The Consolidated Vultee B-32

by Gaither Littrell,
West Coast Editor, Industrial Aviation

History of the design, structure and production of the latest American airplane of the heavy bomber class—

Design work on the XB-32 was begun late in 1941, with the plane destined to be a companion ship to the Boeing B-29 Superfortress. Production of the B-32 had just started in all-out capacity, with a total of 150 planes to be built each month at the company's Ft Worth, TX, and San Diego, CA, plants, when the curtailment order struck — a cutback which came before the plane was officially revealed as being in action.

The B-32 is a four-engined, high-wing, long-range, high-speed craft with a gross weight of 100,000 lb. It incorporates a low-drag Davis wing, with a span of 135 ft. The semi-monocoque fuselage measures 83 ft 1", while the single tail raises 32 ft 2" from the taxi position.

Double-row Wright Cyclone engines, each developing 2200 hp, military rating, power the plane. Each engine is equipped with two exhaust-driven, automatically-regulated turbosuperchargers. Propellers are the huge, four-bladed, hollow steel Curtiss electric props with a diameter of 16 ft 8".

A feature of the B-32 is the new Curtiss automatic synchronizer, which provides constant speed control for each engine by altering the propeller blade angles, thus maintaining accurate synchronization of all engines throughout various flight conditions. This makes it unnecessary to adjust manually a separate control for each engine to maintain synchronization.

Reverse thrust for braking also is incorporated in the B-32. Reversible propellers are attached to the two inboard engines, with the throwing of a switch changing the angles of the prop blades in their hub sockets from a positive to a negative angle.

The plane has two independent sets of flaps, while ailerons are provided with trim tabs operated by electric motors. It is not built for altitude pressurization, as is the Superfortress. A front and aft cabin, and a double bomb bay, are provided.

The B-32 has fully retractable tricycle landing gear, with the main gear dual wheels measuring 56". These retract into inboard nacelle wells. The nose gear is a self-centering caster type with 39” dual wheels operating on a complete 360° swivel. The tail bumper is a fully retractable hydraulic strut.

Engine nacelles house the four primary heat exchangers, while two secondary units are in the fuselage. These provide heat for prevention of icing, and cabin comfort.

Most important overall feature of the plane, which carries a crew of eight, is the extensive use of five types of artificially-aged aluminum alloys, including 24ST-80, 24ST-81, 24ST-84, 24ST-86 and Clad 75ST. It is estimated that nearly 2000 lb were saved in structural weight by the use of these alloys, which also greatly increased yield strengths and reduced ductility.

Methods used in obtaining improved strength characteristics of these alloys over Clad 24ST include one of the following: secondary (precipitation) heat treatment; cold work plus secondary heat treatment, and change in alloying constituents plus secondary heat treatment.

The series of 24ST-80 super-aluminum alloys resulted from a precipitation (artificial aging) treatment applied to the conventional Clad 24ST, Clad 24SRT and extruded 24ST. Nominal composition of the 24S alloy consists of about 4.5 per cent of copper and lesser amounts of magnesium and manganese.

Hardening operation for all 24S material consists of a solution treatment of 910° to 930° F followed by a cold water quench. Precipitation heat treatment to obtain maximum mechanical properties may be accomplished by many time-temperature cycles, the time at temperature depending upon the temperature and the amount of cold work to which the material has been subjected.

At Convair it was found that secondary heat treatment should be 6½ hr at 375° F for all tempers. They have discovered this to be the best compromise of all the factors affecting the aging of the several tempers of 24S aluminum alloy. The material is not affected by prolonged exposure to temperatures at 300° F or less. With higher temperatures, however, time at temperature becomes increasingly critical and causes over-aging.

When a precipitation treatment is superimposed on the cold-work previously given 24ST, 24RT and 24SRT, as is the case with 24ST-81, 245T-84 and 245T-86, the compressive yield is greatly increased and the tensile properties are also increased, though to a lesser extent.

Wing covering of the plane is 24ST-86, while 24ST-84 is used for the stiffeners, which is considered the most effective design. Because of its lower properties, 245T-81 is used where formability requires material having better forming characteristics.

As most of the tempers of 24S aged alloys require a definite amount of cold-work subsequent to the solution heat treat operation, any forming to be done on 24ST-81, 24ST-84 and 24ST-86 is accomplished subsequent to the heat-treat and cold-work operations. Since aging cold-worked materials reduces the ductility and raises the yield strength, it is mandatory that all forming be done prior to the precipitation treatment.

Where strong, light-weight structures with high strength-weight ratios are needed, 75S aluminum base alloy is used in the B-32.

This material is among the newer types of alloys which use zinc as the major alloying constituent in place of the copper which characterized the older duralumin-type alloys. Nominal composition of 75S consists of about 6% zinc and lesser amounts of copper, magnesium and manganese. The 75S alloy weighs about .102 lb per cu in, or approximately 2% more than the 24S alloy. It is available in the form of clad sheet, bare sheet, extrusions and later in forgings. Clad portion of the cladded material consists of a one-percent zinc-aluminum alloy. Each surface has approximately four per cent of cladding.

A solution heat treatment and a precipitation treatment comprises the hardening operation for 75S. Temperature for solution heat treatment, as used by Convair, is 920 ± 10° F for sheet material and approximately 860 ± 10° F for extrusions and forgings. Soaking time at the solution heat treat temperature varies with the thickness of the section. The sheet stock remains at temperature 20 to 30 min, while the extrusions and forgings are at temperature from 30 to 60 min. Aging treatment for all shapes of 75S is found best at 250 ± 5° F for approximately 24 hr.

Given a type specification by the AAF for a fast land-based superbomber with maximum bomb-carrying capacity, Convair officials back in 1941 prepared preliminary designs of the XB-32. These were Army-approved and the first of three XB-32s was test flown at 5an Diego on September 7, 1942.

This first experimental plane was equipped with twin tails, resembling the B-24 Liberator, and the fuselage was extremely cigar-shaped and built with pressurized cabins. The plane was tested principally for design stability and engine performance. Meanwhile, the second XB-32, a combat prototype, was completed and flown.

Tests of this combat model resulted in abandonment of pressurization and remote control gun turrets. Pressurization was abolished to save weight, while Convair officials state that "more accurate fire control was achieved by use of individual gun turrets." While these two planes were undergoing nearly 100 flight tests, the third experimental model was completed.

In this plane the fuselage and empennage, power plant installations and the entire armament system were redesigned. It still carried a twin tail, however.

Artificially aged and stretched aluminum alloy was used on the redesigned Davis wing on this model for the first time, affecting a total wing weight saving of more than 1000 lb.

The No 3 experimental plane became the workhorse of the flight research and development program. Flight logs of No 3 show that the first 13 flights were made to check power plants and surface controls. As these proved out, the bomber was taken to Muroc Army Air Base.

There company crews tested for overload takeoffs and landings. Loads were increased until the plane was taking off with a gross weight of 120,000 lb, or 20% greater than initial specification requirements.

After the 25th flight, the plane was returned to San Diego where the experimental department installed for the first time the single-fin type empennage in place of the twin rudders. Directional and horizontal stability and control with the new tail was tested in the next 75 flights.

It was from the designs of the No 3 plane that the production model came into being, incorporating the best features of all three prototypes.

Redesigning of the third XB-32 into the production model B-32 was completed at Ft Worth. The original contract called for 300 planes to be produced at Ft Worth. A supplementary contract was awarded later for several hundred additional planes. A second supplementary contract for several hundred more went to the San Diego division.

But the cutback order sliced deeply into that hopeful program. All B-32 production had been originally scheduled for the Ft Worth plant, where retooling was accomplished while the B-24 contract was being completed. San Diego also began reconversion to B-32 production while winding up the B-24 program.

One of the production features was the modification of the new planes before they left the assembly line. There were no outside modification centers required because any changes ordered in the planes were incorporated in the modification department, set up at the end of the assembly line itself.

Modification within the factory was found to be the best method to increase delivery rate to training and combat bases. Thousands of changes are made after the plane leaves the assembly line. A considerable slow-down would be created if these changes had to be made at an outside modification center, Convair officials claim.

Modification engineers classify change orders as those required by military necessity (such as a change which affects the fighting ability of the ship) ; those required for increased safety: those required for improvement in the quality of the aircraft, and those required for more efficient production.

The Ft Worth plant was given primary consideration for the production work because it is equipped with an overhead monorail system allowing the removal of completed assemblies directly upward from the jig for transportation to the next operation station without rehandling.

In the construction of the fabrication tooling for detailed parts, as much mechanical knowledge was introduced as possible in order that relatively unskilled labor could be used.

When the B-32 was being designed, complete tactical requirements was not available. Consequently, the tooling program was constantly changing. As a result, the tooling department instituted a method of operation and a type of tooling which provided for the incorporation of changes into the airplane almost immediately.

Changes were constantly made in the tooling program to increase mechanical efficiency, which thus resulted in a completed program, with a minimum number of workers, when the production model began coming off the lines.

This article was originally published in the September, 1945, issue of Industrial Aviation magazine, vol 3, no 3, pp 46, 48, 95-96.
The original article includes 2 photos and one data table. Photos are not credited.