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Aviation History
1931
1931 - 0466.PDF
FLIGHT, MAY 15, 1931 THE DEVELOPMENT OF THE LONG-RANGEFLYING BOAT By MAJOR J. D. RENNIE, A.R.T.C., A.M.Inst.C.E., F.R.Ae.S. (Concluded from page 416) If we now compare these four types of stabilisers withrespect to weight, air drag, and water resistance, it will be found that the retractable inboard float type possessesan important advantage uncommon with wing-tip floats, and with little disadvantage in the other respects. With regard to weight and air drag, the order of merit is: — (1) Wing-tip floats.(2) Fixed inboard floats. (3) Retractable inboard floats.(4) Stubs. Now, other things being equal, the time to take off andthe maximum load which can be taken off depend upon the combined water resistance of the hull and stabilisers,and, taking the hull and wing-tip float arrangement as a basis, the order of merit in this respect is found to be,from the results of tank tests on the '' Sydney '' hull: — (1) Wing-tip floats.(2) Fixed inboard floats. -• (3) Dornier type stubs. With inboard floats or stubs, the increase in resistance,especially of the latter, was found to be considerable up to speeds about ten knots above the hump speed. Henceagain, the retractable inboard floats have the advantage inasmuch as the water resistance need be little, if any,more than for wing-tip floats, as, if the boat is on an even keel and the wing-tip floats clear of the water, thenthe inboard floats may be retracted and so offer no further water resistance. If the practicability and the advantages to be gainedby the adoption of retractable inboard floats be admitted, then we have progressed one step further towards sea-worthiness comparable with the corresponding surface craft. Having arrived at probably the best possible arrangementof hull stabilisers, we must now turn our attention to the remainder of the structure, as clearly, the arrangementshould be such that it is exposed to the least possible chance of damage concomitant with aerodynamic requirenients. To meet this requirement, the wings, propellers, and the tail unit should be placed as high above the hullas possible. This would seem to be the only common- sense arrangement, and the obvious thing to do. Yet astudy of the layout of many modern boats shows that an attempt to attain seaworthiness in this respect, either hasbeen neglected or the importance not realised. We are now in a position to specify the basic featureswhich must be incorporated in the design in an endeavour to comply with this specification. These may beenumerated as under: — • (1) Clean running deep " Vee " hull.(2) Retractable inboard floats. :.: (3) Propeller tips clear of spray and solid water. (4) High-wing monoplane. In the case of item (4), a biplane wing arrangementwould meet the requirement, provided the dimensions were such that the lower plane was at the same height abovethe sea level as the minimum water clearance found neces- sary for the monoplane wing.So far no mention has been made of size. Obviously, other things being equal, the larger the boat the greaterthe degree of seaworthiness. The question then arises, what is the minimum size of flying boat A reliable deci-sion in this respect can only be forthcoming as the result of years of accumulated experience in operations underall sorts of conditions. In these times progress must in- evitably be slow, because, owing to the present world-wideacute economic situation, capital, either publicly subscribed or obtainable through the usual channels, is not available forthe carrying out of an extensive constructional and opera- tional programme. The rate of progress will depend, there-fore, in a great measure, on the ability of the engineer to make full use of all the experience available, and to arriveat the minimum economic size with the least possible expen- diture and delay. While it would be unwise to attempt to predict the mini-mum size to meet this specification, nevertheless, experi- ence would seem to indicate that the normal gross weightwould be between thirty to forty tons. If powered with six engines of about 800-1,000 h.p. each, giving an aggre-gate of about 5,000-6,000 h.p., the normal horse-power loading would be 13-14 lbs./h.p. If the landing speed benot less than 60 knots, then this h.p. loading should lead to a fairly high air performance. The possibility of design-ing to this horse-power loading will depend, of course, on the military load specified and the technique of the struc-ture design. Fig. 8 shows one version of a proposed monoplane designto fulfil the requirements of this specification. It may be- taken as a possible layout for the medium size, six-enginedflying boat outlined above. It will be seen all the funda- mental features determined have been incorporated in thedesign. The deep "Vee" hull bottom, without which no flying boat can have any pretence to seaworthiness, isshown as developed from extensive full-scale and tank tests. The hull is totally enclosed and structurally strongenough to withstand all reasonable sea conditions likely to be encountered. Retractable inboard floats are fitted asshown and operated by the pilot. The wing, engines, pro- pellers and tail unit are situated well above the hull deckand are clear of all solid water. The pilot's control bridge is arranged centrally above, and just forward of, the lead-ing edge of the wing, thus providing a unique view ahead and on the port and starboard bows. Immediately abaftof the pilot is the engineer's cockpit, from which there is access in flight to all engines. With regard to the question of accessibility of the en-gines in flight in order to carry out minor repairs or adjustments, there are two difficulties to be overcome if
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