are practical and less or more so than for land-based operations appear of fundamental 

 importance, and worthy of intensive investigation. 



Spray clearances. — High-speed jet-propelled configurations are characterized by 

 high fineness ratios, small frontall areas, high hydrodynamic loadings, and little or no 

 spray clearance for vital components such as the engines themselves, the flaps when 

 extended, and the horizontal tail. This problem in some respects dominates the basic 

 design, although not necessarily adversely. Spray can be tolerated to some extent, and 

 spray damage can be minimized by auxiliary devices and operating techniques. It must, 

 however, be recognized and lived with as far as aerodynamic considerations permit, and 

 in the treatment of mechanical or electrical gear subject to wetting. 



Stability and control. — Adequate hydrodynamic stability and control during 

 water operation are relatively difficult to achieve because high-speed aircraft are never 

 too well off aerodynamically in these respects and their marginal characteristics reflect 

 on their water behavior more than is generally appreciated. In the case of new and 

 untried hydrodynamic design solutions, which abound in this era, the configuration 

 must be made operable before other characteristics can be evaluated or optimized. 

 "Runability" must therefore receive a great deal of attention in towing tank investiga- 

 tions, and is the basis for much of the work there with dynamically similar models. 



Take-off performance. — Take-off performance is defined in terms of the total 

 time and water distance required and of the ease of acceleration through critical spray 

 or stability regimes. The latter is probably of greatest importance since it determines 

 the adequacy of severly compromised hydrodynamic solutions found desirable from 

 flight considerations. High-speed aircraft are blessed with high thrust- weight ratios for 

 take-off; on the other hand, their high take-off speeds still prolong this phase of the 

 flight. With both thrust and wing loadings determined primarily from mission require- 

 ments, the actual take-off performance remains properly a dependent characteristic 

 except as it may be optimized in the detailed hydrodynamic design. 



Rough-water capabilities. — The surface of the water in nature is seldom per- 

 fectly smooth or, on the other hand, too rough for seaplane operation except in large 

 operating areas with long fetches. It is a moot question what should be demanded of 

 a high-performance water-based airplane in this respect. Even relatively small waves 

 and swells can inflict local damage on light aircraft structures, and at the high water 

 speeds for take-off and landing can impart excessive motions and accelerations to the 

 airplane as a whole. Adequate provision for these contingencies in severe sea states 

 can be made only at the expense of the structural weight-gross weight ratio which is 

 already critical. The alleviation of rough-water effects is recognized as a major goal 

 in hydrodynamic research and some progress had been made, but it is believed strongly 

 that the rough-water capabilities of a transonic or supersonic airplane is a derived 

 rather than a primary specification, subordinate to rigid flight requirements. 



Maneuvering and Handling 



The maneuvering and taxying problems of high-speed water-based aircraft are 

 at this point probably more severe than those of propeller-driven seaplanes because of 

 the relative inflexibility of jet engines, their lack of reversibility, and generally close- 

 coupled transverse locations. Wing-tip floats may still be required for static transverse 

 stability and safe cross-wind taxying, but may have to be retracted or jettisoned in 

 flight because of their drag penalty at sonic speeds. 



While outside the scope of this paper, the subject of basing requirements must 

 be included in any listing of considerations for advanced water-based aircraft. The 

 point of such aircraft is, of course, their ability to operate in unprepared areas and 

 independently of elaborate facilities; nevertheless, they will require bases of some sort 

 where they can be fueled, loaded, and repaired. The airplane's part in these operations 

 must be determined early and provisions made for it, subject, however, to the same 

 flight restrictions that apply to all the factors discussed. 



185 



