Skydiving Fall Rate
Investigating skydiving freefall speeds using a barograph
Major portions of this research were presented at the 1997 PIA
International Symposium in the Parks College Ram-Air Parachute
Study presentation, and in conjuntion with the presentation
The Ups and Downs of Something Called Fall Rate
given by Garry Carter of Body Sport USA.
Graphs of jump data are at the end of the report.
In observing the dates associated with this document, it may seem that
there is not much ongoing research being made with the barograph.
That is because the barograph has simply worked so well in its present
form and using its present software that it is now taken for granted
as a stable item of research equipment used in my other research! G.P.
Skydiving Fall Rate
by Gary Peek
A research project investigating skydiving freefall speeds using a
microprocessor-based barograph (recording altimeter).
from the original text 2/17/97, update 2006
Fall Rate Problems
I first became interested in determining skydiving fall rates
scientifically in the late 1980's when I observed that even after
years of skydiving knowledge and jumpsuit research, skydivers of
various sizes and weights were still having problems with fall
rates when skydiving together. I knew that many of these problems
were due to jumpers abandoning the concept of jumpsuits designed
with a "standardized fall rate" in favor of faster speeds suggested
by competitors. I had also seen problems caused by novice jumpers
who had obtained advice on jumpsuit purchases from some experienced
jumpers who either did not understand or chose to ignore the fall
rate differences caused by subtle differences in jumper size,
weight, body shape, experience level, and geographic location.
In order to help solve this problem, however, I needed a more
precise and more readily available method of determining fall rate
than simply requiring video on jumps in order to time the freefall
with a stopwatch.
(In recent years, the number of skydiving disciplines involving
higher, lower, and widely varying speeds has increased. I have
found that this fall rate information can also take some of the
guesswork out of matching fall rates between jumpers participating
in these disciplines and their video jumpers, along with other
jumpers following along on such jumps.)
The Barograph
To determine these fall rates, what I needed was a device that
displayed or recorded the freefall speed of a skydiver in miles-
per-hour, which seems to be the most often used term with which
skydivers refer to their fall rate. I felt that having a display
would allow a jumper to adjust their fall rate during a jump to
a more favorable rate, or an agreed upon rate. I had heard of
"barographs", which log barometric pressure, being used in record
altitude jumps and knew that they were available, but that they
were large and cumbersome mechanical devices and also expensive.
I began my work on my first electronic barograph in 1990 when I
began to notice that solid-state pressure sensors, which are the
basis of digital barographs and altimeters, had become widely
available and affordable. Microprocessor technology and personal
computer availability had also progressed to the point where
development of a small digital barograph was practical. (These
same advances in pressure sensor and microprocessor technology
have allowed AAD manufacturers to offer microprocessor based
AAD's in recent years.)
My Three Models
I have constructed three versions of the barograph, with each
version getting smaller, more accurate, and easier to write
computer programs for, again due to improving technology. Each
of the versions recorded altitude versus time during freefall,
and from that data, could calculate speed. Much electronic and
theoretical help in working through some of the problems I
experienced along the way was given to me by Ken Mathews of
Mathews Technical Services in St. Louis, Missouri.
The first barograph did not have a display, but was designed to
print a paper copy of the logged time and freefall speeds to a
small printer after the jump, since a portable computer was not
available to me and drop zones did not have computers available
to use. Analysis of these freefall speeds revealed large variations
in freefall speed that could not be accounted for, although the
average of these speeds seemed reasonable. I thought that the
burble of air around a jumper's body might be the cause of these
variations, but the variations were so large that I considered
other causes. I had nothing else to compare these readings with
since skydiving altimeters of the aneroid type are mechanically
damped enough to eliminate any noticeable variations due to the
burble. I tried to eliminate the variations by adding simple
averaging calculations in the barograph program but this was
not enough to reduce the variations to acceptable levels.
The second barograph, developed in 1993, was designed with a
display large enough to see in freefall, however, it was still
rather large and I seldom wore it on my wrist. It could also
transmit a data file to a computer after the jump, so a graphing
program was written to see the jump altitudes and speeds calculated.
The large speed variations could now be seen graphically, and the
burble around a jumper's body was assumed to be the reason.
Various mechanical means were tried to reduce the effect of this
"noise", but with limited success. Some of the things I tried
included enclosing the sensor in many different materials and in
chambers. I also tried connecting a plastic tube to the sensor
port and positioning it in various places on my body. Software
filtering was also attempted at this point, but many program
changes were required to experiment with software filtering tech-
niques, since program development for this microprocessor was too
difficult and time consuming to do at a drop zone considering
that I was busy with other activities.
My Current Barograph
Work on the current version of the barograph began in September
1995 using a microcontroller that is easier to program using the
more complex calculations required to do adequate software filter-
ing. This unit is not much larger than a large altimeter and on
most jumps I have worn it on my wrist. However, while collecting
data from some of the initial test jumps, I sometimes wore it in
my jumpsuit. While looking at graphs of the speed, I became more
aware of the differences in the pressure and speed fluctuations
depending on where the barograph was worn. Major pressure buildups
and releases when it was on my wrist occurred at about 0.7 second
intervals, and when in my jumpsuit, at about 1.25 second intervals.
This information also provided me with the confidence that the
pressure variations were indeed due to the burble around a jumper's
body, since the differences followed the differences in the amount
of my body nearby and "upwind" of the barograph. I also recorded
a King Air descent with the barograph and was surprised to find
that there was still a large amount of noise present even though
there was no burble around a skydiver's body to cause that type
of fluctuation. Experiments proved that the pressure sensor was
also acting as a microphone and picking up the pressures changes
of the many noise sources involved in skydiving and flying. Several
mechanical means were tried to muffle the audible noise picked up
by the sensor but all of them required a great deal of physical
space to implement. At this point I decided that software filtering
was the only way that I could practically deal with the various
pressure differences I had been experiencing and decided to continue
by developing software to handle them.
Around this time I began detailed discussions with Dr. Jean Potvin
at Parks College of St. Louis University, who had been doing ram-air
parachute deployment research for several years. With Dr. Potvin's
help I implemented, but rejected for various reasons, many software
filtering techniques including Fast Fourier Transforms. Currently,
a relatively simple but multi-stage averaging formula is being used.
Unfortunately, all filtering of this type produces a "lag" in the
signal proportional to the amount of filtering. Increasing the amount
of filtering to the point of having a steady speed displayed also
increases the lag in the displayed speed to the point of not being
very useful for body position adjustments by the jumper. Decreasing
the amount of filtering causes speed variations large enough to
eliminate the effectiveness of even having the display. In practical
use however, the lack of a steady display of speed has not reduced
the effectiveness of the barograph in general since I seldom even
have time to look at the display while in freefall due to other more
important activities. Most of the benefit of using the barograph to
record fall rate has come from analyzing the data graphically after
the jump and comparing the speeds to known events during the jump
and to speeds from other jumps. My current graphing program has many
built-in features that make it easy to determine certain freefall
speeds and averages.
True Airspeed versus Sea Level Airspeed
During development of the various barographs I contacted Garry
Carter, founder of Flite Suit, Inc. and owner of Body Sport
USA. Garry created of the concept of a standardized fall rate
in 1980 and has since been manufacturing jumpsuits that are de-
signed to this standard using a "standardized fall-rate formula"
that he developed. This formula came from years of experience
designing jumpsuits as well as some wind tunnel experiments done
by a researcher for Garry using GI-Joe doll models dressed in
various jumpsuit sizes and fabrics. I had noticed that Flite
Suit order forms had listed 100 MPH as the normal speed pre-
ferred by "recreational jumpers" (meaning not competitors),
but the average speeds I had been recording with my barographs
were noticeably faster than that. He reminded me of the fact that
true airspeed increases with altitude and that we fall faster in
the less dense air at higher altitudes. Therefore, he adjusted
these speeds using sea level as a reference point so that the
speeds could be compared by other factors. He also pointed out
that since the 1980's though, fall rates have increased in
general somewhat, so the speeds I was recording were reasonable
even without the adjustment.
Since we are falling faster at the beginning of a jump than
at the end, using only the true airspeed (or TAS) for my
study of fall rate would not be sufficient, so after further
discussions with Jean Potvin on the best way to go about this
I added the calculations to my barograph software needed to
adjust the true airspeed at any given point in the skydive to
sea level airspeed (or SLAS). These calculations are based
on data from standard atmospheric charts and airspeed calcu-
lations and use a field elevation entered into the barograph
before the jump.
Freefall Speed Versus Altitude Chart (condensed for brevity)
Freefall speed (MPH) vs. Altitude (MSL), factored for air density
0 ft. 2K 4K 6K 8K 10K 12K 14K 16K 18K
100.0 103.0 106.1 109.3 112.8 116.3 120.1 124.0 128.1 132.4
102.0 105.0 108.2 111.5 115.0 118.7 122.5 126.5 130.7 135.1
104.0 107.1 110.3 113.7 117.3 121.0 124.9 129.0 133.2 137.7
106.0 109.1 112.5 116.0 119.5 123.3 127.3 131.4 135.8 140.4
108.0 111.2 114.6 118.1 121.8 125.6 129.7 134.0 138.3 143.0
110.0 113.3 116.7 120.3 124.0 128.0 132.1 136.4 140.9 145.6
112.0 115.3 118.8 122.5 126.3 130.3 134.5 138.9 143.5 148.3
114.0 117.4 121.0 124.7 128.6 132.6 137.0 141.3 146.0 151.0
116.0 119.4 123.1 126.8 130.8 135.0 139.3 143.8 148.6 153.6
118.0 121.5 125.2 129.0 133.1 137.3 141.7 146.3 151.1 156.2
The Current Barograph and Plotting Software
After a jump, the program running in my current barograph can send
the following data, which is collected every quarter second during
the jump:
- Time since logging began.
- Unfiltered altitude.
- Filtered altitude.
- Unfiltered true airspeed (TAS).
- Filtered true airspeed (TAS).
- Unfiltered sea level airspeed (SLAS).
- Filtered sea level airspeed (SLAS).
(It also sends the software version, filter factor, field elevation,
and other constants set before the jump.)
The graphing program for my barograph data which runs on an IBM PC
compatible computer can use this data to:
- Plot both altitudes versus time on the same screen.
- Plot all four speeds versus time on the same screen.
- Average the speeds between given points on the graph for
purposes of finding freefall speeds during certain
portions of the jump.
Accuracy Information Related to the Barograph Microprocessor Software
Note: This accuracy information is expressed in a narrative
manner and may need to be interpreted differently than if
expressed in a mathematical manner.
Timing accuracy:
The pressure value is being sampled every quarter second. 600
seconds worth of samples are being saved in 598 seconds with the
current program due to software timing variations. 598/600=
.99666666 or 99.66% time accuracy.
Absolute altitude accuracy:
The pressure sensor and/or its circuitry is not linear, but
calculations are done to adjust for this, including adjustments
to various ground levels and barometric pressures. Testing was
done by comparing the barograph altitude to reference altimeters
in an altitude chamber and in aircraft. (Aircraft testing is
ongoing.) My Altimaster III was used in the chamber and both
it and my Altimaster II, which agree with each other very closely,
are used while climbing to jump altitude. The altitude varies about
50 feet at some altitudes and has always been in the same direction.
Worst case conditions make it possible for the total altitude change
being timed to be 50 feet off if it was absolutely correct at one
end of a timing period and 50 feet off at the other end.
TAS vs. SLAS calculation:
Barograph processor software calculates Sea Level Air Speed (SLAS)
from True Air Speed (TAS) using a formula developed by Jean Potvin
which is based on a chart of the standard atmosphere and known
airspeed calculations. A computer program was written to display
these speeds in order to verify and adjust the formula. With the
current calculation, worst case error is 0.13%
Time Required to Achieve Terminal Velocity
An added benefit of graphing jumps to determine speed has been
that the length of time to reach these speeds can also be seen.
Due to the extreme pressure fluctuations related to exiting an
aircraft, the measurement of this time is subject to a great
deal of interpretation of the graph. The times, however, seem
fairly consistent, with 10-12 seconds being the average time
when accelerating to face-to-earth skydiving, and as long as 15
seconds for faster fall rate jumps. Since a barograph determines
freefall speed based on pressure measurement as opposed to actual
airspeed measurement, what must be kept in mind is that this
length of time is actually the time required to begin falling
straight down rather than the time it takes to get to terminal
velocity. The times that I interpreted on the jumps I made do
not show any significant differences related to the aircraft
that I jumped from, but the differences could be substantial
when jumping from aircraft with very high exits speeds.
Actual Jump Data
Since beginning work on this project I have made over 25 jumps to
collect barographic data leading up to the point when I developed
a satisfactory software filtering method and when I decided to add
sea level airspeed to the calculations. Before this point the
average freefall speeds at certain points during the jumps were
more difficult to analyze, and to compare to other jumps due to
the lack of SLAS.
The following freefall speeds are from analysis of jumps made AFTER
filtering and SLAS were added. Unless otherwise noted, the jumps were
made with the baragraph on my wrist. In some cases the average speeds
were still difficult to determine due to fluctuations caused by certain
body position changes during the skydive, but in many cases the speed
fluctuations were at a minimum, which increased the confidence level
in the process of determining average speeds.
In the case of face-to-earth formation skydives the speeds reported
will be most often be sea level airspeed since this speed provides
a reference to other jumps and to general fall rate. Since many
face-to-earth formation skydives slow down during the course of the
jump, some jumps have speeds reported corresponding to the beginning
of the jump shortly after attaining terminal velocity, and then after
slowing down.
For other jumps, true airspeed will also be reported. What must be
kept in mind is that a true airspeed reported is only from one part
of a jump where the speed could be accurately analyzed and does not
represent an average or a reference, therefore a true airspeed
should not be compared to any other jump's true airspeed or to any
other part of the same jump where the true airspeed will be different.
Abbreviations: FTE=face-to-earth, FS=formation skydiving, SOLO=solo
jump, usually made solely to record data, TAS=true airspeed, SLAS=
sea level airspeed.
Jumps I made as a Load Organizer at the 1996 World Freefall
Convention, with me always in the base of the formation:
File name Description of Jump SLAS
Start Avg. Later
08031750.JMP 11-way FTE FS 107 105 107
08041230.JMP 18-way FTE FS 105 100
080412WA.JMP ??-way FTE FS 108
080510PM.JMP 10-way FTE FS 106
080515WA.JMP 15-way FTE FS 107
08051800.JMP 10-way FTE FS (unit turned on at 1500) 103?
08061730.JMP 14-way FTE FS 101 (funnel, zoo)
08061910.JMP 14-way FTE FS ??? (3 big changes)
08071000.JMP 11-way FTE FS 106 103
08080900.JMP ?? 109 104
08081200.JMP ?? 110 107
08081400.JMP ?? 109 105
08081915.JMP 9-way FTE FS (base felt fast) 107 109
08090930.JMP ??-way FTE FS (2nd base felt fast) 108
08091300.JMP 12-way FTE FS (big guys, fast base) 108 107
08091510.JMP 9-way FTE FS (base felt fast) 106 (steady)
08091810.JMP 10-way FTE FS 108 (steady)
08101050.JMP 10-way FTE FS (no-contact O at first) 114 108
(Only the very experienced jumpers felt
comfortable at the start of this jump.)
08101330.JMP ??-way FTE FS 108
08111200.JMP 16-way FTE FS 104 103
Notes: At the 1996 WFFC jumpers started going low at 104 SLAS, nearly
everyone felt that 108 SLAS was a comfortable speed.
Other jumps (made in the St. Louis area):
File name Description of Jump TAS SLAS
Fast Avg. Slow Fast Avg. Slow
09021010.JMP Sit SOLO relaxed laid back position 123 118
09021140.JMP SOLO standing up whole time 150
09021250.JMP Sit FS 2-way, slow controlled movement 118
I went slightly slower to help out
09021435.JMP I followed FTE a 2-way SIT FS in my smaller 124
FTE FS suit, "proper" sit positions
09021650.JMP I followed a Strong Tandem, 170+100 pounds 107 104
in small jumpsuits
09021845.JMP 12-way FTE FS, passing me as a no-contact 112
base, building a formation on the other side
09081100.JMP 9-way FTE FS good piece flying 108
09081908.JMP I followed FTE a 2-way SIT FS in my smaller 127
FTE FS suit, "proper" sit positions
09149999.Jmp I took pictures of Jean Potvin wearing the 111
barograph and the strain gauge
instrumentation with belly-mount video
09221054.JMP 11-way FTE FS, me in 2-way (no contact 113 108 108
for a while) base, then normal slowdown
09221750.JMP 9-way FTE FS 113 108
09221916.JMP FTE SOLO, my regular large wing suit 114 100
full swoopcord slow, then full relaxed
FS position
File name Description of Jump TAS SLAS
Fast Avg. Slow Fast Avg. Slow
09291110.JMP SOLO in smaller FTE FS 200 (part 1) 177 (part 1)
suit, tried to go as fast 180 (part 2) 148 (part 2)
as possible straight down 117 (part 3) 107 (p3)
but kept falling out of it
last part as slow as I could
09291854.JMP Borrowed FTE FS suit, 139- 133 125- 114
very tight on me, 144 128
nylon body, first
part full slow, then
relaxed FTE FS position
10051108.JMP Skysurf, board is 48x11 144 125
inches, rounded ends,
normal wing sit-suit,
standing up the whole jump
10051843.JMP 10-way FTE FS, various experience 108
10131257.JMP Sit flying, normal sit suits 126
10131634.JMP Sit flying, one large person in 136 129
jacket and shorts in a somewhat
laid back position
File name Description of Jump TAS SLAS
Fast Avg. Slow Fast Avg. Slow
10131809.JMP I followed a Strong Tandem, 170+130 108 110
pounds in small jumpsuits, good arches
10261116.JMP I followed a Strong Tandem, 165+190 110
pounds in medium jumpsuits
11031522.JMP Me in sit suit, another fast faller 122
FTE in a tight suit
11031649.JMP 4-way FTE FS, 1 intermediate 108 106
jumper and 1 light weight jumper.
11031743.JMP Followed photographer taking stills 115
for a "family photo", no formations,
just FTE posing.
11091406.JMP Followed 150 pound person on Surflite 130
beginner skysurf board, some standing,
some loops. (Note: fall rate was just
as fast when going face-to-earth at
pull time.)
12211634.JMP 4-way FTE FS with one person sit flying, 113
one normally fast faller,
did several formations.
012597MT.JMP 3-way FTE FS with Jean Potvin wearing 119
strain gauge instrumentation and another
very fast faller, no-contact flying to
determine who fell fastest, very relaxed,
cold weather, much clothing underneath
012597OJ.JMP 3-way, same as above but turning points 114
012597RC.JMP 9-way FTE FS, base people kept it fast 111
Jumps made at Skydive Houston the weekend after the 1997 PIA Symposium:
File name Description of Jump TAS SLAS
Fast Avg. Slow Fast Avg. Slow
The following 4 jumps were made with Peter Raymond in conjunction
with his Air Time Designs sit fly/free fly seminar and coaching.
021597KH.JMP Followed P.R. head-down flying with 130 125
one novice head-down flyer
021597LK.JMP Followed P.R. sit flying with one 128
novice sit flyer
021597PJ.JMP Myself sit flying with P.R. and 133
two novice/intermediate sit flyers,
Peter said it seemed slow compared
to his "normal" sit flying speed
021597QO.JMP Followed P.R. and two other intermediate 140
head-down flyers
021597SH.JMP Followed the previous two intermediate 155 145
head-down flyers staying head down
and fast throughout the jump
021697NQ.JMP 11 way face-to-earth formation skydive 115 113 110
021697PF.JMP Myself sit flying with one 130 124
intermediate/experienced sit flyer
Freefall Speeds Chart
The following is a chart comparing the sea level airspeeds (SLAS)
of various types of skydiving based on recent research and data
collected so far.
102 camera suit (Garry Carter data)
104 "slow" Strong Tandem
104 very slow face-to-earth formations
108 "average" Strong Tandem
108 overall average face-to-earth formation skydiving
(Garry Carter and barographic data)
110 "fast" Strong Tandem
110-115 "fast" recreational face-to-earth formation skydiving
114 competition team face-to-earth formation skydiving
(Garry Carter data)
114-120 "fast fallers" in face-to-earth formation skydiving
118-122 sit-flying in a laid back or relaxed position
with a standard design sit-flying suit
125-135 sit-flying in a straight-backed sitting position
with a standard design sit suit
125-130 skysurfing with normal wing sit-flying suits
150 standing up in a sit-flying suit
125-160 head down flying
Summary and Conclusions
Small microprocessor-based barographs make possible the study of
skydiving fall rate in a scientific manner as opposed to guesswork,
and with compensations added that are specific to skydiving freefall
speeds. They also make possible the setting of fall rate by objective
data rather than by arbitrary size, weight, and jumpsuit selection.
- A difference of only a few miles per hour of freefall speed can
adversly affect the ability of jumpers to participate in successful
formation skydives.
- Without an objective fall rate reference, jumpers often compensate
for fall rate differences in a less than ideal manner.
- The difference in "true airspeed" fall rate at different altitudes
is significant enough to require a common reference by which to
compare the fall rates of jumpers trying to fall at the same rate.
- Timing the complete freefall on a jump is useful only for determining
the average "true airspeed" fall rate on that jump and is
not adequate for determining the fall rate for any specific
portion of that jump.
This current barograph and its software, combined with the graphing
program, provides a more accurate system of determining freefall
speed compared to previously available methods. This is mainly
because the complete jump can be recorded and plotted in a variety
of ways, and then analyzed at length after the jump.
Limitations: Plotting jumps to determine freefall speeds during
certain portions of the jump requires careful analysis and the
cross-checking of multiple figures. Personal familiarity with
the jump being analyzed is very helpful in order to prevent values
caused by known extremes in pressure during certain events on a
skydive from affecting the analysis.
Future Research
Other barograph jumps will be made in the future for the purpose of
"filling out" the freefall speeds chart so that it can be used as
a guide to jumpsuit selection, video partner and team personnel
selection, load organizing, and as a general guide to skydivers
attempting to fall at the same rate.
Some of the jumps to be made are:
- Jumps made by other skydivers of varying sizes and weights.
- Other "high speed" skydives, e.g. sit flying, standup, head down, traditional style, etc., or following along on such skydives.
- Following competition teams.
- Following student jumpers on some of their initial long freefalls.
- Following Tandem Vector skydives.
- Following more Strong Tandem skydives.
Example Barograph Plot
The plotting program can plot altitude versus time and speed versus time.
Only the speed plotting is shown below. The altitude plotting is used
mainly to determine the time at which certain events took place, since
it is normal for a jumper to remember the altitude at which an event occurred.
The graphs below are two sections of the same jump displayed
one screen at a time in the (original DOS based) plotting program.
This is jump 08091810.JMP, a typical 10-way face-to-earth
formation skydive that I organized at the World Freefall Convention.
The numbers at the bottom represent the time since recording was started.
The numbers at the right are speeds in MPH, and a minus value means
descending. The program allows the jump to be shifted until the area of
interest extends from the left of the screen toward the right. The
averages shown in the upper right are based on how much of the jump is
included in the averages calculated. This is selected with the line with
arrows, and can be varied.
Notice how the true airspeed and sea-level airspeeds get closer as
the altitude decreases, and how the filtered signal lags the
unfiltered signal. Being able to see all four signals is helpful
in determining speeds because the speeds can be compared with one
another, and when they are similar there can be a high level of
confidence that the speeds that have been determined are accurate.
The graphs below are from the Windows based program. The first shows the basic
speed and altitude graph with a window selected to be zoomed. The second shows
that part of the skydive zoomed in and with the calculated speeds displayed.
Note that there four speeds: Filtered and unfiltered, true and adjusted.
