Which Airfoil Section By John W. Thorp he choice of the best airfoil section depends upon the T scale effect the Reynolds Number. The best airfoil secspeed and size of the airplane. An engineer calls this

tion for a light low speed airplane will be different than the best airfoil section for a large fast airplane. Altogether too much emphasis has been placed upon procedures for the selection of an airfoil for a new design. When the basic requirements for an airfoil for a light airplane are established, any available airfoil that satisfies these requirements will be almost equally as

good as any other airfoil meeting these requirements. Calculating classical ratios of CL max/CD min is virtually a waste of time since the experimental errors introduced in recording and interpreting test results are often greater than the differences between individual airfoils.

Care must be exercised to only compare airfoils tested under identical conditions and at the same scale (Reynolds Number). Reynolds Number=9354VC in a standard sea level air where V is Velocity in miles per hour and C is Wing Chord in feet. A light airplane with a 4 ft. chord landing at 50 mph will have a landing Reynolds Number of 1,875,000 in standard sea level air. The maximum lift coefficient decreases quite a bit as the Reynolds Number decreases (i.e. small plane and

slightly over 3 million. Later tests indicate that turbulence and other factors inherent in this tunnel raised the actual effective Reynolds Numbers to 7 or 8 million. When applied to an airplane the results shown in reports such as NACA Technical Report No. 460 are much

too favorable. The corrections necessary to reduce the reported values to the proper Reynolds Numbers of approximately 2 million are beyond the amateur's reach. It is much more practical to use reports such as NACA Technical Reports No. 586 and No. 824 which show results at more nearly the actual light airplane Reynolds Numbers. However, these later reports slip in a "hooker" which will throw the uninitiated. They introduce "section characteristics". Section characteristics are

very useful, but should not be confused with wing characteristics such as would be obtained from model tests at some aspect ratio such as 6. Section characteristics are shown for infinite aspect ratio. The lift curve slopes shown are much steeper than would be experienced with the relative low aspect ratios of homebuilt airplanes. A small aspect ratio stretches out the lift coefficient vs. angle of attack curve so that a higher angle of attack is required for the same lift. This occurrence is easily seen in Fig. 2.

low speed). Fig. 1 shows the maximum lift coefficiental various Reynolds Numbers for a 23012 airfoil. Note that

the maximum lift coefficient is only 1.49 at the Rn of 1,875,000 of the sample airfoil.

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Figure 1

Tests conducted in the variable density wind tunnel by NACA in the early 1930's show Reynolds Numbers of

Many good aerodynamics texts show the method of correcting the infinite aspect ratio section characteristics to actual aspect ratio wing characteristics suitable for use in design. NACA Technical Report No. 824 also shows how this is done.

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Generally speaking, an airfoil for a light airplane should possess moderate to large camber as these airfoils do not suffer such large maximum lift coefficient losses at low Reynolds Numbers as will symmetrical airfoils or low cambered airfoils. Airfoils with 3% or 4% camber are desirable. Fig. 3 shows the maximum lift coefficient for various cambers for the NACA five digit airfoils with a 12% thickness. Note that the lift coefficient increases as the camber increases.

The airfoil section thickness should be as small as is consistent with housing the structure. Practical thicknesses are from 12% to 15% for cantilever homebuilt monoplane wings depending upon aspect ratio. Fig. 4 shows the maximum lift coefficient that can be obtained at various thicknesses for the NACA 63_2XX airfoils. Note that the lift coefficient is a maximum at 12 and 15%.

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Figure 3

Stall characteristics are an item deserving careful consideration in the design of a small airplane. Actually, wing geometry, airplane directional and longitudinal stability, etc., will have greater effect upon the airplane's stall behavior than will the airfoil alone. However, an airfoil should be selected which will show a smooth peak of its lift coefficient curve at low Reynolds Numbers to avoid abrupt stall. Moderately blunt leading edge radii characterize airfoils suitable for use on small airplanes. The stalling phenomenon is best explained by the point of initial flow separation and its progression. When the separation starts at the trailing edge and moves forward the peak of the CL curve is smooth and desirable. When the stall starts at the leading edge all flow breaks down at once and the CL curve shows an abrubt or detached peak. The higher the Reynolds Number, the greater the flow energy and the sharper will be the leading edge around which the air will pass at large angles of attack without breaking down. Where Reynolds Numbers and

flow energy are low, large leading edge radii are required to insure separation starting at the trailing edge.

Thickness distributions should generally be well forward like the older airfoils as these will be less sensitive to construction irregularities than some of our more modern airfoils with points of maximum thickness from

40% to 50% of chord. 24

JUNE

1960

I find it difficult to pick airfoils better than the

3412, 4412, 4415, Clark Y, USA 35B, Gottingen 398, etc.,

for most homebuilt airplanes. For a 15% thick wing the newer NACA 652 415 with a = .50 is well behaved and does not require unreasonable smoothness. Airfoils of large camber such as are herein advocated possess large pitching moment coefficients. However, the actual pitching moment is proportional to velocity squared and on low speed airplanes no part of the structure will be designed by pitching moment or the related center of pressure travel. Actually, construction irregularities near the trailing edge on low moment coefficient airfoils can make high moment coefficient airfoils of them. Deflecting the ailerons will move published moment coefficients of symmetrical airfoils clear off the chart.

Regardless of airfoil chosen, care must be taken in wing construction to build accurately and to finish smoothly. Wings are almost never as smooth as wind tunnel models and flight results are almost always somewhat poorer than anticipated. The importance of a smooth surface can be readily

understood when one realizes that the maximum lift coefficient for the NACA 63-415 decreases 20% for an airfoil with standard roughness. Where drag is concerned it is relatively unimportant which airfoil of a given thickness you choose. What is important is to build the wing smoothly.