## Air Drag

Espouse, rant, and rebut -- politely.

### Air Drag

A craft has two types of drag: subsonic and supersonic (below and above the speed of sound). Also factoring in is the amount of roll the craft has.

In Subsonic the Force of drag is:

Fd = Cd * airdensity * velocity^2 / 2 * Area
where
Cd is the drag coefficient
airdensity is 0.001275g/cm^3 at equatorial sea level then falling as altitude gets greater.
Area is Affected Area

In Supersonic I am not sure what the equation is and I would like to know. It is also function of the Roll.

If someone knows about the Space Shuttle, Saturn V or any other craft. It would be helpful.
Philosophaie
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### Re: Air Drag

Philosophaie wrote:A craft has two types of drag: subsonic and supersonic (below and above the speed of sound). Also factoring in is the amount of roll the craft has.

In Subsonic the Force of drag is:

Fd = Cd * airdensity * velocity^2 / 2 * Area
where
Cd is the drag coefficient
airdensity is 0.001275g/cm^3 at equatorial sea level then falling as altitude gets greater.
Area is Affected Area

In Supersonic I am not sure what the equation is and I would like to know. It is also function of the Roll.

The equation is the same for motion in any fluid, and does not change with speed. It does not matter if the motion is subsonic or supersonic. What does change with speed is the coefficient of drag; you need to choose a value appropriate for the conditions (the Reynolds number, which is equivalent to speed) and you may need to integrate the solution with a varying CD if the speed is changing during flight (as is the case for the meteors I analyze). For simple shapes, the value of CD can be calculated; for complex shapes, it needs to be determined empirically, either by experiment or simulation.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris Peterson
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### Re: Air Drag

Chris Peterson wrote:
Philosophaie wrote:
A craft has two types of drag: subsonic and supersonic (below and above the speed of sound).

In Subsonic the Force of drag is:

where
Cd is the drag coefficient
airdensity is 0.001275g/cm3 at equatorial sea level then falling as altitude gets greater.
Area is Affected Area
In Supersonic I am not sure what the equation is and I would like to know.

The equation is the same for motion in any fluid, and does not change with speed. It does not matter if the motion is subsonic or supersonic. What does change with speed is the coefficient of drag; you need to choose a value appropriate for the conditions (the Reynolds number, which is equivalent to speed) and you may need to integrate the solution with a varying CD if the speed is changing during flight (as is the case for the meteors I analyze).
http://en.wikipedia.org/wiki/Drag_coefficient wrote:
The drag coefficient Cd
Code: Select all
`0.250    Toyota Prius0.044    F-4 Phantom II (supersonic)0.021    F-4 Phantom II (subsonic)`
Art Neuendorffer

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