offer efficiencies somewhat superior to those of hydro-skis, and their rough-water 

 advantages over planing hulls have been further demonstrated by recent surface-craft 

 applications. There is therefore a need for further basic research on high-speed 

 cavitated and ventilated flows, and new design and operational concepts leading to their 

 effective integration into high-speed aircraft configurations. 



Struts. — Underwater lifting elements for aircraft must be supported by strut 

 systems extending below the hull or fuselage. When completely submerged, the struts 

 are a source of drag predictable from airfoil data, except when the speeds are high 

 enough for the onset of cavitation. Rake reduces the section drag coefficient and 

 increases the incipient cavitation speed as would be expected because of the reduction 

 in effective thickness ratio (ref. [40]). 



When piercing the surface, struts produce wave drag also predictable from theory 

 (ref. [41]), and are subject to ventilation effects. As for the underwater plates, the 

 mechanism of the ventilated flow and the laws of similitude that apply are not com- 

 pletely understood and merit further investigation. 



Impact Loads and Motions 



The loads and motions resulting from wave encounter increase with forward 

 speed and must continue to be dealt with in the design of high-speed water-based 

 aircraft. Theoretically, the acceleration resulting from an isolated wave impact varies 

 as the square of the vertical velocity, and, with the same flight-path angle as the square 

 of the forward speed. Actually, other things do not remain equal and maximum 

 accelerations increase at a slower rate (unpublished experimental data), but even the 

 first power is prohibitive when landing speeds are being doubled or tripled. 



The results cited have shown that rough-water loads and motions of hulls are 

 greatly reduced by increased beam loadings, afterbody lengths and angles of dead rise 

 (ref. [5]). They have also been ameliorated in practice by the use of reversible thrust 

 during landing and thrust augmentation during take-off to reduce the number of 

 critical impacts. Perhaps the greatest improvements are offered by the use of heavily- 

 loaded elements, to achieve high orders of penetration and localization of the loads. 

 The development of auxiliary systems for this purpose constitutes a fruitful field 

 for further investigation in order to hold high-speed water-based aircraft competitive 

 with their land-based equivalents with regard to structural weight while retaining a 

 reasonable degree of seaworthiness. 



High-Speed Test Facilities 



Along with increases in airplane landing speeds, there is an obvious need for 

 a corresponding increase in the water speeds available for experimental investigations 

 of hydrodynamic elements and configurations. The maximum towing speeds of the 

 NACA seaplane tanks (50 knots) are now regularly employed in fundamental experi- 

 ments and Froude model evaluations. Full-scale experience has approached 160 knots 

 for seaplanes and 200 knots for racing craft. The latter speeds are still below the 

 runway speeds of current high-speed landplanes. 



Planing data has been obtained at the NACA's Langley Aeronautical Laboratory 

 up to 120 knots by the use of a free-water jet propelled by compressed air (ref. [42]) 

 as shown in figure 21. This apparatus has proved useful and capable of further 

 development; it is subject, however, to boundary corrections and free-surface effects 

 not present in a still-water towing tank. 



This laboratory has recently placed in operation a high-speed hydrodynamics 

 facility capable of test speeds up to 130 knots. A view of the facility is shown in 

 Figure 22. It consists of a concrete tank 2,000 feet long, 8 feet wide, and 5 feet 

 deep adjacent to an existing track and towing carriage provided for research on wheel 

 landing gears. The hydrodynamic model and its instrumentation are suspended from a 

 temporary boom on the carriage overhanging the water tank, and data are obtained 

 during decelerated runs after the carriage is brought up to speed by a water-jet catapult. 



204 



