‘G’ is the ratio between a given acceleration and the acceleration due to gravity. The term ‘G force’ is used sometimes to describe a force, which produced acceleration, which is a multiple of the acceleration due to gravity (9.81 m/sq sec). Thus, an acceleration of 98.1 m/sq sec would be 10 G. High sustained G, that the modern day combat aircraft are capable of, is G forces of 7G or more sustained for 15 seconds or more.
Conventionally there are three axes of acceleration, as per the inertial forces acting acting on the human body:-
- Gx : acceleration acting in antero-posterior axis.
- + Gx = when inertial force acts from front to back
- – Gx = when inertial force acts from back to front
- Gy : acceleration acting in side to side axis
- + Gy = when inertial force acts from left to right
- – Gy = when inertial force acts from right to left
- Gz : acceleration acting in head to feet axis
- +Gz = when inertial force acts from head to feet
- -Gz = when inertial force acts from feet to head
Schematic diagram of the Axes of acceleration due to inertial forces
Effects of acceleration on the human body depends on the following factors:–
- Magnitude: Higher the magnitude, more the effects.
- Duration: Shorter the duration, lesser the effects
- Rate of onset/offset:
- Gradual onset – circulatory reflexes are effective;
- Rapid onset – circulatory reflexes bypassed;
- Slow offset – longer to recover; and
- Faster offset – shorter time to recover.
- Site of application: if force is dissipated over larger area, there is less distortion of tissue.
- Direction: Human tolerance is poor in Gz axis but best in Gx axes.
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1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.
Acknowledgement. Image courtesy Wikimedia Commons