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Quest for Performance: The Evolution of Modern Aircraft

Part I: THE AGE OF PROPELLERS

Chapter 8: Boats in the Sky


Biplane Flying-Boat Developments, 1920-30



[179] The slow pace of aeronautical development during the first half of the 1920's was briefly mentioned in chapter 3. If anything, technical advancements in flying-boat design during this period lagged behind [180] those of contemporary landplanes and did not experience the rapid acceleration that characterized landplane developments in the latter half of the decade. The biplane configuration dominated flying-boat design, and most efforts were aimed toward military applications. In the following section, developments in Navy patrol boats are discussed, after which two significant amphibian aircraft developed in the 1920's are described.

The Refined Patrol Boat

According to reference 118, the United States Navy had an inventory of 1172 flying boats at the termination of hostilities in November 1918. By the middle of 1925, this number had shrunk to 117 aircraft consisting of outdated wartime H-16, HS-2L, and F-5L boats. Fortunately, the stagnation in military flying-boat development was relieved, to some extent, by the Naval Aircraft Factory, which continued design refinement of the biplane flying boat. The wartime F-5L, redesignated PN-5, formed the starting point of these activities, which resulted in a number of improved aircraft designs. Until 1928, prototypes of each of the improved designs were built in the limited facilities of the Naval Aircraft Factory, but lack of funds prevented placing production contracts with industry.

One prototype, the PN-9, was a much refined development of the PN-5 equipped with Packard engines and a hull of aluminum alloy construction. This aircraft achieved a dubious place in aviation history by its failed attempt to fly nonstop from San Francisco to Hawaii. With a crew of five under the command of Commander John Rodgers, the PN-9 left San Francisco on August 31, 1925, and came down at sea after flying 1841 miles, a new distance record, about 200 miles short of Maul, Hawaii. A higher than expected fuel consumption, coupled with the lack of anticipated tail winds, resulted in fuel exhaustion and the unanticipated landing at sea. Crude sails were fashioned from fabric torn from the lower wings, and the flying boat was literally sailed 450 miles to the Island of Kauai, which was sighted on the 10th of September. (Marginal steering capability prevented the aircraft from reaching the much nearer island of Maui.) Whatever may have been lacking in flight planning or in understanding of engine performance, the seaworthiness of the new all-metal hull and the seamanship of the crew were clearly demonstrated by this remarkable venture. Reference 80 is cited for a succinct description of the flight.

By 1928, the Navy had both a flying-boat design that it liked, the PN-12, and sufficient money to place production contracts with several [181] aircraft manufacturers. Martin, Douglas, Keystone, and Hall Aluminum ultimately participated in the program, and aircraft produced by these companies were designated PM, PD, PK, and PH, respectively. The aircraft produced by each company were based on the Navy-designed PN-12, but they differed from this aircraft and from each other in a number of details that are not discussed here. Detailed descriptions of each of the aircraft can be found in references 109 and 118.

Typical of the patrol boats produced in this program was the Martin PM-1 illustrated in figure 8.8. The aircraft had the classic twin-engine biplane configuration, similar in concept to the F-5L, but was much cleaner than the earlier aircraft. The number of drag-producing interplane struts and wires had been reduced, and the tip-bracing arrangement on top of the upper wing had been eliminated. Neatly cowled nine-cylinder radial air-cooled engines on the PM-1 contrasted with the exposed in-line engines and clumsy radiators on the F-5L. The data in table IV show that the PM-1 had more power, was somewhat heavier, and had slightly less wing area than the F-5L it was designed to replace. The Martin, however, was nearly 30 miles per hour faster than the earlier aircraft and had a 31-percent lower zero-lift drag coefficient. The higher value of the lift-drag ratio of the F-5L resulted from the higher wing aspect ratio of this aircraft as compared with the PM-1. The lower aspect ratio of the Martin boat probably resulted from a design trade-off between aspect ratio and empty weight, in....




Figure 8.8 - Martin PM-1 Navy patrol boat; 1929. [USN via Martin Copp]



[182]....combination with the effect on C_D,O of more interplane bracing which would probably have been required for higher aspect ratio wings. The hull of the PM-1 was a refined version of the sponson-type, two-step employed on the F-5L and was of all-metal construction; the wings were of metal-frame structure covered with fabric and had thicker airfoil sections than those used on the F-5L.

Flying boats based on the PN-12 design, such as the PM-1, served the Navy until well into the 1930's. The U.S. Coast Guard also employed these aircraft and ordered several of those produced by the Hall Aluminum Company, the PH-3, as late as 1938.

The last and also the largest and highest performance biplane flying boat developed for the U.S. Navy was the Hall XP2H-1 shown in figure 8.9. (The largest biplane flying boat ever built was the Short Sarafand launched in England in 1932. It had a gross weight of 70 000 pounds, a wing span of 150 feet, and was equipped with six engines, mounted pusher-tractor style in three nacelles, totaling 5500 horsepower. The Sarafand, after service with the Royal Air Force, was scrapped in 1936.) The XP2H-1 was equipped with four in-line Curtiss V-12 engines of 600 horsepower each; the engines were configured in a ....




Figure 8.9 - Hall YP2H-1 four-engine Navy patrol boat; 1932. [USN via Martin Copp]


[183] ... pusher-tractor arrangement in two streamlined nacelles mounted atop support pylons. The wings of the single-bay biplane were tapered, had a metal internal structure covered with metal sheet, and were braced with struts and wires. The single-step hull was of all-metal construction and had enclosed accommodations for the crew of six. Figure 8.9 shows a complex tail assembly featuring two fins and rudders mounted on top of a single horizontal surface, which in turn was attached to the hull by a single, low-aspect-ratio fin or pylon.

The data in table IV give a gross weight of 35 393 pounds for the XP2H-1 and a maximum speed of 139 miles per hour. The zero-lift drag coefficient of 0.0291 was about 40 percent lower than that of the Martin PM-1, and the maximum lift-drag ratio of 10.2 was about 30 percent higher than that of the Martin PM-1. At a much reduced speed, maximum endurance could be obtained by cruising on two engines, and the maximum range was estimated to be 4560 miles. Although the Hall XP2H-1 had very creditable performance, it arrived on the scene too late to compete effectively with the new monoplane flying boats that began to appear in the 1930's.

Ordered in 1930 and delivered to the Navy in the fall of 1932, the XP2H-1 was a one-of-a-kind aircraft. In 25 hours and 15 minutes, the aircraft made a notable nonstop flight from Norfolk, Virginia, to Panama in 1935. The pilot on this flight was Lt. John S. Thatch, who was destined for fame in World War II. The aircraft met an ignominious end later in 1935 when it sank during an open-sea landing attempt. The XP2H-1 represents the last in a long-lived line of United States designed biplane flying boats.