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S-40
S-40
	Sikorsky S-40 PanAm 1931
General information
Type	Amphibious flying boat
National origin	United States
Manufacturer	Sikorsky Aircraft
Designer	Igor Sikorsky
Primary user	Pan American Airways
Number built	3
History
Introduction date	August 7, 1931
First flight	1931
Developed from	Sikorsky S-38
Developed into	Sikorsky S-42

The Sikorsky S-40 was an American amphibious flying boat built by Sikorsky in the early 1930s for Pan American Airways. During WW2 they were used by the United States Navy for training.

This was the first of flying clippers, large flying boats of the 1930s used for long distance air travel. More advanced designs soon followed, but the S-40 was a big step forward as it could carry 38 passengers as opposed to the S-38's eight.

Design and development

Sikorsky designed the S-40 in response to a request from Juan Trippe, president of Pan American Airways, for a larger passenger-carrying airplane. The S-40 could carry 38 passengers, a significant increase over the S-38's eight passengers. Wind tunnel testing of the S-40 started in October 1928, and models of the hull were tested in April 1929.^[1] The aircraft featured a pantry with an electric refrigerator and stove as well as a smoking lounge with book-ended mahogany wood paneling. Six life rafts were carried.^[2]

Despite a significant size increase over the preceding S-38, the S-40 design was a conservative iteration of the smaller aircraft;^[3]^: 62 the numerous flying wires and strut braces that were used in the exterior support framework caused significant drag and prompted Charles Lindbergh, retained as a consultant for Pan American, to tell Sikorsky "it would be like flying a forest through the air."^[4]^[5]^: 57 Only three were built as Sikorsky began designing the more advanced and streamlined S-42 shortly after the S-40 entered service,^[3]^: 62–63 based partly on input from Lindbergh.^[5]^: 57 All three S-40s were built by the Vought-Sikorsky Aircraft Division of the United Aircraft in Stratford, Connecticut.

The first test flight occurred on August 6, 1931.^[6]^[7] The first aircraft was christened American Clipper by Lou Henry Hoover, wife of President Herbert Hoover, on October 12, 1931 at NAS Anacostia;^[8] after its christening, American Clipper flew around Washington, D.C.^[9]

In 1935, all three aircraft were upgraded and re-designated as the Sikorsky S-40A. Upgrades included replacing the original 575 hp (429 kW) Pratt & Whitney R-1860 Hornet B engines with smaller displacement but more powerful supercharged 660 hp (490 kW) R-1690 Hornet T2D1 engines,^[10] eliminating the landing gear, and increasing the maximum weight slightly. Another source states the landing gear was removed soon after the type certificate was issued.^[11]

Operational history

Passenger carrying service was initiated on November 19, 1931, with a S-40 piloted by Charles Lindbergh and Basil Rowe, flying from Miami, Florida to the Panama Canal Zone with stops at Cienfuegos, Cuba; Kingston, Jamaica, and Barranquilla, Colombia. Igor Sikorsky, the aircraft's designer, was on board as a passenger; during that trip, Sikorsky and Lindbergh began working on concepts that were used in the succeeding S-42.^[5]^: 57

The S-40 was Pan American's first large flying boat. American Clipper served as the flagship of Pan Am's clipper fleet and this aircraft model was the first to earn the popular designation of "Clipper" or "Pan Am Clipper".^[12] The three S-40s served without incident during their civilian lives, flying a total of over 10 million miles.

Navy Service

They were retired around 1940 and turned over to the US Navy during World War II, who used them as trainers for four-engined flight instruction. Under Navy service, the aircraft were designated Sikorsky RS-4.^[13] (see also Sikorsky RS) Two of the S-40 were pressed into Navy service, and called RS-4.^[13]

In July 1943, an RS-4 (752V) was coming in for beaching when it struck an unforeseen submerged rock, which damaged the wheel strut, which in turn caused a list that led to a pontoon striking a rail at the edge of ramp. Beaching is when the flying boat is on the water, but brings its wheels down to come on to land, usually by a seaplane ramp. Another was damaged at Dinner Key in April 1943, and then returned to Pan-American by the Navy, however it was not returned to service but was used for spare parts. ^[13]

List

Sikorsky S-40 aircraft of Pan American Airways^[14]^[15]^[16]
Name	Registration	Delivered	Notes	Ref.
American Clipper	NC80V	October 12, 1931	Christened by Lou Henry Hoover on October 12, 1931 at NAS Anacostia. Acquired by Navy in April 1942. Damaged beyond repair on April 3, 1943.	^[8]^[13] ^[17]
Caribbean Clipper	NC81V	November 1931
Southern Clipper	NC752V	August 1932	Damaged on July 9, 1943 in Navy service while being beached.	^[13]

Specifications (S-40)

Data from American airplane specifications^[18]

General characteristics

Crew: 6 crew
Capacity: 38 passengers
Length: 76 ft 0 in (23.16 m)
Wingspan: 114 ft 0 in (34.75 m)
Height: 23 ft 0 in (7.01 m)
Wing area: 1,740 sq ft (162 m²)
Empty weight: 23,000 lb (10,433 kg)
Gross weight: 34,000 lb (15,422 kg)
Fuel capacity: 1,066 US gal (888 imp gal; 4,040 L)
Powerplant: 4 × Pratt & Whitney Hornet 9-cylinder radial engines, 575 hp (429 kW) each

Performance

Maximum speed: 117 kn (135 mph, 217 km/h)
Cruise speed: 97 kn (112 mph, 180 km/h)
Stall speed: 56 kn (64 mph, 103 km/h)
Range: 780 nmi (900 mi, 1,400 km)
Service ceiling: 13,000 ft (4,000 m)
Rate of climb: 700 ft/min (3.6 m/s)

References

Notes

^ "Mammoth Sikorsky Passenger Transport". Popular Aviation. October 1931. p. 22.
^ "Details of The Giant Sikorsky". Popular Aviation. December 1931. p. 89.
^ ^a ^b Knapp, Lewis G. (2002). "2. Industry at the River". Stratford and the Sea. Charleston, South Carolina: Arcadia Publishing. pp. 37–66. ISBN 0-7385-2399-2. Retrieved 26 March 2020.
^ Gandt, Robert L. (1991). "10: The Flying Forest". China Clipper: The Age of the Great Flying Boats. Annapolis, Maryland: Naval Institute Press. ISBN 978-1-61251-424-6. Retrieved 26 March 2020.
^ ^a ^b ^c Leary, William M. (2005). "Sharing a Vision: Juan Trippe, Charles Lindbergh, and the Development of International Air Transport". In Dawson, Virginia P.; Bowles, Mark D. (eds.). Realizing the Dream of Flight: Biographical Essays in Honor of the Centennial of Flight, 1903–2003. Washington DC: National Aeronautics and Space Administration, NASA History Division, Office of External Relations. pp. 47–68. ISBN 9780160831515. Retrieved 26 March 2020. alternate link: PDF
^ "40-Passenger Plane Tested in Connecticut". Santa Cruz Evening News. AP. August 7, 1931. Retrieved 26 March 2020.
^ "The Largest Amphibion in America". Calexico Chronicle. August 13, 1931. Retrieved 26 March 2020.
^ ^a ^b "Photograph of the christening ceremony of Pan Am's 'American Clipper', a Sikorsky S-40 flying boat christened by Mrs. Herbert Hoover on Oct. 12, 1931". UM Libraries Digital Exhibits. University of Miami. September 1985. Retrieved 26 March 2020.
^ "Huge Plane Salutes Capitol". Calexico Chronicle. October 30, 1931. Retrieved 26 March 2020.
^ Johnson, E.R. (2009). American Flying Boats and Amphibious Aircraft: An Illustrated History. Jefferson, North Carolina: McFarland & Company, Inc. pp. 189–191. ISBN 978-0786439744. Retrieved 27 March 2020.
^ Rankin, Robert H. (September 1935). "The Marvelous History of the Pan American Airways". Popular Aviation. Vol. XVII, no. 3. pp. 154–156, 200–201. Retrieved 27 March 2020. After securing a license as both a flying boat and an amphibian, the wheel gear was removed from the machine and the "American Clipper", as well as its sister ships "Caribbean Clipper", and the "Southern Clipper", became full-fledged flying boats.
^ Rankin, Robert H. (December 1937). "The Story of the Amphibian". Popular Aviation. Vol. XXI, no. 6. pp. 11–14, 70. Retrieved 27 March 2020.
^ ^a ^b ^c ^d ^e Campbell, Douglas E. (2018). "37854/37855 - Sikorsky RS-4". VPNavy! USN, USMC, USCG and NATS Patrol Aircraft Lost or Damaged During World War II - Listed by Bureau Number. Lulu.com. p. 321. ISBN 978-1-387-88879-5. Retrieved 27 March 2020.
^ "Sikorsky S-40". Pan Am Clipper Flying Boats. Retrieved 23 March 2020.
^ "The Pan American Clippers". Aerofiles. Retrieved 23 March 2020.
^ "Pan American Airways Clippers 1931–1946". The Flying Clippers. Retrieved 24 March 2020.
^ "Mrs. Hoover Christens the 'American Clipper'". U. S. Air Services. Vol. XVI, no. 13. November 1931. pp. 17–18. Retrieved 26 March 2020.
^ Aviation January 1933, pp. 28–31.

Bibliography

"American airplane specifications". Aviation, Vol. 32, No. 1, January 1933, pp. 28–32. Registration required.
Davies, R.E.G. Pan Am: An Airline and its Aircraft. New York: Orion Books, 1987. ISBN 0-517-56639-7.
Yenne, Bill. Seaplanes & Flying Boats: A Timeless Collection from Aviation's Golden Age. New York: BCL Press, 2003. ISBN 1-932302-03-4.