The general physical state includes adequate
sleep (at least seven hours prior to aerobatics),
absence of infections (never conduct aerobatics
with an illness—viral or bacterial), absence of
hangover or drug effects (never undertake aero-
batics with a hangover or while taking alcohol
or any drug) and good physical fitness. Aero-
batic pilots should not become obese, “out-of-
shape”, or under “crash diets”. Experience leads
to knowledge and understanding and the develop-
ment of additional tolerance to G forces (1-2
G’s). Recency leads to physiologic adaptation
and fitness and an increase of perhaps one to 1.5
additional G. This is lost in a week or so of
layoff, but comes back rapidly after warm-up.

With respect to the physical and mental fit-
ness of aerobatic pilots, one female pilot during
the mid-1960’s had a severe anemia and lost con-
sciousness in a maneuver while practicing for
the international competition. The plane nosed
over and dove into the ground with fatal con-
sequences. Also, severe emotional upsets preced-
ing competitive aerobatics can lead to excessively
low maneuvers and crashes.‘+ Documented loss
of consciousness during (+) Gz G’s in closed
course air racing due to a weakened condition
caused by diarrhea exists(pylon racers can pull
4-5 (+) Gz G’s for several seconds).?* In the
same study, a pilot who suppressed information
concerning an earlier heart attack, died during
the stresses of the race. If at any time the pilot
does not feel mentally or physically up to par,
or if his aircraft, the environment, or the general
circumstances, seem wrong, postponement or can-
cellation should be accomplished. A word of
caution on alcohol: no alcohol for 24 hours prior
to aerobatics, to avoid the hangover effect, and,
once again, never perform with a hangover. Do
the celebrating after the performance!

One additional point of caution. Use only
aircraft designed for aerobatics in conducting
these maneuvers (unless the pilot is a highly
skilled test pilot who knows the limits of the
aircraft and performs for demonstration pur-
poses). Always wear a parachute during aero-
batic maneuvers and plan ahead concerning
escape from maneuver points of possible struc-
tural failure. In inverted flight with (—) Gz
loadings, a double seat belt is a good idea, as is
a shoulder harness. Never leave loose cushions

or other objects in the aircraft which can jar
loose and jam the controls. The above and re-
lated points were also stressed by Duane Cole
on 4 August 1972 at the International Aerobatic
Club Meeting, Experimental Aircraft Associa-
tion 20th Annual Convention, Oshkosh, Wiscon-
sin. With respect to the duration of aerobatic
routines, the late Bevo Howard used 12 maneu-
vers during performances in his Buecker Jung-
meister. These included a series of slow rolls in
a 360° circle, a hammerhead stall and turn, an
8-point sectional roll, a 14% snap roll, an inverted
snap, a double snap, a double snap on top of a
loop, a square loop, a vertical 8 (inside loop to
outside loop), a vertical snap, a spin, inverted
flight with hands off the controls and an inverted
ribbon pickup. ‘The above maneuvers required
approximately 15 minutes and obviously would
be quite fatiguing at the end. Howard kept in
good physical condition by swimming.

At the 26 May —4 June 1972 “TRANSPO”
at Washington, D.C.’s Dulles International Air-
port, the following times were clocked for aero-
baties by the indicated pilots or groups:

Scotty McCray (Glider) -. -.-. 7 minutes
Bob Hoover (Shrike) - 10 minutes
Mary Gaffaney (Pitts) 10 minutes
'T, Poberezny, C. Hillard,

G. Soucy (Pitts—formation) ------ 15 minutes
Walt and Sandi Pierce

(Dual aerobatics) ~- 10 minutes
Hughes and Kaizian (Wing riding) 10 minutes
Dawson Ransome (Pitts) 10 minutes
Bob Hoover (F-51) --- 15 minutes
Art Scholl (Chipmunk) 10 minutes

Average: 10.8 minutes

It is apparent from the above that by historical
precedent and present practice, the average series
of aerobatics covers about 11 minutes. Eleven
minutes of consecutive aerobatics is quite fatigu-
ing and illustrates the necessity for maintaining
good physical conditioning and health.

Figure 14 shows the maximum limits of human
tolerance to G’s as determined in large centri-
fuges** Note that for (+) Gz, the point
where grayout begins for the uninitiated is three
G for five seconds. Eight (+) Gz for 15 seconds
causes blackout and temporary unconsciousness,
even in the most experienced individual. A mili-
tary G-suit (sometimes referred to as the


PULL-OUT +3.5 G's

3 SECONDS

SPIN

3 TURN SPIN
LEFT OR RIGHT
12 SECONDS
1,100 FEET

egjep|\—>

Figure 9.—Spins consume a great deal of altitude in a relatively short period. Therefore, start these at a safe
altitude.

creases resulting in the stretching of their walls.
“Stretch receptors” in the aortic arch and carotid
artery detect the extent of this stretch and send
nerve signals through the “visceral” nervous sys-
tem into the central nervous system where the
signals are pocessed in the lower brain and result
in outflow signals that are carried by the para-
sympathetic nervous system (in this case, the
vagus nerve portion) to the heart. The impulse
is inhibitory and the heart rate slows. The blood
pressure then tends to lower toward “normal”.t

The above reflex are represents a “feed back”
mechanism and is a “cybernetic” governor in the
complete sense on the term. In addition to the
adjustment which keeps the arterial pressure
from rising excessively, there is a parallel balanc-
ing mechanism which comes into play if the
mean blood pressure is too slow. This causes a
reflex nerve arc to send impulses through the

10

sympathetic nervous system to the heart result-
ing in a faster rate and an increase in blood
pressure. The reciprocal relation between blood
pressure and heart rate as controlled by the above
two reflexes is known as “Marey’s Law”. These
reflex adjustments require about five seconds to
come into play. Therefore, if one experiences
a rate of onset of one G per second, it is obvious
that unless some other interim method is used
to compensate, a force generated by five G’s can
be reached before physiologic reflex compensation
results. In the positive G, direction, blackout
occurs unless some other compensation mechanism
is used. (Other compensation methods include
holding the breath and forcefully exhaling and,
or, leaning forward to decrease the distance of
the column of blood between the heart tip and
the eye level (base of the brain)*®.


G EFFECTS ON THE PILOT DURING AEROBATICS

Many prospective aerobatic trainees enthu-
siastically enter aerobatic instruction, but find
their first experiences with G forces to be un-
anticipated and very uncomfortable. The unin-
formed student may actually lose consciousness
at three (+) G’s and incorrectly assume that
he is unfit for aerobatics. If the aerobatic in-
structor does not have a basic understanding of
the physiology of G force adaptation, he will be
unable to clearly explain the phenomenon, and
the student will likely be lost to further aerobatic
activity.

No airplane pilot is “complete” without train-
ing in stalls, which is why proficiency in stall
recovery must be demonstrated prior to solo.
Similarly, unless the pilot has instrument train-
ing and an instrument rating, he is limited to
fair weather operations and is a potential hazard
should he get into loss-of-outside-reference con-
ditions in haze, fog, rain or darkness. This is
why all airline transport pilot certificate appli-
cants must demonstrate instrument proficiency.
Likewise, the complete pilot is proficient in spin
recovery, and demonstration of this capability
is required for the instructor certificate. Many
feel that the complete pilot also must have some
aerobatic training, especially today when wake
turbulence upsets are potentially more severe
than in past years because of the introduction
of large jet aircraft.

Push-over Forces

Let us assume that an aircraft is flying along
a straight course with wings level and that the
pilot pushes over into a 70° dive (Figure 1).
The force (f) necessary to deflect the aircraft
from its prior straight path through the curved
path is directly proportional to the product of
the mass (m) of the aircraft and the rate of
change of velocity (a for acceleration) experi-

enced. In physics this is stated by the formula
f=ma.*

We define one (+) G as the strength of the
gravitational force (which tends to accelerate a
mass toward the center of the earth) all of us
experience when stationary at or near the surface
of the earth (note that a body is not accelerat-
ing when it is standing still or is moving at
constant velocity in a straight line—if it makes
a curve, even at constant velocity it accelerates
because it moves away from the straight path).
This force may be expressed in terms of an in-
dividual’s “weight”.

If an elevator begins to move up while one
is standing in it, one experiences a “heaviness”
feeling during the acceleration phase. If the
upward acceleration is great enough to double
one’s weight should he be standing on scales at
this time, we define the accelerative force as
two (+) G’s. When the elevator reaches con-
stant velocity as it moves up, the acceleration
returns to zero and the individual is back at one
(+) G. As the elevator begins to decelerate as
it moves toward the top of the building (if not,
it might shoot through the roof), the individual,
not strapped to floor, tends to continue going up
and, the scales show less than the individual’s
weight (if the weight shown is half that at one
(+) G, convention refers to this as an accelera-
tive force of 0.5 (+) G.) If the elevator

* Acceleration (a) is the time-rate of change of
velocity. It is a vector quantity, ie. it has direction
and magnitude. With a constant velocity, one can have
acceleration due to a change in the direction of motion
of a moving mass. The case of a uniform change in the
magnitude only of velocity (as in the case of a freely
falling body) is called uniformly accelerated linear mo-
tion. When the direction only is changed this is called
uniform curvilinear motion or translatory motion in a
circle.

Acceleration is equal to V* over R and is directed
toward the center of the circle. R is the radius of the
circle. If the path of the airplane or object does not
follow a perfectly circular route, the motion is not
uniform curvilinear motion.



“Anti-G” suit) can provide a tolerance of 4.5
(+) Gs in the Z axis for five minutes.*

The (—) Gz tolerance for students is two to
five seconds and causes subjective discomfort (as
noted earlier, two (—) G's is a net change of
three G’s from one (+) Gz). The experienced
pilot can tolerance 4.5 (—) Gz G’s for five sec-
onds before head pain (headache—referred to as
cephalalgia), breathing discomfort (the lungs
are pressed by the liver) and other subjective
unpleasant sensations lead the pilot to terminate
the maneuver. Note that the (—) G’s cause
enough discomfort to lead the pilot to terminate
the negative maneuver prior to loss of conscious-
ness. In this sense, outside maneuvers are physi-
ology safer than inside maneuvers. In fact, the
first pilot to ever perform a loop, Adolphe
Pegoud at France, in the summer of 1913 per-
formed both outside and inside loops.?** He
repeatedly and routinely demonstrated outside
loops (flying “over the top”, down-under, and
back up) in a Bleriot monoplane. The airplane,

* During World War Two, German Stuka dive bomber
pilots were able to adapt to 8 (+) G's for a few seconds
on pull out by a combination of the Valsalva maneuver
and bending over to a rather extreme angle (personal
communication, Dr. Harald yon Beckh, former Stuka
pilot).

which used wing warping for bank, was modified
by adding stronger “landing wires” for (—) G
to the wings and a heavier horizontal tail. The
outside loop, as noted, was entered from a dive
with the aircraft pushed forward over on its
back and allowed to curve up and around to the
starting point. Pegoud also performed two other
types of (—) Gz maneuvers and tail slides.

For vertical eight and other maneuvers that
lead to unconsciousness, the pilot should discon-
tinue these at the point where visual sensation.
begins to change. Consideration should be given
to logging the periods and points of unconscious-
ness in various extreme maneuvers in order that
long range corrective action can be taken in
terms of modifying the repertoire.

Figure 15 lists the Gz tolerances in the (+)
and (—) directions of the average healthy ex-
perienced human. Note that the one (+) Gz
axis tolerance is about 20 hours in the immobile
sitting position, and that after this period, the
desire to slouch, to drop the head to one side or
the other, or to Jean on a table, not to mention
to lie down, becomes overwhelming. All of these
changes lead to diminishing the height of the
blood column from the heart to the brain. After
two hours of standing straight with no move-

HUMAN ACCELERATION TOLERANCE LIMITS

ONSET OF EXPERIMENTAL
DIRECTION OF — AIRCRAFT INITIAL HUMAN PHYSIOLOGICAL
TYPE OF G BODY MOVEMENT MANEUVER SYMPTOMS MAXIMUM EXPOSURES UMITS
+ HEAD TO FOOT © PULL OUT ~—= STUDENT: 36 8G FOR 15 SECONDS BLACKOUT
OF DIVE OR 5 SECONDS —(4.56 FOR 5 MINUTES TO
TIGHT LEVEL © GRAYOUT WITH G SUIT) UNCONSCIOUSNESS
TURN
- FOOT TO HEAD © PUSHOVER STUDENT: 2G 4.5 G FOR 5 SECONDS PAIN
5 SECOND (HEADACHE)

SUBJECTIVE DISCOMFORT

Ficure 14.—This chart is for application to aerobatic students.

17

19



fo

OUTSIDE LOOP

-3.5 TO
-4G’'s

—_—

-3.5 10

ome SS

INVERTED

-46's
S/

_—

INSIDE LOOP

2 103
SECOND
BLACKOUT
(ALSO +4 TO
+4.5 G's)

VERTICAL “8”

EXCEEDS
PHYSIOLOGICAL
PRESSURE
COMPENSATION
MECHANISM

1 hy
2

>
UPRIGHT

+470 +4.5 G’s

Ficure 18.—This maneuver is very stressful from the physiologic as well as the aerodynamic standpoints.

the venous blood due to the (—) Gz, force tend-
ing to retard the return of the blood from the
head to the heart. Normally the venous blood
returns to the heart in a gravity-feed system.
In the (—) Gy, position, this venous return is,
therefore, somewhat impaired. There is a reflex
stretch mechanism in these great neck veins
which is designed to speed up the heart and cause
it to pump more blood in the presence of venous
overload (this is known as the “Bainbridge”
reflex). Stretch receptors in the great neck
veins detect excessive venous blood pressure and
send signals through the visceral nervous system
to the central nervous system, with return signals
through the sympathetic nervous system to stimu-
late the heart. This reflex is designed for the
(+) Gz circumstance, and has an adverse effect
in a (—) Gz maneuver since it results in the
heart attempting to pump more blood into an

14

already overloaded head circulation having de-
creased venous outflow. Because of the en-
gorged slow-flowing blood circulation, the brain
becomes somewhat hypoxic since it extracts much
of the oxygen from the slow moving blood dur-
ing the outside maneuver at the 4-5 o’clock and
7-8 o’clock positions.

Following the completion of the outside loop
which required at the 7-8 o’clock position 3.5 to
4 (—) G's along the Gz axis to round-out hori-
zontally, the inside loop is abruptly begun. Here
the (4+) G’s begin to be imposed on the Gz axis,
an imposition for which the blood pressure
physiology is not attuned. On the contrary, all
reflexes have been working against the (—) Gz
axis. There is a five second delay in reflex
response from one (+) Gz to increased (+) G@
loadings, and when the loadings begin from the
(—) Gz side, this response is increased by several


+4

NET 1G
CHANGE

IN STEADY
STATE- 4
NEAR EARTH

G NDIeATOR

NET 2G )°
CHANGE

MAXIMUM
<6 INDICATOR

MY fssdlddtt ties

“MAXIMUM
-G INDICATOR

+5

+7

+8

-4

THE G METER

-3

Fieure 5,—The acelerometer or “G” meter.

ment toward the (—) Gx direction with sudden
decelerations. Physiologically, because of its
design features the body can adapt much more
readily during aerobatic flight to Gx and Gy axis
accelerations than is so with the G, axis, hence,
we will direct our major attention here to the Gz
axis accelerations (+) and (—).

The G Meter

Figure 5 portrays the instrument panel G
meter which indicates the accelerative forces
experienced by the aircraft which, by virtue of
the pilot’s cockpit seating arrangement, are
exerted through the G, axis of the pilot.

Note that one needle stays at the maximum
(+) G’s experienced during a maneuyer, one
needle stays at the maximum (—) G’s, and one
fluctuates continuously with the imposed accelera-
tion force (in straight and level horizontal flight
this will read one (+) G.) An instant reset
button is present allowing then centering to one

(+) G of all three needles prior to the next
maneuver.

Note also that the pilot experiences a net change
of one G during a maneuver from a one (+) G
baseline that pulls two (+) G, while he experi-
ences a net change of two G’s during an outside
maneuver starting from a one (+) G baseline
and going to a one (—) G level.

If the pilot performs a maneuver from a one
(+) G baseline to 4 (+) G’s, his net change
is 8 G’s, while if he performs from the same base-
line a 4 (—) G maneuver, his net change is 5
G's. Negative maneuvers of the same numerical
value as positive maneuvers but with opposite
sign, impose greater physiological net changes,
therefore, on the pilot. In addition, as dis-
cussed later, the body’s physiologic adaptive
mechanisms to combat G load effects in the Gy,
axis is designed primarily to adapt to (+) Gz
changes (since we evolved in a (+) Gz environ-
ment) and function poorly against (—) Gz
accelerations.


AIRCRAFT
MANEUVER

ACCELERATIONS

70° DIVE PULL-UP

————

+4G 3 SECONDS
+66 1 SECOND

Figure 2.—Positive G’s in a “pull-up”.

Pull-up forces

Figure 2 reveals possible force in a 70° dive
and pull-up. These are four (+) G for three
seconds to six (+) G for one second. Note how
the blood and body organs tend to pool toward
the lower part of the body. Obviously, since the
human brain requires essentially continuous blood
circulation from the beats of the muscular pump,
the heart, for maintenance of an adequate oxygen
supply, and since the circulatory system is a
complex network of flexible vessels of which the
major vessels run lengthwise in the body, there
is a physiological limit to the time the pilot can
withstand these higher G forces before losing
consciousness. A brief loss of consciousness in
a maneuver can lead to improper control move-
ment causing structural failure of the aircraft
or collision with another object or terrain.

Steep turn forces

In steep (+) G turns, the centrifugal force
tends to push the pilot through the floor boards,
and as shown in Figure 3, a steep turn of 180°
change of direction will yield two (+) G for
35 seconds and if made in 15 seconds can yield
five (+) G. Every private pilot has been taught
that (by reason of geometry and vector forces)
all 60° banked turns held at a constant altitude
pull two (+) G during the turn.’ * The difference
in this maneuver between fast and slow aircraft
is that the faster aircraft covers more area dur-
ing the maneuver.* The same interrelationships

* Assuming constant acceleration, if one doubles the
velocity of an airplane in a 60° banked turn, the radius
of curvature becomes four times greater. If one were
to triple the velocity, nine times greater. For a con-
stant acceleration the radius of curvature varies as the
square of the velocity. The same applies, of course, to
the diameter of a loop. High speed loops take a great
deal of vertical airspace.



CW,

AIRCRAFT
MANEUVER

ACCELERATIONS

oe

PUSH-OVER TO 70° DIVE

0G FOR 35 SECONDS
—1G FOR 15 SECONDS

Figure 1—Negative G’s in a “pushover”.

rapidly reversed course and began accelerating
downward so that the individual and the elevator
fell solely by the accelerative pull of gravity,
the individual would experience during this pe-
riod zero G (since his body would register zero
on the scales). If the elevator were forced to
accelerate downward faster than the accelera-
tion caused by the pull of gravity alone, and if
the individual were tied to the elevator, so that
he would be pulled with it, he could experience
“negative” G’s. If a spring scale were placed
between the individual and the floor, the foot-
ward tug in pounds on his body could be meas-
ured, and if this tug were to be exactly equal to
his weight, we would call this an acceleration
of one (—) G.

Figure 1 illustrates that a pushover from
straight, wings level, flight to a 70° dive, can
produce forces on the aircraft (and the strapped-

in pilot) of 0 G for 35 seconds to one (—) G
for 15 seconds (depending upon how hard the
pilot pushes over).* Other G/time combina-
tions are also possible, of course. If the pilot
were not snugly strapped in, the one (—) G
acceleration would result in his departing the air-
craft at a tangent to its curved flight path, and,
through a “parabolic arch” free-fall, reach a ter-
minal velocity at one (+) G (not counting the
effect of wind blast, which, depending upon the
pilot’s speedi at exit exerts a certain drag force.)
At the zero G acceleration, the aircraft and the
pilot are both falling solely by the pull of gravity
and the pilot has the sensation of floating. Note
that at one (—) G, the blood and body organs,
expecially the heart, the liver, and the intestines,
tend to move toward the head of the pilot (more
about this, later).


seconds. When the first strong (+) Gz forces
hit at the 4-5 o’clock position, the nervous
mechanisms are stil ltrying to catch up with the
imposed (+) loading and all of the compensa-
tions have to be made in the completely opposite
direction. These are possibly aided by the val-
salva maneuver and by head lowering. The blood
flow slows to the brain and at the 6 o’clock posi-
tion the brain has extracted- much of the oxygen.
Some slight increase in circulation may be gained
at about the 6 o’clock point as the G forces re-
duce somewhat. However, the acceleration
against the pull of gravity at the 7-8 o’clock
position imposes 44.5 (+) G’s upon an already
overstressed, barely compensating, cardiovascular
system. This new load decreases the circulation
once again and, this time, loss of consciousness
occurs at the 7 o’clock—9 o’clock position. Since
the large G forces begin easing off after the
8 o’clock position, and the physiological pressure
maintenance reflexes begin to have an effect, cir-
culation to the brain is reestablished and con-
sciousness returns.

Physiologically, no permanent harm occurs to
the healthy individual in the above maneuver.
World War II dive bomber pilots found that if
they did a (—) G push over they were more
likely to black-out when the (+) G recovery
was made than was so if they rolled over and
made a (+) G entry to the dive In view of
their loss of consciousness during the pull-up
when the aircraft itself was under great aero-
dynamic stress, they feared inadvertent control
movements which might structurally damage the
aircraft under these conditions having the visual
loss of reference “black-out”, prior to, and just
after, the unconsciousness. Also, the relaxed
hold on the stick while unconscious could lead
to a dive into the ground. Accordingly, they
trimmed for a predetermined amount of nose
up flight prior to the point of possible loss of
consciousness. The aircraft, thus, tending to do
the proper thing while the pilot was an “inert
passenger”.

Physiologic Tolerance

It appears desirable that, depending upon the
pilot and aircraft, no maneuver be routinely per-
formed which leads to unconsciousness at any
point. Physiologically, humans progressively

15

adapt within limits to imposed strains and
stresses, and with practice, any maneuver will
have Jess and less of an effect (again, within
limits, depending upon human physiology and
the individual pilot), A lay off of some days
or weeks can result in a lowered G tolerance, but,
normally, this returns rapidly with practice. One
old-time (1930's) aerobatic pilot (known as the
“batman”) fixed a harness in his garage and
hung upside down for a few minutes each day.
This would give a one (—) Gz acceleration and,
possibly, he did maintain a slight increased
physiologic tolerance in the (—) Gz axis. How-
ever, (—) Gz adaptation does not increase as
effectively as is the case with (+) Gz, adaptation,
since, as already mentioned, the physiologic
mechanisms are designed to counteract (+) Gz
accelerations.

Other factors which affect G tolerance are (1)
the skeletal anatomy, (2) the cardiovascular
architecture, (3) the nervous system, (4) the
quality of the blood, (5) the general physical
state and (6) experience and recency.

Short, squat individuals would inherently have
an edge toward G tolerance maximum levels over
tall long-bodied, long-necked, individuals, al-
though superb aerobatic pilots are, of course,
found in the latter category. A highly efficient
heart, free of coronary disease, and capable of
raising the blood pressure rapidly, upon demand,
is a prerequisite to safe, prolonged aerobatics.
Some very young individuals have such elastic
arteries that attempts to raise blood pressure are
partially thwarted by lateral distension of the
vessel walls. Normal aging results in a decrease
in elasticity in arterial walls and, in this respect,
acts to increase the tolerance to (+) Gz accelera-
tions. The nervous system involves tempera-
ment (some persons will never emotionally adapt
to aerobatics because of fears instilled in child-
hood or because of an inability to see the point
of such activity). The quality of the blood
relates primarily to its hemoglobin content (this
is oxygen carrying component and should be be-
tween about 13-17 grams %—females should be
especially wary of iron deficiency anemia), its
salt content (be sure to have adequate Na* Cl
in hot weather) and water content (excessive
dehydration lowers blood volume and the ability
to change blood pressure).



Terminology for Plus and Negative
Accelerations on the Pilot

In the same fashion as with aircraft design
limits for G, the human body has upper limits,
as modified by the necessity to keep the physi-
ologie functions operational. In order that a
clear delineation of the human limits to G can
be presented, certain arbitrarily established
parameters must be utilized. Figure 4 provides
a standardized terminology for describing G
forces acting on the body of a pilot during aero-
batic maneuvers.t It will be noted that since
the top of the heart elastically hangs in the
chest from the aorta and certain nearby tissue
sheets, the direction in which the heart moves
relative to the skeleton when the whole body is

TERMINOLOGY FOR
ACCELERATION FORCES
ON THE BODY

VECTOR DIRECTION NAMED
FOR THE DIRECTION THE
HEART MOVES RELATIVE TO
THE SKELETON UNDER THE
IMPOSED ACCELERATION

accelerated, determines the name given to the
force. For example, an inside loop pulls (+) G’s
and tends to move the heart toward the pelvis,
and since the long axis of the spine is referred
to as the G, axis, this maneuver is said to pull
(+) G’s in the Z axis, and, if 2 G’s were pulled
at a point, this would be written: 2 (+) G,. An
outside loop pulling one (—) G’s at a point
would be indicated by 1 (—) G at that point.
The Gy axis runs through the shoulders and
has some applications in snaps and spins (but
requires a specially mounted G meter for measure-
ment). The Gx axis runs through the chest and
has a certain bearing on tail slide maneuvers
and recoveries and crash impacts, where seat
belts and shoulder harness prevent excess move-

-Gz

Ficure 4.—Standardized acceleration terminology.

5



TECHNICAL REPORT STANDARD TITLE PAGE

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
FAA-AM-72~28
4. Title and Subtitle 5. Report Date
; July 1972
G Effects on the Pilot During Aerobatics %. Performing Organization Code
7 Author(s) 8. Performing Organization Report No.

Stanley R. Mohler, M.D.

. Performing Organization Name and Address 10. Work Unit No.

Aeromedical Applications Division
Federal Aviation Administration 11. Contract or Grant No.
Washington, D.C. 20591

13. Type of Report and Period Covered

. Sponsoring Agency Name and Address

Office of Aviation Medicine
Federal Aviation Administration OAM Report

800 Independence Avenue, S.W. 14. Sponsoring Agency Code
Washington, D.C. 20591

” Supplementary Notes

. Abstract

Sport, precision, and competitive aerobatics, and especially air show and
demonstration flying are enjoying a rebirth of interest exceeding that of
the 1930's. Improved aerobatic airplanes and power plants are in the hands
of more civilian pilots than ever before. These aircraft enable the pilot
to easily initiate maneuvers which exceed human tolerances, yet not over-
stress the aircraft. Military aircraft reached this point in World War II
and the G-suit was perfected to protect the pilot. The military groups
still use the G-suit but this equipment is impractical for most civil aero-
batic activities. This paper provides information on (1) the nature of
aerobatic G forces, (2) human physiology in relation to G forces, (3) human
tolerances to various levels and times of exposure to G forces, and (4) means
by which tolerance to G forces may be increased in terms of (a) the general
physical condition and (b) the time during the maneuver when the G forces
are imposed.

17. Key Words 18, Distribution Statement
Aerobatics Availability is unlimited. Document
G Effects may be released to the National Technical
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Virginia 22151, for sale to the public.
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can frighten the initiate if some explanation
and guidance is not given by the instructor or
demonstrator ahead of time. The experienced
pilot in good health knows how to adapt and
what to expect, and at the levels of G shown
for the times given in Figure 6, derives pleasure
from the physiological and optical sensations.
The chances are he wants more. Years ago, for
example, pilots vied with one another for the
maximum number of consecutive inside loops.
Charles “Speed” Holman made 1,453 consecu-
tive loops in 1928 at St. Paul, Minnesota, taking
five hours (this gives an average time of a little
over 12 seconds per loop).?

It is noted also that, initially, persons may
become airsick during aerobatics because of the
unusual stimulations of the inner ear plus the
anxiety of not knowing exactly what is happen-
ing. This tendency is usually rapidly lost (ap-
parently through “habituation” of the semicir-

cular canals) and for most persons never becomes
a serious problem again.

The inside aileron roll is a maneuver between
the slow-roll (which pulls zero to one (—) G)
and the barrel roll (which is sort of a spiral loop
and pulls plus Gs all the way around). The
aileron roll (see Figure 7) takes six seconds for
completion, either to the left or the right and
reaches a maximum of 2.5 (+) G’s.

The inside snap (Figure 8) takes three seconds
to the left and pulls 2.5 (+) G’s. Since the snap
to the right in aircraft having propellers which
rotate clockwise (when seen from the rear of
the aircraft) is entered at a slightly higher air-
speed, the snap takes 2.9 seconds and recovery
pulls three (+) G’s. The propeller spiral slip
stream tends to yaw the aircraft to the left, hence,
the slightly higher entry speed in right snaps to
compensate for this effect. This maneuver is so
rapid that it is over before the novice realizes

-_

NN

INSIDE AILERON ROLL

Z

TO LEFT
TIME: 6 SECONDS

MAX G’s: +2.5

TO RIGHT
6 SECONDS

+25

Fieure 7.—The aileron roll is perhaps the best maneuver to use to introduce novices to aerobatics,

8


Maneuvers

Typical aerobatié maneuvers weré filmed by
the author using an on-board movie camera and
were also timed by a stop watch. Light aero-
batic aircraft of the Beech Musketeer Aerobatic
Sport III and the Beech T-84 type were used.
In addition, the G forces experienced were re-
corded at critical points in the maneuvers. The
data would be modified somewhat by other types
of aircraft and by varying the force of control
applications Appreciation is extended to Mr.
William Kershner who assisted with most of the
maneuvers.

Figure 6 reveals that the inside loop in the
above type aircraft takes about 15 seconds and
pulls 3.5 (+) G for 1 second at the 4~5 o’clock
position and at the 7-8 o'clock position. The

rest of the loop averages about 1 (+) G. The
build-up to, and drop-off from, 3.5 (+) G@ is
necessary to change the direction of the relatively
fast aircraft as it enters the six o’clock position
(f=ma) to an upward direction, and the pilot’s
pull back on the stick is, accordingly, of cor-
respondingly greater force at the 3.5 points. The
pull-out entails the 3.5 (+) G level because the
accelerating aircraft must be changed in direction
from straight down to the horizontal against the
pull of gravity.

The average “naive” person or novice aerobatic
student will “gray out” (loss of vision due to
decreased blood flow through the brain and
retina) at 3.5 (+) G's, especially if the indi-
vidual doesn’t expect the imposition of the accel-
erative force and is unprepared or does not know
how to adapt. Accordingly, this simple maneuver

16

TOTAL TIME:
15 SECONDS

1G

+3.5 G
(1 SECOND)

Fioure 6.—The forces and times in a loop.

4H

46

+3.5G (1 SECOND)

pr

G INSIDE LOOP

In a Beech Musketeer Aerobatic Sport III, the entry is at 140 mph

to get one G at the top at 70 mph.

7



AIRCRAFT

MANEUVER
ACCELERATIONS

My
y Yin
Ny

Ki

My

STEEP TURN OF 180°

+2 G 35 SECONDS
TO +5 G 15 SECONDS

Fieure 3.—Positive G’s in steep turns of 180° direction change.

exist in all aerobatic maneuvers, with aircraft
capabilities varying according to design and
power plant characteristics.**

Aircraft G limits

Most aerobatic maneuvers for demonstration
are variations of the loop, slow roll and snap.‘

** When an airplane is banked 60° with coordinated
controls (left stick, left rudder) and kept at a constant
flight atitude, the airplane describes a circular path to
the left, The resultant vector from the center of lift of
the aircraft is perpendicular to the lateral and Iongitudi-
nal axes of the dircraft and is twice as long as the lift
vector from the same point. ‘The result is that a 30°/
60°/90° triangle exists, having a hypotenuse of two and
an opposite side (with respect to the 80° angle) of one.
‘The sine of 80° is 14. Therefore, the force acting on the
pilot “through the floorboards” In a 60° banked constant
altitude turn is twice that in straight and level flight.
For further details see Kershner’s and Hurt’s publica-
tions.

If the maneuver is accomplished to impose a
(+) G load, it is referred to as an “inside” ma-
neuver, while (—) G load maneuvers are termed
“outside”.

The FAA has established G load design limit
factors for civil certified aircraft which weigh
under 12,500 pounds and these are as follows
(each limit is supplemented by a times “1.5”
safety factor in case of occasional accidental
excess loading) :

Category +@ -—@

Normal 3.8 0.4 times 3.8
Utility 44 0.4 times 4.4
Acrobatic 6.0 0.5 times 6.0

The above limits are minimum design require-
ments under Federal Aviation Regulations 23.337
(see Code of Federal Regulations, Title 14, Aero-
nautics and Space, FAR Part 23, 1972).






4 SECONDS

2 seoanDs ‘

+16

Ne G's

<a,

TOTAL TIME:
24 SECONDS

+4.2 )

INSIDE SQUARE LOOP

pr

Fiaure 10.—The square loop requires a high level of G’s on entry and recovery,

In addition to the stretch “pressure sensors”
(also called baroreceptors or pressoreceptors)
described above, there are certain additional regu-
lator mechanisms including the closing or open-
ing of the tiny arteries just. before the capillary
network, ‘The vast numbers of these tiny arteries
(arterioles), their sensitivity to nerve stimula-
tion, and their critical point of location in the
arterial system, make them powerful additional
controllers of blood pressure. Also, adrenalin
release into the blood in highy stressful (+) Gz
circumstances can raise blood pressure through
its cardiac stimulation effect. In very stressful
(—) Gz maneuvers, this should have an adverse
effect by forcing the heart to send blood to an
already overengorged brain.

11

Figure 12 shows in the center top, a simplified
schematic diagram of the one (+) Gz “normal”
situation, illustrating the tip-of-the-heart to eye
level distance, the position of the liver between.
the diaphragm and the heart, and the relative
size of the aorta and major outflow arteries.
Large veins (jugulars) return the blood from
the head to the heart and a large vein (the vena
cava) runs parallel with the aorta and returns
blood from the lower limbs and trunk to the
heart.

At the right is shown a four (+) Gz force
which results in a pulling away of blood from
the vessels supplying the brain. The heart is
smaller because it cannot fill well since the lower
trunk and leg vessels now have pooled blood.



CIRCULATION DYNAMICS DURING G, ACCELERATION

(uve)

ft

-36, +16,

(we)

=,

TO INCREASE G TOLERANCE
DURING MANEUVER, LOWER HEAD
AND, OR, FORCIBLY EXHALE
AGAINST CLOSED GLOTTIS

_
EYE-LEVEL
TO HEART TIP
VERTICAL
DISTANCE
-“

“F-
DECREASED
VERTICAL DISTANCE

__

(uvER )

Figure 12—The human body is a relatively soft and flexible structure, hence the specific effects of vertical axis
accelerations upon it.

(valsalva maneuver), an increase of (+) Gz
tolerance can be obtanied of an additional 1 to
2 (+) G's. 18

Figure 18 illustrates the vertical “8”, a ma-
neuver that when entered from the inverted posi-
tion and performed well in competitive aerobatics
is good for 40 points (Aresti system). If the
upper loop is “inside” and the lower “outside”,
36 points are possible. The maneuver in Figure
13 combines high aerodynamic performance re-
quirements with pilot physiologic limits. The
figure is shown in the “Aresti” system of “aero-
cryptographic” portrayal, with the broken lines
indicating (—) Gy, forces. The competitive
aircraft with inverted fuel systems, favorable
power/weight ratios, and fairly light wing load-
ings are particularly suited for this maneuver.
Examples are the Moravan National Corporation
Zlin 526A or 526AS (160 hp), the DeHavilland
DHC-1B-2-S$3 Chipmunk (200 hp) and the
Yakovley YAK-18PM (300 hp). The Pitts

1B

Aviation Enterprises Pitts S-1 Special also per-
forms the maneuver well.

In communication with various pilots who have
practiced the maneuver, periods of unconscious-
ness occur at the 7-9 o’clock position on the in-
side loop which follows the outside loop.° If the
inside loop is performed first, followed by an out-
side lower loop, the unconsciousness does not
happen, but, as previously stated, the maneuver
is then worth fewer points.

‘The mechanism of unconsciousness experienced
in the maneuver as portrayed in Figure 13 is as
follows. The maneuver is entered from the in-
verted position which has already resulted in a
certain congestion of the brain and eyes with
blood since there will be a one (—) Gy force.
The pressure receptors in the aortic arch and
carotid arteries (Figure 11) will have sensed the
blood engorgement and will have sent signals
to slow the heart. In addition, the large veins
of the neck will be stretched by the slowing of



ACKNOWLEDGMENT

The author expresses his deep appreciation to Richard M. Hansen of the
FAA who drew the illustrations for this paper. Also, appreciation is
extended to Edward Podolak, Siegfried J. Gerathewohl, Ph.D., and Harald
von Beckh, M.D., for technical comments. William K. Kershner furnished
invaluable aerobatic demonstrations.





INSIDE SNAP ROLL

=

TO RIGHT

TO LEFT

TIME: 3 SECONDS 2.9 SECONDS

MAX. G’s: 42.5 +3

Ficure 8.—The snap roll is the fastest roll maneuver.

what happened. He may be confused and claim
that a left snap was actually to the right.

Three turn spins (Figure 9) require a loss
of altitude of about 1,100 feet in the aircraft
referenced, taking 12 seconds, and leading to
3.5 (+) G’s on pull-out for three seconds. Some
aircraft pitch partially upside down, nose low,
on entry, a frightening experience to some
novices, and the increasingly rapid roll rate in
certain aircraft plus the nose low attitude is also
frightening if not understood. Subtracting three
seconds for recovery, one can see that each spin
turn is three seconds, approximately that of one
snap (the snap is, in effect, 2 horizontal spin,
entered from an accelerated stall at a speed some-
what higher than that of spin entry).

Figure 10 shows the inside square loop which
takes 24 seconds and contains four 4-second legs
and four 2-second vertical ninety degree turns.
Note that the abrupt change of attitude from
level to nose-up and nose-down to level pull
42 (+) G’s for as high a speed safely attainable
by the aircraft (f=ma). This level of G’s will
definitely black out the unprepared person. For
this short period of time, the body of a 170-
pound pilot weighs 714 pounds. The arms and

head are four times as heavy and the novice feels
very uncomfortable, A portion of the top of the
square loop records zero G’s in aircraft without
inverted fuel flow capability and an oil system
suitable for inverted flight, since the aircraft
actually follows a somewhat parabolic falling
maneuver as the engine cannot sustain negative
Gs for more than a few seconds. In aircraft
with systems suitable for inverted flight, one
(~) G would be recorded across most of the
top of the square loop.

Physiologic Adaptation to G Forces

Figure 11 illustrates the arterial blood flow
pattern as the blood leaves the heart. The heart
pumps the oxygenated blood upward through
the body's largest artery, the aorta. The aorta
arches 180° and sends a column of blood down-
ward to the trunk and lower limbs. The carotid
arteries exit from the arch of the aorta and
serve the head, including the brain and eyes.
The subclavian arteries also receive blood from
the aorta and serve the arms.

When the mean blood pressure rises above a
certain “normal” level in the aorta and carotid
arteries, the diameter of these elastic vessels in-



CAROTID ARTERY
BLOOD PRESSURE SENSOR

AORTIC ARCH

BLOOD PRESSURE
SENSOR

SYSTEM REPONSE
TIME TO CHANGE
THE

BLOOD PRESSURE:
5 SECONDS

VISCERAL NERVOUS
SYSTEM RELAYING
PRESSURE INFORMATION
TO BRAIN AND

PRESSURE COMPENSATION
MECHANISMS

BLOOD PRESSURE SENSORS IN PILOT

Figure 11,—Aerobatic pilots should be familiar with the physiologic mechanisms which compensate for G force
imposition during aerobatics.

The veins especially dilate because of their thin
walls relative to arterial walls, and considerable
blood pools in the abdominal “splanchnic” viens.
The liver has slid toward the pelvis. Gray-out
to unconsciousness may occur in the experienced
aerobatic pilot because of decreased blood flow
through the brain and eye retina if a four
(+) Gz acceleration is experienced steadily for
one minute.

At the left, a three (—) Gz force is shown.
The blood in the major arteries and viens to the
head is forced toward the head and the brain
and eyes are engorged. The experienced pilot
feels head discomfort, fullness sensations and
some have reported that the lower lid creeping
over the cornea gives a “reddish” visual sensa-
tion. The whites of the eyes become bloodshot
and at higher (—) Gz forces, tiny capillaries
may rupture (due to high arterial pressure and
high venous pressure, these on both ends of the
thin walled capillaries) giving small red hemor-
rhages. Some persons have experienced tiny

12

retinal capillary hemorrhages, also, resulting in
several days of “spots before the eyes”. Note
that the liver has pressed up through the dia-
phragm against the heart and lungs, making
breathing difficult. Note also that the aorta to
the lower trunk and legs is low on blood volume.
There is no effective way to compensate for the
above (—) Gz changes as there is for the (+) Gz
changes. Unconsciousness may occur if 4.5 (—)
Gz is imposed for five seconds, but, as with
(+) Gz unconsciousness, as soon as the accelera-
tive force is removed, a rapid return to con-
sciousness occurs with restoration of circulation
(due to the highly oxygenated status of the
arterial blood—assuming the pilot is not exposed
to a hypoxic environment).

The lower center diagram of Figure 12 illu-
strates that if the vertical distance between the
heart and the brain is decreased, and, or, the
thoracic and abdominal pressures are increased
prior to, or during, a (+) Gz acceleration by
attempting to exhale against a closed glottis