3. Details of the trailing edge appeared to be very critical for producing 

 reasonable pressure distributions. The calculation procedure assumed the trailing- 

 edge point to be symmetrically located about the zero offset position for the suction 

 and pressure sides of the section. The procedure used to aline the measured offsets 

 with design did not force the trailing-edge points to symmetrical locations. This 

 procedure resulted in sharp kinks in the pressure-side offsets at the trailing edge, 

 which completely altered the pressure distribution. By fairing out the kink at the 

 trailing edge, reasonable pressure distributions were obtained. The modifications 



to the sections are shown in Figure 36. The pressure distributions varied only 

 slightly with the extent of fairing, provided the sharp kink was eliminated. 



4. The pressure distributions were calculated from equivalent two-dimensional 

 measured section shapes. Since the measured sections were true three-dimensional 

 shapes, the measured meanline had to be reduced to its equivalent two-dimensional 

 value. Assuming the design meanline distribution of a NACA 66 airfoil section of a 

 =0.8 meanline, the offsets were adjusted to have the camber producing the required 

 lift coefficient in two-dimensional flow. The coordinates on the suction and 

 pressure side were reduced as follows: 



Y 2D = Y " (f M-W <VV 



After the above modifications were made, potential flow pressure distributions 

 were calculated for each of the measured sections. Figure 37 shows the comparison 

 predicted from measured and design offsets. Also shown is the difference between 

 the measured and design offsets, expanded by a factor of ten. The pressure distri- 

 butions predicted from measured offsets generally matched the design curves over most 

 of the chord. Extreme fluctuations in the pressure distribution occurred over most 

 of the sections caused by the local irregularities in the section shape. The magni- 

 tude of the pressure fluctuations appeared to be dependent upon the change in slope 

 surrounding the irregularity. The measured offsets produced extreme variations in 

 pressure from design at the leading edge, partially due to the approximated leading- 

 edge shape resulting from insufficient measured offsets at the leading edge. 



48 



