Leaderboard

The spar pieces for my Rev XX are too long to fit in a large suitcase, even diagonally. I decided to build a couple of frames so my kites would fit in a piece of carry-on luggage. First, it's important to understand how roll-wrapped carbon fiber tubes are made. Carbon fiber fabric is impregnated with resin, and wrapped around a mandrel. The fabric can be any type of weave, or even a combination of different types in layers. Plain weave Twill weave The fibers of a plain weave make sharp turns in close proximity to each other, which reduces its strength slightly. Twill weave is the most common carbon fiber pattern. Rather than alternating between top and bottom every row or column, the alternations are in a 2x1 pattern, which provides a bit more strength. The finish after some light sanding is matte. Glossy tubes are made by adding a super-fine sanding step at the end. You can also get pultruded tubes, which aren't woven at all. The fibers are parallel to the length of the tube. Pultruded tubes are very stiff against bending, but weak in terms of being crushed. A roll-wrapped CF tube is made by wrapping resin-impregnated cloth around a mandrel, vacuum sealing or taping the assembly, then heat curing it. The mandrel is a highly-polished metal rod, so the inside diameter of the CF tube is precisely controlled. The outer diameter is determined by the initial thickness, and the amount of sanding done afterwards. Medium sanding will produce a matte finish. Following this with fine sanding produces a glossy finish. The spars on my rev XX kites are 9mm outside diameter, and 8mm inside diameter. You can get 50 cm carbon fiber tubes from Aliexpress in a wide variety of outside and inside diameter combinations. Usually they come in pairs, for a total length of 1 meter. This is more than long enough for one spar of the largest quad kites. Just search for "carbon fiber tubes." Be careful - sometimes one vendor will allow you to choose from a wide range of sizes, and also between twill and plain weave. And you may also have the choice of matte or glossy. I prefer matte, as it is better for gluing. You generally have to sand the gluing area anyway, so maybe it isn't important. The outside diameters are usually slightly larger than the specified dimension. This is great, because you can sand the pieces down for a perfect fit. You can cut a carbon fiber tube with a cutoff wheel on a rotary tool. Carbon fiber generates a lot of fine particles, so it's a good idea to have a powerful vacuum near the action. And you may want to wear a mask. Finally, eye protection is a must - not just for CF particles, but the cutoff wheel can break and send shards flying. Mark your length by wrapping a piece of painter's masking tape around the tube. The rotary tool can't quite make a flush cut when the piece you are cutting off is more than a few cm. No problem! Cut it off just a bit longer than the marked length. Don't worry about it being nice and flat. Try to prevent the small piece you are cutting off from going into the vacuum. You may be able to use it for a ferrule on another frame. Now make a precision cut right at the tape. With practice, you can get a clean, flat edge. If you would like to clean it up a little bit, wrap the other end of the spar with tape and put it in a drill. The tape is to provide grip, and to prevent scratches. Now sand the end with one hand, while holding the drill trigger with the other. To make an interior ferrule, sand the outside a little bit at a time, until it has the right fit inside your main tube. Don't go too quickly, or you may remove too much material, and the fit will be sloppy. Once you are happy with the diameter, bevel the edge for easy insertion. Flip the ferrule around and sand and bevel the other side. Before you glue a ferrule into a tube, try to scuff the inside of the tube. Remember that it came off of a very smooth mandrel! Mark the insertion depth of of the ferrule with a piece of tape. Apply some 2-part epoxy to the inside of the tube with a toothpick, being careful not to get epoxy on the outside. If you do, wipe it immediately with a paper towel. And if it gets messy, put some rubbing alcohol on the towel. Slowly insert the ferrule into the tube using a screwing motion. This helps spread the epoxy around. Don't back the ferrule out once you've started to insert it! The epoxy on the ferrule will make a mess on the second insertion. Glue the ferrule into one half spar. Don't glue the other half spar onto it. Remember the goal is to make spars that you can pull apart and fit into a small piece of luggage. I made a frame for my Rev XX kites with 9x8x50mm tubes. The ferrules are made from 8x6x50mm tubes. This is the simplest configuration for that particular kite - an internal ferrule connects two 9x8 pieces, each cut to half the length of the total spar. The center leading edge spar has an internal ferrule on either end as well. Note that the center leading edge is a different length than the other spars. Make all of your measurements carefully! Unfortunately the 9x8 mm tubes are not common, and tend to be more expensive than the other sizes. Another one of my kites has 8x7 mm spars. They are lighter and more flexible than the Rev spars. So an 8x7 set made for my full sail XX seemed like it would be a fun project. The internal ferrules are made from 7x5 mm tubes. Those are used to join each spar half, and also for the joint between the center leading edge tube and the outer leading edge tubes. I didn't want to use an external ferrule anywhere inside the leading edge, as they may rub against the leading edge of the kite. Since the kite still wants 9x8 mm tubes for the plastic fittings at the outside ends, and at both ends of the vertical spars, I chose to cut the 8x7 mm spars short, and extend the length with external 9x8 ferrules. I've flown the 9x8 mm frame in very heavy winds, to the point where the person next to me said that the kite might explode. I haven't tested the 8x7 mm frame in heavy winds yet. The 9x8 mm frame is indistinguishable from the stock Rev frame. My expectation is that the 8x7 mm frame will be more flexible. Here are the weights of the various frames, not including the Reflex springs: Rex XX Stock Frame: 96 g 9x8 mm frame: 100g 8x7 mm frame: 88g The 9x8 frame is just a tiny bit heavier than the stock frame, which makes sense because of the extra ferrules. The implication is that the stiffness should be very similar. The 8x7 mm frame is appreciably lighter than the stock frame, even with its extra ferrules. Once you arrive at your travel destination, you may find it convenient to tape the two halves of each spar together. This make assembly and disassembly much easier. Try building some travel frames, and post your experiences here!