title: Hydrostatic theory and G protection using tilting aircrew seats.
abstract: A hydrostatic theory of blackout is generally supported in the acceleration literature, but there is disagreement as to the correct origin in the thorax for h, the hydrostatic distance to the eye. Our goal was to determine whether representative published data would preferentially support some particular origin for h. Based upon a reanalysis of published data (1) relating acceleration tolerance (T) to seat-back angle (theta), it was found that a simple hydrostatic model, with h measured from the aortic valve to the eye, yields an excellent fit. The nonlinear equation fitted was Ti = Tmin 1/cos(theta i-phi), where phi = correction to back angle. The value of phi derived from the 1975 data (1) (13.74 degrees) was the same as the independent estimate of phi for this same seat (13.06 degrees) derived from published values of h, which were measured by X-ray with h referenced to the aortic-valve and eye (2). The acceleration tolerance data for the other two seats of the 1984 study also strongly supported the model. Future studies should measure h (aortic valve-to-eye), use seat-back angles giving even spacing on a 1/h scale, and analyze data using the required nonlinear methods.
mesh_terms: D000054:Acceleration; D000401:Aircraft / Q000295:instrumentation*; D006112:Gravitation; D006801:Humans; D006874:Hydrostatic Pressure; D008954:Models, Biological; D012044:Regression Analysis