Development
With the Background work completed and a clear goal in mind to build the most advanced Attack plane of the era, Ed Heinemann and Robert Donovan set out and began designing a plane that would out-perform every other bomber of the Second World War.
The first notable difference between the Invader and other bombers of the day is that A-26 Invader was the first aircraft designed to make use of the new laminar flow airfoil wings. This was considered a radical decision at the time, but it proved correct. The wing shape, developed by NACA, allowed for a higher speed – which addressed one of the Army’s points – and it reduced structural stresses, which allowed for a higher G-force tolerance – which addressed a second point. This second point was also address by the decision to use a new aluminum-alloy developed by Alcoa Aluminum called 75S. This new alloy was lighter than other alloys of the time, without sacrificing strength, which allowed for lower weight and faster speed.
On the wings, they designed two powerful 18-cylinder Pratt & Whitney R-2800 double-wasp 2,000 horsepower radial engines. They also added a highly advanced (for the time) double-slotted electrically operated flaps. These new flaps, according to a 1943 memo from designer Ed Heinemann, would give the plane a 30% higher lift coefficient than the older Fowler flaps used on the Havoc. The new flaps also reduced the necessary take-off and landing speed by 10%, which meant a shorter runway could be used, which met another of the Army’s checklist items.
They kept the basic tricycle landing gear configuration of the original Model 7A, as well as the large bomb bay, and wing-mounted bomb racks.
To streamline the craft they moved the defensive firepower into 2 remotely operated turrets that a single gunner could operate with a periscope. This was a direct response to one of the Army’s requirements.
Determined to get the design right, Douglas shared the design plans with NACA in early 1941 and wanted to get an evaluation of the plans, especially the correct shape for the plane’s fuselage, cross-section, and nacelles. Elton W. Miller, the head mechanical engineer at NACA evaluated the designs. He noted that “It is powered by two 2800 engines in nacelles which extend especially far forward with respect to the wing. The nacelles are below the wing, the upper surface tangent with the upper surface of the wing. The nacelles appear unusually long.” During his final memo Miller said: “With reference to the length of the nacelle, it appeared that the length was determined by the position of the propellers with reference to the pilot’s cockpit and the personnel quarters in the fuselage and by the space needed for retracting the landing gear. It did not appear possible to make it any shorter for practical reasons and nobody suggested making it longer. It was the general opinion that the drag of the nacelle would depend largely on the skin friction and more especially on the number of scoops and protuberances on the surface.”
NACA’s final report was delivered by NACA engineer E. N. Jacobs to a combined group of Douglas, NACA, and military personnel. The final report was that the large nacelles of the Invader would account for a large portion of the A-26’s total drag. The suggested solution was to put the engines inside of the main body of the fuselage and use shafts and gears to drive the propellers. Douglas engineer Dr. M. U. Clauser opined that such a radical design philosophy was not likely to be pursued by anyone during the war.
With regards to the question about the shape of the fuselage, Miller said this: “The main question seemed to be whether it would be better to use a square-section fuselage as contemplated in the preliminary design or whether a circular fuselage would be more efficient.” Miller noted some basic geometry considerations; a square shape has a higher internal volume than an equally sized tube. So, for the circular shape to carry the same volume of space as the square, it would have to be larger. But, a larger circular fuselage would also increase the drag of the plane and would make it slower. On the other hand, the joints required to marry the wings to the square fuselage would also increase drag. In the end, there was no practical advantage to choosing one shape over the other.
Douglas engineers told NACA that they would like to use NACA’s 19-foot wind-tunnel at the California Institute of Technology (Caltech) in Pasadena, CA. They wanted to build a 1:8 scale model of the plane to study the laminar flow wings, the new flaps, longitudinal stability of the plane, stalling, and other related questions. They also wanted to build a large nacelle (Perhaps full-sized) to verify engine cooling capabilities. Finally, they wanted to build large wing surfaces to test in the low-turbulence tunnel. These tests were completed under codename: “Project 98”.
The 1/8 scale model was tested in many different configurations, with flaps at different positions, and with the wheels up and down. Ultimately the engine nacelle was built to 1:3 scale and included an electric motor turning a scale propeller. The engine had to be water-cooled and this took a lot of the city’s water pressure, which elicited complaint from the local residents. The decision was made to conduct the engine tests at night so as to use the water when everyone was sleeping, which was successful in quieting the dissent.
The standard practice for NACA at the time was for NACA to build its own models for the wind-tunnel tests. But Ed Heinemann convinced NACA’s Dr. Carl Lewis to allow him to build the big 10-foot wing section because Heinemann wanted to use it for further testing at Langley.
NACA and Douglas partnered together and designed new engine cowlings. While most people do not consider the covers as more than aesthetic piece or some basic form to protect the engine from debris, they were actually very advanced. Ed Heinemann said of the cowlings that the Invader’s engines “are cooled with the latest high entrance velocity cowling…. This cowling not only has less aerodynamic resistance but has resulted in lower engine temperatures than heretofore experienced.”
With the design laid out on paper and initial wooden scale wind-tunnel tests completed, work began building a full-scale wooden mockup. In April 1941, the Army Air Corps evaluated the mock-up and were pleased with the presentation. On 2 June 1941 the Department of Defense awarded Douglas with a contract to build two fully-operational prototypes. By the end of June, Douglas had a contract for a third prototype.
Due to great design work, Heinemann and his team had developed an initial plane design that was easy to modify. Thus, when the prototypes were being built at the Douglas El Segundo, California manufacturing plant, very little had to be done in order to accommodate the changes the Army wanted.
XA-26 was built as a 3-man attack bomber with a transparent nose for a bomber/navigator.
XA-26A was built as a 2-man night fighter.
XA-26B was built as a 3-man attack plane with a hard nose with interchangeable armament configurations.
The Army, seeing that war was inevitable and having confidence in Douglas’ design, issued a contact for 500 Invaders in Oct 1941, even before the prototypes were completed. In the meantime, the contract for A-20s was also increased to give the Army more equipment in the meantime.
The inclusion of the US into the war slowed development of the prototypes and the first prototypes were not ready until June 1942. But, eventually they did move on to Testing.