Sikorsky S-40
The Sikorsky S-40 was a four-engine amphibious flying boat built by Sikorsky for Pan American Airways and first flown in 1931. Only three were built, all for Pan Am; the first, christened American Clipper, began the airline's Clipper naming tradition and served as its flagship. The three aircraft carried 38 passengers on Pan Am's Caribbean and Latin American services, flew more than 10 million miles without incident in civilian service, and finished their careers as US Navy trainers in the Second World War. None survives.
Why It Mattered
The S-40 was Pan American Airways' first large flying boat, and the first aircraft to bear the airline's famous Clipper name. The Sikorsky Historical Archives records that when it was built it was the largest aircraft built in America, and the largest amphibious aircraft in the world. It carried 38 passengers, where the airline's earlier S-38 had carried eight. American Clipper, the first of the three, served as the flagship of Pan Am's clipper fleet. Only three were built, because Sikorsky was already designing the more advanced S-42 that would take its place.
Design Notes
The S-40 began with Juan Trippe, Pan American's president, asking Sikorsky for a larger passenger aeroplane than the S-38. Wind-tunnel testing started in October 1928, and models of the hull were tested in April 1929. The design stayed conservative: a much larger development of the S-38 rather than a clean-sheet aeroplane. Four Pratt & Whitney Hornet radial engines of 575 hp each provided the power. Its structure depended on a mass of external wires and struts, which added significant drag; Charles Lindbergh, a Pan Am consultant, told Sikorsky that flying the S-40 felt like flying a forest through the air. Inside, passengers were carried in liner comfort, with a pantry fitted with an electric refrigerator and a smoking lounge panelled in mahogany. In 1935 all three aircraft were upgraded and redesignated S-40A, with more powerful 660 hp Hornet engines and the landing gear removed.
Operational History
The S-40 first flew in August 1931. On 12 October 1931 the first aircraft was christened American Clipper by Lou Henry Hoover, wife of President Herbert Hoover, at Naval Air Station Anacostia. Passenger service began on 19 November 1931, when Charles Lindbergh and Basil Rowe flew from Miami to the Panama Canal Zone, calling at Cienfuegos, Kingston and Barranquilla. Igor Sikorsky travelled as a passenger on that flight, and during it he and Lindbergh discussed ideas that later helped in creating the S-42. In Pan Am service the three aircraft flew without incident, accumulating more than 10 million miles between them. Retired around 1940, they were given to the US Navy during the Second World War, which used them to train pilots for four-engine aircraft and designated them RS-4.
Afterlife
Only two of the three S-40s were pressed into Navy service as RS-4s. American Clipper, acquired by the Navy in April 1942, was damaged beyond repair on 3 April 1943. That April another RS-4 was damaged at Dinner Key; the Navy returned it to Pan American, which kept it only as a source of spare parts. On 9 July 1943 Southern Clipper struck a submerged rock while being beached in Navy service and was damaged. American Clipper and Caribbean Clipper were sent for scrapping in 1943, and Southern Clipper followed in 1944; no S-40 survives.