When a solid body is moved through a fluid (gas or liquid), the fluid resists the motion. The object is subjected to an aerodynamic force in a direction opposed to the motion. This force is called drag.
As with aircraft lift, there are many factors that affect drag. We can group these factors into (a) those associated with the object, (b) those associated with the motion of the object through the air, and (c) those associated with the air itself
Aircraft geometry has a large effect on the amount of drag generated. As with lift, the drag depends linearly on the size of the object moving through the air. The cross-sectional shape of an object determines the form drag created by the pressure variation around the object. The three dimensional planform shape affects the induced drag of a lifting wing. If we think of drag as aerodynamic friction, then the amount of drag depends on the surface roughness of the object; a smooth, waxed surface will produce less drag than a roughened surface. This effect is called skin friction and is usually included in the drag coefficient.
Motion of the Air
Drag is associated with the movement of the aircraft through the air, so drag will then depend on the velocity of the air. Like lift, drag actually varies with the square of the velocity between the object and the air. How the object is inclined to the flow will also affect the amount of drag generated. If the object moves through the air at speeds near the speed of sound, shock waves may be formed on the object which create an additional drag component called wave drag. The motion of the object through the air also causes boundary layers to form on the object. A boundary layer is a region of very low speed flow near the surface which contributes to the skin friction.
Properties of the Air
Drag depends directly on the mass of the flow going past the aircraft. The drag also depends in a complex way on two other properties of the air: its viscosity and its compressibility. These factors affect the wave drag and skin friction which are described above.
We can gather all of this information on the factors that affect drag into a single mathematical equation called the Drag Equation. With the drag equation we can predict how much drag force will be generated by a given body moving at a given speed through a given fluid.
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