AIRFOIL TERMINOLOGY

                    Air foil Terminology

What is an airfoil? How an air foil is described in modern science, what are the parameters that determines its shape?

An Air foil is primary building block of an aircraft which has two edges namely leading edge and trailing edge, the straight line connecting the leading and trailing edge of the air foil is called as chord line. The line which is drawn equidistant from the upper and lower surfaces of the air foil and connecting the leading and trailing edge is called as mean camber line.The distance between the leading and trailing edge is called chord of an air foil.

 The maximum distance between the mean camber line and chord line is referred as maximum camber. This is one of the variables that determine the aerodynamic characteristics of the wings.

Maximum thickness to chord ratio is expressed in terms of percentage. For subsonic wings the ratio is normally 12-14%

 Depending on the camber (curvature) of the air foil it is defined into two categories

•  Symmetrical Airfoil
•  Asymmetrical or Cambered Airfoil.

 Symmetrical air foil has identical upper and lower surfaces and the mean camber line is coincident with the camber line i.e. wing camber is reduced to zero.

 Asymmetrical air foil has wing camber and two different types of wings are used normally. A thick well cambered wing will produce high lift at slow speeds where as a thin long wing will produce good high speed characteristics.

National Advisory Committee for Aeronautics (NACA) has developed numerous air foils which are used in various aircrafts around the world. They are usually of four or five digits. If the first two digits of the air foil is zero then it is a symmetrical air foil. 

 How to identify the parameters of a four digit air foil?

 In a four digit Airfoil

First digit represents the maximum camber as the percentage of chord.

Second digit represents the distance of maximum camber from leading edge of the Airfoil in tenths of chord

Last two digits describe the maximum camber as the percentage of chord.

Let us consider two examples here

 NACA 2412 – Cambered Airfoil

Maximum Camber is 2% located 40% (0.4 chords) from the Leading edge

Maximum thickness is 12% of chord

 Four-digit series air foils by default have maximum thickness at 30% of the chord (0.3 chords) from the leading edge.

 NACA 0015- Symmetrical Airfoil

00 indicates that it has no camber.

15 indicate that the air foil has a 15% thickness to chord length ratio, it is 15% as thick as it is long.

Applications:

 Symmetrical Airfoil:       

 Symmetrical airfoils have identical upper and lower surfaces. They are suited to rotary-wing applications because they have almost no center of pressure travel. Travel remains relatively constant under varying angles of attack, affording the best lift-drag ratios for the full range of velocities from rotor blade root to tip.

 However, the symmetrical airfoil produces less lift than a Asymmetrical airfoil and also has relatively undesirable stall characteristics.

The helicopter blade (airfoil) must adapt to a wide range of airspeeds and angles of attack during each revolution of the rotor. The symmetrical airfoil delivers acceptable performance under those alternating conditions. Other benefits are lower cost and ease of construction as compared to the Asymmetrical airfoil.

 Asymmetrical Airfoil:

Non symmetrical (cambered) airfoils may have a wide variety of upper and lower surface designs. The advantages of the Asymmetrical airfoil are increased lift-drag ratios and more desirable stall characteristics. 

Asymmetrical airfoils were not used in earlier helicopters because the center of pressure location moved too much when angle of attack was changed. When center of pressure moves, a twisting force is exerted on the rotor blades. Rotor system components had to be designed that would withstand the twisting force. Recent design processes and new materials used to manufacture rotor systems have partially overcome the problems associated with use of Asymmetrical airfoils.