Perturbations in Boundary Probe Data

[j...@oxaero.com]

Jari,

Some interesting stuff in the perturbations in the probe data.  

Fig. 1: RMS of 4 Lowest Prob Pitot Perturbations at Each Speed w Clean and Taped LE

This is the RMS level of the perturbations in the lowest three probe Pitots for indicated airspeeds 40, 45, 48, 49, 50, 51, 52 and 55 kts, with clean leading edge (left chart) and taped leading edge (right chart).  At the highest level, where the speed is about double the three lower levels, the amplitude of the perturbations peaks on each side of the 50 kt region.  The tape seems to stabilize the flow at that level.  Something is happening around 50 kts where the flow is naturally more stable at the 4th level.

Fig. 2. Lowest 1st & 3rd Probe Pitots at 49 KIAS with Clean and Taped LE

At 49 kts, a little correlation can be seen between the 1st and 3rd levels.  The tape messes it up.  This is on the low side of the 50 kt region where the 4th level stabilizes.  Curious, but not sure what it means, if anything.

Fig. 3: Lowest 2 Probe Pitots at 52 KIAS with Clean and Taped LE

At 52 kts, good correlation can be seen between lowest two levels with clean leading edge.  The tape messes it up.  This is on the high side of the 50 kt region where the 4th level stabilizes.  Seems to indicate that the scale of the instability on the surface .125 chord behind the LE is approaching the height of the top of the 2nd Pitot, .110 inches.

Fig 4: Lowest 2 Probe Pitots at 55 KIAS with Clean and Taped LE

At 55 kts, nearly perfect correlation can be seen between lowest two levels with clean leading edge.  The tape messes it up.  This is farther on the high side of the 50 kt region where the 4th level stabilizes.  Seems to indicate that the scale of the instability on the surface .125 chord behind the LE is as high as the top of the 2nd Pitot, .110 inches.  Since the correlation does not appear in the 3rd level, the scale of the instability seems to be well defined.

Notes:

The tape was not placed correctly to match prior performance measurements that show large performance increases in the 48-52 KIAS range, so, for that reason, the taped LE measurements may not show the changes I am looking for. 

Sample rate = 1 Hz, so no idea about the frequencies of the perturbations, but the amplitude swings are well defined.

Jim

Figure 1. RMS of Four Lowest Prob Pitot Perturbations at Each Speed with Clean and Taped LE

Figure 2. Lowest Two Probe Pitots at 49 KIAS with Clean and Taped LE

Figure 3. Lowest Two Probe Pitots at 52 KIAS with Clean and Taped LE

Figure 4. Lowest Two Probe Pitots at 55 KIAS with Clean and Taped LE