Air Permeability of Parachute Gloths

Abstract

The air permeability of 8 standard nylon parachute cloths was determined, using samples 6.05 in. in effective diameter. Fifty-nine experimental nylon cloths manu factured by the Bally Ribbon Mills were subjected to the same test procedure, as were 2 experimental fabrics of Orlon and Dacron, and 4 different samples of wire screen.Upon assuming that the pressure gradient in the flow through a parachute fabric is proportional to the arithmetic sum of an inertial (βρυ2) and the viscous contribution (αμυ), the existence of a parameter, β/α (whose measure is length), can be inferred. This length can be employed to characterize the geometry of the cloth. Experimental work to date in the case of the 8 standard nylon cloths, the Orlon and Dacron fabrics, 14 of the Bally Ribbon cloths, and the 4 wire screen samples indicates a justification of the assumption.Employing the characteristic length so determined permits writing a single relation common to all cloths between a "flow-through-drag coefficient," Cr, and a Reynolds num ber based on the characteristic length, viz., Cf= 2 + (2/ NRe).A procedure is suggested for estimating the parameters α and β from physical meas urements for plain-weave cloth; thus an indication of the permeability is predicted from construction details of the fabric.