Myth Debunked – G’s

G forces in upset recovery training
By Ben Filippini, Chief instructor for Advanced Pilot Training at the NASTAR Center
As an instructor of upset prevention and recovery training, I spend a lot of time talking to pilots about different flight conditions and parameters. Perhaps one of the most misunderstood concepts we discuss is the idea of G forces. Specifically, different G force loads are seen during different flight maneuvers, but what are they? This article seeks to dispel some of the myths of G forces.
First, we must examine the concept of G-force. Simply put, G-force, or “G,” is a unit of measurement which quantitatively describes the force of acceleration acting upon an object. One G is the accelerative force affecting any object on Earth at sea level and is also known as “standard gravity.” In other words, it is the force that is constantly pulling all objects towards the center of the Earth and can be further quantified as 9.8 meters per second squared (9.8m/s2). Because of this, it is actually correct to say that all objects are falling towards the center of the planet at all times. However, they never actually reach the center because the ground or other surfaces hold them up in equilibrium, effectively neutralizing the accelerative effect of gravity. If a surface holding an object were to be removed, said object would immediately fall towards the center of the planet. Effectively this is what occurs when someone jumps out of a tree or an airplane; the surface holding them in equilibrium with gravity is removed, and they accelerate towards the center of the planet.
More specifically, G’s measure this acceleration as a multiple of this standard gravity, or 1G. If an object accelerates at twice the rate of standard gravity, it is considered to be experiencing 2G’s of G-force. Likewise, 5G’s would describe acceleration equal to five times the force of gravity. But how does this force actually affect the object itself? Well, another way to denote gravity is through weight, which technically is a measure of gravity on an object. Knowing that G and weight are directly and linearly related, it is true that 2G’s equaling twice the force of gravity would likewise equate to two-times an object’s weight. In other words, an object weighing 200 pounds at 1G would weigh 400 pounds at 2G’s and so forth.
Continuing with the discussion, we often hear about zero-G and that at zero-G an object is weightless. This is not entirely accurate. More precisely an object experiencing “weightlessness” means that whatever accelerative forces it is experiencing is in equilibrium with other forces. Therefore it is a relative weightlessness, not truly without weight. For example, it is common to say that astronauts experience weightlessness while in orbit in space. They are not actually without weight, though. Instead, the force of Earth’s gravity acting upon their body is countered in equilibrium by the centrifugal force generated by their orbit. In fact it is most accurate to state that an object in orbit is perpetually falling to earth.
Having said this, you may wonder why you see them floating around weightless in space if they are just in equilibrium. Well, as mentioned before, “weightlessness” is just perpetually falling. Since the spacecraft and all of its contents, including the astronaut, are falling at the same rate, there is no relative difference in velocity and no rate of change is perceived. Therefore both seem to not be moving relative to each other. This same principle can be seen when stopped next to another car. If the car begins to inch forward, one can feel as if they are moving backwards even though they are not moving at all. That is because of a relative change between the two vehicles. Interestingly, this “weightless” effect can be experienced by skydivers jumping with other people or objects likewise falling to the ground.
Just as one can experience positive and zero-G’s, there are also negative-G’s. But what exactly does this term mean? It really has to do with the idea of difference in relative acceleration. Technically there is no difference between positive and negative G’s, they just act in the opposite vector direction from each other relative to an object. As such, whether a G-force is positive or negative is entirely determined by the object upon which the forces are acting. We, as humans, recognize the difference between “up” and “down” because of our physiology; negative G’s would be a force pulling us “up.” This can be seen most readily if you do a handstand. You have now inverted your orientation to the Earth, and thus your relation to its 1G, creating a relative G-force of -1G. However, for an object with no delineated “up” or “down,” there is no such thing as negative G’s. A basketball, for example, will experience 1G regardless of what side is up or down.
Hopefully this article has helped clear up some of the confusion surrounding G-forces. Standby for future articles discussing G’s and their effect on pilots and aircraft in flight.