Sport Aviation 8/81 pp 26-29 Dragonfly: Curves - Composite Construction Made Simple By Bob Walters (EAA 110084), President Viking Aircraft P. O. Box 9000 Suite 234 Carlsbad, CA 92008 ANY VEHICLE MOVING through a fluid has to deal with drag. That's just the way Mother Nature made things work and everyone knows that you can't fool Mother Nature. You don't have to be a genius to see that sharks or porpoises are fast swimmers. Even the uneducated eye is a pretty good judge of streamlining. You don't have to be an aerodynamicist to see that a fighter is probably a faster flyer than an ultralight. They all have to contend with drag, but because drag becomes more important at higher speeds, the high speed vehicles pay more attention to drag reduction than their low speed relatives. For reasons we won't get into here, Mother Nature seems to like curved surfaces. You can either argue that form follows function, or vice versa, but few people would argue with the statement that all other things being equal, curves are good. In designing the Dragonfly, it was decided to reduce drag as much as reasonably possible by producing a sophisticated fuselage shape. Being able to produce this low drag shape at a low cost was the key to meeting the design goal of building a two - seater that exceeded 150 mph with a small engine at less than $5000 total cost. Let's begin with a few words about curves. For this discussion let's consider one - way curves as simple curves, and two - way curves as compound curves. Take a flat sheet of some material and bend it along one axis. You now have a simple curve. Sheet aluminum, plywood, sheet foam and other materials commonly used in homebuilt construction bend rather easily to form these simple curves. Now try to bend the sheet along the other axis at the same time. Not so easy, is it? It's because these materials do not readily form these two way, or compound curves. The reason for this is because these materials do not easily stretch. A thin sheet of rubber or heated plastic can be stretched into a compound shape quite easily, which is one reason that a lot of curved things are made of rubber or plastic. Sheet metal is also formed into various compound curved shapes by employing relatively expensive machinery to force the metal to stretch and shrink. General Motors makes Chevrolet fenders using giant machines; however, you don't often see people making Chevy fenders in their garages. Consider for a moment the Polen Special, a rare and exciting homebuilt. In fact, it's so rare that many readers may not be familiar with it at all. It's the Ferrari of homebuilts and most people would agree that a good one word description would be "sexy." The Polen Special has nice curves. Now consider the Aerosport Quail. It's hard to find any curves on the Quail, much less compound curves. The Polen Special and Aerosport Quail are both single seat, single engine, conventional metal aircraft. Why do they look so different from one another? The answer is curves. Does this mean that Mr. Polen is a better person than Mr. Woods? Of course not, but it is obvious to even the non - homebuilder that the curves found on the Polen Special are more difficult to fabricate than the flat surfaces found on the Quail. The Quail sacrifices some aerodynamic efficiency in the interest of reduced construction cost and ease of fabrication. The Polen Special sacrifices construction costs and ease of fabrication in the interest of increased aerodynamic efficiency. Everything is a compromise when you make something as complicated as a homebuilt aircraft. This is simply a fact of life. The problem of curves is further complicated by the fact that homebuilders are generally unskilled workers. I'm not being blasphemous when I say unskilled. Don't confuse talent with skill. The average homebuilder is not a professional airplane maker. He may be a welder, mechanic, or some other skilled worker, but very few homebuilders are skilled at every aspect of airplane building. More often than not, the homebuilder is a talented individual who is able to quickly learn enough about the various aspects of airplane making to produce a nice finished product in spite of his lack of skill. Few EAA members can weld titanium, but plenty of us can make strong, if not pretty, welds using 4130 steel. It's easy to achieve a skill level sufficient to weld 4130, thanks to the help available from the EAA. Does that mean that you can quickly learn to form compound curves in sheet aluminum? There aren't very many aluminum forming seminars at Oshkosh because forming compound curves in aluminum is a skill that is not so easy to learn. If it were, more airplanes would look like the Polen Special. At any rate there are numerous examples of beautiful homebuilt aircraft each year at Oshkosh that are examples of a triumph of talent and perserverance over lack of professional skill and experience. What about foam/fiberglass composite construction? Here the situation is a little better than with metal. It's actually easier to make a foam fiberglass box with rounded corners than to make one with square corners. Sheet foam easily bends to tnake simple curves, so the combination of simple curves and rounded corners can produce fairly nice fuselage shapes. It turns out that rounding the corners of a foam fuselage is a skill that is easily learned and most homebuilders can quickly master it. However, the curved box with rounded corners has its limitations. If the designer does not have an appreciation for the materials or if he lacks aesthetic sense, the rounded box technique can produce structurally sound, but ugly airplanes. The rounder the corners the better the shape. But larger corner radii not only require more foam but at some point the corner rounding process turns into a shaping process. Shaping foam is not a skill that is easily learned. It is actually sculpturing. Carving the statue of David was done by simply chipping away everything that didn't look like David. Writing the instructions is easy, but achieving the desired result isn't! Carving foam to a pleasing shape isn't so easy either, and composite homebuilders prove it with regularity. Shaping blocks of foam into compound curves is an expensive, time consuming process that requires a skill not easily learned. The end result is usually a composite aircraft with compound lumps rather than compound curves. The designer has done the builder a disservice by implying that carving pleasing shapes is easy. Why not have a skilled shaper make a perfect part, then make a mold, and then sell the molded parts? That way the homebuilder is not faced with trying to make a difficult shape. This, of course, is commonly done with engine cowlings, wheel pants, etc. and is a good approach for single skin, non - structural parts. The production of structural components usually requires more expensive tooling, more expensive materials, more complicated techniques, and more skilled labor. This isn't necessarily bad, but it certainly isn't cheap. Unless the manufacturer can produce a lot of parts, the cost of each individual part will contain a significant part of the tooling and development cost. Skilled labor isn't easy to find nor is it inexpensive. Does that mean that the composite builder is faced with a choice of a lumpy aircraft or~an expensive pre - molded one? What about the little guy? Has the EAA member on a budget been left behind by the composite designers? Fortunately, this is not the case. Viking Aircraft has developed an easy, inexpensive way to make compound curved fiberglass/foam structural members that do not require any skill more advanced than the ability to read. We certainly didn't invent either the material or the process. The process has been used for hundreds of years. The core material used on the Dragonfly fuselage is l Yz" thick, 4Yz pound per cubic foot polyester based urethane foam manufactured by Clark Foam Products. This core material is the same formulation as is used in the famous Hobie Cat sailboat and is very similar to PVC foam in performance at a fraction of the cost. It is superior to PVC as far as shapability is concerned, and is available in a variety of densities and thicknesses. Full details concerning the construction method are available in the Dragonfly construction manual, however, the following short synopsis will illustrate the basic techniques. The Dragonfly fuselage bottom and sides are cut out, fiberglassed on the inside, and assembled along with several foam/fiberglass bulkheads. This threesided structure is like the curved box with rounded corners discussed earlier. The fuselage top covers are made of the same l/z'' urethane foam core material, but with one very important difference. Clark Foam Products has a special gang saw that cuts narrow saw kerfs halfway through the foam sheet to produce a 1" scored grid in a checkerboard pattern. The Yz" sheet of foam, which previously behaved much like a piece of plywood, is now able to conform to a surprisingly small radius compound curve. All that is required is a method to control this advantageous property. The Dragonfly plans contain full size patterns for the mold formers which are cut out of chipboard or plywood and fastened to the workbench. Next a number of flexible wooden battens are fastened to the formers sort of like stringers to make what amounts to a simple female mold. In the case of the Dragonfly, one mold is required for the aft fuselage cover, and another mold for the forward fuselage cover. Each mold costs about $7 and takes about four hours to fabricate. They can be re - used a number of times if several builders are located in the same area. Next, a sheet of scored foam is pushed into the mold with the saw cuts on the inside (the side that will eventually be the inside of the fuselage.) The foam is temporarily nailed into position so that it exactly matches the mold. Hundreds of tiny dabs of automotive polyester body filler (Bondo) are used to glue the foam core to the wooden stringers. After the Bondo cures, the nails are removed and the inside of the fuselage cover is prepared for fiberglassing. A mixture of epoxy and glass bubbles used as a lightweight filler is first squeegeed into the saw kerfs to fill any voids. Then the appropriate number of layers of glass cloth is applied. After cure, the part is removed from the mold. The Bondo dabs produce little or no damage to the bare foam exterior surface during removal. The fuselage cover is next positioned on the three - sided box - like fuselage and the edges of the molded part are sanded to match the fuselage sides. The fuselage exterior is now ready to receive its fiberglass skin. This unique method produces a nicely shaped fuselage without overtaxing the skill level of the average homebuilder. The Dragonfly received the EAA Outstanding New Design Award for 1980, partly in recognition of this building technique. Some have claimed that the Dragonfly is too pretty and should have received a workmanship award rather than a design award, but in fact, the workmanship on the Dragonfly prototype is quite ordinary. The truth is that the pretty shape of the Dragonfly is a product of its design, not a product of superior workmanship. Using this technique, even a beginner can achieve good results at a price anyone can afford. Part of the key to good design is an understanding of the building materials available. What does this mean to the average EAA member? It means that if a designer has a feel for the materials that he specifies then he can design a pretty aircraft and expect that a builder can produce the desired shape without spending a lot of money for expensive premolded parts. It also means that a "one off' part can be produced for your custom dream ship cheaply and easily. Any good book on boat lofting will provide sufficient information so that amateur designers can produce any required shape using this scored foam/wood mold technique. An entire fuselage could be made in several parts, for example. Of course, proper structural engineering is mandatory and it must be made clear that a piece of foam with a bit of fiberglass cloth on each side does not in itself guarantee a structurally sound fuselage. Contact Wicks Aircraft Supply, 410 Pine St., Highland, IL 62249 if you are interested in purchasing the materials necessary to use this building technique. Viking Aircraft does not stock or sell any building materials, nor are we able to provide any engineering data to the would - be designer. Captions: The Dragonfly on approach to landing at Oceanside Airport. Dragonfly plans contain full size patterns for making these chipboard mold formers. After being cut to shape, they are mounted to an alignment keel on a level workbench. A mixture of epoxy and a lightweight filler (glass bubbles) are first squeegeed into the foam core to fill any voids in the saw cuts, then the glass cloth is applied. After cure, the part is removed from the mold. The aft cover is positioned on the fuselage and then sanded to match the sides. Finally, the cover is fiberglassed, making it an integral part of the fuselage. Wooden stringers are fastened to the chipboard formers, forming what amounts to a simple female mold. It takes about four hours to make - at a cost of around $7.00. The mold shown is for the aft fuselage cover. A similar mold is used to make the forward fuselage cover. This is the curvatious - and uniform - result of making the aft fuselage cover in the female mold. A sheet of scored foam is placed in the mold, with the saw cuts on the inside. It is temporarily nailed into position so that it will exactly match the shape of the mold. Small dabs of Bondo glue the foam to the mold. After it cures, the nails are removed so the inside surface can be fiberglassed.