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NACA 6-Series
The 6-series airfoils were designed to achieve desirable drag, compressibility, and Clmaj performance. These requirements are somewhat conflicting, and it appears that the motivation for these airfoils was primarily the achievement of low drag. The chordwise pressure distribution resulting from the combination of thickness and camber is conducive to maintaining extensive laminar flow over the leading portion of the airfoil over a limited range of Q values. Outside of this range, Cd and C|max values are not too much different from other airfoils.
The mean lines used with the 6-series airfoils have a uniform loading back to a distance of x/c = 2. Aft of this location the load decreases linearly. The a = 1 mean line corresponds to the uniform loading for the series-16 airfoils.
There are many perturbations on the numbering system for the 6-series airfoils. The later series is identified, for example, as j
NACA 65,-212 a = 0.6
— v 6.0
Here 6 denotes the series; the numeral 5 is the location of the minimum pressure in tenths of chord for the basic thickness and distribution; and the subscript 1 indicates that low drag is maintained at Cj values of 0.1 above and below the design G of the 0.2, denoted by the 2 following the dash. Again, the last two digits specify the percentage thickness. К the fraction, a, is not specified, it is understood to’equal unity. The 65r212 airfoil is shown in Figure
3.7.
T ift and drag curves for the 65i-212 airfoil are presented in Figure 3.8. Notice the unusual shape of Q versus CJ, where the drag is significantly lower between Q values of approximately 0 to 0.3. In this region, for very smooth surfaces and for Reynolds numbers less than 9 x Ю6, extensive laminar flow is maintained over the surface of the foil with an attendent decrease in the skin friction drag. This region, for obvious reasons, is known as the “drag bucket.” In practice this laminar flow, and resulting low drag, is diflicult to achieve because of contamination by bugs or by structurally transmitted vibration that perturbs the laminar boundary layer, causing transition. Chapter Four will discuss the drag of these airfoils in more detail.
