Stripping Down the Upper Rear Fuselage:
This is our 40th restoration update covering the Museum's Douglas SBD Dauntless. We hope our readers have enjoyed following the progress which Pioneer Aero’s engineers have made so far. This particular report covers major progress with the aircraft's upper rear fuselage. Pioneer Aero's William Lowen has successfully de-riveted its skins and begun restoring the structure, much of which is in excellent condition thankfully.
These are the first skin panels which William Lowen removed from the upper rear fuselage. With Douglas part number 5090628, these fairings sit either side of the fuselage around the aft rim of the rear gunner’s cockpit, the broader end facing the aircraft’s tail. There is some corrosion damage, so William decided to bead blast them to reveal the extent of the problem and thus learn whether remediation was possible. (image via Pioneer Aero Ltd.)
The underside of the aforementioned skin panels following media blasting. Some corrosion was present, but Lowen will determine an appropriate repair plan in the near future. (image via Pioneer Aero Ltd.)
The port aft skin almost ready for removal; only a handful rivets are yet to be drilled out. (image via Pioneer Aero Ltd.)
William Lowen is seen here lifting the skin’s forward edge from underneath the skin immediately to its fore. (image via Pioneer Aero Ltd.)
Rivet Removal:
More than a thousand rivets had to be removed in order to deskin the upper rear fuselage. While seemingly a straightforward and mundane operation, drilling out rivets requires considerable skill to execute properly. Any carelessness can easily cause a rivet hole to be enlarged beyond design limits, which might result in a part being rendered unsalvageable.
The key to removing a rivet without damaging the mounting hole is to use the correct, nominal-size drill bit to match the rivet shank diameter. First the head is drilled to the start of the rivet shank. In the case of a raised rivet, like most of the rivets on the Dauntless fuselage, this is the level of the skin. The head is then broken off with a pin punch of the same nominal diameter. If a smaller drill is used the head will not break off and if a bigger drill is used it will damage the skin. Once the head is removed the shank can be drilled a little further using the same size drill to release the skin from the structure. If a smaller sized drill is used, the tension in the remaining rivet shank will remain too great for releasing the skin from the shank. Forcing it free will likely damage the skin and underlying structure. Once the skin is removed, the same process of drilling is repeated to remove the remaining shank from the structure. One has to make sure that the drill is perfectly aligned with the rivet center and its axis when removing rivets.
WWII aircraft rivets are typically formed from a tempered aluminum alloy, made even harder through the work hardening which occurs during the riveting process. As a result, without proper care, a drill bit can skip off the rivet's head during drilling and mar the surrounding structure. This is especially true for non-flush-mounted rivets, and the Dauntless is full of these!
Typically, an engineer will use a center punch to create a divot at the center of a rivet head so the drill bit can dig in at the correct location more easily. That being said, AD aircraft rivets, the most commonly used variety in the USA during WWII, already have a small dimple at their center for identification purposes. A skilled aircraft engineer can typically use this dimple as a starting point for drilling through it, without the need of a center punch.
Interestingly, however, most of the fasteners which hold the SBD's upper rear fuselage skins to the underlying structure are AN455D4 brazier head rivets (AN = Army-Navy standard, 455 = brazier head, D = 2017 aluminum alloy, 4 = 1/8" shank diameter). D series rivets have a roughly 25% higher work-hardened tensile strength than the AD series made from 2117 alloy. These rivets are identified by a raised dot/nipple on the rivet head, which makes it a little trickier to drill precisely at the center of the head, although the raised dot is usually crushed mostly flat during the riveting process.
Another view of the denuded upper rear fuselage. The upper longeron is the large structural member running from near the upper right of the image through to Frame #11. Note the whitish areas at several positions along its length. These are telltale signs of aluminum corrosion. It is uncertain yet whether this will render the part unusable, but an inspection following its removal from the fuselage should determine its future airworthiness potential. (image via Pioneer Aero Ltd.)
A closeup view of the upper rear fuselage structure between Frames #11 and #13 looking from the left side towards the aft end. The stringers in this area show very little evidence of corrosion which means that there’s a good chance that most of them will be able salvageable. The horizontal frame at the top left of the image is the doubler which surrounds the opening for loading parachute flares into their tubes. (image via Pioneer Aero Ltd.)
A view of the denuded aft fuselage looking towards the rear cockpit. Seen in the foreground is Frame #15, the aft-most bulkhead in this subassembly. The SBD’s horizontal stabilizers attach to either end of the heavy duty beam running horizontally across its face. (image via Pioneer Aero Ltd.)
This is a close up of the upper fuselage structure between Frames #13 and #15. The top edge of Frame #15 shows evidence of corrosion (note whitish area) which will require further investigation to determine part viability. (image via Pioneer Aero Ltd.)
Skin Doubler Markings:
As you can see in the image above, when the outer layer of skin was removed from the SBD's upper rear fuselage, it revealed a doubler at either side of the aft end. The doubler on the righthand side had some interesting markings inked upon it with a roller, presumably at the Douglas factory during WWII. These markings, as can be seen below, indicated the specifications for the material used to create the part. In this case ".025" indicates that the sheet metal is .025" thick. There is then a curious marking of unknown origin... We shall provide a closeup image of it below, and would love to hear from readers who might have a better idea of what it signifies. Even the guys at Pioneer Aero were stumped!
Following after the curious marking, the characters "24STALC" are visible. In this case, 24 refers to the specific aluminum alloy used - its modern equivalent is 2024. ST is shorthand for "State Tension" which indicates that the metal has undergone heat treatment (tempering) to increase its strength; its modern equivalent designated as "T3". As for "ALC" this means that the material is "Alclad", or clad in pure aluminum. Alclad was a relatively new development during the WWII-era. Essentially it involved coating each side of the aluminum alloy sheet in a very thin layer of pure aluminum. When the pure aluminum oxidizes it actually forms a corrosion barrier for the aluminum alloy lying beneath it. These layers of pure aluminum were typically no more than 5% of the material thickness for sheets less than .062" thick, and 2.5% of the material thickness for all other sheet sizes. So if pure aluminum has such great corrosion resistance, why not make an entire aircraft from the substance... well, it is very soft. 2024T3 has more than four times the tensile strength of pure aluminum!
A closeup of the lettering inked onto the fuselage doubler revealed following the removal of the outer skin. These markings were applied crudely, likely with a roller, and they detail the material's properties. The part was fabricated from 0.025" thick 24ST Alclad Aluminum sheet. 24 refers to the specific alloy, while ST "State Tension" refers to the material's tempered state. The designation "24ST" is no longer used by manufacturers, but it is essentially equivalent to 2024 T3, which is what most replacement parts are fabricated from for this airframe. (image via Pioneer Aero Ltd.)
This is a closeup of the unusual symbol printed on the material markings noted earlier. While it is possibly an inspection stamp, it seems improperly placed for such a marking. Hopefully some of our readers will be able to help decipher its meaning! (image via Pioneer Aero Ltd.)
Component Restoration:
With the skin removed, it was now time to disassemble the underlying structure and assess each component for its future airworthiness potential. The restoration of some fuselage frames has already begun, as the images below will reveal.
The upper half of Frame #11 soon after its removal from the airframe. Initial inspection showed no major corrosion issues, so the attached brackets were removed, with the area underneath being checked. The light-colored, bead-blasted areas on each side highlight where the pulley mount brackets sat prior to their removal. (image via Pioneer Aero Ltd.)
The upper half of Frame #12 soon after its removal from the fuselage. Initial inspection showed no major corrosion issues, so the attached brackets were removed, with the area underneath being checked. There is one minor area of corrosion on the forward face (not visible) which requires a patch repair; that will be completed soon. (image via Pioneer Aero Ltd.)
The upper half of Frame #13 soon after its removal from the fuselage. Initial inspection showed two areas of corrosion concern, but it was decided to investigate further. These areas are identified by the lighter hued regions at Frame top. This frame subsequently underwent bead-blasting, with the corrosion damage being found to be within repairable limits. As a result, a curved channel patch will be fitted to the affected area. (image via Pioneer Aero Ltd.)
This upper half of Frame #14 soon after its removal from the fuselage. Initial inspection showed no major corrosion issues, so it was then time to remove the attached bracket and check the area beneath it. (image via Pioneer Aero Ltd.)
A close up of the forward face of Frame #15 following the removal of all the stringers. This image was captured just prior to Frame #15's removal from the fuselage jig. The pulleys and mounts which you can see are for flight control cables, while the phenolic block is a guide for the trim cable. (image via Pioneer Aero Ltd.)
The arrestor hook cable tension spring mount bracket on a section of stringer. The stringer is airworthy but, as can be seen, the steel bracket is heavily corroded and needs replacing. Interestingly, while the stainless steel cable end fitting is still present, the cable itself had completely corroded away. (image via Pioneer Aero Ltd.)
Two examples of the arrestor hook cable tension spring mount bracket. The bracket on the left is the original from B-22 (as seen in the previous image). It is obviously corroded beyond design limits, but will be replaced by the cleaned-up example on the right, which came from a donor airframe. (image via Pioneer Aero Ltd.)