Comparison of human impact response in restraint systems with and without a negative G strap

A test program to assess the influence of a negative G strap on restraint dynamics and human impact response was conducted at AFAMRL. There were 131 experimental-level impact tests with volunteer subjects performed in eight different test conditions. Forward-facing (-Gx) impacts were carried out on a horizontal accelerator, while vertical (+Gz) impact tests were done on a vertical drop tower facility. In both axes, the experimental exposure was an approximate half-sine waveform with peak acceleration up to 10 G and velocity change up to 9.2 m X s-1. Subjects were restrained to the test vehicle using either the PCU-15/P torso harness and lap belt, which is used operationally in such aircraft as the A-20 and F-15, or a conventional double shoulder strap and lap belt configuration. In one half of the test conditions, fixed-length negative G straps were incorporated into these restraint systems. In the other test conditions, the unmodified restraint systems were evaluated. Adding the negative G strap to either restraint system had clearly beneficial effects. These included decreasing the tendency toward submarining in forward-facing impacts, providing better occupant-seat coupling during free falls, and improving vertical impact protection. Sufficient benefits appear to result from use of the negative G strap to warrant a recommendation for its incorporation into selected USAF restraint systems, such as the PCU-15/P torso harness and lap belt. Additional data analysis revealed that the conventional double shoulder strap and lap belt restraint provided better forward-facing and vertical impact protection than the PCU-15/P torso harness and lap belt configuration. Further research at AFAMRL is planned to identify restraint harness features which may improve the performance of current and future impact protection systems.