pitch and camber changes. The benefit of this concept is shown in Figure 6 



which presents cavitation inception data for a pair of propellers, one 



with and one without a 10 percent pitch reduction at the tip. As shown, 



the variation of cavitation index a with advance coefficient J. is lower 



A 



for the reduced pitch propeller by approximately ojo^ ~ 4/3 for J = 0.80, 

 which corresponds to a ratio of free-stream speeds of 1.15, i.e., an 

 increase in tip vortex cavitation inception speed of approximately 15 

 percent. Although this concept has proven effective, the propeller 

 efficiency suffers a decrease on the order of 5 percent. 



PLANFORM: DELTA, SWEEP, AND OGEE TIP 



A change in planform shape can alter the wing spanwise load distribu- 

 tion, and hence the strength of the tip vortex. This is the primary differ- 

 ence between the delta planform and the rectangular planform. Also, for the 

 delta planform, a strong spanwise flow exists along the sweptback leading 

 edge, which significantly alters the characteristics of the vortex rollup. 

 Apparently, the rollup occurs over a much larger area, resulting in a less 

 concentrated vortex. In the case of marine propellers, this same effect is 

 obtained with blade skew. However, model experiments failed to demonstrate 

 any improved tip vortex cavitation performance with increased propeller 

 blade skew. One difficulty reported in Reference 7 is distinguishing 

 between tip vortex cavitation and leading edge surface cavitation. 



The OGEE tip planform (Figure 4) is a concept specifically designed 

 for helicopter rotors to increase rotor overall efficiency through re- 

 duction in the induced drag associated with the tip vortex rollup. This 

 improved performance is achieved by a rather complex flow process where 

 secondary vortices interfere, destructively, with the primary tip vortex. 

 The results from OGEE tip investigations are somewhat conflicting: opti- 



Q 



mistic reports indicate a 75 percent reduction in vortex core peak tan- 

 gential velocities with a 5 percent increase in efficiency, but less opti- 



9 

 mistic reports indicate decreases in vortex core velocities with decreases 



in rotor efficiency. In addition, the effectiveness of the OGEE tip is 

 reduced significantly for rotor off -design operation. Based upon the 



12 



