close-up look at probe data
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j...@oxaero.com
Oct 3, 2012, 10:38:44 AM10/3/12
to Jari Hyvärinen, Deturbulator Participants, Roy Arnold, Nathan Murray, George Bennett
Jari,
Fig. 1 is a close-up look at the probe data from the two flights last Saturday. The dashed lines are from the clean wing flight and the solid lines are from the flight with tape around the LE. Because the fourth level above the surface shows essentially the same speed for all cases at about double these speeds, it has been omitted in order to expand the data from the lowest three levels.
The vertical axis is the height of the center of each probe Pitot tube above the surface. The horizontal axis is the uncalibrated airspeed indicated by the dynamic pressure in the Pitot tubes. Each curve is for a given glider indicated airspeed as shown in the legend.
The differences are small, probably because the tube IDs are too large to reveal the fine structure of the boundary flow. Nevertheless, there is an indication of changes that correspond to the notched performance peaks shown in Fig. 2. Notice that the two glider speeds where the tape caused the greatest speed-up of the boundary flow are on each side of the 45 KIAS glider speed which is mostly unchanged. This appears to correspond to the performance peaks on each side of the notch at 49 KIAS shown if Fig. 2. I am not confident that this is a real correspondence, because the 45 KIAS speed does not look as I expect based on my idea of what is happening at the notch. But, of course, I may be wrong. More and better data is needed.
I did not take enough care to get the tape edge exactly where it was when the performance data were taken. No doubt, this shifted the glider speeds in Fig. 1 where the significant differences occur. It appears to have slowed the pertinent speeds in Fig. 1, causing the notch speed to be around 45 KIAS instead of 49 or 51 KIAS as shown in Fig. 2. This makes the 40 KIAS point too slow to catch the left performance peak and the 48 KIAS speed a bit too fast. This can contribute to the diminution of the speed changes measured.
Each data point in Fig. 1 is an average of 40 measurements sampled at 1 sec. intervals while flying a constant airspeed.
Jim
Fig. 1 is a close-up look at the probe data from the two flights last Saturday. The dashed lines are from the clean wing flight and the solid lines are from the flight with tape around the LE. Because the fourth level above the surface shows essentially the same speed for all cases at about double these speeds, it has been omitted in order to expand the data from the lowest three levels.
The vertical axis is the height of the center of each probe Pitot tube above the surface. The horizontal axis is the uncalibrated airspeed indicated by the dynamic pressure in the Pitot tubes. Each curve is for a given glider indicated airspeed as shown in the legend.
The differences are small, probably because the tube IDs are too large to reveal the fine structure of the boundary flow. Nevertheless, there is an indication of changes that correspond to the notched performance peaks shown in Fig. 2. Notice that the two glider speeds where the tape caused the greatest speed-up of the boundary flow are on each side of the 45 KIAS glider speed which is mostly unchanged. This appears to correspond to the performance peaks on each side of the notch at 49 KIAS shown if Fig. 2. I am not confident that this is a real correspondence, because the 45 KIAS speed does not look as I expect based on my idea of what is happening at the notch. But, of course, I may be wrong. More and better data is needed.
I did not take enough care to get the tape edge exactly where it was when the performance data were taken. No doubt, this shifted the glider speeds in Fig. 1 where the significant differences occur. It appears to have slowed the pertinent speeds in Fig. 1, causing the notch speed to be around 45 KIAS instead of 49 or 51 KIAS as shown in Fig. 2. This makes the 40 KIAS point too slow to catch the left performance peak and the 48 KIAS speed a bit too fast. This can contribute to the diminution of the speed changes measured.
Each data point in Fig. 1 is an average of 40 measurements sampled at 1 sec. intervals while flying a constant airspeed.
Jim
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