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Flying Boat Design Considerations

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.

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 the wings to minimize spray problems and reduce aerodynamic drag. Many flying boats have 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 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.

The voluminous hull is usually designed with from 70- to 100-percent reserve buoyancy. 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 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.

The flying boat 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 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 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 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.

At speeds well below and above the hump speed, a large margin exists between the drag and the engine 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.

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.

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