Figure 4-5.  Typical airfoil section.  from the leading edge to the trailing edge, is the mean camber  line. This mean line is equidistant at all points from the upper  and lower surfaces.  An airfoil is constructed in such a way that its shape takes  advantage of the air’s response to certain physical laws. This  develops two actions from the air mass: a positive pressure  lifting action from the air mass below the wing, and a negative  pressure lifting action from lowered pressure above the wing.  As the air stream strikes the relatively flat lower surface of  a wing or rotor blade when inclined at a small angle to its  direction of motion, the air is forced to rebound downward,  causing an upward reaction in positive lift. At the same time,  the air stream striking the upper curved section of the leading  edge is deflected upward. An airfoil is shaped to cause an  action on the air, and forces air downward, which provides  an equal reaction from the air, forcing the airfoil upward. If  a wing is constructed in such form that it causes a lift force  greater than the weight of the aircraft, the aircraft will fly.  If all the lift required were obtained merely from the  deflection of air by the lower surface of the wing, an aircraft  would only need a flat wing like a kite. However, the balance  of the lift needed to support the aircraft comes from the flow  of air above the wing. Herein lies the key to flight.  It is neither accurate nor useful to assign specific values to the  percentage of lift generated by the upper surface of an airfoil  versus that generated by the lower surface. These are not  constant values. They vary, not only with flight conditions,  but also with different wing designs.  Different airfoils have different flight characteristics. Many  thousands of airfoils have been tested in wind tunnels and in  actual flight, but no one airfoil has been found that satisfies  every flight requirement. The weight, speed, and purpose  of each aircraft dictate the shape of its airfoil. The most  efficient airfoil for producing the greatest lift is one that has  a concave or “scooped out” lower surface. As a fixed design,  this type of airfoil sacrifices too much speed while producing  Applying Bernoulli’s Principle of Pressure, the increase in  the speed of the air across the top of an airfoil produces a  In a wind tunnel or in flight, an airfoil is simply a streamlined  object inserted into a moving stream of air. If the airfoil  profile were in the shape of a teardrop, the speed and the  pressure changes of the air passing over the top and bottom  would be the same on both sides. But if the teardrop shaped  airfoil were cut in half lengthwise, a form resembling the  basic airfoil (wing) section would result. If the airfoil were  then inclined so the airflow strikes it at an angle, the air  moving over the upper surface would be forced to move  faster than the air moving along the bottom of the airfoil.  This increased velocity reduces the pressure above the airfoil.  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 airplane off the ground.  Thus, modern airplanes have airfoils that strike a medium  between extremes in design. The shape varies according to  the needs of the airplane for which it is designed.  Figure 4-6  shows some of the more common airfoil designs.  lift and is not suitable for high-speed flight. Advancements  in engineering have made it possible for today’s high-speed  jets to take advantage of the concave airfoil’s high lift  characteristics. Leading edge (Kreuger) flaps and trailing  edge (Fowler) flaps, when extended from the basic wing  structure, literally change the airfoil shape into the classic  concave form, thereby generating much greater lift during  slow flight conditions.  Low Pressure Above  4-7  C a m b e r  of  u p e r s u r f a c e  C am ber  of  l o w e r  s u r f a c e  Trailing edge  Leading edge  Mean camber line  Chord line  Early airfoil  Later airfoil  Laminar flow airfoil  (Subsonic)  Circular arc airfoil  (Supersonic)  Double wedge airfoil  (Supersonic)  Clark 'Y' airfoil  (Subsonic)  Figure 4-6.  Airfoil designs. 