The 6-series airfoils also offer better lift coefficients at higher Mach numbers than the earlier airfoils.ДЄirfoils for vertical axis wind machines are normally symmetrical in shape, meaning that both upper and lower surfaces have the same shape. However, their critical Mach numbers are lower at higher lift coefficients than are the same types of airfoils. The NACA 6-series airfoils have higher critical Mach numbers than the earlier airfoil types at typical cruise lift coefficients.

This lends support to the importance of understanding the impact of poor surface quality and leading edge contamination on the overall characteristics of the airfoils and, thus, the performance of the aircraft when designing wings. The leading edge contamination also reduces the lift curve slope, especially for thicker airfoils that have the location of minimum pressure farther aft than do thinner ones. The magnitude of the reduction is similar for both. Leading edge contamination (roughness) causes large reductions in C lmax for plain and flapped airfoils. It exceeds the theoretical value (2ПЂ) for thin airfoils. The lift curve slope for smooth 6-series airfoils is slightly steeper than that of the 24-, 44-, and 230-series airfoils. However, the thinner 6-series airfoils have substantially lower C lmax with flaps. The maximum lift coefficient with flaps is about the same for moderately thick 6-series airfoils as it is for the NACA 23012 with flaps. The NACA 230-series airfoils with thickness ratio less than 20% generally achieve the highest maximum lift coefficients. The maximum lift coefficient for moderately cambered 6-series airfoils are as high as those achieved using NACA 24- and 44-series airfoils. Thus, at high Reynolds numbers where laminar flow is no longer achievable, drag can be kept low by ensuring smooth surface qualities. In fact, the C Dmin depends more on the surface quality than on the chosen airfoil. Wings of moderate thickness ratios with such surface qualities can achieve a minimum drag coefficient of the order of 0.0080. Wind tunnel tests have shown that extensive laminar flow is possible on smooth three-dimensional wings if the surface quality is smooth and like that provided by sanding in the chordwise direction with No. However, these characteristics are realized only if the quality of the lifting surface is smooth.

These are paraphrased below.ДЄirfoils permitting extensive laminar flow, such as the NACA 6- and 7- series, have substantially less drag at typical cruise lift coefficients than other kinds of airfoils. lists a number of things to keep in mind when selecting airfoils. Snorri Gudmundsson BScAE, MScAE, FAA DER(ret.), in General Aviation Aircraft Design, 2014 NACA Recommended Criteria

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