speed in a straight line, force must be applied to increase or  decrease the speed.  According to Newton’s law, since air has mass, it is a body.  When an aircraft is on the ground with its engines off, inertia  keeps the aircraft at rest. An aircraft is moved from its state  of rest by the thrust force created by a propeller, or by the  expanding exhaust, or both. When an aircraft is flying at  uniform speed in a straight line, inertia tends to keep the  aircraft moving. Some external force is required to change  the aircraft from its path of flight.  The three laws of motion that have been discussed apply to  the theory of flight. In many cases, all three laws may be  operating on an aircraft at the same time.  Bernoulli’s principle states that when a fluid (air) flowing  through a tube reaches a constriction, or narrowing, of the  tube, the speed of the fluid flowing through that constriction  is increased and its pressure is decreased. The cambered  (curved) surface of an airfoil (wing) affects the airflow  exactly as a constriction in a tube affects airflow.  [Figure 2-2] Diagram A of  Figure 2-2  illustrates the effect of air passing  through a constriction in a tube. In Diagram B, air is flowing  past a cambered surface, such as an airfoil, and the effect is  similar to that of air passing through a restriction.  Newton’s second law states that if a body moving with  uniform speed is acted upon by an external force, the change  of motion is proportional to the amount of the force, and  motion takes place in the direction in which the force acts.  This law may be stated mathematically as follows:  If an aircraft is flying against a headwind, it is slowed down.  If the wind is coming from either side of the aircraft’s  heading, the aircraft is pushed off course unless the pilot  takes corrective action against the wind direction.  Newton’s third law is the law of action and reaction. This  law states that for every action (force) there is an equal  and opposite reaction (force). This law can be illustrated  by the example of firing a gun. The action is the forward  movement of the bullet while the reaction is the backward  recoil of the gun. As the air flows over the upper surface of an airfoil, its  velocity increases and its pressure decreases; an area of low  pressure is formed. There is an area of greater pressure on  the lower surface of the airfoil, and this greater pressure  tends to move the wing upward. The difference in pressure  between the upper and lower surfaces of the wing is called  lift. Three-fourths of the total lift of an airfoil is the result of  the decrease in pressure over the upper surface. The impact  of air on the under surface of an airfoil produces the other  one-fourth of the total lift. An airfoil is a surface designed to obtain lift from the air  through which it moves. Thus, it can be stated that any part  of the aircraft that converts air resistance into lift is an airfoil.  Airfoil Force = mass  ×  acceleration (F = ma) Figure 2-2.  Bernoulli’s Principle. S a m e m a s s o f a i r Mass of air Velocity increased Pressure decreased (Compared to original) Normal pressure Normal flow Increased flow Normal flow Normal pressure A B 2-4 Bernoulli’s Principle and Subsonic Flow