G-force

thumb|In straight and level flight, lift (L) equals weight (W). In a steady level banked turn of 60°, lift equals double the weight (L = 2W). The pilot experiences 2 g and a doubled weight. The steeper the bank, the greater the g-forces.

thumb|This [[Top Fuel dragster can accelerate from zero to in 0.86 seconds. This is a horizontal acceleration of 5.3 g. Combining this with the vertical g-force in the stationary case using the Pythagorean theorem yields a g-force of 5.4 g.]]

The g-force or gravitational force equivalent is a mass-specific force (force per unit mass), expressed in units of standard gravity (symbol g or g0, not to be confused with "g", the symbol for grams).

It is used for sustained accelerations that cause a perception of weight. For example, an object at rest on Earth's surface is subject to 1 g, equaling the conventional value of gravitational acceleration on Earth, about .

More transient acceleration, accompanied with significant jerk, is called shock.

When the g-force is produced by the surface of one object being pushed by the surface of another object, the reaction force to this push produces an equal and opposite force for every unit of each object's mass. The types of forces involved are transmitted through objects by interior mechanical stresses. Gravitational acceleration is one cause of an object's acceleration in relation to free fall.

The g-force experienced by an object is due to the vector sum of all gravitational and non-gravitational forces acting on an object's freedom to move. In practice, as noted, these are surface-contact forces between objects. Such forces cause stresses and strains on objects, since they must be transmitted from an object surface. Because of these strains, large g-forces may be destructive.

For example, a force of 1 g on an object sitting on the Earth's surface is caused by the mechanical force exerted in the upward direction by the ground, keeping the object from going into free fall. The upward contact force from the ground ensures that an object at rest on the Earth's surface is accelerating relative to the free-fall condition. (Free fall is the path that the object would follow when falling freely toward the Earth's center). Stress inside the object is ensured from the fact that the ground contact forces are transmitted only from the point of contact with the ground.

Objects allowed to free-fall in an inertial trajectory, under the influence of gravitation only, feel no g-forcea condition known as weightlessness. Being in free fall in an inertial trajectory is colloquially called "zero-g", which is short for "zero g-force". Zero g-force conditions would occur inside an elevator falling freely toward the Earth's center (in vacuum), or (to good approximation) inside a spacecraft in Earth orbit. These are examples of coordinate acceleration (a change in velocity) without a sensation of weight.

In the absence of gravitational fields, or in directions at right angles to them, proper and coordinate accelerations are the same, and any coordinate acceleration must be produced by a corresponding g-force acceleration. An example of this is a rocket in free space: when the engines produce simple changes in velocity, those changes cause g-forces on the rocket and the passengers.

Unit and measurement

The unit of measure of acceleration in the International System of Units (SI) is m/s2. However, to distinguish acceleration relative to free fall from simple acceleration (rate of change of velocity), the unit g is often used. One g is the force per unit mass due to gravity at the Earth's surface and is the standard gravity (symbol: gn), defined as  metres per second squared, or equivalently  newtons of force per kilogram of mass. The unit definition does not vary with location—the g-force when standing on the Moon is almost exactly that on Earth.

The unit g is not one of the SI units, which uses "g" for gram. Also, "g" should not be confused with "G", which is the standard symbol for the gravitational constant. This notation is commonly used in aviation, especially in aerobatic or combat military aviation, to describe the increased forces that must be overcome by pilots in order to remain conscious and not g-LOC (g-induced loss of consciousness).]]

Human tolerances depend on the magnitude of the gravitational force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body.

The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may briefly impose hundreds of g locally but not produce any real damage; a constant 16 g for a minute, however, may be deadly. When vibration is experienced, relatively low peak g-force levels can be severely damaging if they are at the resonant frequency of organs or connective tissues.

To some degree, g-tolerance can be trainable, and there is also considerable variation in innate ability between individuals. In addition, some illnesses, particularly cardiovascular problems, reduce g-tolerance.

Vertical

Aircraft pilots (in particular) sustain g-forces along the axis aligned with the spine. This causes significant variation in blood pressure along the length of the subject's body, which limits the maximum g-forces that can be tolerated.

Positive, or "upward" g-force, drives blood downward to the feet of a seated or standing person (more naturally, the feet and body may be seen as being driven by the upward force of the floor and seat, upward around the blood). Resistance to positive g-force varies. A typical person can handle about (meaning some people might pass out when riding a higher-g roller coaster, which in some cases exceeds this point) before losing consciousness, but through the combination of special g-suits and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle a sustained (see High-G training).

In aircraft particularly, vertical g-forces are often positive (force blood towards the feet and away from the head); this causes problems with the eyes and brain in particular. As positive vertical g-force is progressively increased (such as in a centrifuge) the following symptoms may be experienced:

Grey-out, where the vision loses hue, easily reversible on levelling out
Tunnel vision, where peripheral vision is progressively lost
Blackout, a loss of vision while consciousness is maintained, caused by a lack of blood flow to the head
G-LOC, a g-force induced loss of consciousness
Death, if g-forces are not quickly reduced

Resistance to "negative" or "downward" g, which drives blood to the head, is much lower. This limit is typically in the range. This condition is sometimes referred to as red out where vision is literally reddened due to the blood-laden lower eyelid being pulled into the field of vision. Negative g-force is generally unpleasant and can cause damage. Blood vessels in the eyes or brain may swell or burst under the increased blood pressure, resulting in degraded sight or even blindness.

Horizontal

The human body is better at surviving g-forces that are perpendicular to the spine. In general when the acceleration is forwards (subject essentially lying on their back, colloquially known as "eyeballs in"), a much higher tolerance is shown than when the acceleration is backwards (lying on their front, "eyeballs out") since blood vessels in the retina appear more sensitive in the latter direction.

Early experiments showed that untrained humans were able to tolerate a range of accelerations depending on the time of exposure. This ranged from as much as for less than 10 seconds, to for 1 minute, and for 10 minutes for both eyeballs in and out. These forces were endured with cognitive facilities intact, as subjects were able to perform simple physical and communication tasks. The tests were determined not to cause long- or short-term harm although tolerance was quite subjective, with only the most motivated non-pilots capable of completing tests. 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 in which he was clocked in a little over a second from a land speed of Mach 0.9. He survived a peak "eyeballs-out" acceleration of 46.2 times the acceleration of gravity, and more than for 1.1 seconds, proving that the human body is capable of this. Stapp lived another 45 years to age 89 without any ill effects.

The highest recorded g-force experienced by a human who survived was during the 2003 IndyCar Series finale at Texas Motor Speedway on 12 October 2003, in the 2003 Chevy 500 when the car driven by Kenny Bräck made wheel-to-wheel contact with Tomas Scheckter's car. This immediately resulted in Bräck's car impacting the catch fence that would record a peak of .

Short duration shock, impact, and jerk

Impact and mechanical shock are usually used to describe a high-kinetic-energy, short-term excitation. A shock pulse is often measured by its peak acceleration in ·s and the pulse duration. Vibration is a periodic oscillation which can also be measured in ·s as well as frequency. The dynamics of these phenomena are what distinguish them from the g-forces caused by a relatively longer-term accelerations.

After a free fall from a height <math>h</math> followed by deceleration over a distance <math>d</math> during an impact, the shock on an object is <math>(h/d)</math>· . For example, a stiff and compact object dropped from 1 m that impacts over a distance of 1 mm is subjected to a 1000 deceleration.

Jerk is the rate of change of acceleration. In SI units, jerk is expressed as m/s3; it can also be expressed in standard gravity per second (/s; 1 /s ≈ 9.81 m/s3).

Other biological responses

Recent research carried out on extremophiles in Japan involved a variety of bacteria (including E. coli as a non-extremophile control) being subject to conditions of extreme gravity. The bacteria were cultivated while being rotated in an ultracentrifuge at high speeds corresponding to 403,627 g. Paracoccus denitrificans was one of the bacteria that displayed not only survival but also robust cellular growth under these conditions of hyperacceleration, which are usually only to be found in cosmic environments, such as on very massive stars or in the shock waves of supernovas. Analysis showed that the small size of prokaryotic cells is essential for successful growth under hypergravity. Notably, two multicellular species, the nematodes Panagrolaimus superbus and Caenorhabditis elegans were shown to be able to tolerate 400,000 × g for 1 hour.

The research has implications on the feasibility of panspermia.

Different typical examples

{| class="wikitable sortable"

!Object/event

!Details

!g-force

|Gravity Probe B

|The gyro rotors

|rowspan="2" align="right" |0 g

|Triad I satellite

|The free-floating proof masses

|align="right"|0.3075–0.314 g

|Mercury

|Standing at ground level

|align="right"|0.377 g

|Mars

|Standing at equator at mean ground level

|align="right"|0.378 g

|Venus

|Standing at average ground level

|align="right"|0.905 g

|Earth

|Standing at sea level–standard

|align="right"|1 g

|Saturn V

|Moon rocket just after launch

|rowspan="2" align="right"|1.14 g

|Neptune

|Where atmospheric pressure is about Earth's

|Bugatti Veyron

|From 0 to 100 km/h in 2.4 s

|align="right"|1.55 g

|Gravitron

|Amusement ride

|align="right"|2.5–3 g

|Jupiter

|Gravity at its mid-latitudes and where atmospheric pressure is about Earth's

|align="right"|2.528 g

|Sneeze

|Uninhibited sneeze after sniffing ground pepper

|align="right"|2.9 g

|Space Shuttle

|Maximum during launch and reentry

|align="right"|3 g

|Roller coasters

|Peak during high-g roller coaster rides

|align="right"|6.3 g

|Tower of Terror

|Highest g-force steel rollercoaster

|align="right"|6.3 g

|Formula One car

|Peak lateral acceleration in turns

|align="right"|6–6.5 g

|Glider aircraft

|Standard, full aerobatics certified glider

|align="right"|+7/−5 g

|Apollo 16

|Moon mission on reentry

|align="right"|7.19 g

|Sukhoi Su-27

|Maximum permitted g-force in this Soviet fighter plane

|align="right"|9 g

|Mikoyan MiG-35

|Maximum permitted g-force in this Russian fighter plane

|rowspan="2" align="right"|10 g

|Red Bull Air Race

|Maximum permitted g-force turn in planes

|Flip Flap Railway

|Highest g-force wooden rollercoaster

|align="right"|12 g

|Ejection seat

|Jet Fighter pilot during activation

|align="right"|15–25 g

|data-ort-value="Sun"|The Sun

|Gravitational acceleration at the surface

|align="right"|28 g

|Tor missile system

|Maximum g-force

|align="right"|30 g

|Rocket sled

|Maximum for human

|align="right"|46.2 g

|Formula One

|2021 British Grand Prix Max Verstappen crash with Lewis Hamilton

|align="right"|51 g

|Formula One

|2020 Bahrain Grand Prix Romain Grosjean crash

|align="right"|67 g

|Sprint missile

|During acceleration to Mach 10 (12,000 km/h; 7,600 mph) in 5 seconds

|align="right"|100 g

|Car crash

|Brief human exposure survived in crash

|align="right"|> 100 g

|IndyCar

|2003 Texas Kenny Bräck crash

|align="right"|214 g

||Formula One

|2014 Japanese Grand Prix Jules Bianchi crash

|align="right"|254 g

|Plane crash

|The crash of Swissair Flight 111 in Nova Scotia, Canada

|align="right"|≈350 g

|Formula One

|1994 Monaco Grand Prix Karl Wendlinger crash

|align="right"|≈360 g

|Coronal mass ejection

|From the Sun's corona

|align="right"|480 g

|Formula One

|1994 San Marino Grand Prix Roland Ratzenberger qualifying crash

|align="right"|500 g

|Space gun

|With a barrel length of 1 km and a muzzle velocity of 6 km/s, as proposed by Quicklaunch (assuming constant acceleration)

|align="right"|1,800 g

|Mechanical wristwatch

|Shock capability

|align="right"|> 5,000 g

|data-sort-value=Formula One"|Formula One engine

|Maximum piston acceleration of a V8 engine

|align="right"|8,600 g

|Mantis shrimp

|Acceleration of claw during predatory strike

|align="right"|10,400 g

|Artillery shell

|Rating of electronics built into military artillery shells

|align="right"|15,500 g

|Ultracentrifuge

|Analytical ultracentrifuge spinning at 60,000 rpm, at the bottom of the analysis cell (7.2 cm)

|align="right"|300,000 g

|Mandible snap

|Calculated acceleration of the mandibles of the ant species Mystrium camillae

|align="right"|607,805 g

|Nematocyst

|Ejection of a toxin-containing nematocyst: the fastest recorded acceleration from any biological entity.

|align="right"|5,410,000 g

|Large Hadron Collider

|Mean acceleration of a proton

|align="right"|190,000,000 g

|Neutron star

|Gravitational acceleration at the surface of a typical neutron star

|align="right"|

|Plasma accelerator

|Electron in a Wakefield plasma accelerator

|align="right"|

Measurement using an accelerometer

thumb|right|upright|The [[Superman: Escape from Krypton roller coaster at Six Flags Magic Mountain provides 6.5 seconds of ballistic weightlessness.]]

An accelerometer, in its simplest form, is a damped mass on the end of a spring, with some way of measuring how far the mass has moved on the spring in a particular direction, called an 'axis'.

Accelerometers are often calibrated to measure g-force along one or more axes. If a stationary, single-axis accelerometer is oriented so that its measuring axis is horizontal, its output will be 0 g, and it will continue to be 0 g if mounted in an automobile traveling at a constant velocity on a level road. When the driver presses on the brake or gas pedal, the accelerometer will register positive or negative acceleration.

If the accelerometer is rotated by 90° so that it is vertical, it will read +1 g upwards even though stationary. In that situation, the accelerometer is subject to two forces: the gravitational force and the ground reaction force of the surface it is resting on. Only the latter force can be measured by the accelerometer, due to mechanical interaction between the accelerometer and the ground. The reading is the acceleration the instrument would have if it were exclusively subject to that force.

A three-axis accelerometer will output zero‑g on all three axes if it is dropped or otherwise put into a ballistic trajectory (also known as an inertial trajectory), so that it experiences "free fall", as do astronauts in orbit (astronauts experience small tidal accelerations called microgravity, which are neglected for the sake of discussion here). Some amusement park rides can provide several seconds at near-zero g. Riding NASA's "Vomit Comet" provides near-zero g-force for about 25 seconds at a time.

Notes and references

External links

"How Many Gs Can a Flyer Take?", October 1944, Popular Science—one of the first detailed public articles explaining this subject
Enduring a human centrifuge at the NASA Ames Research Center at Wired
[https://aviation.stackexchange.com/questions/6411/does-a-prone-position-for-the-pilot-minimize-g-force-effects]
[https://www.nestofdragons.net/weird-airplanes/proned-pilots/]
[https://www.nurflugel.com/Nurflugel/n_o_d/weird_07.htm]
[https://medium.com/@vaishnavirajesh/a-preface-to-planes-with-pronated-pilots-33f5286cd721]
HUMAN CAPABILITIES IN THE PRONE AND SUPINE POSITIONS. AN ANNOTATED BIBLIOGRAPHY

Unit and measurement

Vertical

Horizontal

Short duration shock, impact, and jerk

Other biological responses

Different typical examples

Measurement using an accelerometer

See also

Notes and references

Further reading

External links