Significance of headrest geometry in +Gz protective seats

An x-ray technique was used to determine the vertical hydrostatic column length (h) between the eye and the prosthetic aortic valve of seven subjects at three different experimental headrest geometries at 30 degrees and 65 degrees seatback angles. The h from these subjects was then used to explain relaxed +Gz tolerance data acquired from other subjects at the same seating geometries. There was a very significant correlation (r = 0.99, p much less than 0.001) between 1/h and relaxed +Gz tolerance. Changing the seatback angle from 30 degrees to 65 degrees resulted in a 35.6%, 19.8%, and 24.1% decrease in h and a corresponding 69.2% (3.9 G to 6.6 G), 30.3% (3.73 G to 4.86 G), and 36.4% (3.98 G to 5.43 G) increase in relaxed +Gz tolerance for the three different headrest geometries, respectively. These data demonstrate that headrest geometry is an important consideration in obtaining the optimum reduction in h within the operational constraints of the cockpit.