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Pushing the Limit on G-forces

  • 30 Sep 2022
  • 3 mins

What really goes on in the body when the invisible and potentially disorientating G-forces act upon the human anatomy?

Dr. Peng Chung Mien explains what aviation medicine tells us about the limits of human endurance in the cockpit of a fighter jet or Formula One car.

What are G-forces?

Starting with the basics, G-force is a measure of the acceleration force produced by gravity on an object or individual.

Further, G-force is experienced every time there is a change in direction or speed of acceleration or both.

In other words, as we grapple with gravity whether in an aircraft cockpit or behind the wheel of a vehicle, the human body has to contend with the impact of G-forces. These invisible forces will come into play with acceleration or directional change.

Getting Serious

G-forces affect the body in a tri-axis. In the air, the significant effect is felt at the top and bottom of the body, from head to toe. We call this vertical G or G-z.

On land, the effect is mainly horizontal, forwards and backwards, felt at the chest and we can term this G-x. The effect is felt laterally as well, side to side: G-y.

The significance of all of this is that an increase in acceleration force or G-force will multiply weight by the corresponding force.

If something weights 1 kg, at 4G, the weight will become 4 kg in effect.

And that is why endurance is a big part of a Formula One driver’s capabilities. To finish a long race on top, the driver has to maintain his concentration and be conditioned to weather well the physical stresses of competing.

For example, the neck will bear the brunt because a helmet’s weight will increase because of G-forces. At 4G turns, a 6.5 kg helmet will produce a load of 26 kg, a significant increase.

Up in the Air

Speaking of 4G, for vertical G, it requires special training to go beyond 4Gs.

Pilots use anti-G suits to improve tolerance to 6-7Gs, and special breathing techniques and suits to reach up to 9Gs in their fighters.

Proper coordinated positive pressure breathing is what is needed to have the ability to match the modern fighter jet’s design limit at 9 vertical Gs.

Why is there a need for these extraordinary arrangements?

The answer: high acceleration turns give rise to the G-effect where blood in a pilot’s body is pulled down to the lower limbs, depriving the brain of normal blood flow.

The special training is required to maintain blood flow from the heart to the brain so that the pilot does not ‘faint’ or G-LOC. In this case, ‘LOC’ stands for ‘loss of consciousness’.

If you are finding yourself feeling a little dizzy from reading these facts, rest assure that by contrast, the human body is better able to withstand horizontal and lateral G-forces.

But to sum up; for both flying and driving, for an untrained person, tolerance can range from 20Gs for less than 10 seconds to 10Gs for a minute.

Your tolerance will depend on both the level of G-forces and exposure time in an inverse relationship; the higher the level for a shorter time or the lower the level for a longer duration.

Getting Back Down to Earth

To be the first past the checkered flag, the elite drivers of Formula One will battle one another and also G-forces.

These drivers would usually experience up to 5Gs while braking, 2Gs while accelerating, and 4-6Gs while cornering, all within seconds.

Singapore’s own Grand Prix offers 23 corners in a total race distance of over 300 km.

The challenging Marina Bay Street Circuit confronts the drivers with a hard-braking section right before the very first turn.

Let us not forget that heat stress is an added factor with the protective clothing the drivers would have on. In essence, great physical fitness is important given also the many upper limb and leg movements needed to control the car.

With the drivers’ heart rates potentially rising, good cardiac fitness is involved too.

Going back to G-forces, try to imagine, the horizontal G load that is produced in straight-line acceleration acting upon the chest, compressing the heart and lungs within the chest wall, causing discomfort and difficulty in breathing.

Eventually, cardiac output could be affected, resulting in a fainting sensation.

Thankfully, the body has the compensatory mechanisms to cope with this.

The Record

The record for peak experimental horizontal G-force tolerance is held by acceleration pioneer John Stapp.

In a series of rocket sled deceleration experiments culminating in a late 1954 test, he was clocked at a little over a second at a land speed of Mach 0.9.

He survived a peak acceleration of 46.2 times the acceleration of gravity and more than 25Gs for 1.1 seconds.

You can understand why he earned the nickname of “the fastest man on Earth”. He ended the test in great pain but lived another 45 years to age 89 without any ill effects.

His example proved that the human body is capable of pushing the limit on G-forces.

Contributed by

Dr. Peng Chung Mein
Farrer Park Hospital