Underwater Lifting Surfaces 



The problem of low-aspect-ratio flat plates moving under and close to the water 

 surface at high speeds has been investigated experimentally by the NACA, and a 

 theoretical method has been developed to predict the lift for unseparated flow at large 

 depths (ref. [23]). The problem remains of taking into account in the theory the 

 influence of the surface, and this phase of the research is continuing. 



In addition to the familiar phenomenon of cavitation at large depths, a dynamic 

 lifting element at shallow depths experiences a type of separated flow brought about 

 by ventilation from the atmosphere as shown in Figure 15. At high angles of attack, 

 air is drawn in through the strong trailing vortices as the speed increases until it reaches 

 the plate and the flow separates from the upper surface. At first the flow reattaches, 

 but which further increase in speed, it completely clears the plate forming the so-called 

 planing bubble. At this point sharp discontinuities in the forces occur as upper surface 

 lift is completely lost (ref. [25]). 



At high speeds, the fully ventilated flow occurs suddenly at low angles of 

 attack without the progressive development described for high angles. The form of 

 the angle of attack speed boundaries for inception of the two types of fully-ventilated 

 flow are shown in the figure for two depths of submersion (unpublished data). The 

 inception speed for the high-angle type does not vary appreciably with angle of attack 

 but increases with depth of submersion; the angle for the low-angle type decreases 

 with speed and varies only slightly with depth of submersion. Between the two 

 branches, the boundaries are essentially independent of speed and submersion as 

 indicated. 



Since the inception of ventilated flow is generally a function of speed, the 

 question arises as to the effects of scale in model tests of submerged elements. 

 Experiments with two sizes of plate have indicated that the high-angle portion of the 

 boundaries at corresponding depths scale according to Froude's Law, a useful result 

 for configuration tests in a towing tank. The low angle portions of the boundaries 



24 



AERATED Jpggg 

 ,- VORTEX-^ ^^^ 



Q.DEG 



.014 



UNVENTED 

 .056 



1 



1 



PARTIALLY 

 VENTED 



FULLY VENTED 



FULLY 

 VENTED 



//////////// A' .' ' ■ ','.■;///// 



SUBMERSION 



PARTIALLY OR UNVENTED 



HIGH-ANGLE 



LOW -ANGLE 



20 



40 

 SPEED, FPS 



60 



80 



Figure IS. Ventilation of underwater lifting surfaces. 

 198 



