First success on upper surface of Standard Cirrus wing (12/3/2004)
December 3, 2004 was another very good day for the Sinha project. After working months to resolve problems with FCSD on the top surface of the Standard Cirrus wing, we finally realized success. Furthermore, we did this at two widely separated wing stations with similar results, indicating that we are beginning to achieve the consistency we seek.
Fig. 20 shows the change in boundary layer flow at the 53" span station. Fig. 21 shows the same for the 167" station. This station is on the aileron, 3" out from the inner end. The drag probe is mounted on aileron, with the hinge gap in front of the probe. The data were taken with the hinge gap open, but with an S-seal inside and mylar on the lower surface to prevent flow through the gap. In these graphs, the dashed line shows the percent improvement, plotted against the right axis.
As usual, the differential pressure between the aircraft Pitot and the drag probe is measured and reported in volts. It is not important to convert to units of pressure for comparative measurements. Note that this is not a measure of the wings drag coefficient, but merely a measure of the change of velocity in the boundary layer near the surface. How this will affect aircraft performance will be seen when we take polar measurements with fully modified wings.
The percent change on the upper surface is roughly half that of the under side (Fig. 13). However, as Fig. 22 shows, at cruising speeds, the upper surface generates about three fifths of the drag. So, in terms of the effect on the drag coefficient, it might be reasonable to expect FCSD to have nearly equal effects on the pressure and suctuon sides. At ninty knots, this would be about 9% reduction, giving an 11% increase in L/D at that speed. Of course, this refers only to the wing, not the entire glider.
However, there is more good news. Dr. Sinha is confident that he can get better improvements on the upper surface. He has already done better on the upper surface of the NLF-0414F laminar airfoil that resembles modern glider airfoils. With the greatest camber at the rear of the wing, these airfoils are easier to treat. The Standard Cirrus wing was a real challenge, but it forced Dr. Sinha to look into issues that will benefit every installation of FCSD and hasten time to market for this new technology.
A second source of optimism is that measurements in the wind tunnel have shown that an increase in lift comes with the drag reduction. So, this is expected to further improve L/D. Evidence of this has been noticed in test flights by the need to apply a little stick pressure toward the treated wing to keep the glider from rolling the other way.
However, my greatest optimism comes from the consistency I see in the results of two very different applications of FCSD. One near-root application and one aileron. Numerically the results are similar. Both show diminishing improvement at the lowest speeds and increasing improvement with airspeed at the highest speeds. Neigher shows an increase in drag at any speed. So, at this point, I am confident that a full span treatment of FCSD will give reasonably consistent performance at every span station and we are now able to move forward toward a full performance measurement on the glider.
Note: the baseline data (black curve) in Fig. 21 was taken in less than ideal conditions. The dips at 50 and 70 kts are likely poor data points. I think the true curve is flatter, like at the 53" station. Thus, I believe true change at 50 kts was about 5% and 8% or 9% at 70 kts.
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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