Overview:

After rebuilding and trial-fitting Fuselage Frame #1 into the SBD's Wing Center Section jig, (covered in Update #90), it then became practical to begin doing the same for the interconnecting structure. This update covers some of that effort, including the installation of the lower engine mount stiffeners, stringers, external skins and the emergency undercarriage release mechanism.

The manufacturer's drawing describing the SBD's lower engine mount stiffener assembly, part number 5063798.
The manufacturer's drawing describing the SBD's lower engine mount stiffener assembly, part number 5063798.

Lower Engine Mount Stiffener Assemblies:

The lower engine mount stiffeners are complex subassemblies formed from several extruded sections of aluminum which extend about seven feet into the airframe aft of the firewall. These components ensure that the engine bearer has sufficiently sturdy fuselage attachment points. The above manufacturer's assembly drawing shows different perspectives of this assembly, whilst the images below reveal the top and side views of a damaged example which Pioneer has had on hand for the rebuild.

An image showing what the lower engine mount stiffener assembly looks like in the flesh. This is a non-airworthy righthand example salvaged from a donor aircraft. Despite the damage, it served as a useful example for the restoration team to work from. Note the substantial-looking bolt at image left; this protrudes through Fuselage Frame #1, where it fastens against the lower right engine bearer attachment point. (image via Pioneer Aero Ltd.)

Another view of the same component portrayed in the previous image, albeit this time from a different angle which expands our understanding regarding the complex nature of these components. Note the curve in the extrusions as it moves further to the right in the image so it can follow the camber of the SBD's Wing Center Section to which it mounts. (image via Pioneer Aero Ltd.)

Drilling Off Parts:

Pioneer Aero's Rod Hansen was responsible for the process of trial-fitting the lower engine mount stiffener assemblies. He started with the lefthand unit, attaching the replacement extrusions to the stiffener plate and engine mount fitting. Hansen then drilled off the assembly against associated parts on the SBD's Wing Center Section, after which he pursued the same process for the righthand example.

A view of the replacement lower lefthand engine mount stiffener assembly during a more recent trial fitting against the now-almost fully restored Fuselage Frame #1. Here the forward ends of the replacement extrusions have been temporarily attached to the stiffener plate and engine mount fitting. (image via Pioneer Aero Ltd.)

A closeup view of the replacement lower lefthand engine mount stiffener assembly where it meets Fuselage Frame #1. Here, the forward end of the replacement extrusions have been fitted to the stiffener plate and engine mount fitting. This assembly was in the process of being trial-fitted against the SBD’s Wing Center Section and Fuselage Frame #1. (image via Pioneer Aero Ltd.)

Another closeup of the replacement lower lefthand engine mount stiffener assembly, this time in the middle section where the extrusions pass beside/through Fuselage Frame #3. The fuselage longeron (primer-painted stringer at image right) aligns with the aft end of the stiffener plate. (image via Pioneer Aero Ltd.)

Another view of the replacement lower lefthand engine mount stiffener assembly on its way rearwards atop the SBD’s Wing Center Section. Here we see the aft ends of the extrusions as they conform to the wing's surface curvature. The fuselage longeron (primer-painted stringer towards image right) is a refurbished original component from B-22. (image via Pioneer Aero Ltd.)

A lower face view of the replacement lower lefthand engine mount stiffener assembly after trial fitting. Note that many of the attachment holes have been transferred to the part, and that the engine mount bracket is also temporarily attached at the far left. (image via Pioneer Aero Ltd.)

A topside view of the same structure depicted in the previous image. The subassembly's complexity is far more obvious now. (image via Pioneer Aero Ltd.)

The extrusions for the replacement lower righthand engine mount stiffener assembly are seen here during trial-fitting against the SBD’s Wing Center Section a couple of months ago. Note how the extrusions follow the wing's surface curvature as they move towards the trailing edge past Fuselage Frame #3 (the primer-painted beam protruding from the wing about halfway up the image). With the restoration of the lower half of Fuselage Frame #1 (see Update #90), including the positioning of the lower engine mounts, fitment of these stiffener assemblies could continue to progress. (image via Pioneer Aero Ltd.)

A view of the replacement lower righthand engine mount stiffener assembly during its trial fitting. This closeup shows where the forward end of the replacement extrusions and stiffener plate are in the process of being fitted to the engine mount bracket itself. (image via Pioneer Aero Ltd.)

Another view of the replacement lower righthand engine mount stiffener assembly during trial-fitting. Here we see the aft end of the extrusions in the process of being positioned and drilled-off to match the attachment holes in the SBD’s Wing Center Section. (image via Pioneer Aero Ltd.)

Skins & Stringers:

While completing trial-fitting for the Lower Engine Mount Stiffeners, Pioneer Aero also began trial-fitting stringers and skin to the SBD's Wing Center Section between Fuselage Frames #1 and #3. The sections of fuselage skin, also referred to as "plating", overlap one another, so it is critical to attach them to the airframe in a highly specific sequence. This is why Pioneer began the process with the small skin section which begins at the forward edge of Fuselage Frame #1 and extends back a foot or so aft of Fuselage Frame #3. You can see this section of skin (identified by the callout numbers 01 and 02) in the image below from the Illustrated Parts Manual; odd callout numbers typically represent parts on the righthand side of the airframe and even numbers their mirror image on the left.

This drawing from the SBD's Illustrated Parts Manual depicts the airframe's lower fuselage, with each callout number representing a specific section of skin on the airframe. Each of the items depicted has the root Douglas Aircraft part number 5063493, with a dash number representing the specific component. Note that each plate has two callout numbers associated with it, an odd number for the right side of the airframe and an even number representing its mirror image on the opposite side. In this case, the callout numbers 01 and 02 represent plating with part numbers 5063493-6 and -7 respectively. These are the two skin sections which Pioneer Aero has remanufactured recently for the Museum's Dauntless project, trial-fitting -7, or "skin #7" in recent weeks.
This drawing from the SBD's Illustrated Parts Manual depicts the airframe's lower fuselage, with each callout number representing a specific section of skin on the airframe. Each of the items depicted has the root Douglas Aircraft part number 5063493, with a dash number representing the specific component. Note that each plate has two callout numbers associated with it, an odd number for the right side of the airframe and an even number representing its mirror image on the opposite side. In this case, the callout numbers 01 and 02 represent plating with part numbers 5063493-6 and -7 respectively. These are the two skin sections which Pioneer Aero has remanufactured recently for the Museum's Dauntless project, trial-fitting -7, or "skin #7" in recent weeks.

As explained in a previous article, Douglas Aircraft often used the same root number to label groups of related parts, applying a so-called "dash number" to identify individual items within that collection. In this particular case, each of the skin sections in the lower fuselage fields an identical part number, 5063493. The plating called out as 01 and 02 in the illustration has dash numbers "-6" and "-7" for the left and righthand examples respectively. These are the small skin panels which they recently remanufactured and trial-fitted to the SBD's Wing Center Section, beginning with the righthand example, part #5063493-7. As a form of shorthand, Pioneer's engineers refer to this skin panel simply as "Skin #7". The images below reveal the work involved.

A closeup showing the front of the lower righthand engine mount stiffener assembly after trial-fitting completion; the forward end of the replacement extrusions having been drilled off against the stiffener plate and engine mount fitting. Also visible are a newly-remade fuselage skin (skin #7) and a pair of stringers, the painted example being an original B-22 component. Each of the fuselage skins has its own unique number, which makes identifying them relatively straightforward. (image via Pioneer Aero Ltd.)

A slightly broader view of the components depicted in the previous image, albeit this time featuring the replacement lower half of Fuselage Frame #2 in the picture in addition to Fuselage Frame #1. (image via Pioneer Aero Ltd.)

A detailed view depicting the middle of the righthand lower engine mount stiffener assembly after being fully drilled-off against adjacent components. Note that Fuselage Frame #3 is also in the picture The newly manufactured section of fuselage skin (skin #7) which has been completed. The primer-painted longeron just exterior to skin #7 is an original B-22 component. (image via Pioneer Aero Ltd.)

A detailed view depicting the aft end of the righthand lower engine mount stiffener after it has been drilled off against adjacent components. Replacement fuselage skin (Plate #7) is also visible. The primer-painted longeron just exterior to the skin is an original B-22 component. (image via Pioneer Aero Ltd.)

This image gives a clearer view of the replacement skin section (skin #7) during its fabrication. The green longeron on the outside of the skin is an original B-22 component, as is Fuselage Frame #3. (image via Pioneer Aero Ltd.)
This image gives a clearer view of the replacement skin section (skin #7) during its fabrication. The green longeron on the outside of the skin is an original B-22 component, as is Fuselage Frame #3. (image via Pioneer Aero Ltd.)

Emergency Undercarriage Release Mechanism:

While trial-fitting the engine mount stiffeners and related components, Pioneer Aero's chief engineer, Martin Hedley, also temporarily attached the various pulleys which form part of the emergency undercarriage release mechanism. As part of this process, Hedley also threaded string through the network of pulleys in the correct sequence to represent the steel cabling which will eventually follow the same path. He performed this task to ensure that the cabling would remain free of obstructions on its path. The following images show some of the work involved in this effort.

A closeup of the pulley depicted in the previous image; note how it has been threaded with string to simulate the steel cabling which will eventually replace it. The team added this to confirm that the cable will have an unobstructed passage though the cutout in the lower engine mount stiffener extrusion. (image via Pioneer Aero Ltd.)

Another view of the previously-described pulley, provided here to reveal the part’s complexity. This component is a refurbished original from a donor airframe. (image via Pioneer Aero Ltd.)

A view of the lower section of Fuselage Frame #3 on the righthand side of the SBD’s Wing Center Section showing where the two recently-added stringers and skin section (plate #7) attach. Temporally mounted to the forward face of Fuselage Frame #3 we can also see a pulley assembly which forms part of the emergency undercarriage deployment system. (image via Pioneer Aero Ltd.)

Temporally mounted to the forward face of Spar #2 in the SBD’s Wing Center Section, we see a cluster of pulleys which form part of the emergency undercarriage deployment system; the orange string represents the steel cables which will eventually be fitted. These pulleys are located below the example shown in the previous three images, with the string fitted to ensure that proper clearances are maintained throughout the cable run. (image via Pioneer Aero Ltd.)
Temporally mounted to the forward face of Spar #2 in the SBD’s Wing Center Section, we see a cluster of pulleys which form part of the emergency undercarriage deployment system; the orange string represents the steel cables which will eventually be fitted. These pulleys are located below the example shown in the previous three images, with the string fitted to ensure that proper clearances are maintained throughout the cable run. (image via Pioneer Aero Ltd.)