PARACHUTE OPENING SHOCK BASICS
Gary Peek and Jean Potvin
Parks College Parachute Research Group
Presented at the Parachute Industry Association Symposium 2001
The detailed understanding of parachute inflation aerodynamics has been, and continues to be at the forefront of parachute development. New parachute applications as well as recent technical innovations in the area of fabric materials and deployment hardware have created new environments in which inflation forces manifest themselves with unforeseen and sometimes destructive consequences. The recent flurry of letters in skydiving publications leaves no doubt as to the level of interest of the sport jumper community into this subject matter.
As members of the Parks College Parachute Research Group we have been studying the inflation dynamics of sport ram-air canopies since 1997. This effort was partly motivated by our own curiosity as sport jumpers in an event that is so crucial to every skydive. Another motivation has been to provide experimental data to validate a parachute inflation computer program that has been under development since 1995. Our research has been supported by Saint Louis University, the U.S. government and by Performance Designs. We have performed over 300 test jumps on a variety of F-111 and zero-porosity ram-air mains, ranging in size from 120 ft2 to 360 ft2. All canopies but one were of rectangular shape.
The main goal of the study has been to clarify the most basic questions pertaining to parachute inflation, namely the relationships between opening shock and the following factors:
Other factors to be studied in the future include the effects of line bight tightness, suspension line elasticity, and pilot chute sizes.
Our basic approach has consisted in using electronic load cells integrated to the risers to measure the force sustained by each riser during inflation and flight (figure 1). Given the well-known jump-to-jump opening variability of ram-air mains, such measurements were repeated over 20 to 30 times. Each jump was performed with the same parachute-harness container system under the same deployment conditions. Most jumps were performed in a "belly-to-Earth" body position during solo skydives using a standard sport rig.
As such our current database should provide many of the answers jumpers seek in solving their canopy opening problems. As explained below, our results show some very basic and yet significant differences between different parachute systems. Hopefully, these should help us all understanding why some proposed solutions for opening shock control work on one canopy design but not on another.
For obvious reasons, parachute manufacturers have studied such questions for a long time. Most of our results will look very familiar to them. However, our focus has differed in that we are interested in establishing the quantitative relationships, that is design equations, which describe the phenomenon of parachute inflation.
Let us stress that we are talking here about structural opening shock, that is, the amount of peak force sustained by the risers, rather than physiological opening shock which is the peak acceleration felt by specific parts of a jumperís body. The two are rather different and often contradictory: for example a "mild" 3-g structural shock may feel like a 10-g opening to a jumper that keeps his/her head down during parachute deployment.
The Parks College Parachute Research Group utilizes a number of unique electronic devices, instruments, and instrumentation systems to enable them to collect data for their various projects. Our instrumentation has been described in details in the seminar we have presented during the 1997 and 1999 PIA Symposiums. The interested reader can also consult our web site, www.pcprg.com, from which the text printed below has been extracted.
Some of this instrumentation is now available as commercial products from
Industrologic, Inc., founded by
Gary Peek. For more information about the Group's instrumentation systems
and electronics you can contact him at email@example.com
3) Three Stages of Inflation
The aerodynamics of inflation is a complex process that is the reflection of the delicate balance existing between several design factors, a few of which are optimized for canopy flight rather than for inflation. Hard openings can be described as the result of an almost instant full spreading of the canopy, which at a fast fall rate generates a huge amount of air resistance (drag) and riser force on the jumper. The general strategy for reducing opening shock is therefore simple:
to slow down the spreading of a canopy, while at the same time generate enough drag force to cause a small rate of jumper deceleration over a sufficiently long amount of time.
Figure 2 shows the total riser force as measured on 120-, 150- and 230 ft2 versions of a PD Sabre canopy. The three canopies were deployed at 4000 feet AGL following a terminal descent at 120 mph true airspeed. The deployments were all performed with the same zero-porosity pilot chute (27" diameter) and jumper suspended weight (205 lbs). All three graphs show the basic three stages of slider-reefed ram-air canopy inflation, namely line stretch or line "snatch", early pressurization, and canopy spreading/slider descent. Besides showing total riser force, the graphs also display a horizontal line, which corresponds to the moment of slider descent. The line represents the signal of a hand-held switch that is pressed by the test jumper upon witnessing slider descent.
Line snatch occurs after the canopy has been pulled out of the bag and begins re-accelerating to the jumper's fall rate. Re-acceleration occurs because relative to the jumper, the bag actually decelerates during the unstowing of the suspension lines. The moment of line snatch is represented in the figures by the isolated narrow peak showing at the beginning of the inflation sequence. On all the parachute designs tested so far, the magnitude of the snatch force has been relatively small compared to the maximum force developed during the later stages of inflation.
In general, pilot chutes made of low porosity fabric and cut in large sizes will increase the force of line snatch if the lines are not securely stowed. Also, as discussed in the warning inserted in Performance Designsí packing instructions, a high snatch force may shake the canopy out of the bag with enough violence to prematurely undo any special folds (such as nose rolling) that went in packing job. Note finally that stowing the suspension lines with tight bights will delay the occurrence of this event because of the extra energy required to deploy the suspension lines.
Early pressurization corresponds to the temporary sniveling that most ram-airs display after snatch but before slider descent. During this stage each cell begins pressurizing until they reach a volume large enough to initiate slider descent. The duration of early pressurization is very much inlet- and slider width- dependent. It is also very sensitive to canopy pitch, i.e. the angle the wing planform makes with respect to the horizontal. Finally, early pressurization duration also depends on the design of the crossports inside each cell, and on the specifics of the packing method. In the first two frames of figure 2, early pressurization corresponds to the "shoulder" that precedes the main peak. In the last frame of figure 2, this stage is represented by the tallest peaks appearing after line snatch. The difference between the parachutes of figure 2 has to do with size, as further discussed below.
Depending on slider sizes and inlet design, the center cell is usually the first to inflate, followed in some cases by the tip cells and the mid-span cells. The tip cells tend to inflate later than sooner than the center cell because of the inward force the air stream is applying on their exposed side, thereby keeping them closed longer. In general cell pressurization is not an instantaneous process, because it is slowed down by the random opening and closing of the cell inlets which lead to highly fluctuating cell pressurization rates. Such randomness is caused by a variety of factors, which are hard to control, including the turbulence generated by the flutter of neighboring cells and jumper. Finally, cell inflation should be very fast for canopies that are highly pitched-down and/or for those that feature inlets with no top lip.
An important consequence of the finite duration of early pressurization is that the drag generated by the partially inflated canopy already causes a substantial amount of jumper deceleration. This in turns leads to smaller opening forces during the full spreading of the canopy. This is the inflation stage were the packing style matters the most. For example, the packing tips offered by the readers of Skydiving in issues #215 and #220 aim at slowing down the flow of air into the cell by folding or rolling tight the nose of selected cells. Another tip is that of freefall videographer Mike McGowan (Skydiving #220) which consists in pulling the quartered slider toward the packer. This lengthens the duration of early pressurization by requiring extra time and extra air flow energy needed to spread the slider properly.
One of the most frequent sources of hard openings is the well-known case of line dump. As the name implies, line dump occurs during the instantaneous unraveling of all stowed lines soon after the bag has left the container. Such unraveling is caused by a combination of loose line bights and violent bag extraction perhaps caused by an oversized pilot chute and/or a too fast of a freefall. Line dumps thus leads to the premature opening of the bag and deployment of the parachute before the full stretching of the suspension lines. In such conditions, early pressurization is likely to begin before line snatch and to ends soon after line snatch. Effectively, the result is a very short early pressurization during which no jumper deceleration has occurred: the canopy thus begins spreading fully when the jumper is still falling at terminal speed, a situation which leads to a very hard and quick opening.
Slider descent marks the final and full spreading of the canopy. Slider descent marks the time of maximum opening forces on mains approximately of 200 ft2 surface area or less (figure 2). Slider descent occurs when the cells have reached a volume large enough to "fan" the suspension lines above the slider in an almost perpendicular direction, thereby generating a down-pushing force on the slider. Because of its own drag, a slider will slide down the suspension lines in a sufficiently long time to reduce further the amount of maximum force. In principle, increasing the slider size should reduce opening shock. However, readers should be aware that too large of a slider may cause large opening forces during the early inflation stage because of the increased exposed canopy surface area that it generates, thus negating the sought benefits. A way out of this conundrum is to keep slider size small but to increase slider drag by adding enough fabric to make it look like an inverted bag, as done on some tandem canopies.
A careful study of figure 1 will also reveal an interesting fact: that riser loading during inflation is not uniform, but involves higher loads on the rear risers. It is also interesting to observe that towards the end of the canopy spreading/slider descent stage, the front risers sustain more loads in order to tow forward the jumperís body during canopy glide.
4) Other Factors
Suspension line stretching
The riser loading graphs shown here do not address the consequences of using suspension lines which stretch (such as Dacron) over those that donít (Spectra). In general "stretchy" lines will act as shock absorbers and therefore reduce the rate of force buildup sustained by the risers during opening. According to the letters in Skydiving #215, several videographers had their canopy fitted with "stretchy" lines as a result. The price to pay for such a conversion is the increase in line drag caused by the greater diameter of the lines, which ultimately translate into a reduction of canopy performance during flight.
Line trim changes and canopy warping
Another important factor for opening shock is the change of line trim which occur over time (Brian Germain, Parachutist , August 1996). Line trim corresponds to the relative length of the suspension lines and defines a canopy angle of attack which, combined with the exact airfoil shape, determine flight performance. During opening and flight all the suspension lines will elongate some, by a different amount spanwise as determined by the relative loading applied on each cell. The lines return to their original length after landing. Performing hundreds of jumps means hundreds of such elongation/compression cycles, which in the process generate a lot of friction between the fibers of each line. Such friction generates heat and ultimately leads to line shrinkage. Such shrinkage is further enhanced on the outboard lines by the friction of the slider, which also generates heat. Depending on the specific design of a canopy, line trim variation will change the opening characteristics of a canopy in either harder openings, or worst, off-heading openings. Not much can be done to remedy this problem except to send the canopy back to the manufacturer or to a master rigger for relining.
Effects of stowed braking
Our research has shown that changing the locking of the brakes somewhere between the quarter- and half-brake settings during packing will somewhat change the force of the opening. As all jumpers know, pulling the toggles for braking involves "cupping" the shape of the canopy thus generating more air resistance. Manufacturers do use this extra setting to further reduce jumper discomfort during opening.
Premature slider motion
The three stages discussed above can be merged into one big disastrous hard-opening stage in a situation when the slider is not stowed against the slider stops but is left several inches below the stops at the end of packing. With the slider lower down the suspension lines, more canopy surface area is exposed to the relative wind after line snatch. Moreover, the inlets of the mid-span cell are allowed to open wider than during normal openings. The combination of these factors leads an opening canopy that has a slider descending during the early pressurization of the cells.
5) The Moment and Amount of Opening Shock
Skydivers have asked us this question all the time: When does a jumper feel the greatest force from his/her risers? Does opening shock occur during the slider-descent phase or during the other two phases of parachute inflation? As mentioned before, the answer depends on the specifics of the parachute design being flown. In particular, it depends on:
1) The size of the pilot chute and whether it is of a collapsible type;
2) The size of the slider;
3) The size of the main parachute;
Other factors may also enter into the picture, but to a lesser extent, such as: the amount of steering line paid out by brake stowing, fabric porosity, suspended weight, etc.
We can answer the question by studying the designs that generate the riser load time curves of the type shown in figure 2. Using a large slider (i.e. 24" by 36" ) on a large canopy (i.e. 230 ft2 ) is shown in the last frame of figure 2. In such a case the maximum force (i.e. 950 lbs) occurs during the early pressurization phase, just before slider descent. This occurs here because the combination of large slider and overall canopy size exposes a much greater canopy surface area to the relative wind thereby generating greater air resistance. It is to be noted that the maximum force sustained during slider-descent, namely 625 lbs, is smaller than the absolute maximum force by 30%!
We can contrast this case with that of a smallest canopy of the same design shown in the upper frame of figure 2, which corresponds to a 120 ft2 main fitted with a 16" by 25" slider. The riser load curve clearly shows an opening shock (i.e. 922 lbs) that occurs during the slider descent phase. The same can be said about the 150 ft2 version equipped with a 19" by 29" slider as shown by the middle frame of figure 2, except that the maximum loading is slightly higher, i.e. approximately 1100 lbs.
In principle, one could imagine an opening shock occurring during line snatch when a large enough (non-collapsible) pilot chute combined with suspension lines that are not stowed securely are used.
In general, the most comfortable openings will be those which occurs over a "long" time interval, namely 2 to 3 seconds. Our measurements have shown that on average, sport canopies tend to generate opening shocks averaging to the 3 to 6-g level after a skydive performed at terminal velocity. Hard (fast) and painful openings have been documented at about 9 to 12 gís.
6) On the technical side: scaling laws for parachute design
So far we have neglected to mention two other factors which obviously have something to do with opening shock: jumper weight and fall rate. Also, we have not addressed the quantitative relationship between opening shock and canopy size.
Our study of the influence of these factors has been rather indirect and involved the use of a computer model that we have been developing over the past five years. The way we have used such a model to study a parachuteís complex behavior is typical of the scientific method:
Using a (validated) computer program is cheaper and faster than performing the hundred jumps that would be necessary to obtain the same information.
Over the years we have developed a computer program aimed at describing the deceleration of a slider-reefed ram-air parachute during the slider descent phase. This model has been validated by comparing with our measurements of the riser forces. Samples of such a comparison is shown in Figure 3 (PD Sabre 150) and figure 4 (PD Sabre 120). Overall, the model has been able to duplicate the evolution of the opening forces on 90% of all the jump recorded, a fact that gives us great confidence in its accuracy. In the paragraphs below we show some interesting predictions on how weight and speed influence opening shock. We will also come back to the issue of canopy size as well.
We have run our computer program over thousands of scenarios where weight, parachute size, opening altitudes, fall rate at line snatch, etc. were varied to reflects a large sample of deployment conditions. The results are condensed in what are called "scaling laws", which can convey simple answers to rather complex questions. Be aware that the laws discussed below are valid only in cases where opening shock occurs during the slider descent phase of inflation.
Our model indicates that the maximum deceleration sustained (in gees) is proportional to the square of the jumperís speed prior to slider descent. For example, comparing two equal-weight jumpers wearing identical parachutes gives:
This means that the maximum deceleration would increase four-times if the speed of one jumper is doubled compared to the speed of the other jumper. This explains why opening shock is so sensitive to fall rates.
As an example let us compare the case of a jumper falling head-down with another jumper who is falling in a belly-to-Earth position. Suppose that the head-down jumper has been opening his chute in this body position and that he has been falling at 165 mph prior to slider descent. On the other hand assume that the belly-to-Earth jumper has opened his parachute at about 120 mph. Typically, he will be falling at about 110 mph or less by the time of slider descent because of his deceleration that began as soon as the parachute started inflating. The v2-law predicts that the maximum deceleration sustained by the two jumpers would go as (165/110)2, i.e. a ratio of 2.25. In other words, the head-down jumper would feel twice as many gees as the belly-to-Earth jumper. This simple scaling law explains why a jumper can experience a wide range of opening shocks with the same parachute equipment, when body position and fall rate change on a skydive-to-skydive basis. For example, the differences on opening force time evolution shown in the graphs of figure 3 or 4 can be explained mostly by fall rate variations on a jump-to-jump basis.
Canopy size scaling
Our model predicts two types of size scaling laws depending on whether the opening is one of two categories:
In the case of instant openings/line dumps, one has the following scaling laws for the maximum deceleration sustained (in gees) and the time of maximum acceleration. These laws compare two parachutes, one being identical to the other but having its dimensions (i.e. chord, span, and line lengths) greater or smaller by a factor s :
Interestingly, these scaling laws suggest that instant openings should give higher opening shocks on large parachutes than small ones (at the same jumper weight and fall rate). The reason is that bigger parachutes have a bigger surface area and thereby produce more drag than smaller parachutes at the same rate of descent. A parachute that is twice as big (i.e. span, chord, suspension line etc. twice as long) would open twice as hard during line dumps. On the other hand the opening time would be about twice as short.
Regarding nominal openings, the model depicts a more complex picture where scaling down in parachute size reduces the rate of canopy surface opening while at the same time increases the value of the initial velocity. Implementing these proportions gives the curve shown in figure 5, which relates the maximum deceleration ratio amaxchute2/ amaxchute1 to the scale factor s. Here the reference parachute (labeled "chute 1") is characterized by the parameter set characterizing a Sabre 150 used by a 205lbs jumper falling at 176 ft/sec at deployment time and at 125 ft/sec prior to slider descent (deployment altitude was 4000ft MSL). The figure shows that for most values of the scale factor, smaller parachutes actually open harder than the larger ones, a trend which is opposite to that of instant openings. For example, if all the dimensions of this particular chute are doubled, the resulting 600 ft2 parachute would feature a maximum deceleration that is 40% smaller than the original 150 ft2 canopy (with same jumper!). For small-enough scale reductions however, figure 5 shows that the opening shock reverses trend and decreases at smaller s , a result of having a parachute so small that it is generating little drag from that point-on.
Let us add that the trend shown in figure 5 is very specific to the canopy, jumper weight and jumper fall rate at deployment time. To illustrate this point we use the following scaling formula which gives the curve shown in figure 5 when the scale factor is large:
Here vT is the terminal speed of a jumper falling under a canopy that is in a "slider-up" configuration. v0 is the fall rate of the jumper-canopy combination just prior to slider descent. This result shows explicitly that the effects of size scaling wont be the same from one jumper to the next since vT and v0 will change on a jumper-to-jumper basis. Note also that v0 depends on the canopy design.
7) What about packing style?
There are three basic main parachute packing methods in use nowadays: Pro, stack and roll. Each has a multitude of variants which aim at exploiting specific features of specific canopies. Here again, generalities are difficult to make because the efficiency of a given packing style is so intimately connected to the specific design of a canopy. Last summer we have begun to study this topic in the most straightforward manner by packing an old PD Sabre 230 using roll-pack and pro-pack. The specifics of the roll pack approach is shown in figure 6. The pro-pack method was that published by Performance Designs.
Figures 7A, 7B, 7C, and 7D show force graphs of the 12 jumps so far performed. All jumps involved the same test jumper and jump profile prior to deployment (same parameters as those described at the beginning of this article). One sees that, for this particular canopy, pro-packing gave more consistent openings than roll-packing. Most interestingly, the average peak force for the pro and roll packs were measured at 4.6 and 4.8 gees respectively, a small difference. This finding clashes with other investigations, in particular those performed on canopies made of F-111 fabric. The difference may be explained by the fact that the Sabre 230 is made of zero-P fabric, a more slippery material which does not retain folds as long as F-111 fabric when the canopy is extracted out of the bag.
The authors thank W. Coe and W. Downey from Performance Designs Inc. for fruitful discussion. They are also grateful to Performance Designs and to the U.S. Bureau of Land Management for providing financial support as well as loaning and rigging the test canopies for this project. We finally thank Becky Brocato, future aerospace engineer and competition 4-way skydiver, who has helped us processing the large amount of data.
Gary PeekGary Peek is an active jumper with over 5000 jumps, an FAA rated Master Parachute Rigger, a Commercial Pilot and jump pilot, and a skydiving photographer and videographer. He holds USPA Static Line, AFF, and Tandem Instructor/Examiner ratings and actively teaches student skydivers in all of these training methods. He has worked in the electronics field in the St. Louis area for many years and has combined his knowledge of electronics with skydiving for a number of those years. He co-founded Industrologic, Inc., an industrial electronics company that now manufactures equipment related to parachute drop testing. In addition to writing computer programs and providing clip art related to skydiving, he also is an author of many articles that have appeared in aviation and skydiving publications. He is the author of the Equipment Color Programs published by Para Publishing and has been doing research on skydiving fall rate using electronic devices since 1990. Gary has been a speaker at 3 previous PIA International Parachute Symposiums on a variety of topics.
As a professor at Saint Louis University, Jean Potvin has been involved in parachute research since 1994. His main interest is the study of the aero-physics of the inflation process. Lately, he has also been involved in several wind tunnel studies and flight tests of sub-scale and full-scale parachutes. In 1995 Dr. Potvin co-founded the Parks College Parachute Research Group with Gary Peek and acts as its science and data analysis coordinator. Jean is a Canadian citizen and a permanent resident of the USA. He has obtained his Ph.D. in Physics in 1985 at the University of Colorado in Boulder, CO. and is a US Parachute Association-rated instructor with over 1900 jumps to his credit. He is also a FAA-rated Senior Parachute Rigger. Jean has been a speaker at two previous PIA International Parachute Symposiums.