Bolt In Cross Brace Explained

There is indeed a bolted-in section of tubing in the upper (top) plane of the forward-most fuselage “bay”, located immediately between the firewall and the opening of the front cockpit – the area immediately above the front rudder pedals. This bolted-in section is V-shaped, with a single bolted fitting forward, at the center of the forward-most fuselage left-to-right cross tube (immediately aft of the firewall). From that centered forward fitting, two tubes welded to that fitting run diagonally toward the rear of the fuselage, one to the left and one to the right, with their rearward ends each welded to individual bolted-in fittings. These left and right fittings are secured to the upper-most left and right fore-and-aft fuselage tubes respectively, just forward of the junction of the second fuselage cross tube. The base of the forward cockpit instrument panel is clamped to that second fuselage cross-tube.

All Model 75 Stearman aircraft were originally designed and built with a single tube welded into the forward-most fuselage “bay” in that location, oriented and running diagonally from the right side of the fuselage to the left side, across the top plane of that “bay”.  When these aircraft were “retired” from military service and were sold to the public as military surplus at the end of WW II, many were converted from their original two-cockpit configuration into a single-seat configuration in order to be used as crop dusters and sprayers.  That single diagonal tube was cut out without replacement, and the dual controls of the front cockpit were removed in order to mount a tank or hopper into the space that was the front cockpit.  This alternation of the fuselage was made to many Model 75 Stearman aircraft.  In fact, it is commonly accepted that their conversion and use as dusters and sprayers is one reason why so many Model 75 aircraft survive to this day.

In restoring a Model 75 Stearman to its original two-cockpit “military” configuration, a new diagonal tube was commonly welded into place to replace the cut-out diagonal tube, and the front cockpit controls re-installed.

In the case of N49680, that single diagonal tube was not replaced to restore the original fuselage configuration, but rather, a V-shaped two-tube replacement was bolted into place.  Instead of a fuselage tube running in a single diagonal direction from one side of the fuselage to the other, two tubes were put into place. one running diagonally in one direction (for example, from center to right), and a second tube running in the opposite direction (from center to left).  That is, the diagonal bracing in the top plane of the forward-most bay of the front cockpit is double that of the original configuration, resulting in bracing of the fuselage not merely in one direction, but two opposing directions simultaneously.

This non-standard bolted-in two-tube configuration was inspected during the restoration, and has been inspected during every annual inspection since, and found to be acceptable.

Nevertheless, this bolted-in two-tube replacement could easily be removed, and a new single diagonal fuselage tube welded into place to return the fuselage to its original “factory” configuration, as has been done with many “restored” Model 75 Stearman aircraft.

Those who are “very knowledgeable” about the Model 75 Stearman aircraft, their design, configuration, and history, including their extensive use as dusters and sprayers after the war, should be very familiar with this fuselage modification and its restoration.  Indeed, it is widely known among Stearman restorers, experienced owners, and enthusiasts that a Model 75 fuselage that has not been cut is a true rarity.

To substantiate my statement that this Model 75 aircraft was originally designed and built with a two-cockpit configuration, we have copies of Boeing engineering drawings, numerous published reference documents, and the military history for this serial numbered aircraft, prepared by Ken Wilson, Stearman Restorers Association Historian.  Boeing Model A75N1 PT-17 serial numbered 75-2242 was assigned to and served training military pilots at the Contract Pilot School, Bennettsville, South Carolina, after its delivery from the Boeing Wichita factory on 01 October, 1941.  All Model 75 aircraft were originally built with a two-cockpit configuration.

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