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G Force – What is?

‘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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Reference

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

3 comments

  1. Geoff McCarthy

    Viewers should recognize this as a predictable GLOC. This rider – usually a local reporter – with the Blue Angels was not wearing a G suit (I assume) since the Blue Angels do not wear anti-G garments of any kind. The F-18 is limited to 7.5Gz, and the pilot commented that they had achieved 7.5.
    The Blue Angels lost a pilot and aircraft a few years ago to GLOC. They still do not wear G suits. I believe this is foolish and dangerous, and I speak as a retired USAF F-16 Pilot-Physician.
    In fact, for a 7.5G aircraft, wearing a full coverage G suit is virtually complete protection against GLOC, and reduces AGSM effort to a minimal level. Needless to say, the US Navy is not buying them…

    1. Clayton Taylor

      Considering that the Blue Angels use stick controls between the legs, the G-suit would be unsafe for accurate mobility while flying. The Blue Angels do many airshows and have a routine they fly every time. They know when the G-force will be at its highest, and therefore, can perform breathing and muscular exercises to prevent GLOC. One reason pilots, even in aircraft that can’t handle higher Gs, wear a G-suit is because they may have to perform sudden and unplanned maneuvers. This would not allow for time to prepare and perform GLOC avoidance techniques. The Blue Angels lost a pilot to GLOC, but a G-suit isn’t a save-all either.

      1. Av Med

        Thank you for your comments.

        While I may partially agree with your comments about “a G-suit isn’t a save-all either”, particularly among aerobatic teams who know the Gs they pull in each manoeuvre, yet there may be times a pilot may not be fully up to ‘it’.

        Hence, one of the most important teaching that we, aviation medicine specialists, must impart to combat pilots is about the factors that lower G tolerance. And of course, knowledgeable pilots are responsible for their personal state of fitness – physical and mental- and must not push themselves and their aircraft beyond the tolerance limits.

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