Parasitic drag


Parasitic drag

Parasitic drag (also called parasite drag) is drag caused by moving a solid object through a fluid. Parasitic drag is made up of many components, the most prominent being form drag. Skin friction and interference drag are also major components of parasitic drag.

In aviation, induced drag tends to be greater at lower speeds because a high angle of attack is required to maintain lift, creating more drag. However, as speed increases the induced drag becomes much less, but parasitic drag increases because the fluid is flowing faster around protruding objects increasing friction or drag. At even higher speeds in the transonic, wave drag enters the picture. Each of these forms of drag 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 at or close to its optimal efficiency. Pilots will use this speed to maximize endurance (minimum fuel consumption), or maximise gliding range in the event of an engine failure.

Form drag

Form drag, profile drag, or pressure drag, arises because of the form of the object. The general size and shape of the body is the most important factor in form drag - bodies with a larger apparent cross-section will have a higher drag than thinner bodies. Sleek designs, or designs that are streamlined and change cross-sectional area gradually are also critical for achieving minimum form drag. Form drag follows the drag equation, meaning that it rises with the square of speed, and thus becomes more important for high speed aircraft.

Profile Drag (Pxp): depends on the longitudinal section of the body. A diligent choice of body profile is more than essential for low drag coefficient. Streamlines should be continuous and separation of the boundary layer with its attendant vortices should be avoided.

Interference drag

Interference drag arises from vortices. Whenever two surfaces meet at a sharp angle on an object, the airflow has a tendency to form a vortex. Accelerating the air into this vortex causes drag on the object, and the resulting low pressure area behind the object also contributes.

Thus, the primary method of reducing interference drag is eliminating sharp angles by adding fairings which smooth out any sharp angles on the aircraft by forming fillets. Interference drag is also created by closely spaced parallel surfaces such as the wings of a biplane or triplane, or the facing surfaces of an external load (such as an external fuel tank or weapon) and the fuselage or wing. As with other components of parasitic drag, interference drag follows the drag equation and rises with the square of the velocity.

kin friction

Skin friction arises from the friction of the fluid against the "skin" of the object that is moving through it. Skin friction is a function of the interaction between the fluid and the skin of the body, as well as the wetted surface, or the area of the surface of the body that would become wet if sprayed with water flowing in the wind. As with other components of parasitic drag, skin friction follows the drag equation and rises with the square of the velocity.

Skin friction is caused by viscous drag in the boundary layer around the object. The boundary layer at the front of the object is usually laminar and relatively thin, but becomes turbulent and thicker towards the rear. The position of the transition point depends on the Reynolds number of the object. [ [http://selair.selkirk.bc.ca/aerodynamics1/Drag/Page3.html Skin friction ] ]

References


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