Dynamic investigation of the angular motion of a rotating body-parachute system

Abstract

The modern design of parachute-pay load systems that undergo specific trajectories has to cope with dynamic behavior characteristics, which were of secondary importance in the past. Static and dynamic measurements, as well as computational simulations, are being employed to help the designers in converging to an optimal solution. However, some aerodynamic dynamic data are often impossible to obtain either computationally or experimentally through direct measurement. Novel experimental techniques have to be implemented in order to expand the analysis capability or to validate the design of specific configurations. A test technique that allows three degrees of freedom for investigating experimentally the dynamic behavior of parachute-pay load systems is presented in this article. The system is utilized to investigate the effect of the parachute geometrical variables on the dynamic stability of the parachute-payload system. The cross-type parachute-payload systems that were tested exhibit three zones of different dynamic stability modes and the occurrence of dynamic instability for statically stable configurations. These results show the need for obtaining more dynamic data for the complete understanding of the dynamic behavior of closely coupled parachute-payload configurations . OTATING parachute-payload systems have become common configurations for smart submunitions that are required to perform a complicated dynamic maneuver, such as searching for targets while descending. New goals were set for the aerodynamic design in order to obtain the desired performance characteristics that are mainly control of the rate of descent, the spin rate, and the instantaneous spatial po- sition of the store. To help meet those goals, both compu- tational and experimental tools were developed. Prediction of the dynamic behavior is used to shorten the design process and validate the performance of chosen configurations. In the past when only the trajectory of the e.g. of the system was required, a simulation based on the dynamics stability analysis of a single rigid mass was sufficient to provide adequate re- sults.1-2 The aerodynamic static coefficients required for such a simulation were obtained via conventional wind-tunnel tests, and verification could be achieved by direct measurements in drop tests. However, when the instantaneous position of the store, as well as its Eulerian angles are sought, the relative motion between the parachute and the payload has to be considered. This motion can be accounted for through an analysis based on two mechanically coupled masses. The na- ture of the coupling joint dictates the number of the degrees of freedom (DOF) necessary for the simulation. This number may vary from 9 DOF (6 DOF for the store and 3 for the parachute3 in the case where the joint is assumed to behave as an ideal hinge), to 15 DOF when the joint is represented by a friction plate that has its own 6 DOF.4 The multi-degrees- of-freedom simulations require inputs of the mechanical and aerodynamical static and dynamic properties of the configu- ration. The aerodynamic data should either be calculated or measured in controlled tests. Computations are unfortunately lagging in their ability to provide most of the aerodynamic data. Measurements in wind tunnels, in tow tanks, and drop tests are therefore the main source of static and limited dy- namic data.58 Innovative test techniques9 have been used to obtain dynamic data that contributed to the simulation ability and to the verification of the predictions. Further modification of these techniques,10-11 enabled the acquisition of the dy- namic coefficients regarding the spin rate of rotating para- chute-payload configurations. With the availability of the dy- namic coefficients, the prediction of the dynamic behavior of some parachute-payload configurations in the steady-state de- scent conditions could be obtained. These predictions are based on a linear dependence between the aerodynamic coef- ficients and the spatial position of the system and its elements. The simulation can handle both stable and unstable config- urations, and, as presented recently,3 can incorporate the sto- chastic nature of the parachute-payload aerodynamics, by forcing stochastic disturbances into the wind velocity. However, advanced parachute-payload systems, which re- quire the parachute to perform as a flight control mechanism, exhibit dynamic patterns that cannot be fully apprehended by the approach of two coupled masses, and the dynamic data and analysis described in the previous chapter. The nature of the connection between the parachute and the payload is such that moments are transferred about the three axes and that the parachute cannot be regarded as a rigid body (elasticity of the suspension lines and that of the canopy play an im- portant role in the dynamic behavior of the system). In such systems, more dynamic data is required in order to success- fully predict the dynamic behavior. Such behavior patterns were monitored with a 3 degrees-of-freedom rig, and the ef- fect of the geometrical variables on the dynamic stability of the systems was investigated.