Second Sink-Rate Measurement: Some Encouraging Data (2/26/2005)
Before this flight, we filled the two gaps in the deturbulator tested on 2/18/2005. Again we took two sets of sink-rate data, graphed separately in Figure 29. This time we saw much less scatter. Figure 30 is an average of the two data series taken in this flight. Because there is so little scatter in the two series, this curve is probably a fairly reliable indication of performance. If so, then we are seeing a nice performance improvement between 50 and 90 kts. It's too bad that we don't have data at 55 and 70 kts to better define the curve in that region. But, our purpose was only to verify that the deturbulator was working before proceeding with a further application of deturbulator to the wing tips.
Two caveats apply to this data. First, both sets of data were taken in the same flight on the same day, so atmospheric conditions could bias the results. Second, temperatures were not noted at the measurement altitudes. These are needed for correcting the sink rates to sea level. However, temperature has a small effect on the result, so standard atmosphere temperatures, based on the ground temperature when the data were taken, were substituted.
Figure 31 plots the L/D from the two data sets averaged versus the baseline L/D. The best L/D is 38.7 at 50 kts, an increase of 7% over the baseline value of 36. If only half of this is true, it's a good showing for a 60% deturbulation of the top wing surfaces.
The unusual low speed stick position of 2/18/2005 was gone, as was the sink rate hump at 50 kts. On this day, however, my first impression was that no forward stick pressure was needed on tow. The trim springs on this glider favor low speeds and I always have to apply an annoying amount of forward stick against the forward-most trim position while on tow and at high speeds. But that was not the case for this flight, except for speeds over 80 kts.
The high speed increase in sink rate is thought to be due to the wing flying with a negative adjustment in AOA because of a changed suction-side pressure profile.
Looking for evidence that the spanwise lift distribution was shifted inboard by the partial application of deturbulator, I photographed the left wing while in a banked turn of about 45 degrees at about 50 kts. This image is shown in Fig 32 along with a similar image of the same wing without deturbulator in a shallower bank. The bending at the tip appears to be less on this flight than before, even though the wing loading is higher. I take this as evidence that the wing was flying with a more inboard lift distribution.
Presently, we have the full span deturbulated on the top surface. We will gather enough data to get a clear indication of performance before working on the pressure side of the wings.
Airspeeds shown in graphs are instrument calibrated. The aircraft airspeed system is not calibrated. Errors in the Standard Cirrus static/Pitot system bias the data towards higher speeds. This makes polars seem better than they really are. However, this is not an issue when the purpose is only to show comparitive data on the same glider.
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