Aerofoil / Airfoil
(The shape of the wing)

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Model Aircraft Aerodynamics

Aerofoil - a device that provides reactive force when in motion relative to the surrounding air; can lift or control a plane in flight. Aerofoils work using the principle of the Bernoulli effect, that a faster flowing fluid (air) exerts a lower pressure on its surroundings than a slower flowing fluid. Due to the shape of aerofoils the air that passes above the wing as well as taking a longer path, is accelerated backwards, this means that the air flowing over the top surface of the wing is moving over the aerofoil surface much faster than that flowing over the lower surface. Thus the air below the aerofoil exerts a larger force on the surface of the wing than that flowing over the top, resulting in the aerofoil generating an upwards resultant force, lift. The acceleration of the air flowing over the top of the wing also causes downwash, which increases drag: Aerofoils are described by their: Chord, Camber, Thickness and Thickness distribution.

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Alternatively, an airfoil in an inverted position will create a downward pressure on an automobile or other motor vehicle, improving its traction and reducing its likelihood of becoming airborne. Airfoils are also found in propellors, fans, and turbines. Flying Radio Control Aerobatics (Radio Control Handbooks) Charles Allison, Andy Nicholls Flying Radio Control Aerobatics (Radio...

Changes in Aerofoil Design since the dawn of Powered Flight
This article is reproduced in condensed form by kind permission of
Helen Alderson, MEng Aerospace Engineering . See the full article

The first serious studies into aerofoil (the shape of the wing) design were in the late 19th Century and showed that a flat plate could create lift. However, studies into the shape of birds’ wings showed that a curved shape was more efficient as the air is forced over the wing at a higher speed than the air passing below the wing. This means that the air below the wing is at a higher pressure than the air above and therefore the air below the wing pushes upwards causing lift.

The first sustained and controlled powered flight was achieved by Orville and Wilbur Wright, on 17th December 1903 at Kitty Hawk, North Carolina and flew a distance of about 120 feet in approximately 12 seconds at an elevation of 10 feet. They had conducted lots of research into the best aerofoil design testing numerous different styles in their wind tunnel. They achieved four successful flights that day with the final one, flown by Wilbur, covering 852 feet in 59 seconds. Just two years later, the Wright Brothers were making flights in excess of 30km lasting around 30 minutes.

Early Aerofoil Designs

The aerofoil, which the Wright Brothers used, was thin and highly cambered. This was most probably because they did tests of aerofoil sections at extremely low Reynolds numbers, where thin sections appear to behave better than thick aerofoil sections. They thought that efficient aerofoils had to be thin and highly cambered (curved) and therefore their aircraft were biplanes.

It was also soon noticed that having a sharp leading edge lead to the aircraft stalling at very small angles of attack, which limited the lifting capabilities of flat aerofoil designs. Also the thin sections needed to be supported by external struts, which create lots of drag especially in the case of biplanes. However, the Wright Brothers did not have problems as they travelled at such a low speed that drag was minimised.

The first improvements that were made increased the thickness at the leading edge of the wing to limit the risk of stalling

The next improvements were made basically on a trial and error basis tending to use thicker aerofoil sections than the Wright and Bleriot designs. They also reduced the camber of the lower edge of the section, which increased the lift produced by the wing. The Gottingen 398 section developed in 1919 and the Clark Y section developed in 1922 were particularly successful and later became the basis for the NACA Four-Digit Series tested in 1932.

The NACA Four-Digit Series

In 1932 NACA (National Advisory Committee for Aeronautics) tested what is known as the Four-Digit Series, a group of aerofoil sections. These aerofoil sections were designed using approximations to wing sections in use at the time, which were known to be efficient, such as the Clark Y section.

The Four-Digit aerofoil geometry is defined by four numbers; the maximum camber of the chord, the location of the maximum camber in tenths of the chord and two numbers for the maximum thickness in percent of the chord.

The NACA 2412, for example, has a 2% camber located 40% from the leading edge and a thickness of 12%

The NACA Five-Digit Series

In 1935 NACA developed the Five-Digit Series, which uses the same thickness distribution as the Four-Digit Series but a different mean camber line so that the position of maximum camber is further forward and therefore the design lift coefficient of the aerofoil section is increased.

The numbering for the Five-Digit Series is more complicated than that of the Four-Digit Series with the first digit when multiplied by 3/2 giving the lift coefficient in tenths of the aerofoil, the next two digits represent twice the position of maximum camber in percent of the chord and the final two numbers represent the percentage thickness. The NACA 23012, for example, has a design lift coefficient of 0.3, the maximum camber located 15% from the leading edge and a thickness of 12% (figure 3.2).

The NACA 1-Series (Series 16)

In 1939 NACA developed the 1-Series but unlike the Four-Digit and Five-Digit series the 1-Series was developed using aerofoil theory instead of geometrical relationships. The theory was to specify the required pressure distribution over the aerofoil as this controls the lift characteristics of the shape and then use this to find the geometrical shape that best produces the pressure distribution.

1-Series aerofoils are also defined by five numbers, the first represents the series, the second the location of the minimum pressure in tenths of the chord, the third the design lift coefficient in tenths and the final two the maximum thickness in percent of the chord. The NACA 16-212, for example, is in the 1-Series with the minimum pressure located 60% back from the leading edge. The design lift coefficient is 0.2 and the maximum thickness of chord is 12%

However, as the 16-XXX aerofoils are the only ones that have really been used they are often referred to as the 16-Series instead of as a sub-group of the 1-Series.

The NACA 6-Series

NACA produced the 2-Series through to the 5-Series using experimental theoretical methods but it was not until the 6-Series that they produced the desired aerofoil behaviour using improved theoretical methods. The aim of the 6-Series was to reduce the drag whilst achieving the maximum design lift coefficient. This was achieved by maximising the region over which the airflow remained laminar.

Many variations of numbering conventions exist for the 6-Series. The most common is the same as is used for the 1-Series with the addition of a subscript between the second and third numbers which indicates how the drag relates to the design lift coefficient. The NACA 641-212, for example, is in the 6-Series with its location of minimum pressure 40% from the leading edge. The subscript represents that low drag is maintained at lift coefficients 0.1 above and below the design lift coefficient of 0.2 and the thickness is 12% of the chord

The NACA 7-Series

The 7-Series was an attempt on improvement of the 6-Series to maximise the areas of laminar flow over each aerofoil by changing the locations of minimum pressure on the upper and lower surfaces.

The numbering convention for the 7-Series is the same as that used for the 1-Series with the addition of a number to differentiate between the location of the minimum pressure on the upper and lower surfaces of the aerofoil and a letter, which represents the thickness distribution and mean line forms used. The NACA 747A315, for example, is in the 7-Series and its location of the minimum pressure is 40% from the leading edge on the upper surface and 70% from the leading edge on the lower surface. The ‘A’ represents a series of standardised forms derived from the earlier series. The design lift coefficient is 0.3 and the thickness is 15% of the chord

The NACA 8-Series

The NACA 8-Series is the final variation on the 6- and 7-Series. These aerofoils were designed for flights at supercritical speeds. Again the aim was to separately maximise the laminar flow on the upper and lower surfaces of the aerofoil.

The numbering system for 8-Series is the same as that of the 7-Series. The NACA 835A216, for example, is in the 8-Series and its location of minimum pressure is 30% from the leading edge on the upper surface and 50% from the leading edge on the lower surface. The ‘A’ represents the series of standardised forms derived from the earlier series. The design lift coefficient is 0.2 and the thickness is 16% of chord .

Modern Aerofoil Developments

Recently, systematic series of aerofoils have decreased as specialized aerofoils are designed to meet the specific requirements. These aerofoils are designed with the help of computer systems and theoretical relationships, which have been developed through extensive research using wind tunnels and mathematical models. The Supercritical aerofoil, which is designed to fly at supersonic speeds, has a well-rounded leading edge, is relatively flat on top and a drooping trailing edge. For a supercritical aerofoil of constant thickness of 12%, wind tunnel research shows a possible increase of 15% in the drag-divergence compared to the more conventional 6-Series. The well-rounded leading edge also improves the design lift coefficient compared with the 6-Series.

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