More success on upper surface of Standard Cirrus wing (12/12/2004)
Today's flight yielded repeated success on the upper wing surface. At the 53" span station, the original FCSD was removed and replaced with another strip that was prefabricated, rather than put together on the wing surface. The new sample produced a solid improvement at low speeds but a slight loss at high speeds compared to old one. We are encouraged to see that reinstalling a strip produced consistent results (Fig. 22).
Fig. 23 shows the aileron station, nine days after the last reported data. No change was made at this station. The improved performance may be due to different aileron deflections.
This was the first time that I saw an unmistakable difference in flight characteristics due to the unbalanced treatment of the wings at high speeds. Previously I had noticed a tendancy to roll left at stall. Today this was more pronounced and began well before the stall. I think this is due to the improved low speed performance of the FCSD at the lowest speeds. However, also today I noticed a definite need to apply greater right stick as the glider increased speed. When applying no side pressure to the stick while diving, the glider consistently rolled left at a rate that increased with speed. The amount of this unbalanced behavior was surprising, considering that only two 12" strips of FCSD were used and only on the upper wing surface. The moment arm for the strip at the aileron station, gives it an advantage in turning the glider.
Since the baseline data for the aileron station was taken with too much turbulence, and since there is a difference in two flights possibly due to aileron deflection variations, I thought it would be interesting to see what the performance would look like if the 12/032004 and 12/12/2004 curves were averaged and the baseline straightened. This is plotted in Fig. 24. I believe this is a pretty good indication of what is actually happening at the 167" span station. We'll see when I get better clean wing data.
At this point, with a number of drag data plots for different surfaces and span stations (Figs. 13, 16, 20, 21, 23, 24), it is interesting to note that the graphs are all generally similar. The best cases show moderate, flat, single digit percentage improvements at low speeds, and performance increasing with airspeed. Generally, the faster you fly the better it works.
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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