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.
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.
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!
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.