NOTE ON PRELIMINARY DESIGN PROGRAM 

 As for subcavitating propellers, the preliminary design calculations form a 

 separate program; however, it is necessary to briefly explain this subject to help 

 understand the final design procedures. 



The present preliminary design of a supercavitating propeller is more or less a 

 combination of subcavitating lifting-line theory and two-dimensional supercavitating 

 cascade theory. The cavity drag-lift ratio will be iteratively fed from the latter 

 into the former so as to compute the circulation and pitch of the blades needed to 

 produce a given thrust. In addition, the foil shape is determined so that the drag- 

 lift ratio is not too large for a reasonably thick cavity. Both the cavity thick- 

 ness near the leading edge, and the minimum cavity length must be prescribed because 

 supercavitating propellers are supposed to have a clean cavity covering the suction 

 side of the blade. However, this cavity length is a function of not only foil shape 

 but also the cavitation number, which is determined by the design conditions. To 

 design such a foil, it is convenient to consider three elementary foils: a basic, 

 cambered low-drag foil, such as the two-term camber foil; a flat plate to supply 

 angle of attack; and the leading-edge singularity to supply the leading-edge cavity 

 thickness. Called a point drag because it produces no lift, the leading-edge 

 singularity is especially useful for creating a long cavity. This is so because the 

 infinite cavity cavitation number of a supercavitating cascade is linearly propor- 



1 fi 



tional to leading-edge thickness; see Figure 2. Figure 2 shows the relationship 

 between cavitation number and leading-edge thickness for a supercavitating propel- 

 ler. Model 3770 in Figure 2 will be used for the numerical test and is discussed 

 in more detail later. 



In general, the specification for the leading-edge cavity thickness is based 

 upon the strength requirement. When the cavitation number is relatively large, 

 however, the leading-edge cavity thickness may have to be greater than the strength 

 condition requires. This greater thickness is needed to allow cavity lengths that 

 are more than 50 percent longer than the chord. Each section has different cascade 

 parameters, i.e., the solidity and the stagger angle; see Figure 3. The present 

 preliminary design method allows the option of using either a specified basic load 

 distribution or a specified basic camber shape for the blade section. In an actual 

 design, the given basic camber shape of the blade section seems to be more conve- 

 nient. Given the basic camber shape (the two-term camber in an infinite medium 



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