Ski-body combinations. — The nature of the relatively high hump resistance of 

 an aerodynamically-refined hydro-ski water-based airplane is shown in Figure 19. 

 The hydrodynamic resistance of the aerodynamically-shaped fuselage alone rises 

 rapidly with speed to a prohibitive value (refs. [35 to 38]). The planing resistance of 



RESISTANCE 



TAKE-OFF 



SPEED 



Figure 7 9. Take-off resistance of hydro-ski airplane. 



the skis alone intersects that of the fuselage at a speed depending on ski size and angle 

 of attack, and this speed is selected to transfer th water-borne load to the skis before 

 the resistance becomes too high for the available thrust. The resistance of the com- 

 bination is somewhat higher than the intersection because of spray interferences, the 

 resistance of the ski gear before emergence, and the energy required for the dynamic 

 emergenc maneuver itself. Above emergence, the resistance decreases rapidly with 

 speed until it approaches that of the skis. 



The hump resistance can obviously be decreased by designing the skis for earlier 

 emergence, but they then approach the proportions of a hull forebody and become 

 more difficult to retract in flight. It can also be decreased by hydrodynamic treatment 

 of the fuselage, which can be accomplished to some extent without a large departure 

 from an aerodynamically clean body. Even with a complete forebody and afterbody 

 on the body, however, the hump resistance is considerably higher than that of a hull 

 alone. 



Because of these considerations, the application of skis to airplanes with low 

 thrust- weight ratios (below 0.3) is not strictly in accord with the concept. The 

 emegence resistance of practical ski-body combinations remains a research problem 

 of some complexity, and means for reducing it to the level of the hump resistance of 

 a hull have not yet been found. 



The extensive involvement with spray of the aerodynamic components of a 

 hydro-ski airplane raises the question of how the forces due to spray vary with scale. 

 A preliminary experimental investigation (ref. [39]) has indicated that the lift force 

 generated on a flat plate by impinging spray from a planing surface can be scaled by 

 the conventional Froude methods. The spray drag forces, however, are relatively 



202 



