smaller for a larger model, evidencing a Reynolds number effect similar to that for 

 turbulent skin friction in solid flow. 



Land-water gears. — In the case of a land-water airplane, the hump resistance of 

 the hull or emergence resistance of the ski-body combination are by-passed entirely 

 by the use of the ramp and taxiing wheels as shown in Figure 20. Here the governing 



RESISTANCE 



MIN. 

 PLANING\; 

 SPEED I 



RAMP 

 FRICTION 



SPEED 



Figure 20. Take-off resistance of land-water airplane. 



factor is the minimum planing speed below which "water stall" occurs and the resist- 

 ance is very high. Above this speed, the resistance decreases rapidly as shown, and the 

 transition speed must be enough higher to keep the resistance to an acceptable value 

 after leaving the ramp. In general the planing resistance will be higher than for the 

 larger skis of a completely water-based combination because of the associated heavier 

 ski-loadings and lower hydrodynamic aspect rations. Thus although the problem is 

 alleviated by the mode of operation, it cannot be neglected entirely, particularly in 

 the choice of design transition speeds and in the need for a hydrodynamically clean 

 configuration in the water-borne part of the operation. 



Hydrofoils and Struts 



Hydrofoils. — The use of submerged hydrofoils as lifting elements for water- 

 based aircraft has always been an interesting idea because of their potentially high 

 lift-drag ratios and favorable rough-water qualities as compared with flying-boat hulls. 

 The hydro-ski is essentially a low-aspect ratio hydrofoil that can be integrated into 

 airplane design and operation, and avoids the flow problems associated with negative 

 pressures by planing on the surface at high water speeds. Apart from early Italian 

 applications, however, competitive high aspect-ratio hydrofoil systems for aircraft have 

 not yet appeared, although fundamental research on the subject has been continued 

 (ref. [28 to 31]) and applied in fields other than aeronautics. 



The incentives for the development of practical hydrofoil configurations remain 

 high, since even with moderate aspect-ratios and separated flows they continue to 



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