AIRFOIL TERMINOLOGY
Air foil terminology
An Air foil is primary building block of an aircraft. How an air foil is described in modern science, what are the parameters that determines its shape? Here is the answer
An air foil 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 air foil
· Asymmetrical air foil;
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 aircraft's 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?
For example, the NACA 2412 air foil has a maximum camber of
2% located 40% (0.4 chords) from the leading edge with a
maximum thickness of 12% of the chord.
Four-digit series air foils by default have maximum thickness at
30% of the chord (0.3 chords) from the leading edge.
The NACA 0015 air foil is symmetrical, the 00 indicating that it has no camber. The 15 indicates that the air foil has a 15% thickness to chord length ratio: it is 15% as thick as it is long.
Symmetrical air foils have identical upper and lower surfaces. They are suited to rotary-wing applications because they have almost no centre 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 air foil produces less lift than a non symmetrical air foil and also has relatively undesirable stall characteristics.
The helicopter blade (air foil) must adapt to a wide range of air speeds and angles of attack during each revolution of the rotor. The symmetrical air foil delivers acceptable performance under those alternating conditions. Other benefits are lower cost and ease of construction as compared to the non symmetrical air foil.
Non symmetrical (cambered) air foils may have a wide variety of upper and lower surface designs. The advantages of the non symmetrical air foil are increased lift-drag ratios and more desirable stall characteristics.
Non symmetrical air foils were not used in earlier helicopters because the centre of pressure location moved too much when angle of attack was changed. When centre 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 non symmetrical air foils.
