Davis and English 



Fig. Z9 - Comparison of torque between bronze and 

 epoxy resin Models — screw T95 



The gauge rosettes were mainly of the star type with 120-degree separation 

 between the arms, and were orientated with an arm of each rosette on a cylin- 

 drical section and pointing towards the leading edge. Rosettes were positioned 

 at 0.5, 0.65, 0.8, and 0.9 of the propeller radius and 20 percent of the local chord 

 from the leading edge on the blade in question. Some of the rosettes and blade 

 wiring can be seen in Fig. 31 before the waterproofing was applied. 



The recorded strain levels were reduced to principal stress values for a 

 full size screw assumed to be 44 inches in diameter and rotating at a speed of 

 1700 rpm. The elastic modulus of the prototype was taken as 30 x 106 ibf/in2 

 and Poisson's ratio was assumed to be the same for model and prototype with a 

 value of 0.3. Graphs showing the results of these experiments are given in Figs. 

 32, 33, 34, and 35 where the maximum and minimum tensile principal stresses 

 and the maximum shear stresses are plotted against the radius fraction for the 

 20 percent chord position. Stress directions are given in Figs. 36 and 37. The 

 results are plotted for a range of advance coefficients and two cavitation num- 

 bers which can be considered typical of the takeoff and flying conditions for a 

 craft employing 44-inch diameter propellers. 



In both conditions the maximum principal stress at the 20 percent chord 

 position occurs around the 60 to 65 percent radius. The stress increases with 



998 



