Basic Aerodynamics  Pressure The Atmosphere Aerodynamics is the study of the dynamics of gases, the  interaction between a moving object and the atmosphere  being of primary interest for this handbook. The movement of  an object and its reaction to the air flow around it can be seen  when watching water passing the bow of a ship. The major  difference between water and air is that air is compressible  and water is incompressible. The action of the airflow over  a body is a large part of the study of aerodynamics. Some  common aircraft terms, such as rudder, hull, water line, and  keel beam, were borrowed from nautical terms.  Many textbooks have been written about the aerodynamics  of aircraft flight. It is not necessary for an airframe and  powerplant (A&P) mechanic to be as knowledgeable as an  aeronautical engineer about aerodynamics. The mechanic  must be able to understand the relationships between  how an aircraft performs in flight and its reaction to the  forces acting on its structural parts. Understanding why  aircraft are designed with particular types of primary and  secondary control systems and why the surfaces must be  aerodynamically smooth becomes essential when maintaining  today’s complex aircraft.  The theory of flight should be described in terms of the laws  of flight because what happens to an aircraft when it flies  is not based upon assumptions, but upon a series of facts.  Aerodynamics is a study of laws which have been proven  to be the physical reasons why an airplane flies. The term  aerodynamics is derived from the combination of two Greek  words: “aero,” meaning air, and “dyne,” meaning force of  power. Thus, when “aero” joins “dynamics” the result is  “aerodynamics”—the study of objects in motion through  the air and the forces that produce or change such motion.  Aerodynamically, an aircraft can be defined as an object  traveling through space that is affected by the changes  in atmospheric conditions. To state it another way,  aerodynamics covers the relationships between the aircraft,  relative wind, and atmosphere.  Before examining the fundamental laws of flight, several  basic facts must be considered, namely that an aircraft  operates in the air. Therefore, those properties of air that  affect the control and performance of an aircraft must be  understood.  The air in the earth’s atmosphere is composed mostly of  nitrogen and oxygen. Air is considered a fluid because it  fits the definition of a substance that has the ability to flow  or assume the shape of the container in which it is enclosed.  If the container is heated, pressure increases; if cooled, the  pressure decreases. The weight of air is heaviest at sea level  where it has been compressed by all of the air above. This  compression of air is called atmospheric pressure.  Atmospheric pressure is usually defined as the force exerted  against the earth’s surface by the weight of the air above  that surface. Weight is force applied to an area that results  in pressure. Force (F) equals area (A) times pressure (P), or  F = AP. Therefore, to find the amount of pressure, divide  area into force (P = F/A). A column of air (one square inch)  extending from sea level to the top of the atmosphere weighs  approximately 14.7 pounds; therefore, atmospheric pressure  is stated in pounds per square inch (psi). Thus, atmospheric  pressure at sea level is 14.7 psi. Atmospheric pressure is measured with an instrument  called a barometer, composed of mercury in a tube that  records atmospheric pressure in inches of mercury ("Hg).  [Figure 2-1]  The standard measurement in aviation altimeters  and U.S. weather reports has been "Hg. However, worldwide  weather maps and some non-U.S. manufactured aircraft  instruments indicate pressure in millibars (mb), a metric unit.  At sea level, when the average atmospheric pressure is  14.7 psi, the barometric pressure is 29.92 "Hg, and the metric  measurement is 1013.25 mb.  An important consideration is that atmospheric pressure  varies with altitude. As an aircraft ascends, atmospheric  pressure drops, oxygen content of the air decreases, and  temperature drops. The changes in altitude affect an aircraft’s  performance in such areas as lift and engine horsepower. The  30 25 20 15 10 5 0 Inches of Mercury Millibars 1016 847 677 508 339 170 0 29.92" Standard Sea Level Pressure Hg 1013 Standard Sea Level Pressure mb Atmospheric Pressure Vacuum 0.491 lb Mercury$^{1"}$ 1" 1" Figure 2-1.  Barometer used to measure atmospheric pressure. 2-2