Douglas Sbd Dauntless Dropping A Bomb, Circa In 1942
A wartime image of a Douglas SBD Dauntless with its dive flaps deployed while dropping its weapon circa 1942. (image via Wikimedia)

SBD Dive Flaps:

One of the SBD's defining features is its set of perforated flaps which runs along the entire wing trailing edge between each aileron. When fully deployed, these flaps were critical to the aircraft's success as a dive bomber. They regulated the aircraft's velocity while also providing greater stability in a dive, so the Dauntless pilot could make aiming adjustments more precisely to deliver their ordnance on target. Indeed, the type proved so effective in its role that it was Dauntless crews who executed the fatal blows for each of the four Imperial Japanese Navy aircraft carriers sunk during the Battle of Midway in June, 1942 - an engagement which turned the war in the Pacific inexorably in the Allies' favor.

SBD-3 Dauntless dive bombers in formation over the Pacific Ocean at around the time of the Battle of Midway. The perforations in the outer wing split flaps are evident here. While stowed during normal operations, the top set of flaps would deploy upwards (and the lower set downwards) during a dive bombing run. (US Navy Image)

Flap Configuration:

There are actually five major control surface components in an SBD's set of flaps. One flap lies beneath the wing center section, while each outer wing panel has a pair of perforated flaps laid one on top of the other. During normal take off and landing operations, only the lower units deploy (as conventional flaps). However, during a bombing run, the SBD's pilot will actuate the upper flap panels on the outer wings as well.

Sbd Flap Bay Illustration
An illustration of the center and outer wing panel flap panels from the SBD's Illustrated Parts Manual. The upper panels for the outer wings are usually only deployed during a dive bombing run, whereas the lower units serve a dual purpose as traditional aircraft flaps as well.

Rebuilding the Center Flap:

Over the last few months, the team at Pioneer Aero has begun rebuilding the central flap panel for the Museum's Dauntless. This panel is substantial in size, extending the entire eleven foot width of the SBD's wing center section. While BuNo.36175 was recovered with its set of flaps,  the center unit was too badly damaged for reuse unfortunately.

Thankfully, a donor unit was available. Although this too has significant serviceability issues, some of its key components are reusable. The restoration team began the rebuild process by fully disassembling the flap and then assessing each component for its potential future airworthiness.

Pioneer's engineer, Jayden Malloy bead-blasted each of the parts he deemed  salvageable to clean away any paint, dirt and minor corrosion. Once cleaned, however, a number of these pieces revealed substantive cracks and/or oversized rivet holes - damage too severe to effect airworthy repairs.

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Some of the center dive flap's leading edge channel sections had damage too severe to repair - as shown in this image. (image via Pioneer Aero Ltd.)

Leading Edge Channel Sections:

Upon his initial inspection, Jayden felt that the flap's original leading edge channel sections appeared to be in decent condition, with only minor, repairable dents to contend with. Upon closer inspection, however, all but three of these components featured enlarged rivet holes, rendering them unusable. Thankfully, these parts are not overly complex to remanufacture - for those with the requisite skills, that is.

Each channel section is formed from a piece of flat aluminum, with a specific cross-sectional profile folded into it using a sheet metal brake. Jayden decided that it made sense to create a small production run of these channel sections, rather than build them piecemeal. Even with the correct dimensions from manufacturer's drawings and the reference parts on hand, Jayden still needed to determine the correct method for folding the new aluminum sheet into the correct profile. A trial run using a piece of scrap aluminum sheet showed that he needed to adjust his setup slightly, so he tried again. The third trial yielded the perfect profile.

With this information in hand, Jayden noted: "I cut my six final pieces to size, inked my four fold-lines on each of them, and folded them - all in one hit - knowing they were going to come out within dimension!" 

Thankfully, most of the brackets and doublers on the center flap proved to be in good condition, and required minimal panel beating to regain their proper shape. Having these original parts - with their rivet holes as guides - makes it easier to put the flap back together since the holes will help align new material with old.


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Some of the original parts from the center dive flap which proved reusable. The three salvageable channel sections are partly visible at the bottom of image. (image via Pioneer Aero Ltd.)

Remanufacturing Dive Flap Ribs:

Some of the ribs which stiffen the central dive flap structure were too damaged to save, so Pioneer Aero's Jayden Malloy set about their remanufacture. This section reveals the process involved.

Malloy started by machining a forming block with the correct profile for the rib out of heavy-duty plastic. He then cut an appropriately-sized blank for the new part from a sheet of un-heat treated, 2024 aluminum (2024 T0).

Using a hydraulic press, Malloy then drove the forming block into the aluminum blank, deforming it against a heavy rubber matt. This creased the sheet metal with the basic outline of the new rib.

Shaping the Ribs:

Clamping the forming block and the partially-shaped rib firmly against one another in a vice, Malloy began using a mallet to bend the aluminum further over the plastic. He could only do this part way down each side, as the plastic forming block had insufficient stiffness for such pummeling where the ribs taper to a point.

Leaving the tapered ends for later, Jayden returned to shaping the apex and central channel section. The apex needed shrinking to get rid of the wrinkles caused by the excess aluminum which concave bends produce. A shrinking machine sometimes best suits this task, but Jayden also employed a lead slapper, basically a dense piece of softer material which grips the aluminum and folds (shrinks) it in upon itself. After forming the apex, Jayden then worked his way down the channel side away from the center as far as was practical. He then folded up the rib's bottom edge to its proper angle as well.

With this completed, Jayden then separated the partially-formed rib from its plastic forming block. To finish shaping the sections where it came to a thin taper, Jayden manufactured two, partial forming blocks (from steel) specifically tailored to each end of the rib. These steel blocks were strong enough to withstand the forming process without bending. Since they were only partial blocks, they were also easy to remove from the rib once it was fully formed. With the rib complete, it was now ready for heat treating to bring it up to the correct temper (strength).


The fully formed center wing flap rib awaiting heat treatment. (image via Pioneer Aero Ltd.)
Some of the newly formed ribs and channel sections for the wing center flap alongside some of the salvaged original parts - note the mounting holes. (image via Pioneer Aero Ltd.)

Flap Skin Fabrication:

After heat-treating the flap's newly-remanufactured ribs to bring them up to full strength, Jayden began straightening the donor flap's original spar. The spar, sometimes referred to as a beam, forms the dive flap's backbone, providing the bulk of its strength and rigidity. It must conform perfectly to its intended shape, as any kinks will transfer easily to the whole structure and distort it. The dive flap's skin is incredibly thin - just .020" thick - so this same principle must also be applied to each of the joggles on the ribs and stringers, as any deviations will cause visible deformation.

The flap's original skin was too damaged for re-use, but it was sufficiently intact to serve as a template. To fabricate a new skin section which matched the original as closely as possible, Jayden placed the original material atop a new sheet of aluminum. He then back-drilled it to replicate the original attachment holes for the spar, ribs, stiffeners, etc. in the new skin, being careful to limit this process to only those areas of the original skin which remained sufficiently intact.

The original center flap skin sitting atop a new sheet of aluminum which will replace it. Back-drilling is underway to transfer the relevant mounting holes, with the flap’s original spar (now fully restored to airworthy) already clecoed in place. Alignment of the flap’s characteristic lightening holes still needed confirming, but these will eventually be drilled and flanged. Note: the blue-tinted aluminum sheet visible at right in the image is the replacement flap bay skin under remanufacture - as covered in a previous article. (image via Pioneer Aero Ltd.)

Trial-fitting Flap Components:

Jayden chose to rebuild the flap on a frame he referred to as a "window jig" - a term which the images below make self explanatory. As Jayden noted, the window jig "keeps [the flap] up off the bench, so that it doesn't get any marks on the skin from swarf or any loose debris. It also makes both sides of the flap accessible, so that I can drill through the holes in the skin onto the new ribs while the skin is held straight. The jig also prevents any twisting from developing as the build progresses."

An overview of the flap components being trial-fitted together. Once Pioneer's engineers are confident that everything fits correctly, the flap will be disassembled so that the skin can receive its lightening holes and each component can be painted with primer. Once that is done, everything can be riveted together. (image via Pioneer Aero Ltd.)