When a body is acted upon by a constant force, its resulting  acceleration is inversely proportional to the mass of the body  and is directly proportional to the applied force. This takes  into account the factors involved in overcoming Newton’s  First Law. It covers both changes in direction and speed,  including starting up from rest (positive acceleration) and  coming to a stop (negative acceleration or deceleration).  Since air is recognized as a body, and it is understood that  air will follow the above laws, one can begin to see how  and why an airplane wing develops lift. As the wing moves  through the air, the flow of air across the curved top surface  increases in velocity creating a low-pressure area.  Although Newton, Bernoulli, and hundreds of other early  scientists who studied the physical laws of the universe did  not have the sophisticated laboratories available today, they  provided great insight to the contemporary viewpoint of how  lift is created.  Newton’s Third Law: “For every action, there is an equal  and opposite reaction.”  In an airplane, the propeller moves and pushes back the  air; consequently, the air pushes the propeller (and thus the  airplane) in the opposite direction—forward. In a jet airplane,  the engine pushes a blast of hot gases backward; the force of  equal and opposite reaction pushes against the engine and  forces the airplane forward.  An airfoil is a structure designed to obtain reaction upon its  surface from the air through which it moves or that moves  past such a structure. Air acts in various ways when submitted  to different pressures and velocities; but this discussion  is confined to the parts of an aircraft that a pilot is most  concerned with in flight—namely, the airfoils designed to  produce lift. By looking at a typical airfoil profile, such as  the cross section of a wing, one can see several obvious  characteristics of design.  [Figure 4-5]  Notice that there is  a difference in the curvatures (called cambers) of the upper  and lower surfaces of the airfoil. The camber of the upper  surface is more pronounced than that of the lower surface,  which is usually somewhat flat.  A half-century after Newton formulated his laws, Daniel  Bernoulli, a Swiss mathematician, explained how the pressure  of a moving fluid (liquid or gas) varies with its speed of  motion. Bernoulli’s Principle states that as the velocity of a  moving fluid (liquid or gas) increases, the pressure within  the fluid decreases. This principle explains what happens to  air passing over the curved top of the airplane wing.  A practical application of Bernoulli’s Principle is the venturi  tube. The venturi tube has an air inlet that narrows to a  throat (constricted point) and an outlet section that increases  in diameter toward the rear. The diameter of the outlet is  the same as that of the inlet. The mass of air entering the  tube must exactly equal the mass exiting the tube. At the  constriction, the speed must increase to allow the same  amount of air to pass in the same amount of time as in all  other parts of the tube. When the air speeds up, the pressure  also decreases. Past the constriction, the airflow slows and  the pressure increases.  [Figure 4-4]  NOTE: The two extremities of the airfoil profile also differ in  appearance. The rounded end, which faces forward in flight,  is called the leading edge; the other end, the trailing edge, is  quite narrow and tapered.  A reference line often used in discussing the airfoil is  the chord line, a straight line drawn through the profile  connecting the extremities of the leading and trailing edges.  The distance from this chord line to the upper and lower  surfaces of the wing denotes the magnitude of the upper and  lower camber at any point. Another reference line, drawn  Figure 4-4.  Air pressure decreases in a venturi tube.  4-6  8  6 4  2  0  I0  VELOCITY 8  6 4  2  0  I0  PRESSURE  8  6 4  2  0  I0  VELOCITY 8  6 4  2  0  I0  PRESSURE  8  6 4  2  0  I0  VELOCITY 8  6 4  2  0  I0  PRESSURE  Airfoil Design  Bernoulli’s Principle of Differential Pressure 