In Figure 19, the predictions by PSP are compared with other predictions 



represented by the envelope covering all the predicted results at the design 



advance coefficient, J = 0.833. The predicted values by PSP are within the 



envelope of the predictions by other methods. 



In Figure 20, comparisons are made between predictions by PSP and by a lifting 



9 A 

 surface method presented by Brockett for a propeller similar to DTNSRDC Propeller 



4498 at J = 0.888. The propeller is warped with 72 degrees warp angle at the tip. 



The section meanline is similar to the NACA a = 0.8 meanline. 



The predictions made by the two different methods are in good agreement at 



r/R = 0.254, but the discrepancies increase toward the tip region, as it did for 



the experimental data of Jessup. 



SUMMARY AND CONCLUSIONS 



The discrete vortex/source lattice lifting surface method has been used for 

 the prediction of steady pressure distribution on a rotating propeller blade sur- 

 face. A computer code, PSP, has been developed by extending the existing propeller 

 global analysis program, PSF, developed at M.I.T. 



For pressure computations on the propeller blades, the velocity jump across 

 the vortex/source sheet must be carefully treated and include the effects of both 

 the spanwise and chordwise vortices. In PSP, the effect of the chordwise vortices 

 at the pressure point, the midpoint of each spanwise vortex, was accounted for by 

 interpolating from the four adjacent chordwise vortices. 



Comparisons of the predictions by PSP with experimental measurements and pre- 

 dictions by other methods on selected model propellers generally showed good 

 correlations. The correlations near the tip region, especially for skewed pro- 

 pellers, i.e.. Propellers 4718 (20 degrees tip skew) and 4498 (72 degrees tip skew), 

 are not as good as those for the inner region. Possible explanations may be that 

 near the tip region of skewed propellers, viscous effects may be large or that the 

 current numerical modeling in lifting surface representations may not be accurate 

 enough. 



*The predictions by Brockett shown in Figure 20 are taken from Figure A 

 (linear 3D method) in "Discussions and Authors' Closures" section of Reference 9, 



12 



