A photograph from back in August 2023 clearly illustrates the pyramidal structure which comprises the Rollover Frame. Referred to in the A-24's Illustrated Parts Manual as the Overturn Structure, this subassembly helps protect the aircrew should an SBD flip onto its back during landing or takeoff. Amongst many other details, Pioneer Aero has recently been working to restore this section of the airframe. (photo via Pioneer Aero Ltd.)
A photograph from back in August 2023 clearly illustrates the pyramidal structure which comprises the Rollover Frame. Referred to in the A-24's Illustrated Parts Manual as the Overturn Structure, this subassembly helps protect the aircrew should an SBD flip onto its back during landing or takeoff. Amongst many other details, Pioneer Aero has recently been working to restore this section of the airframe. (photo via Pioneer Aero Ltd.)

Overview:

In their work on the SBD's upper fuselage, Pioneer Aero's restoration team has made significant progress with the dive bomber's Rollover Frame. This significant subassembly sits between the two crew compartments and attaches to the upper sections of Fuselage Frames #4 and #5 (see illustration below). It is a sturdy, pyramidal structure designed to help protect the SBD's occupants should the aircraft flip over during landing or takeoff.

A depiction of the SBD's Rollover Frame from the aircraft's Illustrated Parts Manual. This structure attaches to upper Fuselage Frames #4 (front) and #5 (reverse), with its four struts bound together at a common node via four attachment forgings, described in the above text.
A depiction of the SBD's Rollover Frame from the aircraft's Illustrated Parts Manual. This structure attaches to upper Fuselage Frames #4 (front) and #5 (reverse), with its four struts bound together at a common node via four attachment forgings, described in the above text.

Rollover Forgings:

As the above illustration reveals, the four struts which provide strength to the Rollover Frame meet at a common node, cinched together with bolts through forged aluminum attachment fittings. All four of these fittings from our SBD's fuselage proved to be restorable, with only minor corrosion issues to remedy. Removing some of this corrosion, however, proved quite challenging!

The bolt holes in each fitting are all counterbored to provide a flat surface for the mounting bolts to press against. But it was at the base of these counterbores where minor corrosion pitting turned up in some instances. Shaving away a thin layer of aluminum would remove this pitting and still leave the part within design tolerances, but for the mounting bolt to function properly, it was critical for the resulting face to remain perfectly orthogonal (i.e. at 90º) to the original bolt hole. While seemingly simple to execute in theory, such a cut was best achieved by using a 'reverse spot facer'. These unusual tools have a shaft which rises up through the bolt hole from the opposite side of the face being machined to ensure that any cutting is precisely orthogonal to the bolt hole. After a short delay waiting for this specialized tool to arrive at their workshop, the team at Pioneer was able to complete the restoration of these critical fittings.

This closeup features one of the four fittings which mount together at the top of the fuselage rollover structure right behind the pilot’s seat. This image shows the forging part way through the process of gently shaving away a thin layer of the bolt hole spot face (using a reverse spot facing tool) to remove minor corrosion. At the time this image was captured, two of the three visible holes had already been cleaned up (note the shiny metal). William Lowen was able to remove all of the corrosion from around these holes whilst keeping the parts within drawing tolerances, allowing the parts to be returned to service. (image via Pioneer Aero Ltd.)

Here are three of the four forged rollover structure top fittings after receiving attention from the reverse spot facing too - note how shiny the metal is around the bolt hole mounting surfaces. These parts are fully refurbished and just await a coat of primer paint before they can be safely stored for reassembly when the rollover structure goes back together. (image via Pioneer Aero Ltd.)

The forgings for the rollover structure after being returned to airworthy condition and receiving a coat of primer paint. (image via Pioneer Aero Ltd.)

Trial-fitting the upper sections of Fuselage Frames #5 and #6 (aft faces) along with the aft extrusions for the rollover structure onto the fuselage jig. All of these components (barring the doublers) are original to B-22. Note that two of the forged fittings are attached at the top of the Rollover Frame. (image via Pioneer Aero Ltd.)
Trial-fitting the upper sections of Fuselage Frames #5 and #6 (aft faces) along with the aft extrusions for the rollover structure onto the fuselage jig. All of these components (barring the doublers) are original to B-22. Note that two of the forged fittings are attached at the top of the Rollover Frame. (image via Pioneer Aero Ltd.)

Rollover Ribs:

To help stiffen the Rollover Frame, a pair of horizontally-positioned ribs are link Fuselage Frame #4 to Fuselage Frame #5 on either side of the airframe. The original units from B-22 proved to be restorable, with the lefthand example only requiring minor corrosion mitigation. The righthand rib, however, had one area of deeper corrosion. Martin Hedley devised an approved repair technique, which required him to cut away the damaged section from the rib. He replaced the missing material with a custom-made insert riveted against a larger backing plate acting as a stiffener. The following images describe the repair process.

These are the original ribs which sit horizontally between Fuselage Frames #4 and #5 at the lower end of the rollover channels. This image shows the ribs following bead blasting and inspection, but prior to the removal of the small clips you can see still riveted to them. The lefthand rib (top) only required treatment for minor corrosion, however the righthand example required a more invasive procedure to fix. The drawing inked onto that rib shows the onset of the repair plan. (image via Pioneer Aero Ltd.)

The original righthand rib part way through the repair process. The damaged section has been cut away, while an insert and a backing plate (doubler) have been fashioned to replace it. Note that the rivet layout has already been planned and drilled out. The "T" shaped doubler will sit on the visible side of the rib. (image via Pioneer Aero Ltd.)

Trial Fitting the Ribs:

To ensure that the ribs which tie Fuselage Frame #4 to Fuselage Frame #5 do so properly, the parts were trial-fitted to these structures on the upper fuselage jig. This took place before all of the remanufactured pieces for Frame #4 were available, so the original, albeit corroded parts, were used as a temporary substitute.

The righthand rib linking Fuselage Frames #4 and #5 is seen here during trial-fitting. Note that Fuselage Frame #4 (at left) is the original, non-airworthy subassembly. It is being used here purely to speed up the fitting process. (image via Pioneer Aero Ltd.)

A general view showing both inter-frame ribs temporarily fitted in position. Note that most of the other upper fuselage frames are also temporarily fitted to the jig. (image via Pioneer Aero Ltd.)

Another view of the two ribs during trial-fitting. (image via Pioneer Aero Ltd.)

The above images show several other upper fuselage ribs in the process of being trial fitted to the jig, so we thought our readers would enjoy seeing the following images describing that work with a little more detail. Great progress is clearly being made!

A repair doubler is seen here in the process of being riveted to the upper section of Fuselage Frame #13, beginning with the forward face. The outer edges, clecoed together to hold the doubler in place, will be permanently fastened together during the process of riveting the skin to the airframe. (image via Pioneer Aero Ltd.)

With its repair doubler riveted in place, the upper section of Fuselage Frame #13 was temporally fitted onto the fuselage jig. (image via Pioneer Aero Ltd.)

The upper fuselage jig is seen here with the rear fuselage frames and tail fin temporarily fitted. Note the newly-fabricated right and lefthand sections of Fuselage Frames #7 and #8 (bare aluminum). These have now undergone heat treatment and also had the cutouts for the stringers and plumbing transferred from the originals. (image via Pioneer Aero Ltd.)