Early use of seamed steel tubing in U. S civil aircraft can be traced back to the 1920’s when Waco, Travel Air and Stearman began to replace early wood fuselage structures with welded steel. Before SAE 4130 “Chromoly” tubing became the norm, SAE 1020 and 1025 was widely used. Comparing strength of 1025 and 4130 would indicate that the tensile strength of 1025 is 55,000 psi while 4130 in the normalized condition is 90,000 psi. While many early fuselage frames were gas welded from SAE 1025 steel, 4130 Cond. N (normalized) is the tubing of choice. If a frame was originally constructed of SAE 1025 steel, it can be repaired using 4130 tubing. For the Boeing model 75 aircraft, this is a moot point because all frames were constructed of 4130 Chromoly tubing and plate.
All early fuselage frames were oxyacetylene welded because the inert gas process had not been invented. After welding welded joints are normalized to relieve the stresses of weld bead areas by heating to slightly above the critical temperature and allowed to cool in still air. When making repairs a mechanic will heat the welded joint using a “rosebud” attachment to the torch and apply heat until the area is bright red but not melting. Once this has been achieved the repaired area should be allowed to cool in still air.
There are two types of steel tube frames – the Warren and Pratt Truss. Fuselage frames are generally a combination of the two types of trusses, the Pratt Truss being used in forward fuselage construction and the Warren Truss used in aft fuselage construction. Both types of construction use a diagonal tube to carry both tension and compression loads while the aircraft is in flight. Below in figures 1 and 2 are the two types of fuselage structures.
The Boeing model 75-fuselage frame is sturdy in every sense of the word when compared to earlier structures. Not only is it constructed from the superior SAE 4130 seamless steel tubing but also it was built to eventually be powered by a 600 hp Pratt and Whitney radial engine, a true test for its sturdy construction. When these type structures are built they are fitted into a welding fixture that holds all the clusters (stations) in place for welding. This assures that all frames are virtually the same, given minor differences from a welder’s technique. All the model 75 fuselage frames were interchangeable from one aircraft to another and the factory welds were impeccably done by true craftsmen of the day. Having accomplished gas welded repairs to Boeing 75 fuselage frames, I can attest to the skill required when gas welding. There are many clusters that take two torches to complete the weld – one to preheat and one to actually weld.
Above in figure 3, a special fuselage fixture designed in 1928 by Albert Vollmecke for Command-Aire, Incorporated of Little Rock, Arkansas. The Boeing fixture was very similar to this early manufacturing aid. The Command-Aire fuselage was constructed from SAE 4130 steel tubing and was actually very similar to the later Boeing model 75- fuselage frame. In fact I had a Command-Aire fuselage frame that had been modified to incorporate a Boeing 75 main and tail landing gear assembly.
h Interesting enough the lower longerons and cross tubes of the model 75 frame would fit perfectly into a Command-Aire frame and was secured by outer sleeves. And the entire last bay of the model 75 fuselage would splice directly into the last bay of the Command-Aire. I asked Albert if Lloyd Stearman copied his fuselage and fixture design and he replied, “We all copied each other in those early days.”
Each cluster weld (referred to as a station) is carefully fitted to eliminate large gaps that are difficult to weld and actually degrade the strength of the structure. Below in figures 4 and 5 are typical cluster welds on a steel tube fuselage frame. Note how tubing centerlines intersect in the center of the longeron.
While most manufacturers of the day did not protect the inside of tube structures from corrosion, the Boeing factory drilled all clusters with a #30 (1/8”) drill bit before assembling and welding. This allowed linseed oil to be pumped into the entire frame under pressure and, although it took some time, the boiled linseed oil would eventually fill the entire frame. The linseed oil was fed into the frame by removing the engine mount stud on opposite corners of the frame (actually we removed all the studs because we were converting stock frames into 450 hp agricultural airplanes and needed to oversize the studs from 7/ 16” diameter to 1/2 “ diameter. Once the linseed oil was inside the tubes we placed the frame with the aft end on a high saw horse and rotated the frame to drain out all the linseed oil we could. This process took several days as we kept raising the tail, eventually standing the frame on its nose to drain the last little bit of oil out. Then we set the frame on the 3-point attitude and allowed all residual linseed oil to drain to the aft lower longerons, then installed the engine mount studs.
The frame was then sandblasted and coated with epoxy primer and paint, usually white. The first coat was a red oxide primer, the second coat was a yellow topcoat and the third coat was white. We used different colors to be sure the frame was completely encapsulated in epoxy paint due to the toxic environment in which the airplane was to operate. In figure 6 is the Boeing Model 75 fuselage frame.
The business of agricultural aviation was where I got my start both as a pilot and mechanic. That was in 1956 at the Hanford, California airport, working for my late uncle George Baldrick. It was here that I learned the Stearman biplane from the frame up. I built 3-sets of 4412 high life ag wings and covered them with Grade A fabric with butyrate dope, then assembled and rigged the airplanes. It was a great learning experience for a young man just out on his own. Below in figure 6 is George’s fleet of 450 hp Stearman biplanes, purchased after WW2 for around $400.00 each. He had 6- airplanes but kept only 4-flyable. One or two airplanes were always under rebuild.
In figure 7 the ship in the foreground has the NACA 4412 high-lift wings while the other 3-ships have stock wings. The 4412 wings would lift a hopper load of 2,000 pounds.
In figure 8, a Chevron advertisement from the July 1967 issue of FLYING magazine, that is George Baldrick with pilot Carl Corwin in the Stearman with my 4412 wings. And the guy crouching directly in front of Carl’s low pass for the camera is me at age 27. This was strictly a pose for this magazine. We had removed all the spray nozzles so water coming from spray booms would be visible to the camera.