Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/118927
S a f e t y W ir e that those who are less tolerant have a "margin of error," so to speak. A few other factors come into play with regards to the severity and nature of injury produced by accelerations. Among these are the duration of the acceleration. Most crash impulses have durations of a quarter of a second (or 250 milliseconds) or less. The general rule is that the shorter the impulse, the better it is tolerated. This is why, for example, an aerobatic maneuver pulling 8g to 10g for a few seconds can cause unconsciousness and possibly a crash, but a similar impulse for 250 milliseconds in a crash impact would likely have little to no effect on most occupants, unless the cockpit structures or restraints fail. How rapidly the impulse is applied (rate of onset) is another factor that can determine the nature of injuries sustained. Assuming the same magnitude (how many g's are applied) and duration, impulses with a slower rate of onset will tend to be better tolerated. This makes sense if you think of crashes as being a series of inertial changes (more on this in a minute). The other point to keep in mind is that the direction the forces are applied will have a significant factor on how well one tolerates abrupt deceleration. While there is a fancy engineering vector system based off of a three-dimensional coordinate map, almost everyone— including engineers—in all but the most formal of settings uses what might be best called the "eyeball" method. Keeping in mind Newtonian physics (specifically the first law of motion), objects in motion tend to stay in motion until acted upon by an outside force. When applied to the eyeballs, which are not all that firmly anchored in the orbits save for a few muscles and the optic nerve, it means that the eyeballs will—at least for a split second—continue moving forward until their "tie-down straps" are stretched to the point that they counteract the movement. That is why if you are in a car and slam hard on the brakes it would be considered an "eyeballs out" deceleration (or negative acceleration). To answer the obvious question: It is at least theoretically possible to decelerate rapidly enough in this direction to literally tear the eyeballs out of the skull. I have seen a lot of extreme trauma in my research experience, but I have never seen it actually happen so far as could be reliably determined. If you are facing in the direction opposite you are traveling, the deceleration would be considered "eyeballs in." If you were a World War II paratrooper riding in a C-47 looking sideways across the aircraft when it suddenly slows, then you would have an "eyeballs right" 30 Vol.2 No.4 / April 2013 or "eyeballs left" deceleration, depending upon which side of the plane you are sitting. Just remember that the eyes keep moving, albeit briefly, in the direction you were traveling before the situation changed. When it comes to vertical accelerations (both positive and negative), the results are a little different. The best way to visualize it is to think about the "stomach jump" you feel when you hit turbulence or ride a roller coaster. What is happening is actually the opposite: Your stomach (or in this case, the eyes) is staying put for a split second in terms of its relative vertical position. Hence, a vertical impact as we think of it in the aviation community (so long as the aircraft is not impacting upside down) is an "eyeballs up" deceleration. Welcome to the principle of the inertial response. It might seem odd that we are judging this in reference to something moving opposite the applied acceleration. (Remember that an acceleration is not necessarily an increase; it is simply a change in velocity, direction, or both.) The reason for this is that, in the vast majority of cases, what causes injury is the body's inertial response to it, and not the acceleration itself. That brings us to the final point one needs to understand to grasp and apply human tolerance factors. As pointed out in Table 1, an "eyes in"/rear-facing deceleration is the best tolerated. This is why, if your design will tolerate it, you should strongly consider using rear-facing seats for nonpilot occupants. Forward-facing seats are less well tolerated; but obviously pilots need to see where they are going, so we are restricted in this particular regard. Traditionally, sideways-facing seats (which produce lateral loads during crashes) have been viewed as being the least well tolerated. However, some crash data from the Indy car circuits have called this into question. The next article in the series will look at the longitudinal ("eyes in" and "eyes out") events. Until then, fly safely. Stephen L. Richey is an aviation safety researcher who has been involved with flying for the better part of two-and-a-half decades, starting with his time as a "junior hangar bum" with a local EAA chapter while a child in Indiana in 1988. He has logged about 700 hours thus far, including time in ultralights and as a perennial student pilot in light singles. His current project is the design of a new homebuilt known as the Praetorian.