Selecting Sport Kite Spars

Spar Comparison Table, Radar and Bar charts were updated on 26-Aug-1998.
This is a (pretty technical) article about sport kite carbon spars. It was first published in Drachen Magazin 2/93. This is an update article, which was published in DRAma 4/1997.

Referencies:

  1. Mark Cottrell, "Swept Wing Stunt Kites", 1990.
  2. David Lord, "Selecting spars for a new kite design", SKQ Vol. 3 No. 4, 1992,.
  3. Simo Salanne, "Mass-stab", Drachen Magazin 2/1993.
  4. Michael Graves, "Spars, Making your best picks...", KiteLines Vol 11. No. 2, Winter-Spring 1995.
  5. Simo Salanne, "Die Qual der wahl", Drachen Magazin 4/1997.

Selecting Spars for a Sport Kite

DRAma published my spar article first time in 2/93 issue. It was followed by one updated spar table. Since that a lot of new spar types have been introduced to the market - and many has disappeared. It's time for a refresh. We also publish a revised text, explaining the concepts and the use of data.

My experiments, described in this article, were inspired by David Lord's article in Stunt Kite Quarterly (not published any more) Vol. 3 No. 4, 1992, "Selecting spars for a new kite design". Dave had measured spar deflections and calculated the relative stiffness of several kite spar types used in U.S. I used similar setup for my measuments of common European spar types and kept the results compatible. Later Michael Graves continued spar studies and published the results in KiteLines, Vol 11. No. 2, Winter-Spring 1995.

Measurement setup

In my original setup two spars were placed on a table, under a weight, and a normal 330 cl (360 g) beer can was hanged on the other end of a spar in my interest at 0.6 m distance from the edge of the table. The deflection was measured between the spars. I measured at least five samples of every spar type and calculated the average deflection.

In the present setup the spar is supported on two points and the load hangs in the middle, where the deflection can be read from a millimeter scale. Distance between the supporters is adjustable 60-80 cm and the weights in use are 500-2000 g. This variable setup gives more accurate results as each spar type can be loaded to deflect similar to one in a flying kite. The measurement setup parameters, spar names, deflections, weight and diameter information are entered in a computer, which calculates the final comparison results.

Originally Dave Lord selected Classforms K75 (glassfibre) as the refence spar material. K75 was then used e.g. by Top of The Line to frame Spin-Off and Hawaiian Team kites. The refence spar is used as the base to which the other spars are compared, that's why we speak about "relative" stiffness and weight. Actually the refence spar could any spar type, but it makes things more concrete if it's widely used and known. In 1992 Spin-Off and Hawaiian were THE refence kites, others were compared to. I have not seen or heard about a new sport kite using K75 frame for many years. It's time to change the refence to a more contemporary one!

I selected the new 6 mm Exel Extreme as the new refence. Because:

Exel Extreme is a new product coming to the market in spring 1997. It has improved strenght against impact forces. Thus it is well suited for trick kites. Unfortunately my laboratory methods don't measure the robustness of the spars.

Stiffness vs. Flexibility

The relative stiffness is the deflection of the reference spar divided by the deflection of target spar. E.g. relative stiffness of Beman Carbon Strong 14 is 0.75. It means it bends 25% more than Exel Extreme 6. Usually a stiff spar is more desirable than flexible. It is easy to interpret the relative stiffness number as "more" means "better". This works particularly well in the bar diagram, where relative stiffness is combined with relative weight. Weight is naturally considered a "less" means "better" matter in a kite. In the diagram the difference in the height of relative stiffness and relative weight bars gives an overall indication of spars characteristics with respect to these two phenomenon.

Selecting a replacement spar.

Let's take an example: you break an AFC2300 spar, you do not have any spare stick around and your kite shop can't keep all spar types in stock. In the table you can see that both Exel RCF-6 and Avia 2200 have almost the same stiffness, they weight about the same. Exel is 0.1 mm larger in diameter and Avia 0.2 mm smaller. If Exel doesn't fit in the connector you sand it a little bit. Avia needs some tape around to fit snug.

Dave Lord's Scale Factor.

Dave has developed a scale factor, which helps you to scale kite designs. Let's suppose you have a Speedwing which have RCF-6 frame. You decide to build 25% larger Speedwing having similar charasteristics. 25% means the leading edge will 1.25 times longer. From the table you will find that RCF-6's scale factor is 0.96. Calculate 1.25 x 0.96 = 1.2, which is the scale factor of the spar you need for the larger Speedwing. From the table you will find that SkyShark 5P, Exel RCF-8 and Beman Ultra Light 21 have scale factor practically close to 1.2. Depending on the availability, price and compatibility with the connectors you have, you can select any of them for your larger Speedwing. They all will result to a frame with similar bending charasteristics as you have in your current Speedwing.

Other way to work it out, is to study the table and then size your new kite based on particular spar. Example: you decide to use 5 mm Exel RCF-5 to build a smaller Speedwing. How large should it be? You take RCF-5's scale factor divide it by the scale factor of RCF-6: 0.84 / 0.96 = 0.87. This means the small one should have a leading edge 0.87 times the lenght of your current Speedwing.

The scale factor can be derived from the formulas used to calculate deflections of loaded beams. I bypass the theory here and just give the relation of scale factor S and relative stiffness R. 

        S =  R to the 1/4 power

or
                             4
        R = S x S x S x S = S  = S to the 4th power

Charts

Both the bar and radar chart have same spars but due to scaling and space required, only the middle section of the table is included in the charts. Spars are identified in the diagram by abreviated names, for the full names see the comparison table.

The Radar-chart diagram represents relative stiffness and weight in graphical (and very compact) format. The spars are sorted by increasing stiffness.

Strength

Relative stiffness does not tell anything about the "strength", "durability" or "robustness" of a spar. A spar with a good relative stiffness might break in use more often than another with a smaller relative stiffness. In general wrapped spars stand better axial hit loads, e.g. wing tip and globe interaction, than pultruded spars of similar weight.

Price

One important characteristic of a spar is the price. Wrapped spars are in general more expensive than pultruded.

Size

Pultruded spars are available in more variety of diameters and lenghts. A single piece pultruded leading edge can be more durable than wrapped two piece and ferrule construction.

Errors

I have done hundreds of measurements and entered the values into a computer. I have checked things out, but there might still be some mistakes. Another possible source of deviation from similar charts or tables is that manufacturers have made changes in their products or product names.

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