Cruciform parachute aerodynamics
Effects of changing the ratio of the arm length and width, the arm ratio, on the static and dynamic characteristics of cruciform parachute canopies are described. Forces and moments were determined from measurements made when fabric canopies were towed under water in a ship tank as well as from integrated pressure distributions determined by using specially designed pressure transducers fixed in fabric wind tunnel models. These techniques, designed to aid in the examination of general principles concerning the dynamics of bluff bodies in viscous flow, were used to investigate time and acceleration dependent variations in aerodynamic characteristics. Flow visualization techniques were utilised to determine the flow field around cruciform canopies. From numerical analysis of fluctuating aerodynamic forces and the determination of the characteristics of the cruciform canopy flow field, it is shown that strong jets of fluid through the four gaps between adjacent canopy arms cause a gross momentum defect in the canopy wake resulting in a high degree of aerodynamic drag. From consideration of solutions of the equations of motion of a parachute system, the value of the first angle-of-attack derivative of the normal force component function is most significant in determining a given system's dynamic characteristics. Experimental results were input into the current Leicester University parachute performance computer model and the variations of dynamic performance characteristics determined as a function of arm ratio. It is shown that a unique optimum arm ratio, corresponding to the most acceptable compromise between aerodynamic drag and dynamic characteristics does not exist for all canopy-payload configurations but will always be greater than 3:1 for any specific configuration.