Wind and Water Tunnel Measurements of Parachute Suspension Line

This experiment was conducted to support recent efforts by the US Army Research, Development, and Engineering Command (RDECOM), Natick Soldier Center, to characterize drag on the parachute’s many systems. These efforts included testing off-theshelf 600 lb flat Dacron suspension line and scaled rigid models made to resemble the actual line. This experiment served to confirm vortex shedding behavior and identify possible mechanisms driving flow-induced vibrations of the line. The Dacron models, with rectangular cross sections, were tested in a wind tunnel at 49, 73, and 90 fps, with tensions of 10, 38, and 55 lb, and at angles of attack of 90° and 70°. At these speeds, the Strouhal numbers varied between 0.14 and 0.23 for the 90° configuration, and the 70° set-up showed results between 0.15 and 0.25. Seven of 90 wind tunnel tests presented low-frequency, highamplitude, self-induced oscillations in both angles of attack. Data collected from the free response of pluck tests found the fundamental frequency of the lines in the wind tunnel had average values of 38 Hz and 73 Hz for 10 lb and 38 lb tension, respectively. Shedding and vibration data do not support vortex shedding as the dominant phenomenon driving the selfinduced oscillations, but does fit predictions for gallop/flutter. Flow visualization from the water tunnel revealed the fluid profile is susceptible to small changes in incidence angle, resulting in asymmetric boundary layer separation and shear layer vortices which provide the potential for gallop or possibly stall-flutter.