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FINDING DRAG COEFFICIENT FOR A PARACHUTE

by Donald F. Elger

Situation

A typical coefficient of drag for a parachute is about 1.3.  However, it is sometimes useful to determine an experimental value for a specific chute design.    

Goal

Using experimental means, determine the coefficient of drag for a parachute

Method

There are a variety of ways to find the coefficient of drag

method 1

Drop a parachute from a known height and measure the fall time.  Assume the parachute is moving at terminal velocity, and calculate terminal velocity as the ratio of height to drop time.  Next, apply the equilibrium principle (i.e. balance weight with drag force) and use algebra to solve for the coefficient of drag.  Note: this method is only valid if the acceleration period is negligible compared to the drop time.

method 2

Record height and fall time.  Use a model that includes acceleration and adjust the coefficient of drag in the model until the model prediction matches the data.  Note: this method is more accurate that method 1, but involves more effort.

method 3

Place a parachute in a wind tunnel and measure the air speed using a suitable instrument such as a pitot tube.  Measure the drag force using a special instrument called a "force balance."  Directly calculate the coefficient of drag.  

Good experimental practices

1. ) During a drop test, use multiple trials (e.g. 10 trials) and average data.

2.) Report data using experimental uncertainty.  (see section 13.4 of Engineering Fluid Mechanics, 7th Edition, by Crowe, Elger and Roberson, 2001.)

3.) Report data using a Reynolds number

Example

From experimental data, we determined Cd = 1.4  +/- 0.2.  Reynolds number based on the average velocity, and the diameter of the projected area was 11,200.