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Kit-Building the Thatcher CX4

Wings in a Week!

By Peter Beck, EAA 19566, ptrbec@aol.com

Dave Thatcher’s Prototype CX4

My son Ted (who you may remember from theApril 2010 issue of Experimenter) and I decided that we would devote the weekend of the 4th to assembling a set of wings for CX4 and document the process.
How far was that? Beginning the morning of Friday the 2nd, we worked on assembling both main wing panels and were finished by the evening of July 5, leaving only the tapered tip panels to complete. We spent about 40 total man-hours between the two of us through the long weekend. And we were able to mount them on our CX4.

Morning, July 2

Evening, July 5 

Wing Assembly
Everything on the CX4 starts with the spars. The kits provide finished center section spar and main wing spars, which are a complete, riveted, dihedral set, and even holes for rib-attaching brackets correctly located and pre-drilled. These last items ensure that the spar caps, which are pre-drilled to match the skins, align with the pre-drilled ribs, which also match the skins. Many builders opt for the spar kit even if they plan to build the rest of the plane from scratch.

All spar components, especially the 1/8-inch and 1/4-inch 4130 steel wing mounting straps, are computer numeric control (CNC) milled and drilled. All rivets are AN or solid aluminum, and they’re squeezed with a production quality pneumatic squeezer. Because of this precision, it no longer makes any difference whether wings or fuselage are built first. It can be ensured that the main spars and the center spar will mate perfectly, regardless of which is built first. Because plans builders must mate and drill each main spar-to-center spar joint by hand, each joint is unique, and Dave Thatcher recommends building wings first.

Finished spars eliminate the largest single block of assembly time for the entire plane. While this time varies, some builders have reported well over 100 hours to build the spars alone. But the biggest values from the finished spars are safety and reliability, and next, the precise alignment of parts that attach to them. 

CNC machined and pneumatically squeeze-riveted CX4 spar 

Attach Ribs to Spars
Wing assembly begins by attaching the ribs to the spars. The rib-attaching angles or brackets are also pre-drilled and align with the mounting holes in the spar. Bolting them on should take no more than two hours per wing.

When the rib-attaching brackets are bolted and riveted to the spar, the bottom main skin can be clecod to the spar. The main ribs can then be clecoed in place on the spar and the skin; these ribs will automatically align with the rib-attaching brackets. Alignment templates are provided in the kit and ensure the top skin will also align with ribs and spar. At this time, the forward edge of the ribs can be drilled and riveted to the attaching brackets. The bottom skin is its own rib-attaching template. One of the rib alignment templates that are included in the kit should be used to align the top of the rib with the spar both spanwise and forward and aft. This ensures proper alignment of the top skin when that is attached (in the next step).


Ed Christian’s right wing at 18 hours

Attach Top Main Skin
When all ribs are riveted to the spar, the top main skin can be clecoed in place. The rear spar then can be clecoed to the underside of the top skin, forming a complete box.

To this point, our wing was erected on a bench without the use of a jig. Our bench is only 8 feet by 3 feet, and it’s reasonably, but maybe not perfectly, level. We were now able to lift the wing off the bench and set it up vertically in a simple cradle to make it easier to work on. We also gained a lot of workspace by doing this.
At this stage, the wing is a closed box, and twisting it isn’t really possible. Because all skin layouts and pre-drilling is achieved through CNC punching or milling, rivet hole layouts are perfectly squared. Further, spars, skins, and ribs are perfectly matched, and skin panels are self-aligning, eliminating any possibility of twist. This precise layout even makes it possible to pre-punch all skin holes in the leading edge, allowing the builder to bend the leading edges – at most a one- to two-hour job. The result will be a leading edge that aligns perfectly with the spar and likewise eliminates twist.

Forming the Leading Edge
Two methods for pre-forming the leading edge are available. One simple and fast, and the other elegant but not too much more involved. The simple way involves offsetting the top spar edge 3/4 inch from the bottom spar edge and pressing down on the doubled-over skin with a 2 x 4. The other method involves vacuum forming the leading edge around a piece of 1½-inch pipe with a shop vacuum. This second method leaves a more evenly rounded leading edge at all points along its length, whereas squeezing down with a 2 x 4 tends to compress the root and the tip ends a little more than the center. Both methods have been fully illustrated by builders in construction photos on several public access websites. Both methods result in a leading edge that aligns with the spars and leading edge ribs perfectly.

Whichever method the builder chooses to form the leading edge, these skins are pre-punched and shipped without forming first, and this enables them to be coiled with the other skins and shipped in a small carton, minimizing risk of damage and reducing shipping cost.

Attaching the Leading Edge
Returning to the wing assembly process, the leading edge ribs can now be attached to the spar.Use the rib alignment templates provided in the kit to ensure alignment of the pre-drilled ribs with the leading edge skin as well as the spar.

Leading edge and main wing panel assembly in a cradle

Finally, locate and attach the leading edge skin, running pitot air lines and wiring for wingtip lighting before, or install conduit to install them later. Start by attaching the top of the skin to the top of the spar, progressively cleco from the spar toward the leading edge, and finally, after wrapping the leading edge around the tips of the ribs, cleco the bottom edge of the leading edge skin to the bottom of the spar.

A long, thin, machinist’s scribe used as a drift pin helps to find and align skin and rib holes for clecoing and riveting. Link to glossary term

Outer Wing Panel Assembly
The CX4 has a longish (3-to-1 aspect ratio) wing that ends in a 30-inch tapered panel and tapered ailerons, giving the wing a distinctive appearance.

The tapered outer panel requires manual layout and drilling, although all formed parts and skin material (cut generously oversize) are provided in the wing kit.

The general procedure is to drill the tapered, 0.032-inch outer wing panel spar to mate to the web of the main spar. Once mated, the outer wing panel spar is removed and (without leading edge for now) laid out, drilled, and assembled with clecos directly on a bench top. The bottom is flat, like the bottom of the main wing section. When this panel is clecoed together in a rigid, non-twisting box, the panel can be attached to the main wing. The top and bottom skins are left long where they slide under the skins of the main wing. Once it’s attached to the wing, the leading edge can be bent, fitted, and riveted. There’s a simple procedure, outlined in the plans, to mate the leading edges of the wing and the outer panel smoothly.

The outer panel fabricated and assembled flat on a bench top

We spent an additional 15 hours fabricating and fitting the outer panels the weekend following the 4th of July holiday. And just to admire our work, we hung the wings back on the airplane.

The wings are now complete, save for hanging the ailerons and attaching the tip fairings. These wings indicate the substantial construction time savings that are possible with the CX4 kits. They are the parts of the CX4 that can be assembled most quickly because of the many identical parts and because all layouts are essentially square.

Fuselage and Tail Surface Kits
Fuselage and tail surface kits, together with the wing kit, comprise the aluminum airframe of the CX4 and have been assembled in half the time quoted by Dave Thatcher for scratch building (850 hours). Kits have now been assembled in less than 400 hours. While more complex than the wings, thus requiring more time to assemble (on the order of 200 hours), the fuselage also offers a significant number of CNC-milled parts, pre-drilling, and pre-punching. These, like the wings, are largely self-aligning and require minimal jigging.

The CX4 fuselage kit incorporates the center section spar, or spar carry-through, and goes from firewall to tail post. All formed ribs and bulkheads are included in the kits. Skin panels are pre-punched and are the primary means by which the fuselage is aligned and dimensions set. Certain skins, like the forward, under-nose floor and the tail cone belly skins, have been pre-rolled (bent) to fit the curvature of the underbelly.

Rich Carlson’s CX4 kit fuselage at about 200 hours

Tail Surfaces
The tail surfaces kit is the least pre-fabricated, requiring the builder to lay out and drill all spars and skins. All spars and ribs are pre-formed, however. Fin, rudder, and stabilizer skins are pre-bent and cut generously larger than final dimensions. The elevators are pre-formed in a brake and require only to be trimmed to final dimension, per Dave Thatcher’s plans. The good news is that all surfaces can be assembled flat on a bench top. No jigging is required.

This tail surfaces kit is provided to complete the aluminum airframe and eliminates the need to procure or chase after small amounts of raw materials – especially for builders outside North America. For kit purchasers committed to taking both wing and fuselage kits, the tail surfaces kit is shipped as a no-cost bonus. This enables shipment of the complete airframe in a single shipment and a way to minimize importation costs.

Until recently, the tail surfaces haven’t been provided as a standalone kit. It requires the most builder layout, drilling, and assembly of all the kits, although the assembly is a straightforward and usually obvious process. In a couple instances, the kit has been provided to builders who wish to“see if they are capable of constructing the airplane,” without commitments to the fuselage or wings kits. In these instances, tail kit price is $1,000.

Builder Experience
As of this writing, these kits are now being completed by eight builders. Building experience for them varies from none to more than three planes of various types. Shop space and access to tools vary from very limited to spacious shops well stocked with tools. One of the projects now nearing completion has been assembled in two locations – a 10-foot x 16-foot shed in an RV park, the builder’s winter home; and a hangar at an airport near Seattle, the builder’s summer home. One project has been assembled in under a year and approximately 400 hours by a builder who had no airplane building experience but is a pilot.

What seems to make the most difference in assembly time isn’t so much the shop space and tools as the builder’s commitment to start – with whatever is possible and obvious – and to finish, solving little “oh, so here’s how it works” puzzles all along the way, and an enjoyment of the building process – together with consistent effort.

Building results also include true (non-twisted) wings whose skins are smooth and ripple-free and fuselages that are true and straight.

51 Percent Rule
The CX4 is normally registered as an experimental, amateur-built plane, and it qualifies to be flown by a sport pilot. Registration as an experimental, amateur-built allows the builder to apply and qualify for a repairman's certificate for his plane, provided 1) he’s the "primary builder," 2) the majority of the plane was built by armatures who “undertook the construction project solely for their own education or recreation.” The repairman’s certificate allows the owner to perform annual condition inspections.

Having only now developed kits that meet the designer’s standards for completeness and quality, and having completed this effort over the last three years,I can now begin application for FAA acceptance of the kits as compliant with the 51 percent rule, which is slated to happen late this year. Preliminary review of the kits against the revised 51 percent rule checklist indicates that the kits should comply.

In the meantime, builders are now assembling CX4 kits, confident that they can demonstrate they are the builders of their aircraft.

CNC Machining
All of the advantages offered by the CX4 kits would be impossible without modern CNC machining capabilities and low costs. And while CNC fabrication technology is really another story, its importance to the CX4 needs to be acknowledged.

For example, skins are punched for rivet holes on a half-million-dollar turret punch. Spar caps and longerons are drilled on a CNC milling machine that happens to have a 20-foot bed. These machines are owned and operated by local subcontractors who support the American automotive and manufacturing industries. And these houses are willing to make small (by their standards) runs of CX4 parts for us and at a reasonable price. The more their installed machines get used, the more money they make.

Of course, none of this is news to the folks at Van’s, Sonex, or Zenith who have used these basic capabilities for a number of years. But the CX4 has come along at a time when CNC technology’s cost has reached new lows, and broader availability has it popping up in personal garages and shops.

CNC machines are now capable of being controlled by common household or video game players’ computers, which was barely even conceivable before 1990. And while the industrial versions of these machines can cost on the order of $500,000 to many millions and would be unaffordable to a CX4 builder, the costs of CNC technology are dropping by a magnitude every 10 years. Through machines like the ShopBot, the capabilities that used to be available only in a $500,000 turret punch can now be had in a $15,000 machine. And the costs continue to plunge. BuildYourCNC, a small online business, now provides a kit of stepper motors and electronic controllers and instructions that enable the CX4 builder to put together the same capabilities in a garage for less than $1,500!

The fact that we can buy “off the shelf” stepper motors and the electronic controllers that deliver measured cutting movements of mere ten thousandths of an inch repeatedly and reliably is a blessing of our American manufacturing infrastructure. The general, low-cost availability of these machine parts is enabling literally tens of thousands of craftsmen and product developers to produce some of the most innovative, high-value, low-cost, low-volume products – like CX4 airplane kits – that have been possible yet.

We’ll elaborate on this story at another time and in other places.

Costs of the kits are low enough to still allow the CX4 to be constructed for a total of about $24,000. Kits are shipped in cartons using FedEx or UPS Ground (or air freight for international builders), avoiding motor freight costs and minimizing damage risks.

The CX4 has been recognized as one of the lowest-cost, all-metal aircraft in its class. The design objective of the kits was to deliver high value yet keep costs within reason. While the kits are priced at $11,900, they incorporate between $3,500 and $4,000 of raw material that the plans or scratch-builder would have to purchase. Thus, when adding the engine, basic instruments, and other components, the cost of a ready-to-fly CX4 is still only $23,000 to $25,000 depending on builder-installed options.

The website for the Thatcher CX4 is: http://www.thatchercx4.com/
Other public access sites with builder comments and photos are:
http://groups.yahoo.com/group/cx4  and  http://www.cx4.ca/coppermine/index.php 

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