# Never heard of this Aerofoil?

I just came across this aerofoil, 65-2xx?

I searched this but it doesn’t come up with anything like 65-200 for example but instead SG6051 which is the closest I could find to this?

Am I missing something here please?

You are not missing anything. The notation, 65-2xx, is in reference to an NACA Series 6 airfoil having a lift coefficient of 0.2 and an undetermined thickness of 10 percent or less. As a comparison, the designation $${65_3}{-018}$$ indicates an NACA Series 6 airfoil having minimum chordwise pressure at 50 percent of the airfoil chord, favorable minimum chordwise pressure existing at a range of lift coefficients 3 tenths above and below the airfoil design lift coefficient, a design lift coefficient of 0 (i.e., zero), and a design thickness of 18 percent of the airfoil chord. Consequently, a $${65_3}{-218}$$ airfoil would have a design lift coefficient of 0.2.

For clarification of the designation such as 65-2xx, the following is quoted for clarity from p. 6 of NACA Report No. 824, Summary of Airfoil Data -

For NACA 6-series airfoils having a thickness ratio less than 0.12 of the chord, the subscript number indicating the low-drag range should be less than unity. Rather than use a fractional number, a subscript of unity was originally employed for these airfoils. Since this usage is not consistent with the previous definition of a number indicating the low-drag range, the designations of airfoil sections having a thickness ratio less than 0.12 of the chord are now given without such a number. As an example, an NACA 6-series airfoil having a thickness ratio of 0.10 of the chord would be designated: NACA 65-210

For the case at hand, 65-2xx is a generalized NACA designation for a similar group of these thinner airfoils. Series 6 airfoils have been extensively used in applications for wings having low drag achieved through laminar flow.

That is an airfoil from the NACA 6-Series.

I quote the Wikipedia article linked above:

• The number "6" indicates the series.
• One digit describes the distance of the minimum pressure area in tenths of the chord.
• The subscript (which is missing in your designation) digit gives the range of lift coefficient in tenths above and below the design lift coefficient in which favorable pressure gradients exist on both surfaces.
• A hyphen.
• One digit describes the design lift coefficient in tenths.
• Two digits describe the maximum thickness as percent of chord.

That's why looking for 65-200 won't give you any results because that airfoil would have 0% thickness. 65-210, 65-212 or 65-215 would be "valid" airfoils.