Physics Of Lift

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The Physics of Flight One of the most novel forms of physics is that of aerodynamics, the study of flight and the effects of gases over an airfoil. The study of aerodynamics was furthered by a few discoveries, like gravity, the laws of motion, and the creation of molecular physics. One of the most significant physical laws that led to the creation of the airfoil is the Bernoulli Principle, which states the relationship between pressure, fluid flow velocity, and the potential energy of fluids (liquids and gases). This principle, discovered by Daniel Bernoulli (1700-1782), a Swiss scientist, states that: As the velocity of the fluid increases, the pressure in the fluid decreases. Conversely, as the velocity of the fluid decreases, the pressure in the fluid increases. An operating example of Bernoulli s Principle is the venturi tube. This device is a tube which is narrower in the middle that at the ends. As air passes through the tube, it speeds up as is reaches the narrow portion and slows down again as it passes the restriction. This can be explained as follows: if the air flowing through the entrance to the tube has 1,000 molecules per inch in the tube and is moving at a rate of one inch per second, there are 1,000 molecules flowing through the tube each second. Assuming that the area of the narrow part of the tubing is one-half that of the rest of the tube, the molecules in the air must speed up to get 1,000 molecules per second through this restricted area. With one-half the area, the speed must be twice that in the rest of the tube to permit the same volume of air to pass through the restriction. More pressure is imparted to the molecules as they accelerate. This leaves less energy to exert pressure and the pressure thus decreases. The principles applied to the venturi are also utilized to create lift for an airplane. However, in lieu of a venturi, a wing with an airfoil shape is used to create a pressure differential in the air. And airfoil is any shape, which is designed to produce lift. Although the wing is the primary part of the airplane that produces lift, other airfoils find application as propeller blades and tail surfaces. An airfoil has a leading edge, a trailing edge, a chord, and camber. The leading edge is the part of the airfoil that fist meets the oncoming air. The trailing edge is the aft end of the airfoil where the airflow over the upper surface joins the airflow from the lower surface. The chord line is an imaginary straight line drawn from the leading edge to the trailing edge. This line has significance only in determining the angle of attack of an airfoil and in determining wing area. The camber of an airfoil is the curvature of its upper (upper camber) and lower (lower camber) surface. The relative wind is the wind moving past the airfoil. The direction of this wind is relative to the attitude of position of the airfoil and it is always parallel to the flight path of the aircraft. The velocity of the relative wind is the speed of the airfoil through the air. The angle of attack is the angle formed by the chord of the airfoil and the direction of the relative wind. The pitch attitude of the airfoil and the angle of attack are the same only in level flight. In other flight conditions, they are different values. An aircraft in straight-and-level flight is acted upon by four forces: lift, gravity, thrust, and drag. Lift is the upward acting force; gravity, or weight, is the downward acting force; thrust acts in a forward direction; and drag is the backward, or retarding force produced by air resistance. Lift opposes weight and thrust opposes drag. When an aircraft is in straight-and-level flight, the opposing forces balance each other; lift equals weight and thrust equals drag. Any inequalities between thrust and drag, while maintaining straight-and-level flight, will result in acceleration or deceleration until the two forces again become balanced. According to the Bernoulli Principle, there is an acceleration of increases in the velocity of air as the air flows around an airfoil shape; therefore, there is an acceleration of the relative wind as it flows above and below the surfaces of the airplane wing. Because the camber of the upper wing surface is greater than that of the lower surface, air flowing above the wing will be accelerated more than air flowing beneath the wing. The Bernoulli Principle also states that an increase in velocity of a fluid, such as air, results in a decrease of pressure with in that fluid. As a result, the reduction in air pressure above the wing will be greater than the pressure reduction along the lower wing surface. This difference of pressure accounts for the upward force called lift. Lift can be increased in two ways, by increasing the forward speed of the airplane or by increasing the angle of attack. The pilot can increase the forward speed of the aircraft by applying more power. This increases the speed of the relative wind over the airfoil. Increasing the angle of attack will increase lift up to a point. As the airfoil is inclined, the air flowing over the top of the airfoil is diverted over a greater distance resulting in an even greater increase in air velocity and more lift. However, as the airfoil is given a grater angle of attack relative to oncoming air, it becomes more difficult for the air to flow smoothly across the top of the wing. Thus, it starts to separate from the wing and enters a burbling or turbulent pattern. The angle at which airflow separation and turbulence occurs on the upper wing surface is called the critical angle of attack. This turbulence results in a loss of lift in the area of the wing where it is taking place. The angle of incidence is the name to the angle between the wing chord line and the airplane s longitudinal axis. Choosing the right angle of incidence in designing the airplane can improve flight visibility over the nose and reduce drag in cruising flight. Most airplanes have a slight positive angle of incidence so that the wing has a positive angle of attack when the fuselage is perfectly level, as in cruising flight. Gravity is the force that lift has to overcome for the airplane to fly and amounts to nothing more than the weight of the loaded airplane. Forward motion is ess

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