Airplane Bulkhead Component Heat Treating

Metlab comes across many unique and interesting heat treating projects each year. Some have a history lesson to accompany the project. Recently, a newly fabricated structural bulkhead for a Ryan ST-A historic aircraft (circa.1934), was treated in the Metlab facility. The customer, Classic Metalcraft, was referred to Metlab by another heat treater that did not have the equipment to properly process the large part.

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Ryan ST-A (Aerobatic) training aircraft circa. 1934

Ryan Aircraft was the manufacturer of the famous Spirit of St. Louis airplane. The Ryan ST’s were a series of two-seat, low-wing monoplane aircraft. They were used as sport aircraft, as well as trainers by flying schools and the military of several countries. The “ST” series (for “Sport Trainer”) was the first design from the company, introduced in 1933. This aircraft was followed by the “ST-A” (A for Aerobatic) which was developed with a more powerful engine.

“We manufacture aircraft parts for displays and museums,” states David Paqua from Classic Metalcraft. “We recently expanded our practice to accept complete restoration work for antique aircraft. Enter the Ryan STA. We decided to produce an exact replica of the Ryan. We obtained copies of the Ryan factory drawings and proceed to fabricate components for the fuselage, landing gear and wings. The most difficult part that needed to be fabricated was the #2 bulkhead. Not only is it tough to replicate without heavy pressing equipment, but it requires heat treating by a knowledgeable firm to prevent distortion. This is where Metlab came into the picture.”

The bulkhead component is a structural piece fabricated with 4130 steel. This segment was located just forward of the instrument cluster. The #2 bulkhead component carries all the stress of the flying wires, landing gear as well as the wing attachments. It was vital indeed to properly fabricate and heat treat this assembly while maintaining a flat section.

Fuselage
The fuselage jig is allowing accurate positioning of the bulkheads and upper and lower stringers. The bulkheads will be covered by a 2024 alloy aluminum skin of .032 thickness.

Paqua explains, “The skin of the aircraft is affixed to the bulkhead. It is critical for the part to have the proper minimum mechanical properties to support the skin as well as remain in shape through the heat-treating process to maintain the aerodynamic characteristics of the aircraft.”

Metlab developed a special fixture to maintain the flatness of the component during processing. Additionally, Metlab consulted with the customer and advised them to tack weld additional bracing inside the component to keep the integrity of the shape and help with the flatness of the entire component during the heat-treating process.

Bulkhead collage
Bulkhead component prior to heat treating.

The physical dimension of the bulkhead is 26” wide X 39” tall and about 2” in section size. The part is quenched and tempered to 180,000 PSI UTS, minimum, about HRC 40 – 44. The part was processed in one of Metlab’s 4′ diameter by 16′ work zone pit furnaces and then clamped on a flat plate for tempering to maintain flatness. Post heat treatment inspection consisted of verification of the hardness and flatness.

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