The Boeing 314 Clipper was an American long-range flying boat produced by Boeing from 1938 to 1941. One of the largest aircraft of its time, it had the range to cross the Atlantic and Pacific oceans. For its wing, Boeing re-used the design from the earlier XB-15 bomber prototype. Twelve Clippers were built, nine of which served with Pan Am. It was the first aircraft to carry a sitting American president, when in 1943 Franklin D. Roosevelt flew from Miami to the Casablanca Conference in Morocco, via Trinidad, Brazil, and The Gambia.
Development
Background As early as 1935 Pan American had identified that a truly trans-Pacific flying boat with unprecedented range and double the passenger payload of the airline's Martin M-130 would be required particularly if they were to provide a service across the longer more difficult Atlantic route and requested proposals from a number of American manufacturers.Trautman, pp. 54-56
In February 1936, not long after the M-130s were introduced into service, Pan American launched a design competition for the first transoceanic airliner. Boeing, Douglas, Consolidated, Martin and Sikorsky were requested to provide preliminary studies and proposals for a long-range, four engine, marine aircraft. The new plane had to be able to transport up to 10,000 pounds of payload with a minimum range of 2,400 miles and cruise speed of 150 mph at an altitude of 10,000 feet. Pan American was also adamant about providing comfort, space, and luxury for their passengers, as well as providing the ultimate in safety. Martin proposed a larger modified version of their M-130 called the M-156 which would offer a longer range and bigger payload with either a 53-day or 26 sleeper configuration. Sikorsky proposed their S-45 whose six engines promised a 52,000 lb payload, but the first aircraft would not be available until late 1939 or early 1940 and it would cost more than the other proposals.
Boeing’s submission After receiving the Pan American request on February 28, 1936, Boeing decided not to submit a proposal. At the time, the company's resources were committed on simultaneous contracts for the United States Army Air Corps.Griffith, pp. 9, 10, 12Bowers, The Great Clippers, Airpower, November 1977 The deadline had passed when a young Boeing engineer, Wellwood E. Beall, became aware of the Pan American project, and thought that Boeing should consider submitting a bid.
Beall had just returned to the United States in 1935 from serving as the company's Far Eastern manager with responsibility for selling the company's fighter and transport planes to the Chinese Government. During his time in China he had often thought about the design of a large flying boat and he now worked in his spare time on a preliminary design.
As Pan American had specified the engines and propellers in advance, Beall proposed to use the wing of the cancelled XB-15 bomber, which had already been designed and wind-tunnel tested; this would allow Boeing to confine the design effort to the hull and the flotation stability and tail assemblies. To supplement his preliminary design study, his then wife, well-known artist and muralist Jean Cory Beall, produced color paintings of the cabin interiors. His proposal was accepted by Boeing management, and he was given permission to request an extension from Pan American, which was granted.
Beall was transferred to the engineering department and allocated 11 engineers to work on the project. The engineering team calculated the optimum wingspan for the flying boat to be compared with the XB-15's . The shortfall was overcome by adding to the width of the fuselage. He combined the wing with more powerful Wright R-2600 Twin Cyclone engines, each of which produced 50% more power than the of the XB-15's Pratt & Whitney R-1830 Twin Wasp engines.Bowers, Flying the Boeing Clipper Complete Operational History , Wings, December 1977 The XB-15's engine nacelles were also retained, as they had originally been designed to house Allison V-3420 liquid-cooled W engines which, since these were not ready, had been replaced on the bomber by the smaller and less powerful Pratt & Whitneys. The engineers estimated that their proposed design would weigh 40,000 lbs and could carry a payload of up to 42,500 lbs.
Sufficient work was completed by May 1936 for Beall, company president Claire Egtvedt, and aerodynamicist Ralph LaVenture Cram (1906–1939) to depart on May 9, 1936, from Seattle to New York, where they made a presentation of their proposal to Pan American.
Pan American accepted Boeing's proposal, awarding it a $4.8 million contract on July 21, 1936, for six aircraft with an option for six more. This order represented the first that Pan American had placed with the company. The first aircraft was to be delivered by December 21, 1937. An amendment to the contract on January 20, 1937, granted a three-month extension to the specified delivery date for the first six aircraft.
The aircraft's introduction was impacted by the complexity of the design and testing and the passage of the Civil Aeronautics Act in 1938, establishing the Civil Aeronautics Board, which had the power to review and certify new aircraft. Boeing missed its contracted delivery date by 13 months.
Testing
There was no dedicated prototype; instead, the first production unit, NX-18601, was used for testing. Once completed it was launched into the Duwamish Waterway and towed to Elliott Bay on Puget Sound for taxi and flight tests. Its wingspan was so great that it had to be turned diagonally to pass through the supports of bridges spanning the waterway. From here it commenced its first taxiing run on 3 June with test pilot Edmund T. Allen at the controls, but he soon identified that it was too lightly loaded when a gust of wind lifted one wing and dipped the other and after increasing the engine power failed to correct the situation it became necessary for all spare crew members to climb out onto the raised wing to balance the aircraft and allow it to safely return to shore.Trautman, pp. 57-60 Following the installation of temporary ballast further taxiing tests continued for a week.
The first test flight was undertaken by Allen on the June 7, 1938, and once airborne it was immediately identified that the aircraft had insufficient rudder control. He was only able to turn the aircraft by increasing the power of the engines on one wing and decreasing that of the engines on the other. This first flight lasted 38 minutes. This directional instability had been observed during wind tunnel tests but Boeing management had rejected any changes to the original single fin configuration to correct it. Someone else who had identified the issue was George S. Schairer, at the time a young engineer at Consolidated Aircraft. After seeing a photograph of NX-18601 with its original tail configuration on the cover of an aviation magazine he had written to Boeing, stating that he believed the tail was too small. NX-18601 was immediately modified into a twin fin configuration, but this was still found to be insufficient and so the centerline vertical fin was restored, which resolved the issue and this three fin arrangement became the production configuration.
Months of extensive testing continued and on January 26, 1939, the 314 was approved the Civil Aeronautics Board for commercial airworthiness.Trautman, p. 63
Design The 314 was a high wing flying boat which used a series of heavy metal ribs and spars to create a robust fuselage and cantilevered wing, eliminating the need for external drag-inducing struts to brace the wings. It was metal skinned with the exception of the control surfaces.
The aircraft were assembled at Boeing's Plant 1 on the Duwamish River in Seattle. The design required the production of 6,000 engineering drawings, 50,000 separate parts, 11 miles of electrical wiring, 576,000 rivetsGriffith, pp. 119-120 and 15,200 bolts.
Engines The original six 314s were powered by four air-cooled, supercharged Wright Aeronautical Corporation Cyclone 14 GR2600A2, two-row, 14-cylinder radial engines. The other six 314A's had the more powerful Cyclone 14 GR2600A2A engines. These engines were subsequently retrofitted to the original 314s during engine overhauls.
Commonly known as "Twin Cyclones", these engines were the first to use 100 octane gasoline in commercial service. The 314 was the first use of this engine, which at the time was not only the largest engine installed on a commercial aircraft, but also offered the lowest specific fuel consumption.
+ Engine SpecificationModel GR2600A2 GR2600A2ATrautman, p. 157Specification 332 579Used in Boeing 314 Boeing 314, 314ASea level rating – Take off at 2,400 revs at 2,400 revs Sea level rating - Normal at 2,100 revs at 2,300 revsWeight Length Diameter (height) Reduction gear ratio 16:9 16:9Supercharger ratio 7.4:17:1Compression ratio 7.1:1 6.8:1Fuel grade (Octane)91/9695
The engines drove Hamilton Standard hydromatic full-feathering constant-speed propellers through reduction gears. The development of these propellers had been fostered by Pan American. In a first for commercial operation, these offered both variable pitch and full-feathering which meant that in the event of an engine fault the three blades of the diameter propellers could be turned directly into the airflow, and so would not rotate and risk further damaging the engine by “windmilling”. Turning the blades into the airflow also reduced drag which was important on long flights over water. On the GR2600A2 engines, the propellers had a diameter of , while on the GR2600A2A, they had a diameter of .Griffith, p. 6 When installed in 1940, they featured the first NACA propeller aerofoil sections on a commercial aircraft.Trautman, p. 59
The 314 featured the first use of rate of flow fuel meters when they were installed in 1939 at the engineer's station as a permanent fitting on an aircraft. Previously they had been only installed as a temporary device during testing. Pan American had supported their development.
Fuel capacity To fly the long ranges needed for trans-Pacific service, the 314 carried of gasoline. The later 314A model carried a further . A capacity of of oil was required for operation of the radial engines.
Wings The wings were constructed in five sections of an aluminum outer skin formed over spars constructed out of bolted square aluminum alloy tube trusses while the ribs were formed out of aluminum alloy tubes and channels. They had a NACA 0018 airfoil profile at their root which changed to a NACA 0010 profile at their tip. The outer ends of the wings were made watertight, which allowed them to provide flotation if the aircraft heeled over in rough conditions.
The wings were thick enough to incorporate a crawlspace, through which each engine nacelle could be accessed by a flight engineer. After removing the two sections of the stainless-steel firewall behind each engine, they could service and if possible repair it in flight, as the fully-feathering propellers made it possible for an engine to be completely shut down. Between June 1939 and June 1941, 431 in-flight engine repairs were performed. The most common problem was fouled spark plugs.
Each nacelle had a side access door on either side which opened downwards and which could be used as work platforms. Each aircraft also carried portable work platforms that could be hung on the outside, forward of the access doors to allow work on the engines. The center sections of each wing contained two fuel tanks as well as compartments for transport of cargo, in particular mail.
The wings were fitted with metal-framed fabric-covered "split" type flaps located under each wings trailing edge. When taking off the flaps were set to 20 degrees and when landing their full deflection of 60 degrees was typically used.