On
earth your body weight on a scale is measured at one unit of gravitational
force, or one G. This is measured in terms of the thrust of your body against
the ground, in other words how much you are pulled towards the earth. When an
elevator, car, or aeroplane accelerates, slows down, or changes direction, you
may experience a slightly increased G-force. The occupants experience the
effects of their body's resistance to the applied force. This happens in
aerobatics, for example if the pilot is recovering from a dive and is pulling
back on the stick the plane continues to turn but the pilot’s body wants to
travel in a strait line, and therefore the pilot is pushed into the seat.
As
the pull up is tightened, the G-forces increase, when the pilot reaches 2 Gs the
pilot seems to be experiencing two times the gravity that is experienced while
static on earth. A 70 kg person would weight two times their weight when
experiencing 2 Gs or 140 kg, at 6 Gs, that same person weighs 420kg. This force
isn’t a gravitational force but it feel like it, it is a centripetal force. A
centripetal force is also the same force experienced in the hammer throw, as the
weight spins faster the weight wants to continue out and not go around and it
seems to get heavier and heavier.
G-Forces
with a positive (+) number in front of them indicates that their direction is
from head to foot. However, imagine being invert or up side down at the start of
a dive, the pilot tends to be thrown upward and outward of the aircraft. The
pilot may experience a sensation of weightlessness, and if the pilot were on
scales the pilot would weigh less than their usual weight. This type of force is
called negative (-) because there direction is from foot to head.
Pilots
planing to participate in aerobatics should be aware of the physical stress of
G-Forces during aerobatic manoeuvres. Many people undertake initial aerobatic
training but find their first experiences with higher G forces can include
airsickness, disorientation and discomfort. These effects are normal and
natural, and with exposure and experience the manoeuvres will have less effects.
Because humans adapt to imposed strains and stress, and with practice,
manoeuvres will have less physiological effects. Tolerance to G forces is
dependent on the individual pilots physiology and any pilot will lose
consciousness if his or her physiological limitations are exceeded.
The
main effect of G-Forces upon the body apart from the weight increase is the
movement of blood about the body. The +G effects will drive blood toward the
lower portions of the body, this will reducing the supply of blood from the
heart and brain. The brain requires a continuous supply of freshly oxygenated
blood for normal operation. The heart also struggles to supply oxygenated blood
to the eyes and then this diminished blood flow to the head can lead to
experience decreased visual acuity then uncoordinated muscular activity and
decreased mental acuity as well as functions, and then unconsciousness. The
later is the most dangerous of all of the effects and can end catastrophically.
But any of the effects can be dangerous even a brief loss of mental performance
during a manoeuvre can lead to inaccurate control movements which could cause
structural failure of the aircraft, or could cause the pilot to be in capable of
avoid a crash.
Knowing the G
related symptoms can be valuable in teaching you limitations and avoiding a
black out. The following +G affects are sometimes described as the following.
Lighter G loading in the eyes of an aerobatic pilot is about 3 to 4 Gs or the
grey out phase. There is a loss of visual acuity or and the start of the tunnel.
The tunnel is the affect of inducing Gs and around the outside areas of your
vision, it starts to go grey and if the Gs continue the greyness starts to move
closer and closer in disenabling you to be able to see things that are right on
the very side of your vision. If this continues the pilot could black out. The
grayout phase can serve as a warning to the fact that there is a significant
reduction of blood in the brain.
Larger
G loading is about 5 to 6 Gs or the blackout phase. Vision can be barely there
or completely lost this occurs when oxygen to the light sensitive cells the
retinal is severely reduced. Some muscle functions and mental activities still
function but there is a very high risk of unconsciousness because of the severe
lack of blood to the brain.
Loss
of consciousness occurs when the blood is reduced enough from the brain, and any
pilot will lose consciousness if his or her physiological limitations are
exceeded. The pilot will slump in his or her seat and their head will flop at
the neck and can have jerking movements at the head and neck if he or she
regains consciousness, (military pilots fondly call this the dead chicken). If
the pilot isn’t properly restrained may fall against the controls and send the
plane out of control.
In
some US Navy and NASA centrifuge studies 50 percent of the pilots loss
consciousness without having any of the warning symptoms. And some pilot’s
lost and then re-gained consciousness without even realizing they had done so.
There
for pilots can only partially rely on these warning effects, and have to rely a
lot on what they know are their limitations.
Some
people can withstand more Gs than others, but everyone has a limit, the limit
varies widely among people but it usually corresponds with their general well
being. It is difficult to predict how much Gs an individual can withstand
because of the number of factors involved. It is very important for a pilot to
know what tolerance to Gs, really is. Tolerance to Gs is related to the rate of
onset of acceleration, how quickly the Gs are put on the pilot and the duration
of exposure, how long they last for. Note that other physiological effects,
effect individual tolerances.
Little
was known about average G tolerances so the US Navy collected data from 1,000
naval aviators and aviation personnel through centrifuge tests. They applied an
on set rate of +1 G per second or how quickly the Gs are put on the pilot and
these were there results.
The
average grey out was at 4.1 Gs but due to individual tolerances it was usually
between 3.4 and 4.8 Gs, but the lowest was at 2.2 Gs and the highest was at 7.1
Gs.
The
average blackout was at 4.7 Gs but it was usually between 3.9 and, 5.5 Gs but
the lowest was at 2.7 Gs and the highest was at 7.8 Gs.
The
average unconsciousness was at 5.4 Gs but it was usually between 4.5 and, 6.3 Gs
but the lowest was at 3.0 Gs and the highest was at 8.4 Gs.
Seeing
as the on set rate is +1 G per second then the G values in this research could
be expressed in seconds. Example you may might expect a gray out at 4.1 seconds,
blackout at 4.7 seconds, and unconsciousness at 5.4 seconds.
With this data you
also need to take into account that there were naval pilots involved in these
test and they have been through special training to improve their tolerance to
Gs so the average tolerances in this data would probably be higher than an
ordinary persons tolerance. But you also need to take into account that the
act of piloting an aircraft can raise G tolerance
and therefore the results of these centrifuge studies during which the subjects
weren’t controlling the aircraft may not apply directly to flight. This
increased tolerance is not so great though.
A
series of further studies were conducted and one of them was into G induced loss
of consciousness or G-LOC. And they found that once the pilot was unconscious
the pilot would stay unconscious for an average of 15 seconds. When they re
gained consciousness they were confused, disorientated and delirious for a
further 5 to 15 seconds. Therefore if the pilot looses consciousness due to +G
forces there will be a 20 to 30 second period where he or she wont be able to
control the plane.
Military
training and high tech equipment only increase the average G tolerance of
pilots. Military pilots have developed ways to reduce the effects of +Gs.
Positive Gs can be counteracted by pulling the head down between the shoulders,
tensing abdominal, chest and leg muscles, closing the glottis by vocalising the
word 'hook', and then exhaling slowly during the period of stress. The pilot
tries to breath as he or she needs to, they don't hold their breath. Any muscle
contractions that tighten the muscles of the trunk and legs will reduce the
space available in the arteries for blood to pool into the lower parts of the
body, therefore increasing the blood pressure to the brain and thus their
tolerance to +Gs. But there is no method to counter the effects of -Gs.
Negative Gs are
encountered when Gs are in a foot to head direction, such as obtained during
inverted flight, or during an outside loop or pushover manoeuvre.
About -1 G
produces an unpleasant congestion and stagnate of blood in the face and head.
The pilot experiencing – Gs, will feel as if he or she were standing on their
head. The – Gs will cause the blood and body organs to be displaced toward the
head.
-2
to - 3 Gs will cause severe congestion of the face and blood vessels will causes
a reddening or flushing of the facial skin, throbbing headache, and the pilot
may also feal disorientation. It will also cause visual disturbances such as
blurring, greying, or occasionally reddening of vision, and a feeling of heat.
Blood vessels in the eyes will become wider or larger and nosebleed may occur.
But severe discomfort is certain. The pilot’s lower eyelids may rise to cover
the pilot's pupil during – Gs, causing the pilot to see only a red glow known
as red-out. After exposure to –Gs there may be tiny haemorrhages in the skin
and eyes and the pilot’s eyelids may be swollen.
The
blood vessels in the brain can tolerate reasonable amounts of – Gs well, but
the increased blood pressure in the chest and neck causes a slowing of the heart
in all pilots. In some pilots there are intervals of several seconds between
beats and after exposure to – Gs some people’s heart may beat irregularly.
The slowing of the heart and varying beats can add to the stagnation of blood in
the brain. Therefore the greatest threat from – Gs is the loss of
consciousness from the slowing of the heart, caused by irregularities of the
heartbeats, and stagnation of blood in the head.
Minus
5 G for 5 seconds is probably the upper limit of tolerance but aerobatic flying
may demand that a pilot spend over half of his or her air time in inverted
flight, and pulling – Gs. And there is no method to counter the effects of
-Gs.
A snap manoeuvre
is a very quick, rapid and jerking manoeuvre and the dangers is that the pilot
may not have enough time to anticipate the different stages of high Gs and can
put the pilot at risk of loosing consciousness. He or she may overstress the
plane and doesn’t have enough time to reduce control inputs.
Going
from + Gs to – Gs is called vice versa, and one of the important aspects of
tolerance to Gs is the effect of rapid changes. In aerobatics such rapid changes
are highly significant. When a pilot is subjected to - Gs, their blood pressure
receptors in their head and chest respond to the increased pressure and cause a
reflex slowing of the heart. A rapid change to + Gs would suddenly drop blood
pressure in these receptors and there would be a rapid speeding up of the heart
to maintain pressure; but because the reflex system requires some time to sense
the change and the need, the heart is delayed in responding to this demand and
the blood flow to the brain may suddenly drop. Therefore visa versa manoeuvres
may be one of the most threatening all aerobatic manoeuvres. Because if the
cardiovascular system isn’t capable enough to react to the rapid change from -
Gs to + G there is a high chance G-LOC or G Induced Loss of consciousness may
occur.
Little
was known about how much was actually endured during a flight so NASA recorded
data from four sequences during a world-class tournament and this is what they
found. They recorded the amount of Gs that was endured and the time it was
endured for this acted like a record of the pilots physiological stress. There
data showed that a range of + 8 Gs to - 6 Gs can occur during an aerobatic
competition. The study showed that in one of the flights the pilot experienced
negative Gs about half of the total time spent in the performance and he spent
approximately 10 percent of the time pulling + 2 Gs or more with highs at + 5.4
Gs. And 20 percent of the time he spent pulling negative Gs he was pulling - 3
Gs or more with lows to - 5.2 Gs. And in an outside 360-degree turn the pilot
experienced – 2Gs or more for 32 seconds. In one manoeuvre over the span of 28
seconds the pilot experienced - 3.4, +2.3, - 3.5, +2.0, - 4.0, and +2.3. And
some of these transitions were at 2.9 Gs per second. The outside/inside vertical
8 manoeuvre, (a visa versa) was physiologically the most demanding of all of the
manoeuvres during the flight. The pilot experienced a - 5.2 Gs turn and then 5
seconds later pulled +5.0 G turn. This in total amounted to 10.2 Gs in 5
seconds, or over 2 Gs per second for 5 seconds. And even for the most G tolerant
pilots these rapid changes from negative to positive Gs are particularly
stressful.
Tolerance to G
forces is dependent on human physiology and the individual pilot. Relevant
factors include the pilot's anatomy, recency of exposure to G Forces and
experience, the height of the person, age, elasticity of the blood
vessels, physical training, the responses of the heart and blood vessels,
health, general well being cardiovascular architecture, nervous system, the quality of the pilot's
blood, general physical state. Lack of recent practise, illness, and poor
physical condition all increase the possibility of loss of consciousness.
Frequent
exposure to G stress may tune the human system, making it less sensitive to
higher G loads.
A pilot’s
physical condition doesn’t generally dictate their tolerance to G loads. In
fact endurance running can actually reduce G tolerance and if an aerobatic pilot
was developing an exercise program he or she would be better of doing resistance
and strength training. Because it increases muscle mass and strength and this
generally improves tolerance to G forces, so an exercise program which
concentrates on muscle-building activities would appear to be the choice for a
person performing aerobatics. But saying this it is also important for the pilot
to have a good well-tuned cardiovascular system because it will allow them to
recover more rapidly from stress.
If
a pilot is used to flying in a costal region and then undertakes the same
aerobatics in a region of higher altitudes the pilot will have a lower tolerance
to Gs. The oxygen content in the blood is lowered by exposure to the higher
altitude, and the oxygen supply to the brain might be reduced to critical levels
during +G loading. And this will reduce the pilots G tolerance.
Anything that
reduces blood volume or cardiovascular response may reduce G tolerance. Such as
dehydration, excessive sweating, severe sunburn, low blood pressure, blood
donations, prolonged standing or sitting, hypoxia, infections and minor
illnesses, and some medications all lower G tolerance. Alcohol and hangovers
will reduce the pilot’s ability to perform safe and satisfying aerobatic
manoeuvres.
If
a pilot has a low blood sugar level it can make he or she very sensitive to G
loading. It can also make the pilot feel unwell, and will reduce their physical
and mental performance. Adequate exercise and an appropriate diet will maintain
the blood sugar at normal levels. The pilot should avoid eating high
carbohydrate meals before aerobatics because the blood sugar will fall in about
an hour, sometimes quicker. He or she should eat well balanced, light meals
before flying but not a large meal because it could cause pooling of the blood
in the digestive tract and decrease G tolerance by reducing the amount of blood
available for general circulation.
Passengers
are particularly vulnerable to disorientation and there is the high incidence of
passenger motion sickness during aerobatics. This is because the passengers are
not as able as the pilot flying to anticipate the movement and handling of the
aircraft. Another factor is the seating position; people seem to do better in
the front seat of an aircraft than in the rear.
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