Sbd Cowl Section Drawing 5191901
A Douglas Aircraft multiple-view assembly drawing for SBD Dauntless part# 5191901 - the upper lefthand cowl for the accessory section.

Accessory Bay Cowls:

Currently, Pioneer Aero Ltd is restoring cowling panels for the SBD’s engine accessory bay. While the project came with a complete set of these hard-to-find components, largely in excellent condition, several issues needed addressing before they were ready for flight. The majority of these repairs involved cracks, dents, failed spot welds and minor corrosion.

In keeping with their remit, Pioneer Aero has carefully thought through and applied a repair strategy to refurbish each of these panels while reusing as much original material as possible.

Lower Righthand Cowl:

The restoration team began first by working on the lower righthand cowl. While it was in remarkably good condition for its age and level of use, there were a number of issues to resolve. The doubler around the cutout at the panel's lower rear edge had a substantial crack, so a replacement part was fabricated. A doubler functions exactly as its name implies, providing an additional layer of material to boost component strength around key areas of stress or wear, such as at panel edges, while allowing thinner skin for the overall structure, thus reducing weight to maximize potential aircraft performance.

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A closeup of the aforementioned data plate for the cowl section, complete with its part and assembly numbers (and inspector approval mark) stamped into it. Interestingly, 5159333 refers to the full assembly of cowl panels for the entire engine accessory bay, whereas 1924 is a truncation of 5151924, the part number for this specific panel (right hand lower). Each of the cowls has a similar data plate riveted to it; yet another fascinating original detail which will be left in place, adding to the aircraft's authenticity. (photo via Pioneer Aero Ltd.)

Some of the spot welds in the lower right hand accessory bay cowling panel had failed. Spot welding was a relatively new technique in aircraft manufacturing when Dauntless production was underway. The process involves bonding two layers of aluminum skin by pressing them together between a pair of electrodes and then passing a strong electrical current through them. This partially melts, then melds the two layers together locally, leaving just a little dimple in the skin to mark the join.

When executed properly, a spot weld produces a robust join, which is both lighter, more rapidly produced and more aerodynamically efficient than conventional riveting techniques. However, if the electrodes or metal surfaces are not fully clean, or if too little/too much current is applied, then the "nugget" (as the welded join is called) can be of insufficient strength to hold long term. Wartime production pressures did occasionally lead to such issues at times, so perhaps that is the case here. In any event, one approved technique for repairing failed spot welds can involve simply drilling through the nugget and then replacing the weld with an appropriately-sized rivet. That is precisely what Pioneer was able to do in this case.

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The cowling edge revealed here marks the locations where spot welds had failed. To fix the problem the spot welds were drilled through to allow rivets to form the join. This image shows the process mid-way through, with the rivet layout marked in pen on the skin surface and some of the rivet holes already cut. (photo via Pioneer Aero Ltd.)
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The fully-repaired lower righthand accessory bay cowling after it had received a coat of primer paint. (photo via Pioneer Aero Ltd.)

Upper Righthand Cowl:

This panel too was in excellent overall condition, barring minor niggles involving a handful of failed spot welds. These were easily dealt with by replacing the spot welds with rivets as done previously with the lower right hand panel.

Top Cowl:

This panel, while in overall decent condition, required a little more effort to repair. In addition to some dents, there were several cracks which also needed addressing. The dents were relatively easy to resolve, using a planishing hammer to gently massage the skin into the appropriate shape.

The cracks, however, required more invasive techniques to resolve. Two of the cracks resulted from faulty wartime fabrication practices (i.e. cutting an access hole with sharp corners instead of using an appropriate relief fillet to prevent crack propagation). While many restoration shops might have opted to rebuild this part from new instead of fixing it, Pioneer Aero accomplished the task admirably, using approved wartime procedures. They carefully excised the damaged material and blended in new sections of skin with appropriately sized doublers underneath - much in the same way that U.S. Navy technicians would have repaired bullet holes during WWII.

Interestingly, the top cowl featured another manufacturing error, as the outer skin did not fully extend to the panel's edge along one side. Although technically incorrect, this transgression did not materially affect the part’s structural integrity. Other than shaving back some of the rough edges where the spot welds overlap the skin edge, Pioneer left this feature as originally manufactured during WWII to preserve its authenticity. Future generations will be able to see this lapse in manufacturing technique to gain understanding for how wartime production pressures sometimes pushed workers (and inspectors!) to cut corners - sometimes literally.

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A closeup of the top engine accessory bay cowling, revealing how the top skin does not properly extend fully to the edge of the part. Note: the shallow dimples in the skin reveal the locations of spot welds attaching it to the stiffener beneath. (photo via Pioneer Aero Ltd.)
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The repairs to the accessory bay top cowling panel are now complete. (photo via Pioneer Aero Ltd.)

Lower Lefthand Cowl:

This panel was in excellent overall condition, but required a little TLC to bring it up to airworthy code. The principal issues with this assembly involved replacing a damaged hinge on an access panel, and replacing a corroded extrusion along one edge.

The image below shows a closeup of the access panel opening and the immediate area surrounding it in the lower lefthand accessory cowl. A crease in the skin is visible between the panel opening and the cut out towards the right of the photo. Thankfully, repairing this defect only required a little planishing to return it to the correct shape. While a little harder to see in the image, it is worth noting that the access panel hinge is broken. Pioneer technicians drilled the defective hinge off, and then replaced it. Interestingly, you will also note the difference between the sheetmetal cutout for the hinge in this accessory cowl section has been manufactured correctly using properly shaped relief fillets rather than the sharp corners seen in the top cowl section described earlier in this post.

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A closeup of the lower lefthand accessory cowl showing the slight crease in the skin and the defective hinge. (photo via Pioneer Aero Ltd.)

The aluminum alloy extrusion which formed the stiffener running along the lower edge of this cowl suffered from intergranular corrosion, as witnessed in the pitting depicted in the lower left image. While media blasting did remove the oxidized material at the surface, the damage was sufficient to require the part to be replaced. Intergranular corrosion typically occurs when there are impurities at the grain boundaries in the aluminum alloy's crystalline structure. The effect, on a microscopic scale, is similar to electrolytic/galvanic corrosion between dissimilar metals.

Replicating the part was relatively straight forward, but removing the old extrusion from the cowling required some patience and significant skill, as it was originally spot welded to the structure (as was the hinge noted earlier). Removing it required the careful grinding down of the old extrusion at the location of each spot weld, but without causing damage to the sheet metal below it. Pioneer accomplished this successfully, as the image attests.

As with each of the other cowling sections for the engine accessory bay, each panel received a brand new set of Dzus fasteners and a coating of primer paint once the repairs were complete.