G-FORCENobody feels the effect of gravity. Gravity acts evenly on every particle of your body, so it does not stretch your muscles or compress your bones the way other forces can. However, when you are in the Earth's gravitational field, you will fall unless something stops you. It could be the pavement or the floor of your house, or the chair you are sitting on. Something supports your weight and provides an equal and opposite force to that created by gravity. This you feel. If you are standing on the ground, the force that counteracts your weight does not act evenly: it acts on your feet, and every part of your weight, from your head or your hands or wherever, must be supported by your legs and your feet. So you feel your weight in the sense that you feel the force transmitted from the ground to your feet to your head.
Similarly, if you are in a jet fighter in a tight turn, you feel the force that accelerates your body in the same direction as the airplane. For comparison, we discuss not the actual force of the aircraft on the pilot, which depends on his weight, but the ratio of the force to his weight. So, in level flight, your seat exerts a force equal to your weight, and you are said to experience 1g (pronounced 'one gee'). The same value equals the ratio of the lift force of the aircraft to its own weight.
Accelerating upwards adds to the initial 1g - you feel heavier, so upward acceleration is said to produce 'positive gees', and downward acceleration to produce 'negative gees'. If you were flying upside down, your up is down, so you would now experience -1g ('negative one gee').
. Space shuttle astronauts experience up to 3g acceleration. A change of 2g is substantial (one feels twice as heavy under 2g acceleration). Normal human tolerance is about 9g and -3g, with the use of air force g-suits' which help to alleviate the effects. When you are pulling high positive g's, your blood is 'pulled' towards your lower body and away from your head. This can lead to a loss of vision and consciousness, known as 'blackout'. Under negative g's, blood pools in your upper body and head, leading to what is known as 'redout'. Additionally, a fast change in acceleration can cause a very fast g-induced loss of consciousness, called G-LOC.
In TFX, blackout and redout are simulated on screen as graphical effects. In the Realism Menu (see TFX CONFIGURATION on page 50), you can define whether you want these effects disabled, and whether you want a realistic loss of control in blackout. "
From the manual