The profile of a conventional wing is an excellent example  of an airfoil.  [Figure 2-3]  Notice that the top surface of the  wing profile has greater curvature than the lower surface.  The efficiency of a wing is measured in terms of the lift to  drag ratio (L/D). This ratio varies with the AOA but reaches a  definite maximum value for a particular AOA. At this angle,  the wing has reached its maximum efficiency. The shape of  the airfoil is the factor that determines the AOA at which  the wing is most efficient; it also determines the degree of  efficiency. Research has shown that the most efficient airfoils  for general use have the maximum thickness occurring about  one-third of the way back from the leading edge of the wing.  The difference in curvature of the upper and lower surfaces  of the wing builds up the lift force. Air flowing over the top  surface of the wing must reach the trailing edge of the wing  in the same amount of time as the air flowing under the wing.  To do this, the air passing over the top surface moves at a  greater velocity than the air passing below the wing because  of the greater distance it must travel along the top surface.  This increased velocity, according to Bernoulli’s Principle,  means a corresponding decrease in pressure on the surface.  Thus, a pressure differential is created between the upper  and lower surfaces of the wing, forcing the wing upward in  the direction of the lower pressure.  Within limits, lift can be increased by increasing the angle  of attack (AOA), wing area, velocity, density of the air, or  by changing the shape of the airfoil. When the force of lift  on an aircraft’s wing equals the force of gravity, the aircraft  maintains level flight. High-lift wings and high-lift devices for wings have been  developed by shaping the airfoils to produce the desired effect.  The amount of lift produced by an airfoil increases with an  increase in wing camber. Camber refers to the curvature of an  airfoil above and below the chord line surface. Upper camber  refers to the upper surface, lower camber to the lower surface,  and mean camber to the mean line of the section. Camber is  positive when departure from the chord line is outward and  negative when it is inward. Thus, high-lift wings have a large  positive camber on the upper surface and a slightly negative  camber on the lower surface. Wing flaps cause an ordinary  wing to approximate this same condition by increasing the  upper camber and by creating a negative lower camber.  Individual airfoil section properties differ from those  properties of the wing or aircraft as a whole because of the  effect of the wing planform. A wing may have various airfoil  sections from root to tip, with taper, twist, and sweepback.  The resulting aerodynamic properties of the wing are  determined by the action of each section along the span.  It is also known that the larger the wingspan, as compared  to the chord, the greater the lift obtained. This comparison  is called aspect ratio. The higher the aspect ratio, the greater  the lift. In spite of the benefits from an increase in aspect  ratio, it was found that definite limitations were defined by  structural and drag considerations.  On the other hand, an airfoil that is perfectly streamlined  and offers little wind resistance sometimes does not have  enough lifting power to take the aircraft off the ground. Thus,  modern aircraft have airfoils which strike a medium between  extremes, the shape depending on the purposes of the aircraft  for which it is designed. The shape of the airfoil determines the amount of turbulence  or skin friction that it produces, consequently affecting the  efficiency of the wing. Turbulence and skin friction are  controlled mainly by the fineness ratio, which is defined as the  ratio of the chord of the airfoil to the maximum thickness. If  the wing has a high fineness ratio, it is a very thin wing. A thick  wing has a low fineness ratio. A wing with a high fineness  ratio produces a large amount of skin friction. A wing with  a low fineness ratio produces a large amount of turbulence.  The best wing is a compromise between these two extremes  to hold both turbulence and skin friction to a minimum.  The acute angle the wing chord makes with the longitudinal  axis of the aircraft is called the angle of incidence, or the angle  of wing setting.  [Figure 2-4]  The angle of incidence in most  2-5 Figure 2-4.  Angle of incidence. Figure 2-3.  Airflow over a wing section.  100 mph 14.7 lb/in 2 105 mph 14.67 lb/in 2 115 mph 14.54 lb/in 2 Chord line of wing Longitudinal axis Angle of incidence Shape of the Airfoil Angle of Incidence