Deceleration Dynamics of Unreefed Cruciform and Flat Circular Parachutes During and After Inflation

We report the results of our study of the drag force and deceleration sustained by unreefed parachutes, particularly after inflation. Using the results of our computer simulations and of our most recent test drop data, we obtain the explicit drag force evolution for decelerating cruciform and flat circular parachutes during their inflation phase and post-inflation phase. Ludtke's fd-drag area law is used to simulate the inflation dynamics, and for the first time is applied to cruciform parachutes. During the post-inflation phase, we have used the new model of unsteady drag recently proposed by some of us (JP, GP and BB). This model does not use the familiar "added mass'* idea but instead rely on the aerodynamic equivalence between a vehicle decelerating to terminal speed in a static fluid, and the same vehicle drifting with the moving fluid while accelerating to the speed of the fluid. It is shown that depending on the swiftness of a given opening sequence, the maximum value of the drag coefficient during the post-inflation phase can be much greater than its steady state value, by up to a factor of three. Our analysis also shows that, as expected, CD(t) rapidly converges back to its steady state value as the parachute-payload system settles into its terminal velocity regime.