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 free fall time calculator of research equipment used in my other research. Skydiving Fall Rate by Gary Peek A research project investigating skydiving freefall speeds using a microprocessor-based barograph recording altimeter. 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 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. 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. 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. Some of the things I tried included enclosing the sensor in many different materials and in chambers. free fall time calculator 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. 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. 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. 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 free fall time calculator 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 free fall time calculator 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 Free fall time calculator contacted Garry Carter, founder of Flite Suit, Inc. 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. 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. It also sends the software version, filter factor, field elevation, and other constants set before 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. Testing was done by comparing the barograph altitude to reference altimeters in an altitude chamber and in aircraft. 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. 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. 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. 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 free fall time calculator. 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. Other jumps made in the St. Jmp I took pictures of Jean Potvin wearing the 111 barograph and the strain gauge instrumentation with belly-mount video 09221054. Note: fall rate was just as fast when going face-to-earth at pull time. 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. 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 numbers at the bottom represent the time since recording was started. 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.
