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Aviation History
1920
1920 - 0592.PDF
JUNE 3, 1930 laying-off. With flexible construction the weight may be roughly assumed to be slightly under -i of the total flying weight, but of course a more correct estimate of the weight must be obtained when the design is far enough advanced to ascertain the skin area of the main hull and planing bottom. These areas are then multiplied by the weights per square foot given in the table of data for similar boats of approxi- mately the same size. As the weight of the whole structure quite as essential to the designer as model tests, which alone are insufficient. Air Resistance of Hulls.—The following tables show com- parative resistance in the wind tunnel of the F.3 and P.5, and one column also shows the latter with the angle between the main hull and the fin-top filled in to a fair curve. It will be seen that there was a slight advantage in this form at the lower speeds, but, at the highest speed, the difference zs 30 35 SPEED //Y KNOTS. Fig. 16.—A set of model resistance curves varies as the cube of the length, subject to a small correction is practically negligible. Various other models were tested f f h i i f b i th d th it f CE ltfor the variation of the proportionate increase of beam ~ from small to larger boats, it is a simple matter to find the weight per square foot for various sizes. Although 10 per ' cent of the total weight is given as a fair estimate of the hull, the examples in the table of data, such as No. 14 and 2so. 2<J, have hull weights of 9 pet- cent, artd 9-3 per cent, respectively, but the hull of No. 14 was too small for the final weight of 2,340 lbs., having been designed for a maxi- mum of 2,100 lbs. In the larger sizes it is possible to slightly reduce the proportion of weight of structure owing to the saving of weight of many details. The accompanying (p. 591) estimate of weights of P.5 not only gives full details for a boat of this size, but also shows the work in detail. Model Trials -~. ~'~ ** v? It is most advisable when designing a flying boat hull—or, in fact, any form of vessel departing from the standard type- that a model should be run in one of the experimental tanks; not only to deter- mine the resistance at varying speeds, but also to ascertain the performance of the model in the matter of wave making, " porpoising " (or longitudinal instability), and whether it will get off at the designed speed. Fig. 16 shows a series of model resistance curves got out by Major Bumpus, R.A.F., for a number of boats of various sizes, from the 45 ft. F.3 and P.5 of about 12,000 lbs. displacement, up to an 84 ft. model designed for 72,000 lbs. flying weight, of which three curves are shown loaded to 60,000, 80,000 and 100,000 lbs. All these curves are reduced to a common scale of resistance per 1,000 lbs. weight with a getting-off speed of 50 knots. Essential as they are, it should be borne in mind that tank tests only give results in smooth water. Consequently, due allowance must be made for sea conditions, and actual experience is py gg in the same manner, and the resistance of CE.i was lowest ; but this model was the only one of the series in which the cockpits were not cut out. The model with the largest cockpits (which were cut well down the sides) had con- siderably greater resistance than any of the others. Resistance of Flying Boats in Wind Tunnel, N.P.L. P.5 Experimental (model supplied).—The model was gouged in four places to a depth of about 0-15 in. to represent Fig. 17.—30-ft. AD boat under bending test cockpits ; the aft one was filled up with plasticine before commencing the experiments. Table I gives the resistance (R in lbs.) of this model at different wind speeds. The coefficient C=R/pV2A is also given where A is the maximum cross-sectional area of the model in ft2, p the density of the air and V the wind speed. Table II gives the resistance of a modified form of this model, viz., the F.E. experimental model, with the angle between the body and the chine filled in. 593
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