Digitizing Procedure

The complete procedure from selecting the airfoil model to the end of the data reduction was standardized. The procedure was as follows: Since measurements

were generally taken at only one spanwise location, the airfoil was examined to be sure that this location was representative of the total section. If the covering had wrinkled, it was smoothed. Small, local distortions were excluded from measuring; however, larger distortions which covered most of the span, such as a flap joint or a long crack in the sheeting, were included.

The airfoil was held in two identical fixtures that supported the model in a level attitude on three points, see Fig. 2.4. These fixtures were designed so that when they were turned over the model was still supported on three points. Because of the impossibility of measuring both sides of the airfoil in a single position, machined reference blocks were permanently attached to the fixtures. These blocks could be touched by the probe with the fixture and model in either position (up or down). Consequently, when the section was turned over these blocks made it possible to maintain a single reference frame for both sides. The leading edge of the airfoil was aligned with one axis of the CMM table so the measured chord direction was perpendicular to the span, and so the same chord would be measured on both sides of the model. All the sections were of a constant chord so this point is not particularly important, but this method made it possible to check that nothing had moved during the measurements. (Because of the relatively non-rigid construction of the models compared to metal parts, the fixturing was free-standing on the machine’s table. There will be more on this later.)

As mentioned above, blocks mounted to the fixtures were used to establish the reference coordinate system for the upper surface. All points on the airfoil were referred by the software to the blocks, so the actual position of the airfoil and fixtures on the machine was unimportant. The chordwise location of the trailing edge was then determined by touching it with the probe from directly behind. This point was used to determine the actual chord of the section. Between 20 and 30 points were then touched on the upper surface. The spacing of points was more or less proportional to the local curvature; near the leading and trailing edges the spacing was small, over the central parts of the airfoil it was as great as ^ in. The final point touched in the upper surface sequence was the leading edge. This was detected by moving the probe vertically past it with the vernier lead screw at progressively closer settings until it touched.

Because of the possibility of distorting the model, the stands could not be rigidly held down to the CMM table. Consequently, any inadvertent movement during the data collection was detected by comparing the leading and trailing edge points as measured from both sides. If they were at the same points with respect to the reference frame established by the blocks on the fixtures, then no motion that could affect the measurement had occurred.

After the upper surface was done, the model and fixtures were turned over as a unit and the leading edge was once again aligned. Measuring continued at the leading edge, the first point here duplicating the last point on the upper surface, and the final point duplicating the first point on the upper surface (the trailing edge).

Of the 67 duplicated leading edge pairs, 46 were within 0.001 in, 52 within

0. 002, 59 within 0.003, 65 within 0.005 and all within 0.0057 in. Part of the difference within any pair is due to the impossibility of finding the exact point that was measured from the other orientation, because of the nature of the lead­ing edge. However, since a chordwise error translates to a much smaller vertical error except at the leading edge, these accuracies imply a general thickness mea­surement error of under 0.001 in. On a 12 in chord this is trivial.

The results of the measurements were collected on a Leading Edge personal computer, which was also used to reduce the data. All the output from the CMM was saved on a disk file. Typical output of the CMM software is shown in Fig. 2.5.

The CMM data are the locations of the center of the ball at the end of the probe, not the surface itself. Consequently, a second program was used to reduce the CMM output to the actual coordinates of the airfoil, to rotate the actual chord so it was parallel to the reference axis, and to normalize the airfoil to coordinates between 0 and 1. This data was also saved as a file. A header was added to identify the airfoil and show the actual chord. A typical output file from this program is shown in Fig. 2.6.