While most aircraft manufacturers have constructed scaled-down versions of their aircraft for testing in wind tunnels, the Glenn L. Martin Aircraft Company went one step further and constructed a 3/8-scale flying prototype model. This novel aircraft served as the basis for what would become one of the most outstanding maritime patrol aircraft of World War Two, the PBM (Patrol Bomber, Martin). The company employed the unusual approach purely for financial reasons, yet the project foreshadowed more modern techniques of flight testing, which employ technology demonstrators that frequently do not match the intended production aircraft in size.
Throughout the early 1930s, the Martin Company competed for new Navy patrol aircraft contracts with the Consolidated Aircraft Company. However, in 1933, Martin abandoned military aircraft for better prospects of the more lucrative Pan American Airways "China Clipper" contracts which led to the remarkable Martin Model 130 flying-boat. Demand for increased passenger capacity soon led to Pan Am's selection of the superior Boeing 314. This forced Martin's flying-boat design team to return grudgingly to maritime patrol aircraft projects. In 1936, their work culminated in a design for the fast four-engined Model 160 patrol bomber.
The company lacked sufficient funds to undertake a large-scale development program without government support. With decreased funding availability, they decided to save on development costs by constructing a 1/4-scale proof-of-concept flying Model 160 prototype. Fearful of losing its monopoly on Navy patrol aircraft, the company exerted pressure on the Navy to hold off on a production contract for Martin's design until the company successfully demonstrated a full-scale prototype, to be designated XPBM-1. In the meantime, Martin's management realized that four-engined patrol bombers were not likely to be cost-effective in the Navy's eyes and revised the Model 160 proposal into the two-engined Model 162. Martin could not afford the time or money to meet the requirement and risked the Navy's disapproval by altering the plans for the 1/4-scale Model 160 and constructing it as a 3/8-scale Model 162. However, the rising tensions in Europe increased the need for maritime patrol aircraft, and the Navy decided to fund the PBM's development without a full-scale prototype. Nevertheless, Martin continued construction on the Model 162 scale prototype, now designated the Model 162A, to ensure that there would not be any lengthy redesigns of the full-scale prototype because of unforeseen engineering problems.
Construction began in mid-1937. The 162A was not a perfectly scaled-down version of the 162. There were a number of compromises in design because of practical considerations such as crew accommodation for the pilot and observer, but the 162A's external features closely resembled those of the production PBMs, including the distinctive gull-wing. Martin engineers did not consider the deviations from the 162 plans to be a significant problem as they were more concerned about the hydrodynamic qualities of the 162A's hull than its flight characteristics, which could be evaluated with small models in wind tunnels.
The 162A's scaled-down engine nacelles could not hold appropriately sized engines, but the project engineers proposed a novel solution. A Martin-Chevrolet engine, developed in the early 1930s for light aircraft, was embedded in the fuselage and drove the propellers, mounted on dummy nacelles, through a system of six V-belts and pulleys. This system was thoroughly tested on the ground before it was installed in the aircraft, but the likelihood of a propeller drive belt failure remained high. This actually occurred during the test program, and the project manager wisely restricted the aircraft to water-handling tests from then on.
Martin did not advertise its work with the 162A, and it was only revealed to the aviation press during the unveiling of the Model 156 "Soviet Clipper" at the company's Middle River Plant near Baltimore. This massive aircraft completely dwarfed the diminutive 162A moored alongside, which caused observers to quickly dub it the "Tadpole Clipper." The nickname became popular with the engineering staff and soon stuck. Even after the 162A had been revealed to the public, details of the PBM program were kept secret, and Martin fostered the general impression that the research was being conducted for a new Clipper design.
On December 3, 1937, the fabric-covered spruce plywood and aluminum aircraft made its first flight with Edward Fennimore at the controls. Along with Martin's Chief of Flight Test, W. Kenneth Ebel, Fennimore put the aircraft through its paces throughout 1938. Ebel had extensive experience testing Martin's Clippers, and was the ideal person to evaluate the 162A. Initially, the cockpit consisted of two tandem seats, but the observer's seat was later removed to make room for a new gas tank.
The wide variety of water conditions on the nearby Chesapeake Bay offered a perfect laboratory to test the aircraft's sea handling qualities, which were essential to the PBM's success as a maritime patrol bomber. The "Tadpole Clipper" did not accumulate a significant amount of flight time over its useful life, but it provided sufficient data to refine the hull design. The most serious problem revealed during testing of the 162 was a tendency for the aircraft to develop an oscillation in pitch, known as porpoising, while planing on the step (the point at which hydrodynamic pressure lifted the hull out of the water). Porpoising was a common problem in flying boat design, and the Martin engineering team solved this problem by lengthening the keel and strengthening the hull to accommodate the increased hydrodynamic pressures. Other research focused on the best bow configuration to keep sea-spray out of the engines and propeller arc.
By the end of 1938, the prototype XPBM-1 was nearly completed. The 162A was retired and hung from the ceiling in one of the main production buildings that produced the 1,366 PBMs constructed during World War Two. In 1953, Martin offered the 162A to the Smithsonian Institution, and twenty years later, the last intact PBM Mariner (see NASM collection) was also accepted by the Smithsonian.
In 1987, a dedicated and enthusiastic team of volunteers, many of whom were former Martin employees, began an exhaustive restoration of the 162A. After more than 10,000 man-hours of labor, the 162 was ready for display while on loan to the Museum of Industry in Baltimore, not far from where the "Tadpole Clipper" had been built and flown. Portions of the rear fuselage undersurface were left uncovered to illustrate some of the modifications made in the 162A's hull during the course of its testing.