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Freefall and P.C. Drag Force

This page is a chapter in 'BASE Wiki Gear'

It is hereinafter reported (and downloadable) an Excel file containing a chart where they can be found freefall data (time (=delay), vertical distance fallen, vertical speed, vertical acceleration) and the (theoretical) Pilot Chute drag (=pulling) forces for various sizes of P.C.'s at each delay. Within the file, at the beginning there are the formulas (governing the successive chart) for: time (=delay), vertical distance fallen, vertical speed, vertical acceleration, as well as the theoretical formula for computing the P.C. drag forces at each delay.

The cart is given for a jumper weighing (=overall=ready-to-jump) 80 kg and having a terminal velocity of 54.5 m/s, but if you follow the tips written just before the chart, any jumper can customize the chart for his/her own data (basically, his/her weight and his/her terminal velocity).

Moreover, the P.C. drag forces are given in the chart as THEORETICAL, but in each formula there is a Fd factor that at download of Excel file is set to 1. Such a Fd factor is a diminishing factor, that once set to values above 1 (i.e., 1.2, 1.3, 1.5 etc.) can keep into account of factors as: manufacturing technique of PC, PC not being an infinitely rigid disk but a piece of cloth and so its section under stress is less large than its section at rest, pilot chute hesitations, etc., for "converting" the theoretical values to more "actual" ones. If in the right cell you change the value for Fd, automatically ALL the values of drag forces are corrected to "actual" values.

Again, consider the chart as a good reference and a good representation of reality (once you put your "true" values), but it is just a chart, it is NOT the "reality" simply because each time you are dressed in a different way, the air temperature is different for each day, each jump you track slightly different from the previous jump, at each jump your P.C. behaves slightly different from previous jump, etc. etc. etc..

What is (nearly) true are the ratios between drag forces of different size P.C.'s at same delay (once you consider that each P.C. is perfectly inflated, etc. etc.).




Categories: Category:Gear
Tags: gear, pilot chute, rest

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