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Old Mar 7th 2013, 06:15 AM   #1
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Post Drag Force & Drag Coefficient

Dear all
I confused about the Definition of drag force and drag coefficient, please help me to understand the correct Def.

thank for any help!!

(See example below)
http://i.imgur.com/s09OaGe.jpg

Matthew
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Old Mar 7th 2013, 01:42 PM   #2
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"Drag" is a force, in units of newtons. A "drag coeffeicent" is a constant that relates elements such as the the object's shape, size, velocity and characteristics of the medium it is moving in to the drag force the object experiences. The formula you cite:


allows you to calculate the drag force based on the object's velocity, frontal area, and density of the medium, but you need to know the vaklue for C_d as well. The other equation:


is a formula to calcute C_d. Once you have that number then you can plug it into the first equation to calculate the drag force acting on the object.
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Old Mar 8th 2013, 12:04 AM   #3
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Drag... it's a *drag*

Chip's input is perfect, however an important distinction about drag, is that it is a vector. In the context of airfoils its direction is not exactly opposite to the direction of travel, but opposite to the relative wind acting on the airfoil. This relative wind is influenced by the angle of attack of the airfoil (it's orientation relative to the direction of travel) and prevailing winds/airflows.

Also in the context of aircraft:

• Profile drag is the drag incurred from frictional resistance of the airfoil passing through the air. It does not change significantly with angle of attack of the airfoil section, but increases moderately as airspeed increases.
• Induced drag is the drag incurred as a result of production of lift. Higher angles of attack which produce more lift also produce increased induced drag. The induced drag is the portion of the total aerodynamic force which is oriented in the direction opposing the movement of the airfoil. Think of it as lift which is in the wrong direction.
• Parasite drag is the drag incurred from the non-lifting portions of the aircraft: the fuselage, tail, landing gear, etc

Hope this helps...

Last edited by Doogli; Mar 8th 2013 at 12:05 AM. Reason: typos
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Old Mar 10th 2013, 09:24 AM   #4
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Originally Posted by ChipB View Post
"Drag" is a force, in units of newtons. A "drag coeffeicent" is a constant that relates elements such as the the object's shape, size, velocity and characteristics of the medium it is moving in to the drag force the object experiences. The formula you cite:


allows you to calculate the drag force based on the object's velocity, frontal area, and density of the medium, but you need to know the vaklue for C_d as well. The other equation:


is a formula to calcute C_d. Once you have that number then you can plug it into the first equation to calculate the drag force acting on the object.
////////////////////////////////

Thank you for your detailed answers. ^^ Matthew
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Old Mar 10th 2013, 09:25 AM   #5
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Originally Posted by Doogli View Post
Chip's input is perfect, however an important distinction about drag, is that it is a vector. In the context of airfoils its direction is not exactly opposite to the direction of travel, but opposite to the relative wind acting on the airfoil. This relative wind is influenced by the angle of attack of the airfoil (it's orientation relative to the direction of travel) and prevailing winds/airflows.

Also in the context of aircraft:

Profile drag is the drag incurred from frictional resistance of the airfoil passing through the air. It does not change significantly with angle of attack of the airfoil section, but increases moderately as airspeed increases.
Induced drag is the drag incurred as a result of production of lift. Higher angles of attack which produce more lift also produce increased induced drag. The induced drag is the portion of the total aerodynamic force which is oriented in the direction opposing the movement of the airfoil. Think of it as lift which is in the wrong direction.
Parasite drag is the drag incurred from the non-lifting portions of the aircraft: the fuselage, tail, landing gear, etc

Hope this helps...
/////////////////////////////////////////////////////////

Thank you for your detailed answers. ^^ Matthew
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