Air Zoo’s PT-23 – The Story of a Restoration

Fairchild PT-23HO_N52020 (3)_Fire Extinguisher_Front Cockpit

The original construction technique for the ribs involved bonding with animal glues, but these bonds had since crystallized. The plywood had absorbed moisture and either separated, or suffered from dry rot. And no one knew what the condition the spar interiors were in. The restoration team built jigs for each  rib, as well as the leading, center and trailing edges. The wings tapered, and the cord did likewise, compounding complexity of the rebuild. Dozens of router bits were consumed chewing off one surface of each spar; hundreds of brass coated steel nails can do a number on router bits! Once the restoration team removed the plywood, they could inspect the interiors. Fortunately, they were OK, and the team didn’t have to fully rebuild them, other than replacing the removed plywood surfaces.

One of the museum volunteers, Lyle Patton, is a master cabinetmaker. This man performs miracles with wood. Both he and Greg, as well as a multitude of other volunteers attended to the long and arduous wing restoration process. Former Ford Tri-Motor pilot, John Allen, was also instrumental in much of the wing building. Once the wing skeleton was complete, the plywood skin had to be glued and tacked, the nails to be removed later.

“Aircraft plywood is perfect throughout…it’s structural, simply structural,” said Greg. “It’s gotta be solid throughout; no voids, no mineral streaks, no knots, no structural defects of any kind. It has to have total glue coverage between all of the veneers.” Aviation-grade plywood, made from a sandwich of ribbon-striped mahogany on the outside, and poplar on the inside, with the grains on the bias,  is very strong and commensurately expensive. All of the wings and empennage were covered with it. Needless to say, none could be wasted, and the restoration team was at the next critical step: forming the leading edge. The PT-23 does not have a very thick wing, and the leading edge is somewhat acute. The team had to build a complicated bending fixture in order to bend the large sheets of 3/32″ thick plywood without cracking it. Essentially they built a long, crude wing with pine ribs and a roll top desk-like surface attached to with a fabric lining. Under the cloth they placed a hot-water pipe with a series of holes along its length. Slowly, over several days, the plywood bent with slowly increased pressure from cargo ratchet-straps cinched around the jig, as hot water sprayed in from beneath and above. Once fully formed, and allowed to dry, a perfect leading edge emerged. Another major problem surrounded rebuilding the empennage. At various stages of the rebuild, the restoration team would assemble associated parts to make sure everything was calibrated correctly. When they tried to mate the completed vertical and horizontal stabilizers, however, they didn’t fit! Unknown to the restorers, the original horizontal stabilizer had been beefed up by 1/16″ at the center, with the plywood tapering towards the outer tips. Therefore the team had to completely strip the horizontal stabilizer, and rebuild it from the skin up to add the increased thickness. (CLICK BELOW FOR THE NEXT PAGE).

Aircorps Art Dec 2019

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