FORCES AND TIP VORTEX CAVITATION INCEPTION 



The parent foil force characteristics are given in Figures 18 through 20. Fig- 

 ure 18 presents the foil lift coefficient C as a function of angle of attack a for 



6 

 Reynolds number R =2.5 and 5.0 x 10 . As seen, the results are quite linear over 



the range of a investigated, -7 deg _< a j< 10 deg, and agree well with the design 



theory. The lift curve slope of m = 0.0525/deg a was computed from existing exper- 



1 f) 

 imental data while the angle of zero lift of a = -2.94 deg and the design angle of 



20 

 attack of a = 0.567 deg were computed from corrected theoretical formulas for two- 

 dimensional NACA a = 0.8 meanline airfoils. The parent foil drag characteristics 

 are given in Figure 19 which shows the variation of drag coefficient C as a func- 

 tion of lift coefficient C for two Reynolds numbers R . In general, the trends 

 observed here are expected; i.e., most of the variation of drag with lift for wings 

 of finite span results from the induced drag, which varies approximately as the 

 square of the lift coefficient for a given wing configuration. The "hump" in the 

 low-drag range is characteristic of the NACA 6-series and results from the sudden 

 shift in boundary layer transition at the end of low-drag range of lift coefficients. 

 In addition, all data for the NACA 6-series wing sections show a decrease of the 

 extent of the low-drag range with increasing Reynolds number. 



The roll moment, or the moment due to lift about the root chord, was measured 

 to establish any possible effects of the various foil tip alterations on the span- 

 wise load distribution. The theoretical value for the roll moment arm Yrm can be 

 calculated according to 



f 



£Ydy 



roll moment o _ „_ . /n itq ^ 



Yrm = ;— TTT — = = 5.25 m. (0.133 m) 



total lift 



1 



My 



or Yrm/s = 0.438 



where i = local spanwise load = "f U ~ \) I ^°'^ elliptic loading, 



27 



