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Some comments on Naca symmetry airfoil data

Full lift and drag data for NACA0012 airfoil from Strickland and Sheldahl at 0.30 and 0.36 million respectively are shown in the figure above. Sheldahl data shows a huge dip after the stall compared to Strickland data. Data below 20° are important for Cp curve result at roughly tsr 3 and above, so Cp curves from both data produce different results especially at tsr 2 to 4. Sheldahl obtained data from testing at Reynolds number of 0.35, 0.5, and 0.7 million and interpolated and extrapolated the rest using a computer program. Beyond 25degree (25 to 180degree), all data are assumed to follow similar curve i.e. influence of Reynolds number can be neglected. Strickland used Jacobs’s data for below 30° and Riegels data for the rest.

Sheldahl NACA0015 lift data at 0.04, 0.36, and 10 million are shown in figure above. The 0.04 and 10 million are extrapolated result from computer program. The problem lies in the low Reynolds data which shows negative lift at the dip after stall. The curves are also not very smooth as the program tries to ‘connect’ the data at 25° (Their original testing data shown in their graphs are smooth). Also, the way the lift coefficient data from changes with Reynolds number differs a lot from Jesch and Jacobs’s data shown in figures below.

Lift data used by Jesch and Walton (1980) for their performance model is shown above. The data source was quoted as Loftin NACA report (Online version not scanned in good condition). Anyway, look carefully at the lift curve change with Reynolds number.

This final figure above is the lift curve for NACA0015 from Jacobs NACA report. Compare Jacobs trend with Jesch and Sheldahl data. There’s definitely some problem with Sheldahl computer generated data but Jesch and Jacobs data exhibit quite similar trend. A useful information is that both Jesch and Sheldahl mentioned that the lift curve after around 25° remains quite constant with Reynolds number.

All the major thin and thick NACA00XX symmetry airfoils lift data shown in the left figure from Sheldahl at 360e3 Reynolds number (in test data Re range). Which airfoil gives better performance or more specific, the highest Cp? Feeding the airfoil data into performance prediction model will give a value but the performance gain between airfoil might be very small that it remains in the error bandwidth (error in input airfoil data and computer model itself). Maybe only testing can verify this.

Related papers

J.H. Strickland (1975) "The Darrieus Turbine: A Performance Prediction Model Using Multiple Streamtubes" SAND75-0431

Sheldahl, R. E., & Klimas, P. C. (1980). Aerodynamic characteristics of 7 symmetrical airfoil sections through 180-degree angle of attack for use in aerodynamics analysis of vertical axis wind turbine. Sandia National Laboratories: SAND80-2114.

Jesch, L. F., & Walton, D. (1980). Reynolds number effects on the aerodynamic performance of a vertical axis wind turbine. Proceedings of third international symposium on wind energy systems. pp.G1-323-332.

Laurence K. Loftin, Jr. Hamilton A. Smith (1949). Aerodynamic characteristics of 15 NACA airfoil sections at seven Reynolds numbers from 0.7 x 10(exp 6) to 9.0 x 10(exp 6). NACA TN 1945

Jacobs, Eastman N Sherman, Albert (1937). Airfoil section characteristics as affected by variations of the Reynolds number. NACA Report 586

Last updated at November 6, 2002
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