AIRFLOW AROUND AN AIRFOIL

BERNOULLI'S PRINCIPLE Bernoulli's principle (fig. 3-2) states that when a fluid flowing through a tube reaches a constriction or narrowing of the tube, the speed of the fluid passing through the constriction is increased and its pressure is decreased. Q3-1. The willingness of an object to stay at rest because of inertia is described by which of Newton's laws of motion? Q3-2. A heavy object will accelerate more slowly than a light object when an equal amount of force is applied. Which of Newton's laws describes this statement? Q3-3. If you blow up a balloon and then release it, it will move in what direction? Q3-4. When fluid reaches a narrow part of a tube, its speed increase and its pressure is decreased. What law does this statement describe? THE AIRFOIL LEARNING OBJECTIVE: Recognize the terms used to describe the various parts of an airfoil section and the terms used in explaining the airflow lift generation. An airfoil is defined as that part of an aircraft that produces lift or any other desirable aerodynamic effect as it passes through the air. The wings and the propeller blades of a fixed-wing aircraft and the rotor blades of a helicopter are examples of airfoils. AIRFOIL TERMINOLOGY The shape of an airfoil and its relationship to the airstream are important. The following are common terms that you should understand before you learn about airfoils. Leading edge The front edge or surface of the airfoil (fig. 3-3). Trailing edge The rear edge or surface of the airfoil (fig. 3-3). Chord line An imaginary straight line from the leading edge to the trailing edge of an airfoil (fig. 3-3). Camber The curve or departure from a straight line (chord line) from the leading to the trailing edge of the airfoil (fig. 3-3). Relative wind The direction of the airstream in relation to the airfoil (fig. 3-4). Angle of attack The angle between the chord line and the relative wind (fig. 3-4). AIRFLOW AROUND AN AIRFOIL The generation of lift by an airfoil depends on the airfoil's being able to create a special airflow in the airstream. This airflow develops the lifting pressure over the airfoil surface. The effect is shown in figure 3-5, which shows the relationship between lift and Bernoulli's principle. As the relative wind strikes the leading edge of the airfoil, the flow of air is split. A portion of the relative wind is deflected upward and aft, and the rest is deflected downward and aft. Since the 3-2 Figure 3-2.—Bernoulli's principle. Figure 3-3.—Airfoil terminology.