Simple Description of Airflow Characteristics Inside an Unfolding Parachute

Abstract

A simple calculation of the aire ow velocity and pressure inside a round parachuteunfolding during theine ation stage that precedes full parachute expansion is described. On the basis of photographic sequences of deployments in thee eld, the parachute is assumed to adopt the shape of a tubularsock as it unfolds from an elongated collapsed or folded state. The effects of the unfolding process are described by the motion of the transition region between foldedand unfolded fabric (called theunfoldingfront ),a region thatmoveslikea traveling pulseuptheparachute’ s longitudinal axis. Such a motion is determined by the energy-dissipating nature of fabric unfolding and is coupled to the motion of the air column entering the sock. An interesting prediction is that the translation velocity of the unfolding front rapidly reaches a constant value, or terminal velocity, which may be signie cantly lower than the air speed of the wind entering the parachute. A comparison with experimental data collected by the U.S. Army and the U.S. Air Force cone rms this feature. The paper ends with a discussion on using the model to generate improved sets of initial conditions for the structure-coupled computational e uid dynamics codes that are used to simulate parachute ine ation.