American Institute of Aeronautics and Astronautics 1 Added Mass of a Model Round Parachute Canopy during Inflation Mohamed Elgabaili 1 California State University, Northridge, Northridge, CA 91330 Kenneth J. Desabrais 2 US Army Natick Soldier Research, Development, and Engineering Center, Natick, MA 01760 and Hamid Johari 3 California State University, Northridge, Northridge, CA 91330 The added mass of a model round parachute canopy during various stages of inflation was computed using a commercial finite element-based solver. Images from an experiment in which a small scale round canopy model was inflated under infinite mass conditions were employed to create a set of solid models. Eight instants during inflation were selected and the potential flowfield about the models at these instants was computed assuming a uniform freestream. Subsequently, the added mass component associated with translation along the symmetry axis was calculated. The computed added mass for the eight cases was greater than the enclosed fluid mass and could not be correlated with the latter. The ratio of added mass to the enclosed mass varied from 1.2 at the ‘sock’ stage to 4.1 at the ‘over-inflated’ stage. A correlation was developed to estimate the added mass as a linear combination of the enclosed fluid mass and the added mass of a disk having the same area as the canopy mouth. I. Introduction HE force required to accelerate an object in an unbounded fluid medium consists of two components: one that is associated with the inertia of the object and the surrounding fluid, and the other with the unsteady drag on the object. The semi-empirical Morrison equation 1,2 uses this approach for the estimation of forces in oscillatory flows. A component of the inertia force is due to the acceleration of the fluid surrounding the object. This component would be present even for an inviscid fluid, and is the product of the acceleration of the body and a term called the “added mass” or the “hydrodynamic mass”. 3,4 The sum of the added mass and the mass of the object is usually denoted as the “virtual mass”. The added mass, which accounts for the inertia of the fluid surrounding the moving object, depends only on the instantaneous body geometry and direction of motion. The added mass may be found analytically for the case of simple, axisymmetric objects; more complex geometries require numerical approaches. The added mass is of particular interest to the aerodynamic decelerator community as it plays a critical role in determining the loads during inflation/deceleration phase and in the dynamic stability of parachutes. Because of the scale of parachute canopies and their light weight, the added mass and the associated moment of inertia are significantly larger than the physical mass and moment of inertia of the fabric canopy. Past studies have not only focused on the effects of added mass on the dynamic stability of round parachutes 5,6 , but also on the dynamics of gliding parachutes 7 . Other studies 8-10 have investigated the dependence of forces and deceleration rates on the added mass during inflation phase of round canopy parachute systems. Moreover, Potvin 11-13 has proposed a number of models to predict the opening loads of both gliding and non-gliding parachutes. At the heart of these models are added mass terms that establish the inertia of the system. However, none of the previous work has expressed the added mass explicitly in terms of the time varying geometry of the canopy during opening. 1 Graduate Student, Department of Mechanical Engineering, 18111 Nordhoff Street, Mail Stop 8348. 2 Research Aerospace Engineer, Airdrop Technology Team, RDNS-WPA-T, 15 Kansas Street, AIAA Member. 3 Professor, Department of Mechanical Engineering, 18111 Nordhoff Street, M/S 8348, AIAA Associate Fellow. T