The behavior of unconstrained parachute fabric under aerodynamic loading

Abstract

As parachute systems are required to make precise drops of increasingly heavy equipment, it is important that the simulation tools that are used to develop these systems have relevant data that they can use for validation. Before a detailed validation experiment can be run, it is necessary to understand how these fabrics behave over a wide range of conditions to determine the proper parameter space for the validation tests. In an effort to determine fabric behavior so that specific test points can be selected for validation, minimally constrained squares of F111 nylon fabric were tested in the United States Air Force Academy’s (USAFA’s) low speed wind tunnel and the Army Research Laboratory’s (ARL’s) Microsystem Aeromechanics Wind Tunnel. The tests at USAFA covered different angles of attack, freestream velocities, and amounts of excess length. Testing at ARL was conducted to obtain particle image velocimetry (PIV) measurements around the fabric while simultaneously tracking the motion of the fabric. The results of the experiments with varying excess length indicated that spanwise excess length had a more significant impact on the fabric mean shape and vibrations than excess length in the chordwise direction. Variations in angle of attack and freestream velocity indicated that the minimally constrained fabric experiences large amplitude vibrations at low angles of attack and vibrations beyond the trailing edge are insignificant at angles above 6°. Data from both the USAFA and ARL experiments indicated that this was true even when the leading edge shear layer was separated and undergoing unsteady oscillations. The PIV results from the ARL experiments indicated that at low angles of attack the fabric often experienced unsteady oscillations causing a leading edge region to flip into an up-or-down quasi-stable position. When this flipping occurred it induced vortex shedding which resulted in highly unsteady forces generated by the model. The lack of vibration of the fabric at angles of attack where the leading edge shear layer was undergoing unsteady separation indicates that the material properties of the F111 nylon make it less sensitive to small disturbances when under load. The presence of spanwise pre-strain eliminated large amplitude vibrations of the fabric at low angles of attack and the vortex formation was linked to leading edge oscillations, indicating that the leading edge state drives much of the fluid–structure interaction for minimally constrained fabric.