Overview:
Pioneer Aero’s engineers have worked on numerous aspects of the SBD’s Wing Center Section over the past year. For obvious reasons, the skin panels have formed a major element of this task. While many parts making up these subassemblies have proved restorable, none of the actual skins were sufficiently intact for reuse; corrosion and/or impact damage had taken a significant toll.
This, of course, has required the restoration team to refabricate the skins from new sheet metal. We have covered some of those efforts in previous articles, albeit without going into much detail. It occurred to us, therefore, that readers might be interested in learning a little more about the processes involved, using the refabrication of the Wing Center Section's skin spanning the top side of Spars #2 and #3.
Build-to-Print?
With a fairly extensive archive of original factory drawings (prints) on microfilm for the Douglas SBD/A-24 family of aircraft, it is possible, in theory at least, to make parts using only this information (i.e. building-to-print). However, these drawings offer only a snapshot of the aircraft’s design over the history of the type, and an incomplete one at that. Indeed, some of the key drawings are missing from the known archive, or illegible, but even when available and sharply rendered, they don’t always represent the version of the part necessary for the aircraft variant in question. This is why original parts, no matter how disheveled, are so important when it comes to aircraft restoration, as they offer a template for creating their replacements.
Templates:
To speed up large-scale manufacturing processes during the war, an aircraft factory would almost always create templates to more easily lay out the starting point for sheet metal parts. That way workers, even those who were semi-skilled, could simply trace around the template with a pencil (or a steel scribe) to accurately mark out a part’s dimensions on the virgin metal, instead of having to undertake the intensive (and potentially error-filled) process of laying it out with a ruler, compass and protractor from the drawing. [One only has to look at the assembly drawing above for the part discussed in this article to appreciate the complexity sometimes involved with the latter.] Templates also made it easier to create a supply of near-identical parts, which both sped up the assembly line and made future repairs more straightforward - since it is obviously a lot easier to fit components together when they conform to the correct dimensions. Few WWII-era templates for manufacturing aircraft parts have survived to the present day, as they were mostly discarded once the manufacture of the type they belonged to had ended. That being said, it is often possible to convert original parts into templates from which their replacements can be fashioned. This is especially true for largely two-dimensional components without any complex curves pressed into them.
Indeed, the team at Pioneer Aero has used a similar process to remanufacture most of the spar web plates and skins for the SBD’s Wing Center Section - other than the leading edges, of course. The following images reveal the process as it unfolded for the upper wing skin which spans Spars #2 and #3. First of all, the skin panel was stripped down to its component parts. The original skin was then laid down over a new sheet of aluminum (identical alloy and thickness), which itself was lying atop a sacrificial plywood panel supported by several sawhorses.
Hole Transfer:
The team made sure to line up the original skin’s aft edge with one edge of the new metal (see image below), clamping the two sheets together at regular intervals down its entire 11-foot width. The original skin has a gentle camber to it, so Pioneer’s engineers weighted it down help it lie flat. They then began transferring holes to the new skin (working initially along those lining the aft edge) using the original skin as a guide to drill through the new metal into the plywood below. The plywood braces the new metal during this process, helping create a clean hole while also preventing it from buckling.
They then followed the same procedure with each subsequent row of rivet holes, systematically advancing across the skin from the aft edge to the forward edge, using Clecos or similar temporary fasteners at regular intervals to cinch down new skin to old. This ensured that the original skin pressed as flush as possible against the new metal, allowing the engineers to trace the original skin's shape onto the new metal, confident that they will align accurately with one another.
Gradually advancing across the skin, transferring successive rows of rivet holes, while making sure to clamp new skin to old with Clecos at regular intervals during the process to ensure that the two sheets stay in the same relative position to one another. Note how the original skin is still weighted down against the new material, since it has a little spring to its surface due to the wing's camber. (image via Pioneer Aero Ltd.)
Practically all of the rivet holes had been transferred from original metal to new at the time this image was captured. Note that the new metal (.050" thick 2024 T-3 Aluminum Alloy) extends beyond the edges of the original skin. In the background engineer Rod Hanson is visible working on one of the smaller pieces of internal structure from between spars #2 and #3. To the right of the photo is the original internal structure which this upper skin mounts against. (image via Pioneer Aero Ltd.)
A close up image showing the rivet hole transfer taking place. The center of the aircraft runs along the row of holes where the masking tape is visible, between the outlines of where the bent stringers mounted. The bulk of the rivet holes have been drilled at this point. There are four different diameter rivets used on this skin, so great care must be used to ensure that the correct sized drill bit is used to create each hole (the Cleco's are color-coded to match specific hole sizes). Note the extensive areas of corrosion (white/light grey patches); this rendered the skin unsalvageable for airworthy purposes. (image via Pioneer Aero Ltd.)
Another closeup of the replacement upper skin between Spars #2 and #3 during its refabrication. There are a number of larger holes visible here, outside of those for rivets. These provide passage for various plumbing and electrical conduits which need to pass through the skin, and will be bored out of the new material (using the correct-sized circular cutters) before the original skin is removed. Furthermore, if you look carefully, you will see that the outer profile of the original skin has been inked onto the new metal with a pen. The excess material will be trimmed away once the original skin is removed. (image via Pioneer Aero Ltd.)
After transferring the rivet holes precisely, and cutting fillet radii into the corners of any interior openings, the team could then remove all of the Clecos and separate new skin from old. They can then finish cutting out the interior openings in the new skin and trim the exterior to an appropriate size - albeit leaving a little excess material along some outer edges to provide a little wiggle room for final-fitting.
A closeup view of the replacement upper skin between Spars #2 and #3 during its refabrication. This is the righthand side, with the forward edge facing towards the bottom right corner. The two interior openings are being prepared for cutting, a process which first sees the fillets in each corner drilled out with circular cutters of the correct radius. (image via Pioneer Aero Ltd.)
This closeup shows the two interior openings in the process of being cut out. The fillets in each corner of the D-shaped opening have already been drilled with circular cutters of the correct radii. The curved edge will be “stitch-drilled” along its arc to cut that profile, while a cutoff wheel will be used to grind through the straight edges. Note that this view shows the lefthand side of the skin, with the forward edge towards the top left corner of the image. (image via Pioneer Aero Ltd.)
The newly-fabricated doubler and packers which attach to the lefthand, D-shaped access hole in the skin (on the inside, of course). These were manufactured using the original examples as a template in a similar fashion to the skin itself. (image via Pioneer Aero Ltd.)
The doubler and packers (from the previous image) are seen here being test-fitted around D-shaped access hole on the lefthand side of the newly-fabricated skin. A hole for a quick-release, Dzus fastener will be drilled into the center of the aft edge of the doubler after the hinged panel is fabricated and test-fitted at this location. (image via Pioneer Aero Ltd.)
Cleco Fasteners - A Brief Overview:
Clecos are a common sight in aviation workshops and indeed in almost any other facility working with sheet metal. For those who may not know, however, Clecos are a form of spring-loaded clamp which aircraft restorers use to temporarily pin sheet metal parts together through the holes which will eventually be filled by permanent fasters such as rivets. When a Cleco’s head is depressed using a specialized pair of pliers, two barbed rods protrude from the base, narrowing together as they extend so they can slip through the rivet hole. Once the Cleco’s head is released, a spring draws the rods back into the body, pushing the barbs sideways so that they catch against the bottom piece of sheet metal and cinch the parts together. There are four basic sizes, each color-coded to fit a specific rivet hole diameter: ø3/32” = Silver, ø1/8” = Copper, ø5/32” = Black, and ø3/16” = Gold.
