About Aluminum Tubing

    Experimenter - 4/97

    By Bob Whittier EAA 1235

    In 1957 the former Soviet Union startled the world by launching Sputnik I, the first earth-orbiting satellite. This spurred the United States to undertake a most energetic space flight research program. So rapid was progress made that in 1969 we were able to land men on the moon.

    Inevitably some people questioned the enormous cost of this program. Spokespersons for NASA responded by pointing out that technical knowledge gained while doing the vast amount of research work involved would, in time, find many worthwhile applications in everyday life. Someone happened to describe this trickle-down process by using the phrase "spin-off." The phrase caught on and is used often today.

    The ultralight airplanes now such an important part of the sport aviation scene are, in fact, a quite visible example of spin-off. In the 1960s no one foresaw that this very different type of aircraft would attain today’s level of popularity and versatility. Their appearance on the scene surprised many people.

    A major problem facing designers of pioneer earth-orbiting vehicles was how to bring them safely back to earth upon completion of their missions. As far back as 1945 Dr. Francis M. Rogallo, a scientist employed by NASA, was studying kite-like devices. The application then in mind was to find a way for large cargo aircraft to drop bundles of supplies to troops on the ground with more control than was possible with parachutes.

    The result was the Rogallo kite. It could be fitted with an elementary radio-control setup which would home in on a receiver in the hands of a person on the ground. This kite consisted of three long aluminum tubes connected together at one end and free at their other ends. When fabric was fitted between them the result was a long, slim package that was easy to stow in a cargo airplane and which would open into a large, V-shaped kite when dropped.

    When the space program came along, NASA looked into various ways of getting space capsules safely back to earth. They investigated the Rogallo carefully, but eventually decided that a large conventional parachute would better suit the stowage and deployment problems inherent in man-carrying capsules.

    People active in the southern California aerospace industry knew that a simple, pleasurable form of low-altitude soaring could be done in coastal areas where steady wind coming in off the Pacific Ocean encountered coastal bluffs and was deflected upward. It occurred to some of them that the simple, inexpensive and readily-transported Rogallo type of kite had sport flying possibilities there. Once launched, any glider has a component of downward motion. Air is expelled from the aft end of the Rogallo configuration, therefore forward thrust is created and so you have the makings of a glider able to soar back and forth along coastal bluffs.

    Hang gliders of the Rogallo type and designed for this kind of flying appeared and quickly became popular. The ready availability of aluminum tubing in many sizes and of nonporous, nonstretching Dacron fabric made light but adequately strong construction possible. Makers of yacht sails quickly learned how to handle new synthetic fabrics to best advantage, but one observer commented that humans could have contrived workable hang gliders a very long time ago using bamboo and silk cloth, except for the fact that they lacked the knowledge of aerodynamics incorporated in even so simple-looking a thing as the Rogallo glider.

    People living far from coastal bluffs learned about the fun of hang gliding. Those living near inland hills and mountains quickly took up the sport. In some areas of flat country, others took to towing large, man-carrying kites behind speedboats. Of trapezoidal shape, these were aerodynamically poor and could not glide on their own. These contrivances gave way to Rogallo-style hang gliders which could be towed off and then cut loose for short but free glides.

    Then people living in country that was both flat and lacking in lakes of adequate size decided that the way for them to get in on the fun would be to install small, light motors of the type associated with chain saws on their hang gliders. Very soon better engines appeared and ushered in the type of ultralight vehicle now so popular.

    At today’s fly-ins, demonstrations of these little flying machines draw much attention. Persons who have never seen modern ultralights in action are often amazed at their quick take-offs, steep climb-outs and short landings.

    We decided that it was time to publish an article on the materials used to construct these fascinating and versatile flyers. Alas, ferreting out the needed information proved to be difficult and frustrating. Most firms in the aluminum tubing and blind rivet manufacturing fields never replied to our requests for information. Like all manufacturers today, they live in dread of product liability lawsuits and become instantly silent when the words "homebuilt aircraft" are mentioned.

    So, this turned out to be a hard article to write. We are much indebted to those people who did help us by providing useful information. What follows is a useful but by no means complete or final coverage of the subject.

    It quickly became apparent to us that ultralights, like many aircraft designs, are still very much in the pioneering and experimental stage. Things change constantly and rapidly. If homebuilt aircraft enthusiasts had limited themselves to traditional aircraft design, materials and construction methods, ultralights as we know them would never have appeared. While development in the field of factory-made airplanes has been in the doldrums for many years, ultralights have progressed very rapidly because designers working in this field are not hobbled by complex regulations.

    Designers and producers of ultralight kits are scattered from one end of the country to the other. Each shop has developed its own ways of doing things. Designers using conventional materials and methods have a huge mass of technical literature to draw from, but ultralight people don’t. So it’s common to encounter differences of opinion.

    The existence of a metal that came to be called aluminum was discovered early in the 19th century. Many researchers were involved in developing ways to produce it in commercial quantities and use it successfully. In 1909 a German metallurgist created an alloy that we now call "Dural." Would you believe, its original purpose was to make light but strong gun cartridge cases! Noting its combination of lightness with springy toughness, Count Ferdinand von Zeppelin saw it as an ideal material for the frameworks of his lighter-than-air dirigibles. German airplane constructors like Junkers and Dornier soon adopted it and after World War I its use spread to other countries.

    Today we use the word "aluminum" very loosely and this leads many to overlook the vital truth that it comes in alloys of many different compositions and characteristics to suit the requirements of a staggering range of applications.

    Some aluminum alloys resist corrosion well, others poorly. Some are much stronger than others. Some are weldable and others are not. There are various ways to heat treat aluminum alloys and they produce different results. Those whose work involves them closely with factory-built airplanes are familiar with the terms "Duralumin" and "Alclad" and the alloy designations 17ST and 24ST. What’s done in the ultralight field often surprises them.

    When one goes shopping for "aluminum," it’s useful to know that some alloys are made with new, pure aluminum while others may contain some percentage of scrap, or "old," aluminum. In the first method, precise alloying is possible. The assorted alloys to be found in a pile of scrap aluminum results in what could realistically be called a mongrel alloy.

    Outboard motors, for example, are made with an alloy tailored to the requirements of the die casting process. When they are junked and added to a foundry’s scrap pile, the resulting castings could be almost anything.

    You begin to see where we are going. The subject of aluminum today is a vast one indeed. To build safe, small aircraft using it, we have to know what’s what.

    Over the years various groups in the industrial and military fields have formulated aluminum alloy designation systems. In 1954, The Aluminum Association developed a standard that uses a four-digit system for identifying various alloys. Each of the four digits has a specific meaning. If you are very serious about making a thorough study of aluminum, the library of some technical college in your area may well have something worth looking into. The catalogs of Aircraft Spruce and Specialty Co. in California and Wicks Aircraft Supply in Illinois contain pages that cover this subject usefully.

    Also, there is a 2200-page book entitled Machinery’s Handbook. It’s worth knowing about because it covers a vast range of shop-related subjects. Its section on Non-Ferrous Metals covers aluminum alloys and processing methods very well. It’s published by Industrial Press, Inc. and costs $95.00. A metals-using manufacturing company or engineering firm or a technical college in your area might have a copy you can see, or your public library could try to find a copy for you through Interlibrary Loan.

    The book Stress Without Tears is a compilation of articles written by Tom Rhodes. This series originally appeared in KITPLANES magazines. It contains data on designing aluminum tube airframes, and is available through EAA’s Order Dept., 1-800-843-3612.

    One very rarely finds aluminum tubing used for primary structure parts in factory-built airplanes. It’s used mostly for such things as small-diameter hydraulic lines and large-diameter, thin-walled ductwork. Early hang glider constructors in southern California may have found odd lots of usable tubing in factory surplus outlets, but for the sake of purchasing and production efficiency and product safety in service, producers of hang glider and ultralight kits have to find and standardize on something that is widely stocked by aluminum distributors and readily obtainable when needed.

    The alloys most widely used by today’s ultralight kit producers are 6061 and 6063, followed by the suffixes T-6 and sometimes T-8, which indicate the temper, or hardness. Some ultralight enthusiasts appear to believe that "aircraft grade" aluminum tubing is used, but this is apparently a matter of misunderstanding or confusion.

    Aluminum tubing can be manufactured by the extrusion process, in which billets of aluminum are loaded into extruding machines, heated to plasticity, and forced out through dies rather like toothpaste coming out of its tube. This is fast and inexpensive and is the process used to make utility grade tubes and shapes used for products such as garden furniture, trim strips, window framing stock and so on. It appears from what we’ve learned that this low-strength tubing is used by some ultralight constructors for certain low-stress applications. To broaden your understanding of the vast field of aluminum, we’ll point out that large-diameter aluminum pipe for such low-stress applications as agriculture irrigation systems can be made by rolling up flat sheet stock and welding the seam.

    The type of tubing considered acceptable for ultralight airframes is called "drawn" in the trade. Billets of suitable alloy are first extruded to form hollow tubes, and this is then "drawn" in a die-and-mandrel setup that makes for more uniform wall thickness while at the same time providing a working of the grain structure such as to increase strength. Of course, tubing processed this way costs more.

    Also, tubing can be manufactured to certain specifications. There is an organization called the American Society for Testing and Materials (ASTM) which was organized in 1898 to bring order to the rapidly-growing world of engineering. Its many committees have over the years formulated and published standards for thousands of engineering materials. We recently had a chance to see the contents of a kit for a Kolb ultralight purchased by a friend. On each tube was stamped the words PLYMOUTH TUBE COMPANY - ASTM - 6063 T6 followed by figures indicating diameter and wall thickness. This might have given some people the idea that it is "aircraft grade." Actually, it is a drawn-type tubing widely stocked and used for a great many applications where something more trustworthy is wanted than the extruded, utility-grade tubing you see in hardware store display racks.

    We should mention here that while these alloys are stronger than utility grades, the drawing process falls a little short of perfection. Responsible kit producers inspect each tube for irregularities before using it.

    Here is a description of the alloys mentioned previously:

    6000 Series — Alloys in this group contain silicon and magnesium to form magnesium silicide, thus making them capable of being heat-treated. The major alloy in this series is 6061, one of the most versatile of the heat-treatable alloys. Though less strong than most of the 2000 or 7000 alloys, the 6000 alloys possess good formability and corrosion resistance, with medium strength. 6061 is the least expensive of the heat-treatable alloys. It can be fabricated by most of the commonly used techniques. In the annealed condition it has good workability. In the T4 condition fairly severe forming operations may be accomplished. The full T6 properties may be obtained by artificial aging. It is weldable by all methods and can be furnace brazed. It is available in the clad form ( "Alclad" ) with a thin surface layer of pure aluminum to improve both appearance and corrosion resistance. This grade is used for such varied applications as truck bodies and frames, screw machine parts and structural components. It is used where appearance and better corrosion resistance and good strength are required.

    6063 is commonly referred to as the architectural alloy. It was developed as an extrusion alloy with relatively high tensile properties, excellent finishing characteristics and a high resistance to corrosion. This alloy is most often found in various interior and exterior architectural applications such as windows, doors, store fronts and assorted trim items. It is the alloy best suited for the anodizing process, either plain or in a variety of colors. It may contain more scrap than 6061.

    The heat treating and artificial aging of aluminum is a subject in itself. In a few words, T6 means "solution heat treated, then artificially aged." Artificial aging consists of heating to around the 300 degree F. range in specific sequences. If left alone, an alloy might age itself with the passage of time.

    Jim Millett, president of U.S. Light Aircraft Corp., told us something that illustrates the peculiarities one can encounter in the field of heat treating aluminum. He suggests taking a small piece of 6061-T6 sheet and trying to bend it with your hands. You will find it to be very strong and springy. Then heat it to 950 degrees. This could be done in a pottery kiln or by using a propane torch to heat it evenly until the surface turns gray. Then immerse it in cold water. This is what is called "solution heat treatment." The sample is now in the 6061-0 condition, the last "0" meaning that it has been annealed to dead soft and easily bent condition. Then bake it in a kitchen oven at 350 degrees for eight hours. This is called "precipitation hardening" and the sample is now back to the 6061-T6 condition and just as strong as it originally was.

    This gives an understanding of why the 6061 and 6063 alloys are the favorites of ultralight designers. Being used for such a wide variety of purposes, it is readily available in lengths from 12 to as much as 20 feet, an important matter when tail booms and wing beams are being designed. Their wide availability in uniform quality and dimensions is a great advantage to kit producers because they can feel reasonably sure of getting prompt delivery of needed dimensions.

    Producers of kits for wooden light aircraft spend much time searching for batches of increasingly scarce aircraft grade Sitka spruce, and once they get it have to reject about 30 percent of the wood in a shipment.

    It’s interesting to compare figures for various alloys known to have aviation applications. 7075-T6 has a yield strength of 67,000 pounds per square inch. For 2024-T3 the figure is 45,000 lbs. per inch. 6061-T6 breaks at 39,000 lbs. while 6063-T6 lets go at only 31,000 lbs. Designers of ultralights know these figures and perform their calculations accordingly.

    From the standpoint of the individual who buys a kit, aluminum tubing has real advantages. Many glues will not set properly if workshop temperatures fall below 70 degrees F. For the built-up wooden beams of cantilever wings, a jig must be carefully constructed and many clamping devices bought or improvised. A pop rivet gun costs much less than does a welding outfit, and it does not require many hours of practice to learn to use skillfully.

    Mr. Millett pointed out an engineering advantage of aluminum-tube-and-gusset construction. This is the ability to design for high compressive strength. Wood, steel tubing and composites remain straight when subjected to tension loads but buckle quite readily in compression. To minimize the tendency for the welding torch’s flame to burn through, steel tubing has to have reasonably thick wall thickness. So to keep a steel tube structure acceptably light, diameters have to be kept as small as possible and that opens the door to relatively easy failure under compression loads. Because aluminum tubing is light in weight and the fact that welding is not used, aluminum tube construction allows the use of thin-walled tubing of greater diameter.

    This makes designing to get good strength in compression possible. You have no doubt noticed that the steel tubes in wrecked fuselages have failed by buckling under compression. Millett tells of one of their Hornets that flew into the side of a steep hill; the most serious damage was to the pilot’s pride!

    Round tubes have the same amount of material in all of their areas, whereas I-beams or box beams built up of wood or metal have most of the material located in their flanges where tension and compression loads are greatest. If we want to make an airplane of metal, we could use a single thick tube for the wing spar. This is in fact done, and while strength in torsion is often good enough to allow the use of a single wing strut, spar strength in bending has to be calculated and proven by static test.

    The problem in trying to design a spar having upper and lower members of steel tubing trussed together with short lengths of steel tubing welded in place is to calculate what tube sizes to use for the flanges to handle loads in various flight attitudes. There can also be a formidable problem in coping with the warping caused by welding heat so as to produce acceptably true spars. A massive jig might have to be built. When aluminum tubing is assembled with gussets and pop rivets, a good combination of strength and ease of assembly is realized.

    Many ultralights feature tail booms made of single, large-diameter aluminum tubes, generally from five to six inches in diameter. Simpler assembly results compared to using many pieces to build up a trusswork structure. The comparatively modest diameter of a tubular boom can be a real advantage in the pusher propeller layout so popular in ultralights, for it allows the engine to be mounted fairly low while still getting adequate clearance between propeller blade tips and the boom.

    The lower the thrust line, the less will the prop’s thrust create a nose-down tendency when the throttle is opened.

    From the strictly engineering standpoint, a single-tube tail boom might calculate out to be inefficient. Due to the long lever arm it forms, elevator and rudder loads put most strain onto it at its forward end, in the region immediately aft from where the tube enters the fuselage structure. If a diameter and wall thickness is chosen to handle stresses there, the aft end can be stronger and therefore heavier than needed. On the other hand, from the standpoint of the economical production of kits, a single large tube calls for much less handling and machining than would the numerous small parts needed to assemble a lighter truss. This is a classic example of the truth of the old saying that "Airplane design is part science and part art!"

    Randy Schlitter, president of RANS, Inc., pointed out something important about using large-diameter tubes to carry major structural loads. Assume we have a tail boom made of large-diameter tubing. If holes are drilled into its top and bottom surfaces, metal will be taken away from the areas that handle significant tension and compression loads. But if holes of the same size are drilled in the left and right sides of the tube, it will retain full strength to handle elevator and tailwheel up-down loads while still being amply strong to handle the lighter rudder loads. Schlitter says that when holes must be drilled on the top and bottom areas of a tube, it’s time to start thinking of installing suitable reinforcing plates. He further points out that the high silicon content of 6061 gives this alloy good fatigue resistance.

    Phillip Lockwood, president of Leza-Lockwood Aircraft, has more to say about drawn tubing in the larger diameters. Because the demand for these is smaller, fewer mills make them and sometimes a desired size can be harder to obtain. A kit builder located near a major commercial center might find it easy enough to get and will go for a single-tube tail boom. But one located in a small town in a sparsely-populated state might prefer to design a truss-type tail boom using more readily-available smaller diameters. The prices asked for kits today reflect the large amount of engineering, tooling-up and materials-purchasing work that’s involved.

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