NACA Five-Digit Aerofoil

After the four-digit sections came the five-digit sections. The first two and the last two digits represent the same definitions as in the four-digit NACA aerofoil. The middle digit represents the aft position of the mean line, resulting in the change in the defining camber line curvature. The middle digit has only two options: 0 for a straight and 1 for an inverted cube. The NACA five-digit aerofoil has more curva­ture toward the LE. Following are the examples of the NACA 23015 and NACA 23115:

2 3 0 or 1 15

Maximum camber Maximum thickness of 0: straight, the last two digits

position in % maximum camber in 1: inverted are maximum t/c

chord 1/10 of chord cube ratio in % of chord

NACA Six-Digit Aerofoil

The five-digit family was an improvement over the four-digit NACA series aerofoil; however, researchers subsequently found better geometric definitions to represent a new family of a six-digit aerofoil. The state-of-the-art for a good aerofoil often follows reverse engineering – that is, it attempts to fit a cross-sectional shape to a given pressure distribution. The NACA six-digit series aerofoil came much later (it was first used for the P51 Mustang design in the late 1930s) from the need to gener­ate a desired pressure distribution instead of being restricted to what the relatively simplistic four – and five-digit series could offer. The six-digit series aerofoils were generated from a more or less prescribed pressure distribution and were designed to achieve some laminar flow. This was achieved by placing the maximum thick­ness far back from the LE. Their low-speed characteristics behave like the four – and five-digit series but show much better high-speed characteristics. However, the drag bucket seen in wind-tunnel test results may not show up in actual flight. Some of the six-digit aerofoils are more tolerant to production variation as compared to typical five-digit aerofoils.

The definition for the NACA six-digit aerofoil example 632-212 is as follows:

An example of the NACA 653-421 is a six-series airfoil for which the minimum pressure’s position is in tenths of a chord, indicated by the second digit (at the 50% chord location). The subscript 3 indicates that the drag coefficient is near its min­imum value over a range of lift coefficients of 0.3 above and below the design lift coefficient. The next digit indicates the design lift coefficient of 0.4, and the last two digits indicate the maximum thickness in percent chord of 21% [4].