• NACA 6409-PT (Fig. 12.66)
The NACA 6409 is considered more of a free-flight airfoil than one for RC soaring. The actual wind tunnel section was the only model that had open bay construction from leading to trailing edge. The ribs were | in (l% chord) thick and had a spacing of 3 in (25%), giving an open-bay cell aspect ratio of 1:4. The sagging of the covering was about 0.025 in (0.2%) worst case, and generally much less—0.005 to 0.015 in. Due to the lack of torsional rigidity, data was taken only up to 200k.
As Fig. 12.66 shows, the airfoil stalls at 8° at 60k; however, in Fig. 12.67, which shows lift data only, the premature stall is not found. The lack of repeatability may have been caused by the gusty weather at the time of the lift run.
As for the performance, the NACA 6409 is an excellent low speed, floater airfoil, but, much like the AQUILA airfoil, the large camber severely limits the high speed performance.
Also see: AQUILA, S2091, SD7043 Digitizer plot: Fig. 10.25 Polar plot: Fig. 12.66 Lift plot: Fig. 12.67
Thickness: 9.00% Camber: 6.00%