NACA Report No. 964

NACA Report No. 964 - The effects of variations in Reynolds number between 3.0 x 106 and 25.0 x 106 upon the aerodynamic characteristics of a number of NACA 6-series airfoil sections was published by the United States National Advisory Committee for Aeronautics in 1950. It contained a series of graphs showing the resulting lift and drag of several NACA 6-series airfoil sections from tests performed in a variable-density wind tunnel, in which the Reynolds number (RN) was set at three different values.

Summary
Results are presented of an investigation made to determine the two-dimensional lift and drag characteristics of nine NACA 6-series airfoil sections at RN of 15.0, 20.0, and 25.0 million. Also presented are data from NACA Rep. 824 for the same airfoils at RN of 3.0, 6.0, and 9.0 million. The airfoils selected represent sections having variations in the airfoil thickness, thickness form, and camber. The characteristics of an airfoil with a split flap were determined in one instance, as was the effect of surface roughness. Qualitative explanations in terms of flow behavior are advanced for the observed types of scale effect.

Introduction
Two-dimensional aerodynamic data obtained at RN of 3.0, 6.0 and 9.0 million are now generally available for a large number of systematically derived NACA airfoil sections. This range of RN is sufficient to satisfy engineering needs for many practical applications, but the recent trends toward both very large and very high-speed aircraft have emphasized the necessity for aerodynamic data at higher values. An investigation has accordingly been made of the aerodynamic characteristics of a number of systematically varied NACA 6-series airfoils at RN of 15.0, 20.0 and 25.0 million. The results of this investigation at high RN together with those for the same airfoils at lower RN are presented in Report No. 964. The NACA 63 series was chosen as the basic group for investigation because these airfoils appear to offer good low-speed characteristics with a minimum of compromise from consideration of the high-speed characteristics. In all cases, only lift and drag were measured.

Apparatus
All the tests were made in the Langley two-dimensional low-turbulence pressure tunnel. The test section of this tunnel measures 3 x 7.5 feet, and the model completely spanned the 3-foot dimension. Seals were installed between the ends of the model and the tunnel walls to prevent air leakage.

Drag
The reaction of the minimum drag coefficient of smooth airfoils to increasing RN is attributed to the relative strengths of two interacting boundary-layer changes. A thinning of the boundary layer with increasing RN gives a gradual decrease of minimum drag. As the RN is increased beyond a certain value, however, the transition point begins to move forward and the drag increases. The flow conditions of the thicker airfoils are more favorable for delaying the forward movement of transition.

The addition of standard roughness to the NACA 63-009 section causes a large increase in the minimum drag at all RN, but increasing the RN has a favorable effect in reducing the drag.

Increasing the RN from 9.0 to 15.0 million resulted in the almost complete disappearance of the low-drag range of all the airfoils except that of 18% thickness.

Lift
The angle of zero lift of the cambered airfoils showed almost no variation with RN. Throughout the range of RN of this investigation, the values of the lift-curve slope for the smooth sections are very close to that predicted by thin-airfoil theory (2π per radian, or 0.110 per degree). The effects of increasing RN on the maximum lift showed two general trends: for airfoils of 12% thickness or less, the maximum lift remains relatively constant over the lower range of RN. Increasing the RN, however, causes a rapid increase in maximum lift; the 18% thick airfoils demonstrated a steady increase of maximum lift as RN is increased.

Concluding remarks
The 18% thick section had a type of maximum-lift variation with the RN that was entirely different from the thinner sections. Any comparison of airfoil maximum-lift characteristics can be made only if the data for the group of airfoils under consideration are available at the same RN. The choice of an optimum airfoil for maximum lift for a given application must be determined from data corresponding to the operating RN of the application.