4th Leading-Edge-Tape-Only Performance Measurement (Revised 10/27/2011)

Below are four sets of sink-rate measurements on Standard Cirrus #60 altered only by the (leading edge tape modification).

  • The red data were take two weekends ago. The was a fairly wide range of values between the beginning and end of the two speed runs that produced the performance peaks. The plotted values are averages in each case. Also, the all-up weight was 32 pounds less for the red flight compared to the others. As you can see, that change had a marked effect on the performance notch, moving it right by 2 KIAS and possibly reducing the depth of the notch. By my estimate, in the 50 KIAS speed range, each knot corresponds to a 0.2 degree change in AOA, so the shifted notch corresponds to a -0.4 change in AOA.
  • There is remarkable agreement between the 1/15/2011 (red) and 4/2/2010 (blue) curves. The agreement at 60 and 65 KIAS indicates that the severe performance loss at 60 KIAS is real, not bad data as I had hoped. Except for the notch shift and irregularity (error, I think) in the 1/15/2011 data, these are essentially the same curves.
  • The overall pattern repeats consistently, indicating, to me at least, that the phenomenon is real. Under the right conditions, very large performance enhancements can be achieved at glider Reynolds numbers in smooth air over limited speed ranges. Therefore, this needs to be taken seriously by the aeronautics community.
  • The reason for the large deviations in the amplitude swings appears to be due to humidity. It is thought that even without condensatioin, water molecules in sufficient concentration adhere to the wing and tape surfaces changing the effective surface friction as seen by the boundary flow. This can be seen in the following graph in which the L/D difference between the maximum and notch is plotted versus the relative humidity at the altitude at which the data was taken. This is a remarkable finding as it seems to indicate that molecular level phenomena are having a large effect on glider performance. It remains to be seen if this relationship will prove to be consistent.

Jim Hendrix

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