Measurements Show 20% Improvement! (10/21/2006)

This report was revised on 1/3/07 after the Johnson flight test evaluation in December when the airspeed system was calibrated, resulting in a reduction of L/D ratios. Also, I pulled more flights from 2005 into the average for that year. Since the deturbulator is still sensitive to high humidity, I did not include flights when it clearly was not working.

(click images for a larger view)

It's been a year since our last report of measured performance improvements on Standard Cirrus #60. In that time Dr. Sinha has modified slightly the way he constructs the deturbulator, aiming for better immunity to moisture and changes in temperature. Also, he has improved his manufacturing method, resulting in greater uniformity of tension and fewer wrinkles in the mylar skin. However, the substrate-skin bonding is not up to production quality yet. A completely new, full-span application has been put on the upper surface of Standard Cirrus #60's wings. The result is an improvement in low speed performance. The temperature/humidity issues, while improved, still remain and Dr. Sinha is devising ways to manage them. It is clear to me that the temperature/humidity problem is an artifact of the design of these early deturbulators, not anything fundamental to the flow control methods Dr. Sinha is developing.

This preference for dry, cool air was demonstrated again this year. Last year, after a summer of disappointing results, performance gains reappeared in October. It happened again this year, as the data taken on 9/27 and 10/21 (right) indicate. Both flights were taken under very good atmospheric conditions. This is evident from a lack of scatter in the data, from vertical air movement, and the way the curves essentially replicate each other. Of particular interest is the movement of the characteristic hump from 50 kts to 70 kts. This movement away from the max L/D cruising speed revealed consistent sink rate improvement through that speed range and elevated the L/D improvement (bottom middle right) by an amazing 20%. The cause of the hump remains a mystery, to me at least, but is clearly seen in many of our earlier measurements (Itís Deturbulation Time Again).

A previous article (A Performance Endurance Issue) plotted the best sink rate change from a number of polars over a four month period. The trend indicated a loss of performance going into the summer months of 2005. That graph has been extended to the present date (right), and inverted so that improved performance is indicated in the positive direction. Points above zero indicate a reduction in sink rate. The trend line indicates that performance was lost over two summers, but returned from fall to spring. A word of caution about this graph is in order. It shows the best sink rate change at any airspeed for individual flight tests with no attempt to average out vertical air movements. So, the points should not be taken as measurements of true sink rate changes. They are plotted only to obtain a trend line. Only the swings in the trend line are meaningful, not it's specific shape.

Changes in performance with atmospheric conditions are sometimes observed in a single flight. During a recent high tow, for instance, the nose of the glider pitched down, indicating increased lift, after passing through the inversion.

This changing of performance with atmospheric conditions, makes taking polars by measuring sink rates, even more dubious than the method already is, due to vertical air movements. Since our object is to show what the deturbulator is capable of, then we must average data for only those flights in which the deturbulator is known to be working. Otherwise, we would be averaging out, not only air movement but deturbulator performance too. For this purpose, I look for measurements showing little or no scatter, reduced sink rates and the characteristic mid-speed hump.

The first four graphs (below) show modified polar and L/D plots for fall flights in 2005 and 2006. Three flights were averaged for 2005 and the two flights in the first graph (above) were averaged for 2006. For the 2005 flights, the glider was equipped with the first deturbulator application and the 2006 flights were taken with the second application which was modified somewhat.

These graphs show, with considerable certainty, how the 2005 and 2006 Sinha Deturbulators modified the performance of the first aircraft to realize large improvements by using flexible composite surfaces.

Perhaps more important than the magnitude of the performance improvement, shown in the first four graphs (below), is the presence of the hump in the polar, notch in the L/D, plots. Clearly it diminishes performance over a wide range of airspeeds on either side. Notice in 2006 how the 40, 50 and 60 kt points together show a trend toward the hump. This suggests that, if the hump can be eliminated, the result might be a smooth polar with slightly better performance at 50 and 60 kts than seen in 2006. This may be "pie in the sky, however, we have shown that the hump can be moved over the airspeed range. Perhaps, it can be eliminated altogether. If so then the data suggest the possibility of performance as illustrated in the last two graphs (blue curves). This is hypothetical, of course, but it is my best guess of the potential of Dr. Sinha's flow control methods for the Standard Cirrus glider.

At this point, we are ready for independent flight testing of this aircraft. This is risky because the deturbulator is still picky about temperature and humidity, but we think we can pick conditions that will show what it can do.

Jim Hendrix
Oxford Aero Equipment

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