Last week I promised an article my great friend Professor Rick Hillum, a physicist and pilot who knows a little bit more about the thorny question of when G-force is positive or negative G than the one-dimensional Wikipedia entry. So here is his explanation, plus another fascinating insight to boot:
Without getting into the laborious detail, g-force is a unit of force equal to the force exerted by gravity; used to indicate the force to which a body is subjected when it is accelerated.
Flying an aeroplane & G force
When flying an aeroplane, much like any other vehicle, be it a car or train, the movement from zero speed, standing-start is normally termed acceleration. Depending on the force of this acceleration, or the speed of acceleration, the person in the plane or train is subjected to a force which is measured and defined by the term G. 2G is therefore twice the gravitational force. G can be expressed in either + or – symbols depending on the direction of acceleration exerted on the pilot, or somewhat less likely, the driver.
In the simplest form, if the acceleration pushes the pilot down in the seat, as would happen when accelerating upwards, then it would be expressed as positive G. If the person was lighter or experiencing weightlessness which was the result of a downward acceleration, such as flying on the outside of a loop manoeuvre, it would be expressed as negative G.
If a pilot was flying straight and level and then banked tight (inside) the curve, then the pilot would experience positive G. However, at the point the pilot entered a rapid decent – as might be experienced in a combat manoeuvre – or was flying outside the curve, then the pilot would experience a negative G.
Another scenario might be in a combat zone where an aircraft is being pushed and pulled around the sky, either attacking or evading an enemy. As the pilots employ different combat manoeuvres, the asserted G can quite rapidly vary from high to low, or positive to negative, caused by changing of flight direction, diving the aircraft, inverting the aircraft (another common manoeuvre), and as the pilots push their aircraft towards, not just the limits of the aircrafts flight envelope, but also their own G force handling capability. Post combat, these turns can also be difficult to subsequently analyse as the accelerating motion is likely to have been in more than one axis (rather than just horizontal or vertical, and possibly a combination of both).
The average modern non-aerobatic Private Pilot in the average GA aircraft (General Aviation) flying around today, would normally experience an exertion of typically 2 to 3G.A WWII combat pilot in a high-performance Spitfire or Messerschmitt would experience up to typically around 6G to 7G. The more important and critical point here, however, is that the more experienced or experten the pilot, the more likely it would be that tighter, steeper, inverted and evasive turns were being flown to avoid the enemy or press home an attack.
Over time pilots would become G-hardened or more tolerant of G forces. Experienced Pilots could often handle really substantial and extreme G forces (towards 9G end of acceleration) in combat flight manoeuvres. In contrast, other inexperienced pilots could have long experienced a red-out, tunnel vision, or even totally blacked out at sometimes as little as 5 to 6G. Pilots in combat could well find they were being out-turned and out-flown by their opponent, not because the aircraft was more capable, but because of the experience and the accumulation of combat hours; the pilot had become G hardened, substantially contributing to their combat survivability and maybe kill rate.