Principles of Flight

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