Form drag

Parasitic drag is drag that results when an object is moved through a fluid medium. In the case of aerodynamic drag, the fluid medium is the atmosphere. Parasitic drag is a combination of form drag, skin friction drag and interference drag. The other components of total drag, induced drag, wave drag, and ram drag, are separate types of drag, and are not components of parasitic drag. Parasitic drag does not result from the induction of lift on the body, hence it is considered parasitic.

In flight, induced drag results from the lift force that must be produced so that the craft can maintain level flight. Induced drag is greater at lower speeds where a high angle of attack is required. As speed increases, the induced drag decreases, but parasitic drag increases because the fluid is striking the object with greater force, and is moving across the object's surfaces at higher speed. As speed continues to increase into the transonic and supersonic regimes, wave drag grows in importance. Each of these drag components changes in proportion to the others based on speed. The combined overall drag curve therefore shows a minimum at some airspeed; an aircraft flying at this speed will be close to its optimal efficiency. Pilots will use this speed to maximize the gliding range in case of an engine failure. However, to maximize the gliding endurance, the aircraft's speed would have to be at the point of minimum power, which occurs at lower speeds than minimum drag.

At the point of minimum drag, C_D,o (drag coefficient of the aircraft when lift equals zero) is equal to C_D,i (induced drag coefficient, or coefficient of drag created by lift). At the point of minimum power, C_D,o is equal to one third times C_D,i. This can be proven by deriving the following equations:

where:

is the dynamic pressure and

where

Form drag or pressure drag arises because of the shape of the object. The general size and shape of the body are the most important factors in form drag; bodies with a larger presented cross-section will have a higher drag than thinner bodies; sleek ("streamlined") objects have lower form drag. Form drag follows the drag equation, meaning that it increases with velocity, and thus becomes more important for high-speed aircraft.

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