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Quest for Performance: The Evolution of Modern Aircraft

Part I: THE AGE OF PROPELLERS

Chapter 8: Boats in the Sky


Design Considerations



[165] A flying boat must satisfy many of the same requirements for performance, efficiency, strength, and reliability as a landplane but, in addition, must possess some qualities of a boat in water and some qualities unique to the flying boat itself. It must be seaworthy, maneuverable, and stable on the water and have low water and air drag. The hull must be designed and the aircraft configured in such a way that the amount of spray passing through the propellers, striking the tail, and passing over the windshield is minimized. The hull must be designed with sufficient structural strength to withstand the various loads imposed by rough water in landing, taking off, and taxiing.

Some of the design features characteristic of a flying boat are illustrated by the Martin Mariner and Grumman Goose shown in figures 8.1 and 8.2, respectively. Both of these aircraft, which may be considered as relatively modern flying-boat designs, feature a high wing mounted atop a deep, voluminous hull, a high tail position, and wingtip stabilizing floats. In both aircraft, the engines are mounted in...




[166] Figure 8.1 - Design features of a World War II flying boat. [NASA]





Figure 8.2 - Design features of a flying boat. [NASA]


[167] ...the wings to minimize spray problems and reduce aerodynamic drag. The Mariner has a gull wing configuration with the engines mounted in the wing break to place them in a high position. The problem of spray ingestion by the engines and propellers is a basic design consideration in the configuration layout of a flying boat.

The tip, or stabilizing, floats are evident in both figures 8.1 and 8.2. These floats are necessary because the narrow beam hull coupled with a high center of gravity make the flying boat laterally unstable on the water. (In terms of naval architecture, it has a negative metacentric height.) The aircraft is usually designed so that it heels about 1° when one float touches the water. When laterally level as in takeoff from relatively smooth water, neither float touches the water. The floats are designed and mounted in such a way as to give a large dynamic lateral restoring moment when one float touches the water on takeoff or landing. Tip floats have historically been the most used form of lateral stabilization; however, a device called a sponson has sometimes been employed. This type of stabilizer is used on three of the aircraft described in this chapter.

The voluminous hull is usually designed with from 70- to 100-percent reserve buoyancy (ref. 46). When floating as a displacement boat, a 100-percent reserve buoyancy means that the hull will support twice the design weight of the aircraft without sinking. The reserve buoyancy is provided as a safety factor, particularly for operation in rough seas. The cross-sectional shape of the forward portion of the hull is usually in the form of a vee or modified vee. The outside angle of the vee is called the angle of deadrise. The larger this angle, the lower will be the impact loads imposed by operation in heavy seas. The friction drag on the forward part of the hull, however, increases with deadrise angle, as does the spray problem. The modified vee bottom of the Grumman Goose is clearly visible in figure 8.2. The intersections of the sides of the forward part of the hull with the vee bottom are called the chines and form a sharp angle. The design of the chines is important in determining the spray characteristics of the hull. To assist in controlling the spray, special spray strips are sometimes attached to the chines as shown by the experimental installation in figure 8.2.

The flying boat in figure 8.1 clearly shows the characteristic manner in which the hull bottom is separated by a transverse step into a forebody and afterbody. At low speeds the hull operates as a displacement boat with both the forebody and afterbody sharing the support of the aircraft in the water. Beyond a certain speed, called the hump speed (more about this later), the hull planes on the forebody [168] with the afterbody contributing little or nothing to the support of the aircraft. The step, acting somewhat like a spoiler on an airplane wing, causes the flow to break away from the afterbody and allows the boat to transition into the planing regime. The step is essential to the successful operation of the flying boat since lift-off from the water is normally not possible without it. This design feature was first introduced by aviation pioneer Glenn H. Curtiss. Two transverse steps have sometimes been employed in the design of flying-boat hulls, particularly on older boats. The more usual practice in later boats, however, is to taper (in planform) the afterbody to a point which effectively terminates the hull. The tail assembly is then carried on a fuselage extension above the hull. Some exceptions to this are pointed out later. The overall length-beam ratio of the hull as well as the value of this ratio for the forebody and afterbody individually are important design variables, as are the height and location of the step.

The design of the hull is important in determining the characteristics of the flying boat in all phases of its operation on the water. The importance of the hydrodynamic characteristics of the hull can be illustrated by considering the influence of hull water drag and aircraft weight on the takeoff distance and on the conditions under which the boat will not lift off at all. As is the case with a landplane, the seaplane must accelerate to a speed sufficiently high, determined by the wing loading and maximum lift coefficient, for the wings to support the weight of the aircraft in flight. The aerodynamic drag of the aircraft together with the rolling friction on the wheels on the runway constitute the resistance to acceleration of the landplane in its takeoff run. In addition to the aerodynamic drag, the flying boat must overcome the water drag associated with the hull. The manner in which this drag varies with speed makes the takeoff problem of a flying boat uniquely different from that of a landplane.

The variation of water drag with speed, along with the accompanying variation of engine thrust, is shown by the conceptual curves in figure 8.3 for a hypothetical flying boat. The water drag of the boat is separated into two distinct speed regimes. Below the hump speed, the speed for maximum drag, the aircraft is operating as a displacement boat with both the afterbody and the forebody assisting in providing the necessary buoyancy. Under these circumstances, the drag results primarily from the generation of water waves. At the hump speed, the boat may be thought of as climbing over its bow wave and beginning operation as a planing hull. In this latter regime, the weight of the boat is supported primarily by the dynamic reaction of the water against the....




[169] Figure 8.3 - Characteristic variation of water drag with speed for a hypothetical flying-boat hull.


...forebody, and displacement buoyancy is relatively unimportant. The water drag in this speed range results primarily from skin friction between the water and the forebody. In addition to the support provided by the planing forebody, an increasing proportion of the aircraft weight is supported by the wings until, finally, the water drag becomes zero as the aircraft lifts off.

Also shown in figure 8.3 is the hypothetical variation of engine thrust with speed. At speeds well below and above the hump speed, a large margin exists between the drag and the thrust. The thrust margin at the hump speed, however, is a minimum, as is the acceleration. If the thrust is less than the drag at the hump speed, takeoff will not be possible. In actual performance calculations, the air drag must be added to the water drag to obtain the total drag as a function of speed.

The magnitude of the hump drag together with its corresponding speed are obviously critically important in determining takeoff performance. For a hull of given geometry, these quantities are approximately related to the length of the hull by the principles of Froude scaling (refs. 46 and 116). According to these principles, the speed V and the [170] length L of two geometrically similar hulls at and below the hump speed are related as follows:



or



where the subscripts refer to the two different hull lengths. For a given value of the parameter,
the wave drags of the two hulls D₁ and D₂ are also related to the length as follows:





where W is the displacement of the hull in pounds and equals the weight of the aircraft when operating as a displacement boat. Thus, the values of speed and drag at and below the hump speed of one hull can be approximately translated to those of a similar hull of different length. The Froude relationships are of fundamental use in sizing the flying-boat hull and interpreting the results of hydrodynamic tests of model hulls. Clearly, the longer the hull, the higher will be both the hump speed and the corresponding wave drag.

In addition to the high drag associated with passage through the hump speed, a longitudinal pitching instability can occur. This instability is characterized by a pitch oscillation in which the boat rocks back and forth between the forebody and afterbody. A too-high or too-low pitch attitude can induce the onset of this instability. The range of stable pitch attitudes varies with speed and is a minimum in the vicinity of the hump speed. Thus, careful control of pitch attitude is required when traversing this critical speed range. The attitude at which the flying boat trims is influenced by both the aerodynamic and hydrodynamic design of the aircraft, the center-of-gravity position, and the pilot's manipulation of the elevator control.

The hydrodynamic characteristics of a flying boat, such as the variation of drag with speed just discussed, depend in a complex way on [171] the detailed configuration of the hull and have been the subject of much study and research. An extensive literature exists on the subject, as can be seen from an examination of technical indexes such as reference 74.

The large body of experimental information available on the hydrodynamic design of flying-boat hulls has been accumulated with the use of a specialized type of experimental facility called a towing basin, or towing tank. Such a facility can be likened to a very long, narrow, indoor swimming pool. The test model is towed in the basin by means of a powered carriage, mounted on wheels, which is located above and across the channel of water. The model is connected to the carriage by struts that contain instrumentation for measuring the pressures, forces, and moments of interest, as well as attitude and position of the model.

Since the latter part of the 19th century, towing basins have been used in the design of surface ships. Although early hydrodynamic studies of flying boats were made with the use of such ship facilities, they were unsuited for that purpose because of the large differences in speed and size between surface ships and flying boats. In 1931, NACA put into operation at its Langley laboratory a towing basin especially designed for the study of the hydrodynamic characteristics of seaplane hulls (ref. 116). This unique facility was 2020 feet long, 24 feet wide, and 12 feet deep, and when filled contained 4 000 000 gallons of water. The test carriage was capable of attaining a speed of 60 miles per hour. (To keep pace with increases in seaplane performance, the capabilities of the basin were expanded in 1936; the length was increased to 2920 feet, and the carriage speed was increased to 80 miles per hour.) Another feature of the Langley basin was the provision of apparatus for producing artificial waves for use in the study of the rough-water characteristics of flying-boat hulls. The Langley towing basin was employed both for basic studies related to hull design and for tests of specific flying-boat designs. During its active life, no large flying boat was built in the United States without supporting tests in the Langley facility. The basin was operated by NACA/NASA from 1931 until the end of the era of large flying-boat development in about 1960.

So far, little has been said about the aerodynamic drag of the flying-boat hull. Yet, this characteristic is critically important in determining the speed and range of the aircraft. Obviously, the drag of the large, bulky hulls equipped with steps and sharp chines tended to be higher than that of the fuselage of a well-streamlined landplane of comparable capability. In recognition of the need to reduce hull aerodynamic drag, both hydrodynamic and aerodynamic studies were made [172] at Langley of hulls that were systematically varied in shape. From such studies, the hull for a given application that represented the best compromise between aerodynamic and hydrodynamic performance could be identified, or at least the direction to take in hull development was indicated. Much progress was made in the reduction of hull aerodynamic drag, while at the same time, acceptable hydrodynamic characteristics were maintained. The high length-beam ratio hulls developed late in the era of the flying boat (ref. 124) represented a large step in narrowing the gap between seaplane and landplane performance.

This has been a necessarily brief discussion of some of the elements of flying boat design. More complete discussions are contained in references 46, 55, 97, and 123, and a discussion of the special problems encountered in piloting a flying boat is contained in reference 81. In the next sections, attention is focused on the evolution of the flying boat in the United States.