Aeronautical engineer and manufacturer Grover C. Loening came up with a novel idea for a military amphibian in 1923. Using the same engine, his amphib could outperform the standard two-seat observation planes that the U.S. Army and Navy were using. There had been plenty of previous amphibians, but they were clumsy flying boats or pontoon seaplanes with retractable wheels attached to single or twin floats.
For his starting point, Loening took the standard military seaplane configuration of the time, the conventional open-cockpit biplane with one main pontoon on struts below it, plus small stabilizing floats under each wingtip. He then filled in the space between the main float and the fuselage to create a single-fuselage float combination that became the hull, and he eliminated the drag of all the connecting struts and wires. He then mounted the wheels with their strut attach points on the chine of the float. When retracted, the wheels were half-imbedded in the float for further drag reduction.
This concept was greatly enhanced by a breakthrough in engine design by Army Air Service engineers. They had developed an inverted version of the famous 400-horsepower Liberty engine that had been the standard Army powerplant since 1918 and would remain in service until 1935. The new arrangement’s high thrust allowed Loening to move the float 18 inches closer to the fuselage than the original upright Liberty allowed. It made for a more compact unit with less drag and, therefore, better performance.
Naturally the new airplane looked rather strange. Regardless of its superior performance, Loening knew that successful sales would depend on pilot acceptance. In design matters, the pilots of the day were a conservative bunch and highly resistant to anything unconventional. Loening decided that the best way to make pilots feel at home in the airplane was to make the view from the cockpit as familiar as possible. To that end he designed thin-section biplane wings that were similar to those of the Army’s principal airplane of the time, the de Havilland 4 that had been developed in England in 1916. In fact, the Loening and DH-4 wings were actually interchangeable.
With no resistance from pilots, plus better performance as an observation plane and the added versatility of being amphibious, Loening won significant orders from the Army for OA-1s (observation amphibians, a new designation that had to be created for the new design) and OLs (observation, Loening) for the Navy. Production continued into 1929. After the “Shoehorn” amphibian had been in production for a few years, Loening tried a more modern thick-section wing with a single bay of struts. It didn’t sell, however, and the services continued to order the old style.
There was a second and unplanned payoff on Loening’s deliberate use of DH-like wings. The Army and the Navy, as well as the Post Office Department that used DH-4s as mail planes, bought quite a few extra sets of Loening amphibian wings and fitted them to the DH-4s to improve their performance.