Complete Analysis
If you want more information, then see Details of Johnson Flight Test.

Background
In 2003, when I began flight testing Dr. Sinha's deturbulator drag reduction device, we anticipated that public acceptance of this new technology would be hard to achieve. A key element in our plan was to obtain independent verification at the earliest possible date. Since Dick Johnson has perhaps more experience testing sailplanes (gliders) than anyone in the world and is known around the world for his uncompromising flight test evaluations, we asked him to do the job. He agreed. Two years later, we finally got my glider (Standard Cirrus, #60, 1970, N2866, QZ) to him.

On 12/13/2006, at Caddo Mills, Texas, Dick and Jeff Baird logged four high altitude test flights in QZ with modified wings using the 2^nd prototype of Dr. Sinha's deturbulator tape which had been installed nearly a year earlier. The next day, they flew two more flights. At that point, Dick felt that he had enough data for a clear measurement of performance. The wings were then cleaned to restore them to standard condition. Then on 12/23/2006 they flew three additional flights to measure the unmodified (baseline) performance. Additional flights also were taken to calibrate the airspeed system.

I had logged our best two flights on 9/27/2006 and 10/21/2006 that yielded optimistic results indicating more than 20% improvement at best L/D speed. These flights encouraged us to take QZ to Dick Johnson immediately rather than delay another year. As reported earlier, we have not yet implemented solutions to our performance consistency (stability) issue, so this was a risky decision. Our purpose, however, was not to present a product to the market, but merely to demonstrate the potential of Dr. Sinha's invention.

The results in a nutshell
Evaluating this deturbulator prototype by time honored means applied to rigid wings yields a respectable 5% improvement. However, at 50 kts (IAS) the data in this test series, as well as prior test flights, are will established at a point far beyond the reach of the traditional 4^th order polynomial fit to glide ratio points. These data indicate a best glide ratio improvement in the range 13% to 18%.

A 4^th order curve fit is not appropriate because the glide ratio curve contains airspeed dependencies that are not found on normal, rigid wings. The deturbulators tested were only the second to be installed on an aircraft and contained some design deficiencies that were not yet corrected. The result was performance variations that cannot be followed by a 4^th order curve fit.

Furthermore, variations in performance with changing atmospheric conditions, have been documented for the early prototypes. Consequently, data that are averaged indiscriminately, without regard to flight conditions, reduce performance information as well as scatter and bias in the data. When only data from equivalent atmospheric conditions are averaged (not done in this test series), then larger performance improvements emerge. This has been demonstrated repeatedly in prior tests. A full analysis of this test series and prior data, taking this into account, can be found in the report Analysis of Independent Flight Test Data. Following is a summary of that analysis.

Traditional analysis
The orange curves in Figs. 8 and 9 are an average of the two best flights (9/27/2006 and 10/21/2006) prior the Caddo Mills tests. (Note: These points are sparse, so the connecting line should not be taken as strictly meaningful since structure between the data points could be missed. It should also be said that some points were dropped from the data because they deviated exceedingly from a curve fit to the data. The dropped points were 2, 2, 2 and 1 from flights 2, 3, 4 and 5 respectively. No points were dropped from flights 1 and 6. A 4^th order fit to the data indicates 15% improved maximum glide ratio. That is a considerable improvement, more than adequate to verify the effectiveness of Sinha deturbulators.

However, the 50 kt point (41:1 glide ratio) falls almost directly over the same point from two or flights (orange). (Click the image for a better look.) Notice that the red line (4^th order fit) is hardly influenced by this high point which is well established in the Caddo Mills data by four flights and by two additional prior flights. Six points are normally enough to smooth data even under poor conditions. A huge residual error would be needed push this point so far above the traditional curve fit. So, this point should be accepted as real and the faired glide ratio should run through it! The improvement indicated by these six data points is then 20%.

Figure 8. Six Flights Glide Ratio

Figure 9. Six Flights Percent Change

Conclusions
Perhaps it is enough simply to walk away with an image of Fig. 25 burned in your memory. The green and blue curves show the glide ratios measured on day 1 (four flights) and day 2 (two flights) respectively. The orange is our best two, nearly identical, flights taken in ideal conditions in September and October before the Caddo Mills tests. Do the Caddo Mills measurements corroborate the our measurements? Considering that the deturbulator design is not yet stable over changes of temperature, humidity and rapid altitude changes, we did not expect the results to match ours. Five features stand out in fig. 25:

1. All eight flights converge around 42:1 at 50 kts (ISA).
2. Day 2 at Caddo Mills follows our data very closely from 50 kts to 65 kts.
3. Day 1 at Caddo Mills follows the baseline from 60 to 70 kts, corresponding to the dip in our measurements.
4. Day 1 at Caddo Mills follows beneath our high speed points.
5. There is a major point of disagreement at the low speed end.

Figure 25. Days 1 and 2 and Best Ever Glide Ratio

Appreciation
Our thanks to Dick Johnson for conducting these tests, Jeff Baird for sharing the piloting duties, tow pilots David Cheek and Howard Hughes, Paula Lara and her staff at Southwest Soaring, Inc. and the Dallas Glider Association (DGA) for funding these flight tests.

Jim Hendrix
Oxford Aero Equipment