April, 2005

Some more construction photos from Dave Christensen, Celerity builder We always get good feedback when we show pictures of our builders' work. This time, feast your eyes on award-winner Dave Christensen's Celerity wing and fuel tank construction!

Leading edge tank-see related photos in the February 2005 newsletter

In case you are wondering, Dave has won awards with his beautifully constructed KR-2. It's easy to see why, when you look at the quality of his work up close, even in a fully enclosed space such as the inside of the wing.

Christensen's aileron counterweight assembly

Dave's aileron counterweight, shown above, is a modified copy of the original-Dave's version is a bit more compact, which will save a little weight. Rather than going along with the plans that show the aileron counterweight's actuator on one side of a rib and the weight on the opposite side, Dave has brought his all together in one small area.

Another view of Dave's counterwight and the aileron itself. The wing tip is to the left, off the picture.

For us perfectionists, please pay close attention to the following three items in Dave's pictures. First, note the plywood doubler on the rear spar where the counterweight is mounted with four screws. It's actually wedge-shaped to put the counterweight in line with the adjoining rib! Second, please note the roller bearing that is carrying the mass of the counterweight assembly's pivot tube. Finally, note that even the hole Dave drilled through the rear spar for the aileron actuator tube has been kept nice and compact. The result? Another excellent installation, executed very nicely. And if you care to look even closer, you can see little if any evidence of excess glue at the corner blocking locations. (Of course, using clear rather than white glue helps somewhat in this respect.) Note also Dave's wing rib caps. As pointed out last month in our newsletter, Dave is going with a slightly different wing skinning procedure, one that he became familiar with while constructing his KR-2 and with which he feels comfortable. Incidentally, that's the same method that was used on the first Celerity prototype. Nice pictures, Dave! Thanks for contributing. -

Composite aircraft construction, from A to Space!

When you received your last newsletter, man had not successfully flown around the world solo without stopping or refueling. Now, as of March 3, 2005, this "barrier" has also been broken. Aeronautical adventurer Steve Fossett flew a small jet, the "GlobalFlyer", around the world solo without refueling. This great achievement has put yet another feather in the cap of Burt Rutan, GlobalFlyer's designer and builder, who hardly needs any introduction to the rest of us. This came on the heels of another of Rutan's creations making two successful flights into space and safely back to earth last December in "SpaceShipOne" and mother ship "White Knight", garnering the $10 million X-prize. And of course by now it's almost ancient history that Dick Rutan and Jeana Yeager flew Burt Rutan's "Voyager" around the world without stopping or refueling over nine days back in December, 1986. These remarkable aviation achievements once again put a relatively quiet man back into the global spotlight, a gentleman whose notoriety stems from his achievements covering the entire array of flight, from homebuilts to spacecraft. Who would ever have thought, back in the 70's while watching Rutan's composite Longeze airplanes take flight, that space flight would someday happen without the government getting involved, using private funding, and that people would be purchasing tickets for future space flights? A common denominator in Rutan's achievements is the use of composite construction techniques. His futuristic looking canard aircraft designs in the early 1970's (the Variviggen, Varieze, and Longeze, which begat the Cozy, Velocity, and E-Racer, among others), were all composite designs. So what is this wonderful stuff we call "composite construction," and why should you use this construction technique to build your airplane? Is it really that easy to work with, and will it hold up over the long term? The answers are; magic, it's the right thing to do, yes it is, and so far so good. Here are some of my thoughts on composites, along with technical information to help put you at ease about using the stuff on your airplane.

Tough enough?

Many times I have shown people my simple piece of one-quarter inch thick Clark foam with fiberglass on both sides. They turn it over and try to flex it--it's so lightweight and yet very rigid! The icing on the cake is when I demonstrate the strength of the piece using my stacked bricks demonstration. That's when they become believers. At first, many onlookers give me the distinct impression that they doubt anything so lightweight could be depended on to hold an airplane with its fuel, occupants and baggage up in the air. They just need to be shown how you can possibly make something so strong out of such lightweight, dissimilar materials. (To be fair, I must point out that most people also lack appreciation for the fact that many airplanes have cloth skin, and that their wing ribs are sometimes hardly thicker than the metal in a Campbells soup can!) Not having a technical background, I imagine that many such observes might think that "these things are made so light and flimsy, they're probably just waiting to fall out of the sky!" But that thin piece of foam and fiberglass is a prime example of the proper application of materials to the intended purpose. (i.e.--Engineering and technology say that you don't need a steel plate to keep rain off your head--a light weight nylon umbrella with wire ribs will do nicely.) And as Burt Rutan demonstrated, you don't necessarily need lumber or aluminum or steel to make structural airplane parts.

Composite airplane skin

Both the Mirage Marathon and Celerity feature composite skin over a wood structure. There are advantages with wood construction that we believe are worthwhile, such as its documented strength, durability, ease of construction, and sound deadening properties. Once we make our wood frame structure, the next problem is deciding what to use for a skin, fabric, more wood, or fiberglass? From a course in human anatomy I learned that the skin is the largest single organ of the human body, in both size and weight. Likewise, the skin of an airplane is large and can make up a lot of the total empty weight. (For ultralights, what's the first thing to go in order to save weight? Fuselage skin, of course.) So the most obvious advantage of composite construction should be realized when it is used to construct the skin. Also, the use of composite materials for the skin allows the builder to craft beautiful, smooth, complex shapes that are much more difficult to achieve using the other types of aircraft covering. The big problem covering an airplane with fiberglass is that fiberglass alone is not a great material for airplane skin--when you make it thick enough to achieve the desired rigidity, it becomes awfully heavy. The solution is to combine it with a material that will make the fiberglass skin stiffer without adding a significant weight penalty. A composite skin is the perfect solution.

Definition of composites

Now what is composite construction exactly? My dictionary defines composite as: "Made up of disparate or separate parts or elements; compound...(Naut.) noting a vessel having frames of one material and shells and decking of another, esp. one having iron or steel frames with shells and decks planked; composite-built." That sounds almost like a tube and fabric airplane, doesn't it? Yes, the definition technically fits. However, in the world of experimental aircraft construction the term "composite" has a particular meaning that has been gained through popular usage. When people say "composite" airplane they are usually thinking of something that is very smooth and fast, made possible by materials such as fiberglass and resin, a combination that yields much more strength than either the cloth or the resin alone. Fiberglass has made that possible. An excellent material for aircraft construction, it is relatively inexpensive, easy to work with, and quite durable. Extensive use of fiberglass to build airplanes has literally changed the face of the homebuilt aircraft movement in recent years and helped foster a new generation of builders that expects classy, hot designs. Now it's also changing the face of commercial small airplane manufacturing with Cirrus, Lancair's Columbia, and numerous other new entries in the field of FAA certified composite airplanes made for the public.

Soft, weak foam makes fiberglass stronger!

Although fiberglass is a wonderful material, some more work had to be done to make it lighter weight and more rigid, especially for large skin panels. Eventually rigid foam came to the rescue. The use of foam for cores was preceded by more expensive core material, which was also difficult to manufacture. It had been common knowledge for some time that lightweight, strong panels could be made by bonding sheets of fiberglass or thin aluminum to the edges of a lightweight honeycomb type core made from thin aluminum or even paper. Unfortunately a significant problem arises in manufacturing with honeycomb--it's difficult to bond the outer skin material to the edges of the honeycomb. Therefore the initial application of honeycomb core was limited to cases where the weight savings was important enough to justify its high cost, such as expensive jets and spacecraft. As an alternative, light weight wood was (and to a limited extent still is) used as a core material. A modern day example is the end-grain balsa wood core used in AeroCore composite strips. The strips are laid over bulkheads and their edges bonded together. This way you can make a fuselage shell rather quickly. In this application the balsa wood grain runs perpendicular to both the length and the width of the strips. A disadvantage with this or any other manufactured component is the cost--you have to pay for someone to make up those specialized core panels. And since it must be purchased in finished dimensions and shapes, it also places limits on the builder's flexibility. The obvious solution for those who want to hold their costs down is to manufacture their own composite parts with foam, fiberglass and resin. Another advantage with foam is that it can be easily shaped by the builder to make any form desired, overcoming the problem of wet fiberglass layups not having any shape or form. In kit airplane factories, huge, expensive female molds are typically used over and over again to make fuselage halves and wing panels. In making these kits, the mold serves a tool. But with "moldless" construction utilizing foam covered with fiberglass, the foam doubles as a form or mold and also as a vital structural component of the piece.

Foam and fiberglass sandwich panels are inexpensive and easy to construct

Remember where I said that the skin is a very large part of an airplane? It certainly is--figure on about 450 to 500 square feet for either the Celerity or the Marathon! Therefore the unit cost (price per square foot) becomes quite an important factor. The unit weight is also significant. Adding one ounce per square foot to the skin will increase total weight 30 pounds. So even minor savings of weight and cost in the skin of an airplane repays the builder with a multiplied benefit. Since there's quite a bit of work involved, especially finishing, the labor costs add up quickly in those kits which have a finished skin. These costs are of course added to the sales price. Many of our builders are on a tighter budget, and they know they can save a lot of money by "making their own sandwiches" instead of buying a kit airplane. Even though three to four layers of fiberglass are required, along with several gallons of resin, the material needed to make the entire skin (including foam) will run about $1,250. You would pay many times that amount for premolded panels. But the real beauty of it is when you start working with these materials, and you find out how easy it is for even a first time builder. And don't worry, because even if you make a mistake (you will--we all do) it's relatively simple to fix because of the nature of the materials used, and the repairs won't cost you very much.

Glass cloth

In a typical composite wing panel, glass cloth provides much of the strength. The glass cloth can be any one of several materials, woven in several different ways, and available in many weights. Glass cloth is woven from fiberglass strands, carbon fibers, or Kevlar® material. Fiberglass is most commonly used in homebuilt aircraft projects. There are two basic types of glass cloth, unidirectional and bidirectional (or "BID"). All the fiberglass strands run in one direction with unidirectional material, and the builder orients these strands in the direction that tension will be applied. A few small cross-strands hold it together during construction. Bidirectional cloth has strands woven in both directions, and it offers many variations such as more strands in one direction than another, satin weave with finer strands, twill, crowfoot weave, etc. The weight (ounces per square yard) largely determines the strength of the fiberglass. We use 3.16 oz and 3.8 oz BID for the Celerity and Marathon. (6 oz is used to make the fuel tanks) There are also two grades of fiberglass cloth, "E" and "S". The E-glass cloth is somewhat less expensive, has adequate strength, and is generally preferred over S-glass. Carbon fiber (sometimes referred to as graphite) is used in special applications where a very strong reinforcement is desired, such as a sailboat mast or a wing spar. I have not worked with carbon fiber, but I am told that it is more difficult to work with than fiberglass. Since our airplanes are designed with the wood structure carrying the loads, we wouldn't need carbon fiber. DuPont Kevlar® (of bulletproof vest fame) is very good for certain limited applications, especially if abrasion resistance is important. It is reportedly difficult to work with and doesn't have enough advantages to make it the material of choice for aircraft construction.

Resin and hardener

The resin that is used to saturate glass cloth protects the fibers and keeps them in proper orientation. Epoxy resin, the most popular type for aircraft construction, is the only resin used for Marathon and Celerity construction. It does not shrink over time like other resins and is easy for the first time builder to use. The other two resin types commonly used with fiberglass are called polyester and vinyl ester. Polyester resin is used extensively for boat building and some mold making. Although it's nice to work with, it tends to shrink over time causing cracks in the paint. While vinyl ester resins are used for some aircraft construction, but they are a chemical stew that's complicated to mix up and use. Mixed and used properly, epoxy resin is the superior material for homebuilt aircraft construction. The mixing directions must be followed strictly, with the proper amount of hardener mixed with the resin, never too much or too little of either component. Unlike polyester resins, you cannot speed up or slow down the hardening process by varying the amount of hardener used. Although the resin and hardener react at room temperature, a process of post curing at elevated temperatures increases its strength. This can be accomplished by parking your aircraft in the sun.