The Tadpole Clipper: A Fully Operational 3/8 Scale Aircraft

Browsing through hobby shops and online catalogs, builders of model aircraft come across several familiar scales. The most common kits are usually 1/48-scale, but modelers can also opt for 1/32-scale and 1/72-scale models depending on their taste for detail and available space. RC plane enthusiasts likewise have a range of recognizable scales to choose from. But how many modelers have ever heard of, let alone built, a fully operational 3/8-scale aircraft capable of carrying an onboard pilot?

In 1937, engineers and craftsmen at the Glenn L. Martin Company in Baltimore, Maryland, did exactly that, constructing a working 3/8-scale model of Martin’s soon-to-be famous PBM Mariner. Affectionately called the "Tadpole Clipper," the pint-sized flying boat weighed in at roughly 2,800 pounds, had a 43-foot, 6-inch (13.26 m) wingspan, and stood just over 12-feet (3.66 m) high. The 28-foot, 6-inch (8.69 m) hull was of metal monocoque construction with internal wooden components—the rest of the scaled-down aircraft consisted of plywood, aluminum, and doped fabric. Because the aircraft was too small to accommodate actual rotary engines, its designers mounted a 120-horsepower four-cylinder Chevrolet engine in the fuselage and connected it via drive belts to a pair of 64-inch propellers attached to mock nacelles on the wings. Though powerful enough to takeoff with two people onboard, the Tadpole Clipper carried only a single pilot and an auxiliary fuel tank in its final configuration.

Impressive as these specifications are, they do not reveal exactly why engineers built the scaled down aircraft. The answer to this is twofold. First, according to a contemporary article, the main purpose of the Tadpole Clipper was "to determine at low cost the performance and characteristics of a projected aircraft which would cost hundreds of thousands of dollars to design, manufacture and test." Company namesake Glenn L. Martin hoped "that the flying model built to the exact scale of the projected airplane will enable him to determine much of the data necessary for its construction, by testing it under natural conditions to eliminate as much design error as it is humanly possible to eliminate." If successful, the Tadpole Clipper promised to offer design data that could otherwise be obtained only from tests in wind tunnels and towing basins.

In addition to providing new engineering insights, the Tadpole Clipper also had a practical second goal. Though famed as an early supplier of military aircraft, Martin had shifted its attention to the emerging commercial market in 1933, a strategy evidenced by its production of flying boats such as the Model 130 "China Clipper." With the storm clouds of war gathering at the end of the decade, company leaders became eager to break back into the production of naval patrol aircraft. Doing so, however, meant going up against formidable competitors, some of whom already had the ears of military officials. Fearing that the time it would take to produce a full-size PBM prototype could cost them a production contract, Martin engineers hurriedly built the trimmed down version as a proof-of-concept for Navy officials.

Construction of the Tadpole Clipper began in mid-1937, and the miniature aircraft first took flight on December 3rd of that year. Over the next six months, Martin's Chief of Flight-Testing W. K. Ebel spent nearly 13 hours at the controls of the scale replica—a daunting task given that the belts driving the Tadpole Clipper’s propellers tended to slip off their pulleys. Though it wasn’t particularly safe as a flying machine, engineers were still able to gain important data from the aircraft's water-handling tests. Reporting on the program, one observer declared that "Ebel can take [the Tadpole Clipper] out on the water in rough weather or on a day when the Chesapeake Bay is like glass, and give it as rough treatment as the Martin engineers think is necessary." Similar to how they would approach testing in a wind tunnel, engineers achieved different hydro patterns on the flying boat by altering the bow and chine lines of its hull with modeling clay and other materials. The Tadpole Clipper also proved useful for investigating the porpoise effect—an oscillation in pitch experienced by flying boats when they accelerate on water—as well as how wave and bow spray patterns might reach the engines on the full-sized PBM's gull-shaped wings.

Recalling its twin objectives—providing design insight and securing military contracts—the Tadpole Clipper was an undeniable success. Engineering data from the 3/8-scale scale model facilitated the process of designing, building, and testing the full-size PBM, a prototype of which (the XPBM-1) first flew in February 1939. Testing of the Tadpole Clipper also confirmed Martin engineers' performance projections for the PBM, leading the US Navy to award the company a $5.3 million production contract for 21 of the new flying boats. Martin went on to build a total of 1,366 PBMs between 1940 and 1949, an impressively large production run for a flying boat with such diminutive origins.

Its work seemingly completed, the Tadpole Clipper was retired and suspended from the rafters of Martin's main assembly building. There it remained until 1953 when the company donated the scale replica to the Smithsonian Institution. Deposited in a large artifact storage facility outside Washington, D.C., the Tadpole Clipper sat mostly undisturbed for nearly 30 years until Martin retiree Roy Shine came across it during a tour. A member of the team that created the Tadpole Clipper, Shine convinced National Air and Space Museum senior curator Bob Mikesh to let him manage its restoration.

In March 1987, Shine and a group of volunteers moved the Tadpole Clipper to a workspace at the Baltimore Museum of Industry not far from where the aircraft was originally built. Directed by Mikesh to both restore the deteriorated aircraft and "[document]…early technology and fabrication methods," the team photographed every component of the Tadpole Clipper as they disassembled it and prepared detailed drawings of its construction. Examinations revealed widespread corrosion on all metal components, as well as rotting and delamination of many of the aircraft's wooden structural members and plywood. The hull and the nose section of the fuselage were in particularly rough shape, necessitating the replacement of nearly all wood and canvas components. Restoration of the gull wing also proved challenging and required the volunteers to replace hundreds of intricately formed spruce spars that had delaminated. The team overcame the final hurdle presented by the severely corroded Chevrolet engine thanks to the timely donation of several parts that were damaged beyond repair. By the time Shine and the volunteers fully reassembled the scale replica in July 1992, they had spent more than 10,000 hours on its restoration. Three decades later, the Tadpole Clipper remains on loan and display at the Baltimore Museum of Industry, testifying to both the workmanship of those who restored it and the importance of the artifact—its unique scale notwithstanding.