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Aviation History
1925
1925 - 0585.PDF
SEPTEMBER 10, 1925 flying boats hitherto built weighed about 30,000 lbs., andthe lecturer expressed the opinion that there was a' con- siderable way to go yet before steel could supplant duraluminfor hull construction. He hazarded the prophecy that in a 60,000-lb. boat steel would be inefficient from the weightpoint of view, but it would be likely to pay in the case of a 100,000-lb. machine. He pointed out that in saying thishe was assuming that all difficulties in the manufacture and working of stainless sheet steel would by that time havebeen removed. The actual constructional design of metal hulls was beingtackled in two distinct ways. In this country we had more or less adhered to recent wood practice by building the hullon transverse frames with longitudinal members, the trans- verse frames being made of duralumin sheets and varyingin depth from a few inches to as much as 2 ft. on the keel amidships. The longitudinals could conveniently be made in U-sectionstrip flanges being provided at the top of the U for riveting the plates. In the designs of Dr. Rohrbach the transverse section of the hull had been made rectangular and the planing bottom forward;of the main step was flat. These external lines clearly indicated a girder construction with the outside Aerodynamic ImprovementTurning to the aerodynamic functions of the flying boat, the lecturer said that aerodynamically improvement hadbeen effected in aerofoil characteristics and in the reduction of parasite drag. Concerning the former the lecturer referred to two newaerofoils recently designed at the Royal Aircraft Establishment of which the section known as the R.A.F. 30 had a centreof pressure which was constant to within 2 per cent, of the chord throughout the normal flying range, and the R.A.F. 33which had a centre of pressure movement of 6 per cent. R.A.F. 30 suffered, however, from a low value of the maximumlift coefficient, which was only 0-46. The maximum lift coefficient of R.A.F. 33 was 0-61. The drag of this sectionwas greater than that of R.A.F. 30. II from these aerofoils there could be designed a new section having a maximumlift coefficient of at least 0-6 and a C.P. movement between kL 01 and kL max. of about 12 per cent, and a minimumdrag approaching that of R.A.F. 30, then something very material would have been achieved. In the reduction of parasite drag the utilisation of thickerwing sections had been of the utmost importance, and it had been left to Dr. Prandtl of Gottingen University toshow that aerofoils could be designed having twice the THE ROHRBACH ALL-METAL FLYING BOAT : The methods of construction employed were referredto by Mr. Simmonds in his paper before the British Association. plating bracing the bays. One advantage of this method was the ease with which transverse bulkheads could be fitted. Bulkheads had been successfully designed for flexible hulls, but they must necessarily be flexible in themselves, and this represented an extra weight over the bulkheads in the rigid boat where the bulkheads were definitely structural members. Even with the small amount of experience possessed they had been able to build good and serviceable duralumin hulls for the same weight as similar wooden hulls, and indeed some designers considered that by using duralumin they had reduced their hull weight, exclusive of hull soakage, by as much as 20 per cent. It was difficult to obtain true comparative figures, but at least it could be said that in choosing duralumin construction an economy in weight as great as 20 per cent, might be expected even for small boats, -while in a flying boat of 60,000 lbs. gross weight the increase of thisjfigure to 30 per cent, was by no means impossible. The lecturer then referred to the difficulty of avoiding •entirely the use of steel bolts and fittings, and pointed out that use of the two materials together might set up electrical tension and corrosion. Protective painting and the use of red lead helped somewhat, but even when every precaution was taken, working between parts was certain and corrosion then became only a matter of time. In this particular respect the stainless steel hull would have very definite advantages. In the meantime it was necessary to show great vigilance in ground inspection of points of contact. relative thickness of the normal thin section without loss, and in certain cases with considerable gain of aerodynamic efficiency. It was in the case of the monoplane that the efficiency of the thick wing was most marked, as it might be possible to dispense with all external bracing. The quantitative values of the absence of bracing was shown in the following table which detailed the resistance of the various component parts of a biplane flying boat of clean design :— Per cent, of total drag. Wings . . . . . . . . . . 40 Wing bracing.. .. .. .. 18 Hull and floats .. . . . . 15 Tail planes, fins and rudder. . . . 9 Power units . . . . . . . . 18 The lecturer pointed out that in practice the saving indicated was not fully obtained, as the monoplane for the same wing area required a greater wing chord than the biplane to obtain the same degree of longitudinal control, and it was therefore necessary either to lengthen the hull or increase the area of the tail surfaces, either of which would increase resistance. For the same performance as the biplane, the internally- braced monoplane wing reduced the horse-power required by no less than 15 per cent. Conversely, for the same engine power the top speed was increased by 6 or 7 per cent, and the climb by about 15 per cent. 585
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