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Quest for Performance: The Evolution of Modern Aircraft


Part I: THE AGE OF PROPELLERS


Chapter 8: Boats in the Sky


Two Amphibian Developments



[183] The amphibian flying boat is a unique type of aircraft equally at home operating from land or sea. Such aircraft trace their origins to early experiments by Glenn H. Curtiss, but the type did not gain popularity until the introduction of a highly innovative design produced in 1924 by the Loening Aeronautical Corporation. The Loening aircraft provided performance comparable to that of a landplane of similar size and performance, the DH-4, but offered the added capability of operation from either land or water. In an era of few airports, this versatility, coupled with high performance, was greatly appreciated. A photograph of a Loening OA-1C amphibian is presented in figure 8.10, and pertinent characteristics of the aircraft are given in table IV.

[184] The Loening amphibian appears much as a conventional single-engine biplane but with the lower part of the fuselage configured as a single-step hull that extended forward of the engine and propeller. As can be seen in figure 8.10, the wheels could be retracted into cavities located on the sides of the hull for water operation. The entire configuration concept was made feasible by an inverted V-12 Liberty or Packard engine that placed the thrust line sufficiently high so that the propeller cleared the forward-projecting hull without, at the same time, causing a fuselage of excessive bulkiness. The two-bay biplane wing configuration was typical of the time period but was unique in utilizing the N-type interplane struts that eliminated much time-consuming effort in rigging the wings. The tip-mounted lateral stabilizing floats also incorporated skids to prevent damage in landplane operations.

The hull was of wood-frame construction covered with aluminum alloy sheets. Wooden spars and stamped aluminum alloy ribs comprised the wing structure, which was covered with fabric. Hand operation was required to retract the landing gear in early models, but actuation by an electric motor was provided in later versions. Open cockpits configured for two or three occupants were usually provided.

A comparison of the physical and performance characteristics of the OA-1C given in table IV with those of the DeHavilland DH-4 in table 1 indicates that the two aircraft are closely similar in size, power, weight, and performance. The added versatility of the Loening amphibian....




Figure 8. 10 - Loening 0A-1C amphibian; 1924. [USAF via Martin Copp]


[185]...assured it of an important place in aeronautical activities in the 1920's and 1930's. Versions of the Loening amphibian were built not only by Loening but, after the demise of that company, by Keystone and even later by Grumman. The modernized version built by Grumman was in production well into the World War II years. These latter aircraft, designated Grumman JF and J2F were known as Ducks. All later versions of the aircraft employed radial air-cooled engines instead of the earlier water-cooled Liberty and Packard power plants. All three services of the United States-Army, Navy, and Coast Guard-as well as a number of civil operators utilized the aircraft. A civil version having a single cockpit and accommodations for four to six passengers in an enclosed cabin was available in addition to the military models. Loening amphibians participated in several record and exploratory flights that, together with descriptions of the aircraft and its development, are given in references 88 and 89.

Today, the name Igor Sikorsky is usually associated with the pioneer development of the helicopter; but in earlier years, he was known as the father of a number of multiengine aircraft, including several excellent flying boats and amphibians. One of these, the S-38 amphibian, first appeared in 1928 and established the great Russian designer in the United States. Serving in a number of pioneer airline operations, notably Pan American Airways, the aircraft was also used by various military services and in several exploratory operations.

A side view of the S-38 is shown in figure 8.11. In configuration concept, the aircraft was reminiscent of the NC boats described earlier and consisted of a short hull with the tail assembly attached to outriggers extending from the upper wing of the sesquiplane wing arrangement. Power was supplied by two Pratt & Whitney nine-cylinder radial air-cooled engines mounted side by side in nacelles located between the wings. A large number of struts integrated the hull, wings, tail assembly, and engines into a unified configuration. Lateral stabilizing floats were located beneath the tips of the lower wings, and the landing wheels retracted into the sides of the hull. A unique feature of the landing gear was the capability for lowering the wheels individually. One might question the advisability of such an action, but turning moments were produced by the water drag of one extended landing gear; thus steering capability, always a problem while maneuvering on the water at low speeds, was enhanced. Another interesting feature was incorporated in the vertical tail surfaces that were cambered and laterally spaced so that one surface was in the slipstream of each propeller. In the event of the failure of one engine, the cambered surface in the slip- stream....




[186] Figure 8. 11 Sikorshy S-38 10-passenger amphibian; 1928. [Joseph P. Juptner via AAHS]


....of the other engine produced a yawing moment opposite in direction to that caused by the power loss. The relative magnitude of the two opposing yawing moments is not known.

Ten passengers and a crew of two could be accommodated in the short, single-step hull formed from a wooden frame covered with aluminum-alloy sheets. Before passage through the hump speed, large quantities of spray passed through the propellers and covered the windshield, effectively blinding the pilots. Although the aircraft was built for a number of years and various fixes were attempted, the spray problem on the S-38 was never effectively solved. Wings of the aircraft consisted of a metal structure covered with fabric.

According to the data in table IV, the S-38B had a gross weight of 10 480 pounds, a wing span of 71.7 feet, and a top speed of 125 miles per hour. Although the Sikorsky S-38 was somewhat smaller than the Martin PM-1, the performance of the two aircraft was nearly the same. The values of the zero-lift drag coefficient and the maximum lift-drag ratio were not those of an aircraft known for outstanding aerodynamic efficiency, but the ruggedness and operational flexibility of the S-38 made it well suited for many diverse roles.

[187] About 120 examples of the S-38 were constructed, and its configuration served as a basis for the single-engine S-39 and the large four engine S-40 flying boat operated by Pan American Airways. The S-41 was a further refinement of the twin-engine S-38, but the high-drag configuration of this series of aircraft was abandoned in favor of arrangements of higher aerodynamic efficiency in subsequent Sikorsky flying boats and amphibians.