Castagneto and Maioli 



is derived from theory but contains unavoidable approximations, simplifications 

 and empirical evaluations which do not yet lead to a univocal procedure well 

 proved by experience (1, 2). 



The basic reasons for such discrepancies seem to be: 



jy , (a) the considerable differences between the true performance in real 

 fluid of the hydrofoils normally used in the design of a naval pro- 

 peller, and their theoretical pesrformance, as deduced by conformal 

 mapping, in ideal flow; and 



(b) the effects due to the blade Avidth; because the blade, in naval pro- 

 pellers, is better represented by a vortex lifting surface rather 

 than by a lifting line. 



The aim of this report is to present some theoretical analyses and to refer 

 to some experimental results dealing with the above-mentioned topics. 



Studies on the dynamics of lifting foils are considered to be a topical ques- 

 tion, because the hydrofoil is the basic element for the majority of naval propul- 

 sive systems, conventional or not. 



2. THE HYDROFOIL'S ACTUAL PERFORMANCE 



2.1 Lift Coefficient in Ideal Flow 



In designing the blades of a naval propeller, extensive use is made nowadays 

 of foils with thickness distribution NASA 16 or NASA 66 mod. (the NASA 66 is 

 less often used because of its thinness at the trailing edge), cambered according 

 to the mean lines NASA a = l, NASA a = 0.8, and NASA 65, and operating at an 

 angle of attack a, measured between the chord and the direction of the undis- 

 turbed flow. 



Tables 1 and 2 show the geometry of the above-mentioned section foils and 

 mean lines, taken from the NASA Report 824. Theoretical values of the velocity 

 increments for the basic thickness forms and mean lines considered and for a 

 wide range of thickness ratios are tabulated in that report for each particular 

 station along the chord, namely: 



(a) A7^ = A'K^ V = function of the thickness ratio of a particular hy- 



drofoil, and proportional to the velocity of the undisturbed 

 flow V ; 



f 



(b) AV^ = A'v f v-f-= function of a particular mean line, and propor- 



tional both to the velocity v and to the camber ratio f^/c ; and 



(c) AV^ = A' Fa vCi^a ~ function of the thickness ratio t„/c , of a partic- 



ular thickness form, and directly proportional, for each thick- 

 ness ratio, to the velocity v and to the lift coefficient c^^, 

 depending on the angle of attack. 



1020 



