Aerofoil, its types and nomenclature

The word aerofoil refers to the cross section of the wing or blade. This aerofoil shape is of two types- Symmetric Aerofoil and Cambered Aerofoil.

Before getting in detail of each type, it is necessary to understand the nomenclature of the aerofoil.

The chord line is a straight line joining the leading edge and trailing edge of the aerofoil. The mean camber line is the one which separates the aerofoil into two equal halves as shown. The maximum distance between the chord line and mean camber line is known as the camber.

The symmetric aerofoil as the name suggests, the upper and lower surfaces are totally symmetrical about the chord line. And the mean camber line, is essentially the same as chord line. A schematic curve of coefficient of lift (C_L) vs angle of attack (α) is shown below:

Now due to the symmetricity of the aerofoil, it produces zero lift at zero degree angle of attack.

Coefficient of lift can be defined as the ratio of Lift force to product of dynamic pressure and planform area, basically a non-dimensional term.

And angle of attack is basically the angle between the chord line and the freestream direction of the air.

More the angle of attack, more is the lift generated up to a certain limit. This certain limit is the critical angle of attack where the aircraft stalls as shown above.

Coming to the cambered aerofoil. It does not have similar upper and lower surfaces and has a greater curvature on the upper part. So the mean camber line and the chord line are separated by some distance.

The C_L vs α curve is shown in the following figure and it can be seen that this aerofoil can produce lift even at zero degree angle of attack since at α = 0 there is some value of C_L hence lift.

To identify whether the aerofoil is symmetric or cambered a good practice is to read the aerofoil’s name. A typical example of NACA Series is taken where-in these aerofoils are named by National Advisory Committee for Aeronautics (NACA). For example, NACA0012, NACA2412, NACA13017, etc.

NACA Aerofoils are designated based on the digit value and the number of digits. NACA series have 4-digit, 5-digit, 6-digit, etc.

For 4-digit:

• The first digit represents the maximum camber in terms of chord length which is the distance between LE and TE.
• Second represents the distance of maximum camber from LE in terms of chord length
• And last two digits represent the maximum thickness of aerofoil in terms of chord length.

 Taking example of NACA 0012 and NACA 2412 aerofoil:

Description	NACA 0012	NACA 2412
Value of maximum camber	Zero	2% of the chord length
Distance of maximum camber from LE	Zero	40% of the chord length
Maximum thickness of Aerofoil	12% of chord length	12% of Chord Length
Type of Aerofoil	Symmetric	Cambered

 For 5-digit:

• The first digit when multiplied with 0.15 gives the design lift coefficient which is nothing but the value of lift coefficient at maximum Lift to Drag Ratio.
• The next two digit when divided by 2 gives the distance of maximum camber in terms of chord length.
• And last two digits again represent the maximum thickness of aerofoil.

 Taking example of NACA 13017 aerofoil:

1^st digit (1) – The value of design lift coefficient is 0.15.
2^nd and 3^rd digit (30) – The value of distance of maximum camber from LE is 15% of the chord length
4^th and 5^th digit (17) – The value of maximum thickness is 17% of the chord. 

 For 6-digit:

• The first digit indicates the series.
• The second digit indicates the chord wise position of minimum pressure point in tenths of percent of chord.
• The third digit indicates the range of lift coefficient in tenths above and below the designed lift coefficient.
• The fourth digit indicates the design lift coefficient in tenths.
• The last two digit again represent the thickness of aerofoil. 

 Taking example of NACA 65(3)-218 aerofoil:

1^st digit (6) – Indicates the series.

2^nd digit (5) – The value of chord wise position of minimum pressure point from LE is 50% of the chord length.

4^th digit (2) – The value of design lift coefficient is 0.2.

3^rd digit (3) – The value of lift coefficient above and below the design lift coefficient is 0.5 and -0.1. This range is governed by the drag bucket region of a drag polar.

5^th and 6^th digit (18) – The value of maximum thickness is 18% of the chord.