however are relatively independent of scale as for cavitating flows. The whole subject 

 is therefore amenable to a better understanding and theoretical treatment before extra- 

 polations of model data involving ventilated flows can be made with confidence. 



Take-Off Resistance 



Typical take-off resistance curves for three types of water-based aircraft are 

 shown in Figure 16. In order to make them comparable, the ordinate is the water 



D-558-H WITH SKIS 

 A - 13,140 LB, V G *- 145 KTS 



R+D 

 A^ 



L/b - 15 LONG A. B. HULL 

 - 75,000 LB, V G = 130 KTS 



C-123 PANTOBASE 

 A = 50,000 LB, V G = 75 KTS 



Z 



.4 .6 



V/V G 



.8 



1.0 



Figure 16. Take-off resistance of water-based aircraft. 



resistance plus air drag of the configuration divided by the gross weight, and the abscissa 

 is the ratio of water speed to getaway speed in each case. The curves are for a high 

 length-beam ratio, long-afterbody, hull-type seaplane with a take-off speed correspond- 

 ing to wing parameters of current interest (unpublished data); the original application 

 of twin hydro-skis to a transonic research airplane (ref. [10]); and the pantobase C-123 

 assault transport (ref. [12]). None of these curves are necessarily optimum, but they 

 represent operable arrangements for widely different purposes. 



The hull configuration has low resistance at low speeds with a hump resistance 

 of 22 percent of the gross weight or an average L/D up to 0.8 V G of 4.5. The pro- 

 nounced second hump near take-off rises to over 30 percent of the gross weight. A 

 part of this rise is attributal to the increasing air drag with speed, but it is largely an 

 increase in water resistance due to wetting of the long afterbody by the forebody wake. 



The hydro-ski D-558 has a very high resistance, 45 percent of the gross weight, 

 at speeds near ski emergence. As mentioned previously, this low-speed peak is charac- 

 teristic since it is related to a small ski size and a deliberate disregard of hydrodynamic 

 concessions to take-off resistance that would increase ski size and weight or the drag in 

 flight. Lower peaks have been obtained, but this type of ski application is properly 

 associated with airplane take-off thrust-weight ratios of 0.4 or greater. The high-speed 

 resistance is generally higher than for the hull because of high ski trims and wetting 

 of airframe components; however, there is a significant improvement near take-off 

 where the afterbody clearance is greater than can possibly be achieved with a hull. 



199 



