the turning action that generates lift are complex. In any case  causing lift, an angled flat plate, revolving cylinder, airfoil,  etc., the flow meeting the leading edge of the object is forced to  split over and under the object. The sudden change in direction  over the object causes an area of low pressure to form behind  the leading edge on the upper surface of the object. In turn,  due to this pressure gradient and the viscosity of the fluid,  the flow over the object is accelerated down along the upper  surface of the object. At the same time, the flow forced under  the object is rapidly slowed or stagnated causing an area of  high pressure. This also causes the flow to accelerate along  the upper surface of the object. The two sections of the fluid  each leave the trailing edge of the object with a downward  component of momentum, producing lift.  [Figure 2-4] Bernoulli’s principle describes the relationship between  internal fluid pressure and fluid velocity. It is a statement  of the law of conservation of energy and helps explain why  an airfoil develops an aerodynamic force. The concept of  conservation of energy states energy cannot be created or  destroyed and the amount of energy entering a system must  also exit. Specifically, in this case the “energy” referred  to is the dynamic pressure (the kinetic energy of the air— more velocity, more kinetic energy) and static air pressure  (potential energy). These will change among themselves, but  the total pressure energy remains constant inside the tube. Lift is generated when an object changes the direction of  flow of a fluid or when the fluid is forced to move by the  object passing through it. When the object and fluid move  relative to each other and the object turns the fluid flow in  a direction perpendicular to that flow, the force required to  do this work creates an equal and opposite force that is lift.  The object may be moving through a stationary fluid, or the  fluid may be flowing past a stationary object—these two are  effectively identical as, in principle, it is only the frame of  reference of the viewer which differs. The lift generated by  an airfoil depends on such factors as: A simple tube with a constricted portion near the center of its  length illustrates this principle. An example is running water  through a garden hose. The mass of flow per unit area (cross- sectional area of tube) is the mass flow rate. In  Figure 2-5, the flow into the tube is constant, neither accelerating nor  decelerating; thus, the mass flow rate through the tube must  be the same at stations 1, 2, and 3. If the cross-sectional area  at any one of these stations—or any given point—in the  tube is reduced, the fluid velocity must increase to maintain  a constant mass flow rate to move the same amount of fluid  through a smaller area. The continuity of mass flow causes  the air to move faster through the venturi. In other words,  fluid speeds up in direct proportion to the reduction in area.  The AOA is the angle at which the airfoil meets the oncoming  airflow (or vice versa). In the case of a helicopter, the object  is the rotor blade (airfoil) and the fluid is the air. Lift is  produced when a mass of air is deflected, and it always acts  perpendicular to the resultant relative wind. A symmetric  airfoil must have a positive AOA to generate positive lift. At  a zero AOA, no lift is generated. At a negative AOA, negative  lift is generated. A cambered or nonsymmetrical airfoil may  produce positive lift at zero, or even small negative AOA. The basic concept of lift is simple. However, the details of how  the relative movement of air and airfoil interact to produce  Figure 2-4.  Production of lift. Bernoulli’s Principle Lift 2-3 For a more in-depth explanation of general aerodynamics,  refer to the Pilot’s Handbook of Aeronautical Knowledge. •  Lift—opposes the downward force of weight, is  produced by the dynamic effect of the air acting on the  airfoil and acts perpendicular to the flightpath through  the center of lift. •  Weight—the combined load of the aircraft itself, the  crew, the fuel, and the cargo or baggage. Weight pulls  the aircraft downward because of the force of gravity.  It opposes lift and acts vertically downward through  the aircraft’s center of gravity (CG). •  Thrust—the force produced by the power plant/ propeller or rotor. It opposes or overcomes the force  of drag. As a general rule, it acts parallel to the  longitudinal axis. However, this is not always the case,  as explained later. •  Drag—a rearward, retarding force caused by  disruption of airflow by the wing, rotor, fuselage, and  other protruding objects. Drag opposes thrust and acts  rearward parallel to the relative wind. •  Speed of the airflow •  Density of the air •  Total area of the segment or airfoil •  Angle of attack (AOA) between the air and the airfoil Increased air  pressure underneath R e d u c e d a i r p r e s s u re U p e r c a m b e r h e l p s t o d f l e c t a i r d o w n Mass of air deflected down