Validation of a Computer Model of a Parachute

Although many sophisticated stability models of parachutes have appeared, advances are concentrated in the simulation of dynamics of the solid body components of multibody systems. For the complex canopy aerodynamics it is routine to quote several standard simplifying assumptions, few of which can be factually justified since none of the models has convincingly demonstrated its validity by detailed comparison with experimental data. Such demonstrations are essential for confident prediction. This study investigates a fundamental three-dimensional parachute model. Fluid accelerative reactions are represented by an idealised added mass tensor, and it is shown that the equations of motion in previous treatments are either inadequately or incorrectly derived and/or implemented. Nonlinear solutions of the six degree-of-freedom equations for a rigid axisymmetric system are obtained, and a parameter sensitivity analysis for dynamic stability of a typical personnel parachute indicates that the most important aerodynamic parameters are the added mass components and the pitch damping derivative, not one of which has been adequately estimated. A systematic validation method is outlined. The kinematics of four free-falling parachute scale models, with canopy flight diameters from 1.4 m to 5.8 m, have been acquired from a strapdown inertial measurement system. Spectral analysis of the transducer signals reveals sharply defined frequencies of oscillation. Comparisons of simulation and experiment demonstrate that satisfactory agreement in frequencies and mean amplitudes of oscillation can be extracted. However, no inherent influence is evident in the model to enable the observed, apparently random amplitude modulation to be reproduced. Sources of the random motion are discussed, one significant and hitherto ignored source being self-excited unsteadiness of the separated flow around the canopy. Measurements of unsteady aerodynamic forces on parachute canopies are needed, also improved estimates of fluid accelerative reactions and aerodynamic damping.