AIRFOIL CHARACTERISTICS AT LOW REYNOLDS NUMBERS

Occasionally one has the need for airfoil characteristics at Reynolds number values much lower than those used by the NACA and others to obtain the majority of the readily available airfoil data. Most of these data

were obtained at R values of 3 x 106 and higher. For remote-piloted vehicles (RPV), model airplanes, and the like, Reynolds numbers as low as 5 x 104-can be encountered. A search of the literature will show little airfoil data available in this Reynolds number range. The most reliable low-Reynolds number airfoil data appear to be those given in Reference 3.35, where tests of five different airfoil shapes are reported for R values as low as 42,000. These tests were conducted in a low-turbulence tunnel.

The five airfoil shapes that were tested in Reference 3.37 are shown in Figure 3.64. These are seen to comprise a thin, flat plate, a thin, cambered plate, two 12% thick airfoils with 3 and 4% camber, and one 20% thick airfoil with 6% camber. The airfoil shapes are similar in appearance to the NACA four-digit series.

The lift curves for these airfoils are presented in Figure 3.65 for four different Reynolds numbers. As one might expect, the flat-plate results are nearly independent of R since the separation point at the leading edge is well defined. To a slightly lesser degree, the same can be said for the cambered plate. The form of the lift curves for the three airfoils is seen to change substantially, however, over the R range from 4.2 x 105 down to 0.42 x 105. Particularly at the very lowest Reynolds number, the C, versus a curve is no longer linear. The flow apparently separates at all positive angles just down­stream of the minimum pressure point, near the maximum thickness location.

The 625 airfoil

A flat—plate airfoil

Curved plate 417A

The N60 airfoil

The N60R airfoil

Figure 3.64 Airfoil shapes tested at low Reynolds numbers.

Figure 3.65 Effect of Reynolds number on airfoil lift coefficients.

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Figure 3.66 Lift curve for the N60 airfoil.

Figure 3.68 Drag polar for the flat-plate airfoil at low Figure 3.69 Drag polar for the 625 airfo

Reynolds numbers. Reynolds numbers.

Figure 3.70 Drag polar for the 417a airfoil at low Reynolds numbers.

This explanation is substantiated by Figure 3.66. Here Q versus a is given for the N60 airfoil. As a is first increased up to a value well beyond the stall and then decreased, a large hysteresis is seen to exist in the curves for the higher Reynolds numbers. Typically, as a is increased, complete separation on the upper surface occurs at around 12°. The angle of attack must then be decreased to around 5° before the flow will again reattach. At the lowest Reynolds number, the lift curve tends to follow the portion of the curves at the higher Reynolds numbers after stall has occurred and a is decreasing. Thus, above an a of approximately 0°, it would appear that the flow is entirely separated from the upper surface for the lower R values of 21,000 and 42,000.

Aerodynamic drag is considered in more detail in the following chapter. Nevertheless, the drag characteristics for these low-Reynolds number tests are presented now in Figures 3.67 to 3.71.

Figure 3.71 Drag polar for the N60 airfoil at low Reynolds numbers.