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.
This is the aft upper edge of the lower righthand accessory bay cowling. The vent allows warm air to escape from the engine accessory bay. (photo via Pioneer Aero Ltd.)
A reverse side view of the same section of accessory bay cowling. (photo via Pioneer Aero Ltd.)
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.
While it isn’t easy to see in this image, the doubler around the cut out (at the lower left of the image) is cracked. (photo via Pioneer Aero Ltd.)
Here is a closeup of the crack in the doubler mentioned in the previous image. The part was too damaged to repair, so had to be removed and replaced. (photo via Pioneer Aero Ltd.)
The replacement doubler resting on the cowling during fabrication, with proposed rivet layout for its installation drawn on the cowling skin to its right. (photo via Pioneer Aero Ltd.)
The replacement doubler in position on the cowling, awaiting final riveting. Also visible on the left is the original data tag for the cowling section. (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.
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.
The righthand upper accessory cowl. The component is mounted in a vice, with riveting in progress to replace failed spot welds. The only repairs this cowl section required are indicated by the clecos pinning the skins together in the upper and lower left hand corners. (photo via Pioneer Aero Ltd.)
An exterior view of the righthand upper accessory cowl following refurbishment. This component is now ready for the application of top coat paint in due course. (photo via Pioneer Aero Ltd.)
An interior view of the righthand upper accessory cowl following refurbishment. (photo via Pioneer Aero Ltd.)
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.
This is the top engine accessory bay cowling. As a point of reference, the forward edge is at the top of the image. The hinged door is the access point for filling the oil tank. (photo via Pioneer Aero Ltd.)
Another view of the top engine accessory bay cowling. As a point of reference, the forward edge is at the left of the image. (photo via Pioneer Aero Ltd.)
A closeup of the top engine accessory bay cowling, with the straight edge of a ruler nestled against the skin. Note how the skin bows away from the ruler near the center of the image, revealing a dent in the skin. The dent will be plannished out in due course. (photo via Pioneer Aero Ltd.)
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.
A closeup of the oil filler access door in the top engine accessory bay cowling. While it is a little hard to see, there is a small crack at each of the sharp corners in the skin cutout for the hinge. Sharp corners such as these should never be used in aircraft manufacture, since they offer a focal point for crack propagation, but we will be able to repair the part. (photo via Pioneer Aero Ltd.)
A closeup revealing cracking damage around both a Dzus fastener mounting hole (left) and at a spot weld location in the top engine accessory bay cowling. (photo via Pioneer Aero Ltd.)
A view of the cowling section (with the repair plan inked onto the skin) showing where the damaged sheet metal will be cut out and a replaced, with a doubler riveted in underneath. (photo via Pioneer Aero Ltd.)
An image revealing the repairs in progress. The damaged sections of skin have been cut out, with replacement sheet metal plugs (and the doubling plates below) clecoed in place, awaiting final riveting. (photo via Pioneer Aero Ltd.)
A view of the same repairs in progress from the reverse side of the cowling. (photo via Pioneer Aero Ltd.)
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.
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.
This is the lower lefthand cowl for the engine accessory section prior to media blasting. (photo via Pioneer Aero Ltd.)
The lower lefthand accessory cowl section following media blasting. Note the overall excellent condition of this component! (photo via Pioneer Aero Ltd.)
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.
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.
Corrosion pitting in the extrusion which runs along the lower edge of the cowl. (photo via Pioneer Aero Ltd.)
The lower lefthand accessory cowl following the removal of the broken hinge and corroded extrusion (which are still visible lying atop/beside the cowl skin. (photo via Pioneer Aero Ltd.)
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.
The lower lefthand accessory cowl during the restoration process. This view shows some replacement rivets already in place just below the cut out. The next step involved riveting in the replacement extrusion to the far side of the skin edge (at the upper edge in this image). (photo via Pioneer Aero Ltd.)
The now fully-repaired lower lefthand accessory cowl, as seen from the exterior skin. (photo via Pioneer Aero Ltd.)
The same cowl, as seen from the interior side. (photo via Pioneer Aero Ltd.)