Simple description of airflow characteristics inside an unfolding parachute

A simple calculation of the airflow velocity and pressure inside a round pa rachute unfolding during the inflation stage that precedes full parachute e xpansion is described. on the basis of photographic sequences of deployment s in the field, the parachute Is assumed to adopt the shape of a tubular so ck: as it unfolds from an elongated collapsed or folded state. The effects of the unfolding process are described by the motion of the transition regi on between folded and unfolded fabric (called the unfolding front), a regio n that moves like a traveling pulse up the parachute's longitudinal axis. S uch a motion is determined by the energy-dissipating nature of fabric unfol ding and is coupled to the motion of the air column entering the sock. An i nteresting prediction is that the translation velocity of the unfolding fro nt rapidly reaches a constant value, or terminal velocity, which may be sig nificantly lower than the air speed of the wind entering the parachute. A c omparison with experimental data collected by the U.S. Army and the U.S. Ai r Force confirms this feature. The paper ends with a discussion on using th e model to generate improved sets of initial conditions for the structure-c oupled computational fluid dynamics codes that are used to simulate parachu te inflation.