matching theory in the trailing-edge region. The measured pressure at midchord may 

 be in error because it was obtained with a questionable surface mounted gage which 

 proved unreliable in unsteady flow. The bracket indicates a more credible result 

 from earlier tests before the surface gage was installed. The measurements on the 

 pressure side have similar irregularities across the chord, with values greater than 

 theory near the leading edge, then dropping below theory further aft. 



Results of this type are questionable, since one expects smoother variations in 

 pressure along the chord. Irregular variations along the chord could be explained 

 by the effect of crossflow and rollup processes of the tip vortex. The leading edge 

 at the 0.7 radius is positioned at the start of the extreme sweepback of the leading 

 edge, which could be the start of streamwise vortex generation and rollup. Strong 

 vortex formation in this leading-edge region will induce high local velocities in 

 the chordwise direction along the leading edge from the 0.7 radius to the tip, thus 

 producing suction peaks in the surface pressures. Also, any degree of crossflow 

 would greatly change the effective blade-section shape traversed by the flow over 

 the gage of interest. It is believed that an extremely complex lifting-surface 

 flow model with advanced numerical-analysis techniques is required to predict the 

 pressure distributions in these regions. 



At the 0.9 radius, measured pressure coefficients on Propeller 4718 are 

 generally less than the theoretical predictions. On the suction side, measured 

 values are less than predictions except near the trailing edge where measured values 

 are slightly greater than predictions. On the pressure side, measured values are 

 less than theoretical predictions uniformly across the chord. On Propeller 4679, 

 theory and experiment are in good agreement on the pressure side; however, on the 

 suction side the experimental pressure distribution is greater than the theoretical 

 prediction. This result is much different than that of Propeller 4718, again 

 implying possible differences in flow patterns near the tip. 



Again, the difference in correlation between theory and experiment for 

 Propellers 4679 and 4718 may relate to different rollup and tip-vortex positions 

 near the tip not accounted for in the mathematical model. The greater skew on 

 Propeller 4679 may cause tip vortex formation further inboard of the tip, resulting 

 in a decrease in pressures on the suction side near the tip. Also tip-vortex 

 separation may occur, influencing the local pressures. 



18 



