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Aviation History
1919
1919 - 0874.PDF
DESIGN AND CONSTRUCTION OF FLYING BOATS' By Captain DAVID NICOLSON Introduction DURING the War information regarding the progress in design and details of construction of aircraft was rigidly guarded by the Defence of the Realm Act regulations, con sequently little has been published on the subject of flying- boat design or construction, and what follows may, to a certain extent, remedy the deficiency at least from the practical standpoint. Within the limits of a paper it is not possible to treat the subject exhaustively, and I merely propose to deal mainly with the construction of the larger types of flying boats, known officially as F.2A, F.3, F.5, P.5, and N.4. Of these, the F.2A has the smallest hull, which measures 42 ft. 6 ins. in length and 10 ft. in breadth, and is 2 ft. 6 ins. shorter than the F.3 and F.5 types. The P.5 and N.4 types, patented by Major Linton Hope, are entirely different in design and construction from those above mentioned. Being of circular cross-section, with fair and easy lines, they offer much less air resistance, con sequently with the same horse-power are driven at higher speeds; they are much stronger weight for weight than the F. type, more seaworthy, and generally show the impress of the trained naval architect's hand. In comparison, the F. type follow more closely the aeroplane body, an attempt having been made to attach a flying-boat fore body to a fuselage tail. Compromises are seldom suc cessful ; the tail proved to be both weak and unseaworthy, and it was found necessary to strengthen it considerably by additional planking. As, however, this class was already slightly tail-heavy, the additional weight did not improve matters. Having rectangular sections, they were actually weaker in transverse strength than the Linton Hope circular design. Development of Flying-Boat Design and Construction The first flying boat in the British service appears to have been the " Donnet Leveque," a French design, which was followed by the Sopwith bat-boat and the Curtiss small " America," which developed later into the White and Thompson and the Norman Thompson (N.T.2B) types. A larger Curtiss boat, with twin engines, built in the United States, was in use for the North Sea patrol, and was probably the worst example of boat building that could be imagined, it having had no less than four consecutive planks butted— not even scarphed—on the same timber, which had a siding of only £ inch, the line of butts being in line with the step, where the boat was naturally weakest. Other early boats were the Saunders small " America," midway in size between the 24 ft. 6 ins. White and Thompson boat and the 33 ft. twin-engined Curtiss, the Nieuport, and the F.B.A. The two latter were of French design, and the last-mentioned was largely used for training purposes ; in this respect also the N.T.2B has done excellent service. The White and Thompson boats were built by Messrs. Saunders, of Cowes. These and other hulls were excellent examples of good work, but the system of their multi-skin- sewn construction proved inefficient, as the wire sewing, sunk flush with the surface of the outer skin, cut into the soft cedar when the boat was subjected to severe landing shocks, etc. Another source of trouble was the combination of a more or less flexible structure, with a series of rigid bulk heads, which did not prevent movement of the parts of the structure between them, with the result that distortion occurred, resulting in considerable damage. The Curtiss boat, as originally built, was wall-sided, t.e, without projecting side fins to the front step and planing bottom, but as it experienced difficulty in getting off the water, the breadth of the step was increased by adding fins, thereby improving the planing efficiency, although the fins in service were very liable to damage. A number of modifications of the Curtiss design were built at the Royal Naval Air Station, Felixstowe, to the plans of Col. Porte, who was responsible for the design of the first large flying boat, known as the " Porte Baby,'" which was built by Messrs. May, Harden and May. Although these " Porte Babies " were all constructed under the best condi tions with respect to material and workmanship, they were inherently weak in the bottom, especially at the step, o wing to the faulty design of the keel, which was of spruce, and very small in section at the tail. American-built Curtiss boats of large size were later on imported in considerable numbers, but although the work- * Paper read before Institution of Engineers and Shipbuilders in Scotland. manship and materials were much improved, they were still weak owing to a lack of continuity in the structural design, and frequent damages were reported, until Lieut.-Com. Hope- Vere (then in command of the Royal Naval Air Station at the Scillies, where these boats were kept afloat at moorings) stiffened up the bottoms with bent timbers and stringers. Boats of the F. type which followed were a great improvement on the Curtiss type, and were fitted with engines of con siderably greater power, but, as in previous boats, the bottoms gave trouble owing to the faulty keel and floor construction. While these various types of boats were being built at Felixstowe and in the United States, the 30 ft. A.D. boat was designed at the Admiralty, and a number of hulls were built. This was the first flying boat to be built on the system of resilient construction, and the hull proved to be extremely strong. Shortly afterwards, during the early summer of 1916, a 41 ft. boat was designed by Major Hope and a model tried in the William Froude Tank at the National Physical Laboratory, Teddington. The chief feature of this design, apart from form and light hull construction, was the much greater proportion of power to weight (15 lbs. per horse power) than in the existing boats, and there is little doubt that this boat would have given a performance far ahead of any existing boat or seaplane, with the exception of the little Sopwith " Schneider." Unfortunately, there was strong opposition to this design, and it was rejected in 1018. The " Phoenix Cork " was built to a somewhat similar but improved design on slightly increased dimensions, and proved very successful, being far ahead of anything previously accomplished. Construction. Type F. The construction of the F. type is on the box-girder prin ciple, with four longerons running right fore and aft. The keel, another fore and aft member, runs from the sternpost right round the nose to form the stem, and finishes at the gun ring. The keel and keelson combined with the floors form the backbone of the hull. That the keel is of faulty design is the opinion of the writer, for many boats were found to leak badly, partly due to the bad connection between the keel and bottom planking, and partly because the keel is too narrow. The keel and planking are fastened as in Fig. I, but only one row of brass screws secure the bottom planking to the keel in the hulls of the F.3 type. Another weak point is the discontinuity of transverse strength caused by running the timbers down to the keel and stopping them there, no provision really being made to hold the centre girder to the bottom planking or sides of the hull. In the F.5 type the timbers are continuous from fin chine to fin chine, as in Fig. 2, forming a much stronger combina tion. The keel was increased in depth to i||ins., the width being kept the same, namely, 2$ ins. It would have been much better if the siding had been increased instead of the moulded depth. The bottom planking is again only fastened with one row of brass screws, which is insufficent. However, a great improvement was made by the continuous timbers, as they were through fastened to the keel by copper rivets. Keelson.—The keelson is fastened to the keel in the F.3 boats by brass screws 18 ins. apart, but in the F.5 types the spacing is 9 ins., and the screws are hove up through bottom of the keel. Although the keelson in some parts measures nearly 12 ins. in depth, it is greatly weakened by having one-third of the depth cut out to accommodate the floors. Floors,—It will be seen from Fig. 3 that two-thirds of the depth of the floor is cut away, thereby sacrificing strength to accommodate the keelson. A built-up floor, as in Fig. 4, would certainly be much more eflective, and would be pro bably 40 per cent, lighter. Timbers.—The timbers are of American rock elm, | in. by in. spaced 4 ins. apart amidships, increasing to 7 ins. at the ends of the boat, and terminate at the lower chine ; but it would have been an improvement had they been carried to the upper chine. The timbers are through fastened to the keel by copper rivets. Bottom Planking.—This is arranged on the diagonal system, the inner skin being of cedar J in. thick at the ends and £ in. thick amidships, fitted at an angle of 4;0 inclination to the keel. The outer skin is of mahogany, -fa in. thick forward, ft in. thick amidships, and % inch thick aft, the planking being at an angle of 300, with the forward end of the planks butting against the keel. This arrangement tends to diminish surface friction in wake of the seams. A layer of varnished fabric is fitted between the two skins, making the structure very strong. The planking is fastmed
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