# Calculating parachute Cd in rocksim

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#### Slickwilly

##### Well-Known Member
I am updating data files in the parachute (PCDATA) file and I wondered if anyone knows how to manually calculate the decent rate (Cd). The setting in Rocksim goes from 0-1.87 and I just needed to understand. :caffeine:

Thanks for anyones help!

I am updating data files in the parachute (PCDATA) file and I wondered if anyone knows how to manually calculate the decent rate (Cd). The setting in Rocksim goes from 0-1.87 and I just needed to understand. :caffeine:

Thanks for anyones help!
The terminal velocity is the descent velocity where the weigh of the body equals the drag of the body.

Drag is proportional to the product of density of the air multiplied by the effective area of the parachute multiplied by the descent velocity squared, The effective area of the parachute is the product of the physical cross-sectional area of the parachute times the Cd (drag coefficent of the parachute). Some parachutes provide more drag force than their physical area while other parachutes have less drag than their physical areas. A flat parachute may have a Cd of 0.75 and a hemispherical parachute may have a Cd of 1.5. Putting in some numbers, a 4 square foot flat parachute with a Cd = 0.75 would have an equilavent drag area of 3 square feet whereas a 4 square foot hemispherical parachute with a Cd = 1.5 would have an equivalent drag ares of 6 square feet.

Check the following reference for the equations.

https://www.grc.nasa.gov/WWW/K-12/airplane/termv.html

Bob

I mentored a TARC team last year. We used Cd=0.75 for flat parachutes and Cd=1.5 for the Fruity Chutes ellipsoid (flattened hemisphere) and Rocksim pretty much nailed the descent times in calm, overcast (read that as no thermals) weather. As I recall, we could not trust the descent rate in the menu where you define the parachute, but the result of the full simulation was correct and included the effect of density variations. Flying here in New England in the winter, the air is a lot denser than your standard 59 degree day as sea level that is the default. You need to set the flight conditions correctly to get good answers from Rocksim.

I mentored a TARC team last year. We used Cd=0.75 for flat parachutes and Cd=1.5 for the Fruity Chutes ellipsoid (flattened hemisphere) and Rocksim pretty much nailed the descent times in calm, overcast (read that as no thermals) weather.

Larry,

For hemi or ellipsoid chutes do you use the 'cloth' diameter or the inflated diameter for the diameter value?

--john

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