Aerospace Textiles

Parachute Canopy Permeability at Deployment Pressure

The permeability on the certificate was measured at 125 pascals. A deployment snatch is sixty times that, and the cloth behaves accordingly.

Certificate Value Laboratory
cfm/ft²
Pa

Around 0.5 for orifice-dominated flow, nearer 1 for viscous.

Deployment In service
Pa
m²

Permeability at Deployment

— cfm/ft²

Extrapolated from the certificate differential

Flow at Pressure

Pressure Ratio
— ×
Permeability Multiplier
— ×
Through-Flow Velocity
— m/s
Velocity at Test Differential
— m/s
Volumetric Flow Through Canopy
— m³/s

Decision-support only, and a substantial extrapolation: the exponent is fitted at low differentials and the cloth also stretches under deployment load, opening the structure further than the power law predicts. Life-safety canopies must be qualified by drop test — never by extrapolated permeability.

Parachute Canopy Permeability at Deployment Pressure — free, with the formula and a worked example, at Textile School.