The pantobase ski example has the benefits of a large existing fuselage with a 

 fiat bottom, large skis with side wings to add planing area, and a relatively low take-off 

 speed. These characteristics add up to favorable resistance levels as compared to the 

 ski D-558 at low speeds and to the hull seaplane near take-off. 



Hulls. — The nature of the afterbody interference problem when hulls are 

 applied to high-landing-speed aircraft is illustrated in figure 17. This is a plot of the 

 optimum (best trim) load-resistance ratio of a high length-beam ratio hull (ref. [32]) 

 plotted against the parameter \ZC A /C V which is the square root of the conventional 

 planing lift coefficient based on the square of the beam. When towing tank resistance 

 data at a given trim or at the best trim are reduced to this form, the points for various 

 loads and speeds fall along a singlecurvewhichextrapolat.es to zero at infinite speed or 

 zero load (ref. [33]). Corresponding data for a prismatic planing surface having the 

 same 20° dead-rise cross section with horizontal chine flare as the forebody of the 

 hull (ref. [14]), and obtained behind a wind screen to eliminate all air drag, show a 

 constant (A/R) max of about 6.0. Adding the air drag of the hull as determined from 

 separate wind tunnel tests (ref. [34]) to the drag of the planing surface gives the 

 intermediate curve, which also goes to zero at zero planing coefficient. Since the hull 

 data include the air drag of the model, the difference represents approximately the 

 decrement due to afterbody wetting. It is because of these two factors that hull efficiency 

 becomes progressively poorer at higher speeds and lighter loads. 



The results of a simple calculation of the water resistance plus air drag of the 

 hull alone for a high-speed water-based bomber, based on the tank data curve of Fig- 

 ure 17, are shown in Figure 18. A design beam loading Ca , of 6.0 was assumed, 

 giving a beam of 7.48 feet for a gross weight of 160,000 pounds. With parabolic 



C A 



unloading, the ratio of the water-borne load to the gross weight, , is shown by the 



Ca 

 dashed line. 



C R 



The corresponding ratios of hull resistance to gross load for various assumed 



values of take-off speed are shown by the solid curves. It is seen that for conventional 

 seaplane take-off speeds, the hull resistance near take-off is relatively low, in fact it 

 has not been a serious problem. At take-off speeds of current interest, it is three times 

 as high, while at take-off speeds which may be associated with supersonic aircraft it 

 could become as much as six times as high. It must be remembered that these curves 

 are extrapolations of the data shown, and are of value chiefly as the formulation of a 

 problem requiring further investigation. 



Tank data similar to that presented indicate that the resistance due to afterbody 

 interference increases with afterbody length-beam ratio, so that the problem is more 

 severe for high-speed hull applications than for low length-beam ratio hulls with short 

 afterbodies. The situation actually is similar to that of seaplane floats for small personal- 

 type aircraft, which also operate at high water speeds in proportion to their size, that 

 is at high Froude numbers, and whose water resistance is critical near take-off (ref. 

 [33]). The resistance in practice is not as high as indicated by tank data which is not 

 corrected for Reynolds number effects on skin friction, but experience with small 

 underpowered seaplanes has substantiated qualitatively the existence of the effects 

 described. 



Tank data for hulls of the planing-tail type having large afterbody clearances 

 (ref. [33]) lie very close to the forebody plus air drag-curve in Figure 17, indicating 

 one solution to the problem if such a hull can be integrated into the overall design in 

 other respects. A second method to increase clearance is the use of auxiliary skis at 

 high- water speeds, extended far enough below the hull for their wakes to clear the 

 afterbody. 



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