Spars-The Heart of the Wing

Laying out the spar, part 2.


We’re looking at spars, the heart of our wings. In our last installment, we took a look at them theoretically, describing the various parts and how they work together to provide the primary load-carrying member of the wing, reacting to loads in the direction of the lift vector. This month, we’ll begin to construct the spar for a Xenos motorglider. It’s an all-metal wing that is as typical as they get, with a built-up metal spar, ribs, and skin. Yes, the Xenos wing is much longer than most, but that is just a matter of design scaling. It is not aerobatic, but the length of the spar makes the root section fairly hefty—heavier, in fact, than many short-winged aerobatic spars.

Building a metal spar can be broken into two parts: assembling and drilling all of the components using temporary fasteners (bolts and Clecoes) and the actual final assembly. This month, we’ll go over the process of fitting together the components and drilling all holes in their proper locations and to final size. The Xenos spar can be bought as a kit, you can (theoretically) build it from plans, or you can buy a completed set of spars from Sonex LLC for an extra $4,000. That sounds like a lot of money, and it is—but I would bet that unless you build a few of them, the number of hours you’ll put in will surprise you. You might find that you’re working for a couple of bucks an hour. But building is what we’re about, and the accomplishment you feel when you’re done is significant!

The wingspan of the Xenos all-metal wing is just over 39 feet. (Photo: Courtesy of Sonex Aircraft)

What’s Included?

The kit parts include extruded and shaped top and bottom cap sections, a large roll of web material (all prepunched), some thick spar web sections (precut and prepunched), and a number of components you have to build from extruded aluminum angle and some sheet metal. Building any Sonex aircraft requires that you fabricate a lot of parts required for major assemblies, and I find it quite satisfying to make the clips, brackets, and other small bits from the drawings. We’ll start the assembly process here with all of the small components already built, which still leaves a huge amount of fabrication to be done during assembly.

The roll of spar web material consists of 6061 T-6 sheets, all cut to approximately 6 inches wide and varying in length. The thick spar web sections are used near the root end and stack up to create a web approximately 3/4-inch thick. The cap extrusions are roughly “T” shaped, with the leg of the “T” to be inserted in slots made up of stacking the web sections. The top of the “T” actually slopes down on the upper spar cap (or up on the lower cap) toward the center of the wing. The caps also taper continuously from root to tip, making for some very complex shapes. The webs actually get in the way during much of the build because of their shape, and we found that it was far easier to assemble as much of the web as possible and insert the caps for drilling near the end of the stack-up.

Liberal use of Sharpies ensured that correct flanges (in this case, ones labeled “J”) were placed and attached at the correct sites. Marking and covering some areas with blue painter’s tape made sure that holes weren’t up-drilled prematurely and that the correct drill bit was used.

Because of the thick web near the root, and considering that many builders buy spars that are completed and don’t have the tools to drive -5 (5/32-inch) rivets, the flanges that attach ribs in this section are part of the spar, with the ribs being attached as lap joints later on in the build process. There are seven pairs of these little flanges on each spar (one fore and one aft in each pair), and they are mostly unique to their station, so you have to label things and make sure they go in the right place. Keep a bunch of Sharpies on the workbench and use them liberally to label parts. Mark things “fore,” “aft,” “inboard,” and “outboard.”

Oh—you’ll be drilling some holes now, others later, and some to different sizes, so label liberally. Acetone will take all the markings off when you’re done. I also like to keep blue painter’s tape handy to cover prepunched holes that are not to be up-drilled until later.

It really helps to have a workbench long enough to lay the spar out from end to end, with no overhang on the ends or sides. Having room to lay out parts parallel is important, too, so having a table wide enough to do this will really keep things organized. With spars about 25 feet long, this makes for a pretty big table! We actually extended our big workbench using some old wooden doors with 2×4-inch framing to create supports. Having a surface that you can drill and Cleco straight into makes things easier, as well.

Building or combining workbenches at the same height that extend at least as long as the wings greatly facilitates the tasks. The ability to work from both sides of the bench also minimizes the need to “swap ends” while work continues.

Parts Puzzle

The drawings for the spar show a schematic of the various web layers. It took a bit of noodling to figure out the jigsaw puzzle, but the prepunched parts gave us lots of clues. If the holes don’t line up, it’s wrong—but don’t let that lull you into complacency, because if they do line up, that doesn’t make it a slam dunk that you’ve got it right. Many hole patterns repeat at various points in the assembly. We also found that even after completing the left spar, getting the right spar assembled was still a thinking person’s game because the layups are not symmetrical left to right. In fact, the right spar is built somewhat backwards to the left!

Frequent and repeated consultation with the drawings proved necessary to ensure proper stacking of the various web layers. As the right and left sides were notably different, builders also have to make sure they consult the correct drawings.

Once you have the layers stacked up, if you look at the end of the root end of the web, you’ll see a groove between layers that will hold the leg of the spar cap “T.” This is very obvious at the root end with its many layers, but once you get out beyond mid-span on the spar, you’ll find that the web has decreased to only a single layer of sheet metal—so the layer sits on one side or the other of the “T.” And that side changes as you continue to go outboard. Learning how to read drawings is vitally important here—especially understanding hidden (dashed) lines so that you know what is in front and what is in back. Never assume you have it right in one glance; look again and again. Spar building is a place to keep the tools in the drawer while you lay things out—for a long time.

Out beyond mid span on the spar, the web decreases to only a single layer of sheet metal and layers sit on one side or the other of the “T” spar cap (bronze colored). That side changes as you continue to go outboard, as shown in this photo.

Drill Time

Once you are confident that you have the layers in place, it is time to drill a few holes—just a few—to pin things together. If you have previously built sheet metal parts, you probably have hundreds of Clecoes at your disposal. Unfortunately, none of them will help you at the root end because the section is too thick for spring-loaded Clecoes. You can use them as alignment aids, but they won’t hold things together. The next step up from a spring Cleco is a wing nut Cleco. They have a long reach and get screwed down without the use of pliers. Unfortunately, they also cost about $3.50 each, and you’ll need lots of them. This is the time to make friends with a long-time sheet metal builder and see if they have some you can borrow. Forget about -3 (3/32-inch) wing nut Clecoes; spend your time and money on a few -4 (1/8-inch) units.

As assembly of the wings approaches the root, wing nut Clecoes begin to replace standard spring Clecoes.

In the Xenos spar, almost all rivet holes will end up as -5. Because many of the holes are very close to the caps or other components, even wing nut Clecoes aren’t going to help; their bodies are too big. In this case, we went to plan B, which turned out to take a little more time, but saved us a whole lot of money! It turns out that a -5 rivet fits in a hole drilled with a #21 bit. And that exactly fits a 4-millimeter-diameter bolt. So, we bought a box of 4×25-millimeter (1-inch) bolts, washers, and nuts. It might sound excruciatingly time consuming to install and remove all those nuts and bolts, but honestly, it just doesn’t take more than a few extra hours—and you’ll be spending hundreds of hours on the spars. We don’t know how the pros do it, but this sure works for those of us that aren’t going to do this more than a couple of times.

The -5 rivet holes were clamped by 4×25-millimeter (1-inch) bolts, washers, and nuts.

Anyway, with the spar web layers in place, we started drilling holes: first enlarging the prepunches to #40 and installing silver Clecoes to hold things in alignment, and then going up to #21 and installing our 4-millimeter bolts and nuts. We left all of the holes that would take other parts—the clips for the ribs, the large blocks for the main wing-mounting bolts, and the holes for spar caps—in their original state. It’s far more precise to leave them this way and drill them when you are ready than to have small and large holes in a stack-up.

Once the main web portions were bolted together, it was time to install the rib attach clips. These have to be match drilled to the prepunched holes in the web components, so we had to take things apart and use one layer of the web as a template for each pair of clips. You clamp one side in place and drill it from the backside—then take it off, clamp it to its mate, and drill the mate. Once you have done this for all of the clips, you can re-assemble the spar web, Cleco the clips in place, then up-drill the holes to final size. Conceptually that is easy—it is just always disheartening to take things apart once they have been put together. Well…that’s tough—it’s all going to come apart again for deburring anyway!

Match drilling the rib attach clips must be done from both sides of the spar

Keep it Square

The large wing attach blocks were next. The holes are finished to inch at this stage, and they’ll be upsized later when the spars are matched together. We found it easy to line up the -inch holes with a temporary bolt, then hold the block straight using a combination square, and match drill the attachment bolt holes. You quickly become adept at figuring out these little processes and making sure that things remain square. Don’t rush—it is just so easy to get the hole to the wrong size, and once you have drilled it too big, you will have ruined a lot of parts.

Inexpensive bookshelf brackets were attached to the workbench to hold and stabilize the spar in a vertical position while the rib attach clips were match drilled.

Once you have matched drilled all of the thick layers and attached the various clips and parts to the web, it is time to think about the spar caps. But first, think about how to line them up. The height of the spar is given as 739/64 inches from root to tip, and getting that right is important if you don’t want to create problems down the road of wing assembly. Unfortunately, there is room for slop in the slots in the web, so you need to position the caps precisely before drilling. Sonex helps you out here with a few drill jigs that get Clecoed into rib attachment holes in the outboard two-thirds of the spar. But they don’t give you many, and we found that taking the time to make a bunch more really sped things up later. We used some 1/8-inch scrap aluminum and drilled them as a stack to give us a matched set.

The other tool we made to give us a precise spar height was a modified combination square. We started with two identical squares and took the slide off one square, turned it around, and put it on the second square. With two slides on one square, we had, in effect, a large, deep and square caliper. We set this for the spar height and slid it on the assembly every time we were going to do some drilling to make sure we kept the dimension perfect. Making tools and jigs can really help when you are working on a large repeating structure. In the end, the time spent on tools will be paid back in simpler and more precise building.

Sonex provides a few drill jigs that Cleco onto the rib attachment holes in the outboard two-thirds of the spar. We found it helpful to make many more for the job.

Adding the Spar Caps

If the layups for the web have been assembled properly, the upper and lower spar caps should slip right into their slots. They did for us, once we blew all of the drilling chips out of the slots! Make sure you clean those out, or you might not get the height that you want. We used quite an assortment of clamps to hold things in position for drilling, but that’s the result of building such long spars. Lining things up and measuring is something you’ll want to do several times. The spar caps are the most critical part of the structure, and in the case of the Xenos, they are not predrilled. So once you start drilling, you are setting the final geometry and edge distance.

As with the rest of spar assembly, we decided to start with a #40 drill bit and silver Clecoes to hold things together. We initially drilled approximately every foot or two to hold things in alignment. The spar caps do flex, and wider spacing will allow them to wave a little bit if you’re not careful. Long drill bits are required in order for the drill motor to clear the edges of the caps, so having a bunch of six-inch bits on hand is good prep for the drilling. Remember that silver spring-type Clecoes will not fit all the way through the stack, so you might have to go right up to a #21 to get a bolt in to hold things together. You can use short #40 drill bits as alignment pins while drilling the large holes until you get enough bolts in to keep everything rigid.

The “T” spar caps slide in nicely once the slots are adequately cleared of aluminum shavings and other debris. The drill jigs ensure they are properly placed before drilling commences.

It took a lot of care to drill precisely perpendicular to the spar web and avoid nicking the caps with the edge of the bit. Take your time, and don’t feel like it all has to be done in one session. Once you have placed bolts or Clecoes every foot or so, both top and bottom, the assembly should pretty much be dimensionally fixed.

When you get outboard a ways, the layers of web material tend to alternate on the front and back of the spar cap flanges. This means that to see the prepunched holes in the webs, you’ll have to flip the spar over to drill many of the holes. Make sure that you have drilled as many of the holes as you can before you flip to hold things in alignment—and then check the alignment and clamps once again after the big flip. If you plan this right, you’ll only have to do this once! We found that one of the problems with this approach is, of course, the Clecoes that you put in on the top. The assembly will want to sit on the Clecoes once you flip it over, so you either need to put in enough of these “legs” to keep everything stable, or you’ll want some 4×4 blocks about 6 inches long to stand the assembly up off the bench in a stable fashion.

Black (-5) Clecoes are your friends when you get to the last dozen feet or so of the outboard ends. The web is thin enough out there that the spring Clecoes will hold, so beg, borrow, or steal about 250 of them, and you’ll be in good shape. This, in combination with the 4-mm bolts, (about 300 of them), should give you enough temporary fasteners to hold a Xenos spar together. If you’re building a shorter spar, you can get by with less.

Next time, we’ll take a look at how you rivet this thing all together. We’ll start with disassembly, deburring, dimpling, and countersinking (the inboard edges overlap in the fuselage and sit flush with each other, so many of the rivets and bolts have to be flush as well), then reassembly and riveting. How do you set -5 rivets? Join us next time and we’ll show you how it’s done.

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Paul Dye
Paul Dye, KITPLANES® Editor at Large, retired as a Lead Flight Director for NASA’s Human Space Flight program, with 40 years of aerospace experience on everything from Cubs to the Space Shuttle. An avid homebuilder, he began flying and working on airplanes as a teen and has experience with a wide range of construction techniques and materials. He flies an RV-8 and SubSonex jet that he built, an RV-3 that he built with his pilot wife, as well as a Dream Tundra and an electric Xenos motorglider they completed. Currently, they are building an F1 Rocket. A commercially licensed pilot, he has logged over 6000 hours in many different types of aircraft and is an A&P, FAA DAR, EAA Tech Counselor and Flight Advisor; he was formerly a member of the Homebuilder’s Council. He consults and collaborates in aerospace operations and flight-testing projects across the country.


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