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Aerodynamic forces


Aerodynamics is the study of the forces and the resulting motion of objects through the air. The two aerodynamic forces are lift and drag. Lift is the force that holds an aircraft in the air and is mostly generated by the wings. In order for lift to happen, two things must be present- fluid and motion. Lift is generated by the contact of a solid body with a fluid, which can either be a liquid or a gas. In addition, lift is generated by the difference in velocity between the solid object and the fluid. If there is no motion between the object and the fluid, lift will not occur. When an aircraft’s wing deflects the air flowing against the path of the plane downward through its curved shape and angle formed with the direction of the motion, the reaction of this action (by Newton’s Third Law of motion) forces the wing upward and produces a lift. Lift always acts perpendicular to the motion.

The second aerodynamic force is known as drag. It acts opposite to the forward motion of an object. Drag is generated by every part of the airplane even the engines. Like lift, drag requires a fluid and motion. There are three types of drag: friction drag, form drag, and induced drag. Friction drag is caused by the friction between the molecules of the air and the solid surface of the aircraft. Form drag is basically the aerodynamic resistance to the motion of the object through the fluid, and the source of this drag depends on the shape of the object. Induced drag is caused by the generation of lift because the difference of pressure above and bellow the wing creates a tendency (called spanwise flow) for air to flow in opposite directions along the length of the wing; this action holds the plane back.

Equations of forces

The aerodynamic forces of lift and drag can be calculated by the following equations, L=Cl*r*1/2V^2*A for lift and D=Cd*r*1/2V^2*A for drag. Cl and Cd stand for lift and drag coefficients. Coefficients contain the dependencies and are usually determined experimentally. For example, lift coefficient is a number that aerodynamicists use to model all of the complex dependencies such as shape and inclination. Adding on, r represents the density of the fluid and is measured in slugs per cubic feet (slugs/cu ft). Density is affected by pressure, temperature, and altitude, and therefore different parts of the atmosphere- upper stratosphere, lower stratosphere, and troposphere- have different densities. As for V in the above equations, it stands for the velocity of the object (aircraft). In addition, A in the lift equation stands for the wing area. For drag, A can represent many different areas depending on how drag in looked at. If drag is looked at as friction drag, then A will be the total surface area of the body, but if form drag is used, then A would be the frontal area of the body. Also if induced drag is used, then A would be the wing area. The amazing thing is that no matter which reference area is used, the drag force at a specific instance will be the same since the drag coefficient (which is related by the ration of the area) will change as well.