with plywood. This stressed skin construction eliminates the need for drag and anti-drag internal bracing because the plywood skins ensure complete rigidity of the wing in all directions.

    Skinning a cantilever wing requires careful jigging and alignment because once the closing plywood skin (top or bottom) is installed, unlike strut braced wings, it will become impossible to twist the wing to compensate for wing misalignment.

    Wood Wing Ribs
    Almost any design wing ribs may be used with wood wings. They may be the built-up strip and gusset variety or may be cut out of a plywood sheet. There are many variations of the two basic types. Essentially though, plywood covered wings require heavier wing ribs than do fabric covered wings. This is due to the need for the ribs to withstand the hammering accompanying the attachment of the clamp-like nailing strips.

    Whenever a greater spar strength is called for, the wing ribs are constructed in two or three pieces to allow for the use of a full depth spar. This complicates the rib alignment and entails much more work.

    In other instances the ribs are fabricated in one piece and are simply slid over the spar to their correct position. Alignment is also simplified.

    Longevity of Wood Wings
    Not commonly known is the fact that wood does not deteriorate with age and may in effect become stronger with the passage of time. This, of course, requires that the wood be properly protected against the intrusion and entrapment of moisture.

    Wood will not suffer dry rot decay if the structure is ventilated and water is not permitted to become entrapped. This means that the wing must have drain holes situated in the low corners of each compartment. Metal aircraft, too, require this simple precaution to minimize the risk of corrosion from entrapped moisture.

    A Bonus Feature
    Unlike the stereotype composite wing structures, wood wings afford a builder considerable variety in the construction methods and details he will encounter. This is a far cry from the monotonous construction techniques imposed on composite aircraft builders. Not only that, a speedy plywood covered wood aircraft can be finished as smoothly as a composite . . . and you don’t have to paint the airplane white to protect the internal structure from the harmful effects of solar radiation that affect composite designs.


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