Scratch Building a Skyray 35

Scratch built Skyray 35

Scratch Building a Skyray 35

Welcome to my first Building Guide. A lot of people seem to be intimidated by building model airplanes. Others think that it takes too much time. I hope to show there is no reason to be intimidated about building a model airplane, even if you've never done it before, and even if you're starting from scratch. While it's true that building a model airplane can take longer than putting together an ARF, you will get a lot of enjoyment from the experience, gain valuable skills (how will you repair your ARF if you don't know how to build?), and end up with a better quality airplane. Well, the better quality will come with building experience...

Why would anyone want to do this?

In the past, it was sometimes possible to save money by scratch building. The Skyray 35 is available as a complete kit with hardware for around only $50.00, and with the current prices of balsa it is unlikely you'll save much money by building one from scratch, so why do it? For one thing, it's good experience. Many Control Line designs are only available as plans, and the Skyray is a good simple design to learn or practice your scratch building skills on. Another reason is to make structural improvements to the kit. As much as I like the Skyray, there are some things about the kit I feel can be changed for the better. The ribs in the kit are die cut Lite Ply. They're heavier than balsa, the fit to the sparrs and leading edge isn't exactly great, and there are four ribs that are slightly different than the other four. They can be sanded to identical shape, but this changes the airfoil a little bit. Subsituting accurately cut basla ribs makes the wing both stronger and lighter, increasing durability in a crash.

The fuselage could also use some help. The stock engine bearers are fairly short and the plywood doublers are a little soft. Putting in longer engine bearers and using aicraft grade plywould for the nose doublers can really improve the stiffness of the front end. The stock fuselage is very skinny and tends to break aft of the wing in a crash. Cutting your own and making it a little taller can really strengthen this area. Replacing the 1/8th balsa stab and elevator with 3/16 results in a stiffer control surface that is also easier to cut hinge slots in. So, although the stock kit builds into a nice airplane, there are lots of good reasons to scratch build one.

Getting Started

The best starting point for scratch building the Skyray is with... a Skyray kit. Why start with a kit if you're going to scratch build the thing anyway? There are a couple of really really good reasons. First, the kit provides you with the plans and an excellent set of full size templates. Second, building the kit will give you a lot of valuable experience with the design. This is especially useful if this is your first building project. I highly recommend starting with a kit. As for tools, the only thing you'll need other than what you'd use to put together a kit or an ARF is either a band saw or scroll saw, for cutting the ribs and various plywood pieces. I suppose you could do it without the saw, but having one makes the job a lot easier.

Critical Parts

The only parts I would consider shape to be "critical" for a good flying Skyray are the wing ribs and fuselage (motor/wing/stab alignment). When you look at the kit ribs, you will notice that there is 2 slightly different sets of 4 ribs each. When making your own rib template, draw a line on a piece of paper and center one of the ribs on it. Trace around one side of the rib, then flip it over so the other side matches up with the line you just traced. How well does it match? Now pick a rib from the other set of 4 and see how well it matches the line. The point of this is to see wich of the 4 possible rib outlines will give you the thickest airfoil. When you find that out, use THAT outline to trace both sides of the rib for your template. Also, make sure that the leading edge and spar slots are perfectly square and not over sized. You want good, wood to wood joints for strength and lightness.

Cutting the Parts

There are two easy ways to get a good, accurate set of ribs. One way is to have 2 templates. Sandwich the rib blanks between them, bolt the whole thing together, and carve/sand the blanks down to match the templates. If you choose this method, make sure both templates are identical and made of a material that is tough enough to stand up to the sanding. I like to use 1/8" aircraft grade plywood and soak the edges with thin CA. 1/8" aluminum also works well. On an airplane with a constant cord wing like the Skyray, an easier method is to spot glue the rib blanks together, trace the template on top, and cut all the ribs at once on a band or scroll saw. The riblets can be cut the same way. Whicherver method you choose, use 3/32" balsa for the ribs and riblets. 1/16" is a little weak and 1/8" is heavier than you need.

When cutting the fuselage, it is important to make sure the engine mounts, wing cutout, and stab platform all line up. I like to draw a line on the fuselage blank centered on the wing cutout to use as a guide line for the engine mount and stab cuts. I also like to build a crutch for the engine so I know exactly how wide to make the engine cuts BEFORE I start cutting. That helps ensure a solid joint between the engine bearers and the fuselage. The rest of the parts are pretty straight forward. If you're using a smooth running engine like an FP-20, you can use 3/32" ply for the nose doublers to save a little weight. If you're running a Fox Stunt 35, it's probably better to stick with 1/8" aircraft grade ply for the doublers.

Building It

If you were careful and accurate when cuttin the parts, the hard part is over. All you need to do is assemble your homemade kit according to the instructions that came with the Sig kit. (You did start with the Sig kit, didn't you?) About the only difference you'll notice is that the wing builds a lot easier with accurately cut ribs. Since you started from scratch, there's also lots of little improvements you could do, but they aren't necesary. Build, fly, have fun!!!

Updateded November 15, 2007

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