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Aviation History
1925
1925 - 0584.PDF
(1) Static trim ; (2) Hump resistance ; (3) Steady running ; (4) Cleanliness. 1. Static Trim.—It was, the lecturer said, desirable when at rest to keep the centre of buoyancy well forward so that the aircraft trims up by the bows. This enabled a boat at anchor to ride well and cleanly, and when opening out the engines, it prevented the thrust moment of the propellers from nosing the hull under water. 2. Hump Resistance.—Hump resistance was originally a point of prime importance to the designer of a flying-boat hull. A slide was shown which indicated that the maximum resistance occurred at a speed of somewhere between twenty and thirty knots, and unless there was a margin of thrust over drag at this critical speed, the machine could not be SEPTEMBER 10, 1925 in use, but all relied upon some form of transverse frame or hoop with longitudinals placed at about 6-ins. centres. Hulls built in this manner had only one great drawback—namely, their absorption of moisture. In a wooden hull weighing 2,000 lbs. the soakage in three months would be of the order of 300 lbs., i.e., 15 per cent, of the hull weight or 2 per cent, of the total weight. It was, the lecturer stated, by no means a pleasant thought that they were carrying about 300 lbs. useless weight, and this fact had been one of the greatest inducements to the substitution of metal for wood in hull construction. Moreover, the wooden hull had so nearly reached the limit of its logical development that for increased efficiency they must turn to other materials. A table was then given showing the THE SUPERMARINE "SOUTHAMPTON": Note the clean running of the machine, reference to which was made in the paper read by Mr. Simmonds. The engines are Napier " Lions." further accelerated. Two factors had combined tr- remove this difficulty : a decreased resistance due to an improve- ment in hull lines, and a lower power loading which was the result of a demand for higher performance in flight. 3. Steady Running.—Steady running was imperative, firstly, because even small changes of angle induced a very large increase in the area of immersion, and secondly, because the longitudinal oscillations were likely to build up until the aircraft was thrown into the air by the water forces before it had attained sufficient speed for the wings to sus- tain it. This vice was known as " porpoising," but experience had now enabled them to entirely obviate the trouble by the suitable location of two steps in the planing bottom of the hull. This enabled aircraft to leave the water at a speed very little greater than the minimum necessary for complete sustentation, and had the added advantage of relieving the pilot of the responsibility of lifting the machine off the water by means of his elevator. 4. Cleanliness.—Cleanliness in taxying on the water and during the get-off and alighting runs was desirable from many points of view. Dirtyness inevitably meant high resistance, and therefore long runs to take off, and it also caused discomfort to the crew and might even break the propeller-blades. Accurate bow design and suitable shape of the planing bottom and chines now serve to get the water well down and out, and a slide was shown of the Supermarine Southampton " taking off, in which the remarkable clean- ness in running was well illustrated. This all-round improvement of water performance combined with increased strength and ability to ride out heavy seas, had been secured without increasing the weight economy £ Weight of hull figure, Weight of aircraft. Hull Construction i Turning to the subject of hull construction, the lecturer referred briefly to the earlier flying boats, which had more or less rigidly braced hulls, and said that the new conception of a boat which would fly meant abandoning this rigid braced hull and supplanting it by a flexible boat-built construction in which timber was utilised throughout, and a multitude of . wires and small metal fittings were entirely obviated. Mr. Simmonds said it was not possible in his paper to enter into the details of flexible wooden hulls, but the main principle was that the shell, which was usually composed of two skins of mahogany planking, was relied upon to transmit the loads. Various methods of supporting and stiffening the shell were relative efficiencies of various suitable metal and alloysheets :—. Material. DuraluminStainless steel AluminiumMild steel Timbers— Max. stress tons'sq, in 27 40 10 26 Max. comor Max. stress, density. 9-6 5-1 3-673.3 es- Percentage efficiency. 100 53 38 34 sive stress, tons sq. in. Spruce .. .. 2-23 5-2 54 Mahogany .. 2.68 5-0 52 The use of the elastic limit instead of the maximum stress did not alter the relative positions of the metallic materials, although clearly it considerably increased the efficiency of the timber, as shown in the following table :— Mater al. DuraluminStainless steel Aluminium Mild steel Timbers— Spruce Mahogany Elastic limit, tons/sq. in. 13-5 32 • 0 7-5 13-0 2 -23 2-68 Elastic limit, density. 4-8 " 4-1 2-75 1-65 5-2 5-0 Percentage, efficiency. 100 85 57 34 108 104 In each case the lecturer had assumed 100 per cent, effi- ciency for duralumin. From the tables it was seen that duralumin outclassed stainless steel in strength for weight, but in other respects stainless steel appeared to have definite advantages. At this stage in the development of stainless steel the degree of malleability that could be obtained was a little uncertain, but, on the other hand, its highly resistive qualities to the corrosive action of sea-water appeared to be well established. With duralumin, plating, painting, or varnishing gave a high degree of immunity from corrosive action, but the slightest puncture of the coating was liable to be a source of trouble. As far as the lecturer was aware all modern aircraft manufacturers in this country and abroad had, for the moment, turned their attention to duralumin in preference to steel. For the size of hull they were at present building the steel sheets for the skin of the boat would have to be thinner than was practically possible in order to realise the full comparative efficiency figure. The largest successful 584
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