Circumferential stresses were measured on 5-ft flat circular block-constructed solid cloth and ringslot parachutes in a wind tunnel during inflation under infinite mass condition and at steady state. Five Omega senors placed along gore centerlines and a force link measured stresses and the parachute force versus time. Radial stresses were measured on a 3-ft solid flat circular block-constructed parachute at steady state. Stress measurements were evaluated in view of maximum and steady-state stresses versus location and time, as well as in respect to the parachute force. The measurements indicated strong stress variations, being different for both types of parachutes. In the 3-ft solid cloth parachute, the radial stress was, at certain locations, greater than the circumferential stress.

Parachute Canopy Stress Measurements at Steady State and During Inflation

There is some evidence to indicate that injuries associated with parachute opening shock are much less frequent than injuries associated with the acceleration loads imposed by ejection seats, even though the stresses presumed to be induced in the human body may often be of the same order. Since all such data is based on operational experience, the details are necessarily vague, and it is hard to draw firm conclusions. The Aeromedical Research Laboratory, USAF, as part of its studies of aircrew injury, decided to see whether there was any difference between the dynamics of parachute opening when used with a rigid anthropometric dummy of the type normally used to obtain opening shock data, and a human parachutist whose body is naturally more resilient.

A new look at parachute opening dynamics

An experimental study of the steady-state stresses in a model ribbon parachute canopy is presented. Using Omega sensors, the distribution of circumferential stress in the horizontal ribbons of the parachute was measured, as well as canopy pressure distribution, shape, and drag. The stress distribution and shape were calculated for the parachute canopy using the parachute structural analysis code CANO. It was found that the stresses calculated using CANO were larger than the measured stresses and that the calculated canopy shape was substantially different from the measured shape. A modified version of CANO was developed that gave a better representation of the canopy shape and stress than the original version of CANO.

Measured and calculated stress in a ribbon parachute canopy

Exploratory parachute canopy stress measurements during inflation and at steady state

Vortex lattice theory applied to parachute canopy configurations

Analyses of maximum stresses in parachute canopies have previously associated maximum stress with opening shock. Loads calculated from such analyses are too small to account for many cases of canopy failure. It is proposed that the maximum stress on a canopy is related to the radial velocity parallel to the plane of the skirt with which each concentric ring of the canopy reaches its maximum diameter. At this instant it must decelerate the radial component of the air inflow. This deceleration results in a transient hoop stress imposed on the ring of cloth. An expression is developed for evaluating this stress, designated snap stress, relating it to diameter and filling time. The significance of this relation is discussed and magnitudes are calculated, showing that it can account for otherwise unexplained test failures. Suggestions are made for means of relieving this snap stress, and recommendations are made for further investigation of this source of canopy stress.

K. J. Asfour

Papers

Analysis of Dynamic Stress in an Inflating Parachute

Analysis of dynamic stress in an inflating parachute