Investigations of Parachute Fabric Permeability Under An Unsteady Pressure Differential

Abstract

A series of different experimental techniques were applied to quantify the effects both steady and unsteady pressure differential across a fabric had on the flow through the fabric. Static permeability measurements were made on four common nylon parachute fabrics over a range of fix pressure differentials to provide a baseline for the other techniques. An examination of the effects humidity had on the permeability of the fabric showed that the permeability had no change or slight increase when exposed to flows with higher humidity. The permeability at high static pressure differentials was examined by applying a weak shock wave to a fabric specimen. The results of this experiment showed the flow resistance increased (i.e. permeability decreased) as the pressure levels increased in all the fabrics except for the low permeability fabric which showed the opposite effect. A computer numerically controlled, piston permeability apparatus was designed and applied to test the dynamic permeability of the low permeability fabric. The results from this investigation do indicate a small effect of unsteady pressure differential on the fabric permeability. The fabric permeability is slightly higher than the static permeability when the pressure differential is increasing with respect to time and the opposite is true when the pressure differential is decreasing. This change in permeability is more pronounced as the pressure is higher and the pressure changes more rapidly with respect to time, suggesting dynamic permeability likely affects highly unsteady phenomena such as parachute opening.