Skydiving Fall Rate

Investigating skydiving freefall speeds using a barograph

picture of 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 techniques, 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 designed 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 preferred 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 calculations 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:

  1. Time since logging began.
  2. Unfiltered altitude.
  3. Filtered altitude.
  4. Unfiltered true airspeed (TAS).
  5. Filtered true airspeed (TAS).
  6. Unfiltered sea level airspeed (SLAS).
  7. 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:
  1. Plot both altitudes versus time on the same screen.
  2. Plot all four speeds versus time on the same screen.
  3. 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.

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:

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.

graph


graph




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.

graph


graph



log file