SPACE AGE WONDER FOR ULTRALIGHT STRUCTURES
OU HAVE ALL heard of graphite tubing by now and know a little, bit of what it's supposed to be capable of doing. I'm sure you've all heard that it will end inflight structural failures and that it is pretty well bullet proof . . . and that it's twice as strong at half the weight of aluminum and very expensive. Well, for the most part, all those things are true. But like most strong statements one hears, they do not tell the whole story. Just for a start, let's take all the points above and deal with them one at a time.
By Eagle Sarmont Grafspan, Inc. 3365 Axford Rd. Santa Cruz, CA 95062
2) A piece of graphite reinforced tubing (aluminum base) can be made to match the design strength and multiply the ultimate strength by two or more of an equivalently dimensioned piece of aluminum at half the weight and only 5-6 times the cost.
Will End In-Flight Structural Failures To be perfectly honest, the only way to totally end in-flight structural failures is not to fly. The laws of probability being as they are, somebody, somewhere will find a way to exceed the load capabilities of any airplane no matter how strong it is if they work at it long enough and hard enough. What graphites can do, though, is to make it much more difficult. That is, if it's used to strengthen a glider versus maintaining the original strength and trimming as much weight off as possible.
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Twice As Strong At Half The Weight And Very Expensive Again this is true, but it does not tell the whole story. In some load situations, graphite fiber is 10 times stronger than an equal sized piece of aluminum. So coupled with the fact that a volume of graphite weighs only 58% of an equal volume of aluminum, it could be said that graphite was 10 times as strong at half the weight. But this would be quite misleading. The problem is that graphite fiber is a unidirectional material (i.e., its strength lies in the direction of the fiber) while metals such as aluminum and steel are homogeneous materials (i.e., equally strong in any direction). So any comparison between them is, to a certain degree, like comparing apples with oranges. To make such a comparison, then, you need to be very precise about exactly what properties and with what conditions you are dealing. A couple of more accurate rules of thumb might be: 1) A piece of 100% graphite/epoxy tubing can be twice as strong as an equally dimensioned piece of aluminum . . . at half the weight and approximately 20 times the cost. 54 JUNE 1979
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Notice the differences between these comparisons. For example, with the 100% graphite/epoxy tube, there is no mention of design strength or ultimate strength. This is due to the fact that with 100% graphite/epoxy tubes, the two strengths are almost identical. The reason for mentioning it with the graphite reinforced aluminum tubes is that, the aluminum that the graphite is wrapped around will start to bend well before the graphite is loaded to a significant level. In cases where a
.060" to .080" or greater thickness of graphite has been laid up on a tube (a deflexorless leading edge, for example) and the design load has been exceeded to the extent that the tube has been bent, the graphite over a period of 24 to 48 hours will pull the tube back to within a fraction of an inch of being straight. This happens as long as the aluminum sub-tube has not buckled. For those of you with more interest in the actual numbers of the strengths and stiffnesses for various amounts of graphite, I've included graphs of some of our test samples. I am sure that you have noticed the different cost multipliers involved. All of this can become confusing. Just be clear that these are only approximations and that the spread between the two cases and their magnitudes will vary noticeably with different sizes of tubing, the numbers given being more of an average than anything else. The reason for the big difference in price multiples
has to do with the unidirectional nature of graphite and the fact that a tube needs strengths in a lot more directions than one. One last thing on the cost is the price of the graphite itself. There are many types of graphite (almost as many as flavors of ice cream) with prices ranging from $32 a pound to $300 a pound. It just so happens that as pilots we are lucky. There is a type of graphite that can be obtained for $32 a pound, when bought in sufficient volume, that is suitable for our needs. Cheap, isn't it? Is It Really Bullet Proof? If by bullet proof you are referring to a tube's ability to withstand impact loads, it isn't. And neither is aluminum. For example, on 100% graphite tubing the epoxy is all that is capable of dealing with any impact loads. Epoxy, being as brittle as it is, doesn't compare too well with T-6 aluminum. On the other hand, the graphite reinforced tube with the aluminum acting as a stiff and springy backing did better than either of them alone. For example, one of our tests included checking to see if non-visible damage could be done to the epoxy that would seriously degrade the performance of a graphite reinforced piece of aluminum. The results were that we were unable to create any non-visible damage that weakened the tube any amount that we could measure. In fact after going at the tube with a claw hammer on one side in the most critical structural location and making very sizable dents, we were only able to degrade the performance by about 15%. To make a long story short . . . graphite will almost always work best as a reinforcement rather than by itself, and at a 70% savings over 100% graphite tubing! The purpose behind all this is to supply some information and an introduction to a new product that you'll shortly be coming in contact with. The product, graphite reinforced a l u m i n u m tubing, is being produced by Grafspan, Inc. of Santa Cruz, California. What we do is custom design and build tubular spars to meet specific design loads, both bending and axial (axial — compression and tension) for aircraft and other products. We have developed an extensive set of computer programs to assist us in making our tubes the lightest
and cheapest possible for whatever requirements they need to meet. These programs show us what the various loads are across the length of the spar, allowing us to build the tube with the exact amount of graphite/epoxy necessary at every location along its length. In most cases all we would need to design and build a set of deflexorless leading edges for a microlight would be the lengths and various hole locations on the tubes, the tubes sizes, the planform of the sail and its twist curve and what tubing would be an acceptable size for us to roll the graphite epoxy on. If extra strength is required on a specific part of the glider or just an overall upgrading of the maximum design, G load, all we need to know beyond the above mentioned information is how much of an increase you want. Because of the cost of developing these programs and the amount of time involved to work with a new design and interact with a designer, we have to charge a bit more for developing a set of prototype tubing. Also due to uncertainty of the accuracy of any data given us regarding sail shape and twist curves on flexible wing style aircraft (this is due to the fact that it's difficult information to get) we would require H.G.M.A. certification of the design before making any production runs with their accompanying lower price. As a result of this we expect to be dealing mostly with either manufacturers or designers, and occasionally groups of individuals who have joined together to share the cost. For those of you who do not want to put out the money to buy a new glider built with graphite, but would like to get some for your currently owned aircraft, here is what you can do. Contact the manufacturer or the designer of your craft, whether show room new, or out of production, to let them know of your interest. In most cases it is possible to make retro fit kits that will both strengthen and improve the performance of your glider or ultralight at a reasonable price. It is only through such contacts that they will be able to determine if there is enough interest to justify the development of such a kit. Some of the potentials are: . . . down tubes, king posts and cross bars with half the weight of your current one. (We can work with straight tubes only.) . . . deflexorless leading edges stronger than anything you can make out of aluminum, with, in most cases, a decrease in weight. . . . struted gliders . . . shorter cross bars . .. bigger spans .. . higher aspect ratios . . . cantilevered flex wings . . . lighter weights . .. acrobatic capabilities where they were not before due to strength considerations.
All of these are possibilities that become available with composite materials. To give you some idea of who is doing what in graphite: Seagull Aircraft is working
on lighter and stronger struts and cross bars along with deflexorless leading edges; Highster Aircraft is working
with deflexorless leading edges on their double surface design; Ultralight Products is working with stronger and lighter down tubes on their new designs; Flight Design — this is a new company that is producing the 'Lancer', a design from New Zealand — is working on
deflexorless leading edges and a cantilevered landing gear for motor wings; Pterodactyl — James McCornack, writer of Tech Tips' (Glider Rider) and innovator par excellence — is working on a motor glider designed to make the most of graphite/epoxy tubing. SPORT AVIATION 55
