Similarity criteria for canopy porosity and environmental impact analysis of air permeability

The porosity is one of the important properties of canopy fabric, which affects the stability, aerodynamic performance and inflation performance of parachutes. The similarity criteria for canopy porosity were presented on the principle of similarity analysis. The Ergun theory of porous medium was introduced to establish the model of air permeability and differential pressure between canopies fully considering the air flow property during decelerating process of parachutes. And the differential pressure equation of micro-pore jets of fabric was established based on Bernoulli's theorem. Then, a novel model of air permeability for parachute canopy under different flying environment was proposed. The air permeability calculated by the model is in good agreement with the experiment. According to different environmental conditions, the impact analysis of air permeability and effective permeability of parachute canopies was studied. The results show that geometric and dynamic similarity are sufficient conditions for porous similarity of canopy. The air permeability and effective permeability of canopy are positively correlated with flight velocity and air density, negatively correlated with aerodynamic viscosity and less affected. There exists blockage effect in micro-pore jets of fabric.