Can YOU see something wrong here??
What's wrong with THIS opening? http://www.triaxproductions.com/trailers.htm
this is one of many similar [ahem] jumps at Bridge Day this year.
edit: changed link by request of original poster ~TA
Basejumper.com - archive
General BASE
http://www.triaxproductions.com/trailers.htm
this is one of many similar [ahem] jumps at Bridge Day this year.
edit: changed link by request of original poster ~TA
Re: [pope] Can YOU see something wrong here??
Looks like he/she has way too small PC (or too short delay for that sized PC) resulting in a serious PC -hesitation and / or PC in tow. Secondly there is horrible snivel and oscillation of PC, both together resulting in an unpreferable opening sequence in base-enviroment and also some offheading. Thirdly he/she has modified skydiving gear with d-bag and packed slider up which in my books is not preferable configuration for any BASE-jump but at least not for 0-1 sec delays.
Re: [pope] Can YOU see something wrong here??
Or this guy (see attached pic). Check out his kneepad....
Re: [base428] Can YOU see something wrong here??
just curious............... what are the rules for gear at BD?
Re: [BASE813] Can YOU see something wrong here??
The rules for gear are pretty much the same as they've been for 17 years, and they probably won't change anytime soon (read the FAQ for more info).
In reply to:
just curious............... what are the rules for gear at BD?
Re: [pope] Can YOU see something wrong here??
This looks very similar to the oscillating PC and scary opening that was on C2. Also there was a scene from a movie shown at bridge day on sat night that had two jumpers in wing suits shown flying with a large cliff to the jumpers left, the second jumper filming the first and a shot that showed a jumper dragging his toes off a cliff face. What movie was this from?
Re: Pope
Does it also look to you that the pilot chute has a bungee cord or something in its design to prevent its inflation until a critical speed, used to collapse the pilot chute while under canopy in the skydiving environment?
Re: [pope] Can YOU see something wrong here??
Looks like the guy was using a collapsable pilot chute for it seems to hesitate to even inflate.
Re: [rendezvous] Can YOU see something wrong here??
I have several videos of "normal" PC's doing exactly what you saw in that video (hesitate for 1-2 secs). Just ask Tim C. from Spiders from Mars about his Tombstone 2 sec PC hesitation in the late 90's. *Any* PC can do this, as PC's work the majority of the time. Don't think that this couldn't happen to your brand new $100 PC.
Re: [base428] Can YOU see something wrong here??
What would cause that PC oscillation? It looked like it was pulling straight until the canopy started to come out the bag and then it just went all over the place. Could the initial opening shock of the PC contribute to that?
Re: [pope] Can YOU see something wrong here??
In reply to:
What's wrong with THIS opening?
I'm no expert, but I wouldn't call this "oscillation". The pc looks grossly asymetrical, causing it to kite around, maybe due to design/construction flaws, or due to being connected incorrectly. It certainly has a lot of horizontal drive.
hmmm... what do you suppose would happen if you larks-headed your bridle around only one load tape?
Re: [Treejumps] Can YOU see something wrong here??
In reply to:
The hesitation was likely caused by simply dropping the PC instead of throwing it.
I disagree - he did about a .5 second delay (so very little burble), and his pitch doesn't look too bad.
Re: [pope] Can YOU see something wrong here??
Just my opinion 
… Because we don’t know the gear configuration, I believe we are all shooting in the dark on this one… But I know it’s good to speculate on what could cause this. If I knew that the right PC was used for the given delay, that a BASE bridle was being used, that this was a BASE canopy, that this was a BASE rig being used, and that the proper delay was being taken for the use of a slider, I would be freaked the hell out by what I just watched. But from the looks of it, and I may be wrong, everything was wrong with this picture/jump… What I saw wrong with this jump... The wrong gear was being used with the wrong gear configuration for the given delay. In my opinion, this person had two options to prevent this… The first one being as simple as taking the proper delay for the given gear configuration (Smaller PC and Slider being used…). This would have decreased the time taken on the slider opening and the time the PC had to oscillate. Or, for this given delay, remove the slider and attach the appropriate PC. Again this would decrease the opening time for the canopy and the time the PC had to oscillate. The more time the fabric (PC and Canopy) sits out in the wind, the more time it has to cause problems, like PC oscillation and other canopy malfunctions (e.g. PC Hesitation, Line Twists, etc...).
Again, I wish we knew more about the actual gear used. But the #1 thing that sticks out in my mind is the fact that the wrong delay was taken for a slider jump…
… SBCmac
Re: [pope] Can YOU see something wrong here??
In reply to:
What's wrong with THIS opening?
http://www.triaxproductions.com/triax-BD04.mp4
(right click and "save as") or try:
http://www.triaxproductions.com/triax-BD04.avi
That pilot chute sure did have some wicked rotations in both directions. I agree with whats already been said that either the PC is attached uneven and/or the PC has a defect.
The hesitation doesnt look like it was caused by a bungee kill-line. You can see that the apex of the PC is not pulled down like it would be with a bungee.
Hard to tell if the mesh is small hole or large hole.
If you notice how the mesh is all squeezed together and just stays that way for nearly 2 seconds. This type of PC hesitation was one of the primary reasons I developed the Super Mushroom PC packing technique. I have seen several PC hesitations like this with good quality BASE pilot chutes while using the regular mushroom both going hand held and stowed. When packing using the regular mushroom technique, you stretch out the mesh, which also collapses it together, and then you s-fold it. The opening to the rip-stop is closed for the most part at the top end of the mesh column, and seems to stay closed until the s-folds unfold and the mesh starts to expand. From my observations through the years, I think on occasion it just takes a while for the air molecules to work their way into the collapsed mesh and subsequently into the canopy part of the PC. The Super Mushroom has the mesh already expanded for the most part. Also with the Super Mushroom, the mesh/rip-stop seam starts to expand on its own once it leaves your hand.
It also looks like the initial pilot chute rotation put 1 or 2 line twist into the lines below the slider before the slider started coming down which was probably a contributing factor to the long snivel. Since he had a d-bag, I wouldn’t be surprised if he had a sail slider as well.
Re: [pope] Can YOU see something wrong here??
It looks as if the bridle gets hung up on something near the top of the container, which doesn't allow the bridle to get full extention thus putting the PC in the jumpers burble. It seems to clear from the top of the container then the PC gets to full extention and clean air then pulls everything else out of the container. One thought that comes to mind on the point of snag are the tuck flaps on the rig(if any). I can see an inexperienced jumper using the tuck flaps to 'stow' his excess bridle so it will have less of a chance to entangle with his arm. Maybe he used the tuck flap like the little piece of velcro some BASE rigs have to keep the bridle free. Maybe he had the bridle stowed elsewhere on the rig and it failed to release at the designated time. It seems to be trailing him from the middle of his shoulder blades I also have to agree with the asymetrical PC too... what an oscillating bitch that is
. Although it may have started to oscillate when it released from the snag point and had to find a new point to pull everything out from. Jason
BASE 570
So does someone know what really happened or do we all have to guess and come to our own conclusions??
Re: [base570] Can YOU see something wrong here??
Quite a few jumpers like to tuck a bit of bridle in the edge of a closing flap at the top of the rig when going hand-held. The primary reason for doing this is to keep the bridle from snagging around the bottom corner of the container. Perhaps a better way is to tuck a bit of bridle in the closing flap at the bottom of the container. What I see on the video is that the PC inflates and then pulls the tucked bridle out.
Re: [Treejumps] Can YOU see something wrong here??
Hi Tree, Two opinions are better than one. I posted simply to offer mine to maybe benefit someone else.
Is the reserve PC you are thinking of from a RW Vector? The vector PC does have a big hole in the bottom (same diameter as the spring coil). I do understand how a round parachute begins to open with a low pressure created on the outer surface of the canopy. For that low pressure to happen airspeed is required. On a BASE jump, I personally like my PC to open immediately even if there is zero to very little airspeed.
Once the round starts to open then air does rush into it inflating it quickly at that point. The round parachute is a drag device at that point and it works by capturing air.
I just dont agree with these two statements of yours.
In reply to:
Pilot chutes do not capture air to inflate.
a PC is not a air capturing device.
I hope there are no hard feelings if my opinion differs from yours.
Re: [Treejumps] Can YOU see something wrong here??
In reply to:
Quote: "From my observations through the years, I think on occasion it just takes a while for the air molecules to work their way into the collapsed mesh and subsequently into the canopy part of the PC."
Hi Johnny,
Just to avoid any confusion about PCs, they do not inflate by capturing air inside the PC. Let me repeat this, as to be 100% clear: Pilot chutes do not capture air to inflate. A pilot chute generates lift exactly as a ram air canopy does, by having the air flow over the wing (around the round canopy). There are many reserve pilot chutes that use no mesh, but simply use F-111 where mesh normally is, and they inflate just fine. The mesh may improve some performance aspects, and it may take away others, but a PC is not a air capturing device.
Otherwise, I agree with what you posted. The PC delay appeared to be casued by simply releasing the PC instead of throwing it.
Cya.
So you're saying a round parachute doesn't open by air forced into it from beneath? Rather, that the fabric is "sucked open" by airflow past the fabric on the outside? Where does the air that ends up inside the canopy come from and how does it open this way if it does not fill the inside from below?
Just trying to better my understanding of how rounds open.
Gardner
Re: [base311] Can YOU see something wrong here??
This is exactly what is being said Gardner. The Bernoulli principle dictates that most of the initial opening stages of pilot chutes and parachutes are from air flow on the outside, NOT THE INSIDE. This fact is well known among the parachute design community. Additionally, a detailed study of round openings will show that the canopy inflates from the top down, and at some point the bottom skirt has opened enough to finish the final stages of inflation. I believe this effect also predominates in squares, inflating from the top down resulting in a generally tail first inflation. Only problem is that the tail has the longest lines and line overs therefore can be a significant risk factor. Therefore, we use items like tailgates, etc to control the tail first opening sequence. Johnny has a point though: knowledge of the very low speed environment is limited and anything that can be done to improve pilot chute performance can only help, obviously. Maybe we should consider adding taschengerts to pilot chutes too instead of just old ratty rounds.
I am also skeptical about any analysis of this video. It is just too difficult to see any real detail.
Re: [460] Can YOU see something wrong here??
In reply to:
In reply to:
This is exactly what is being said Gardner. The Bernoulli principle dictates that most of the initial opening stages of pilot chutes and parachutes are from air flow on the outside, NOT THE INSIDE.
previously:
In reply to:
Just to avoid any confusion about PCs, they do not inflate by capturing air inside the PC. Let me repeat this, as to be 100% clear: Pilot chutes do not capture air to inflate. A pilot chute generates lift exactly as a ram air canopy does, by having the air flow over the wing (around the round canopy).
uh. no.
Bernoulli and his predecessors don't deserve such abuse.
Bernoulli's principle describes how total pressures balance and are a sum of static and dynamic pressures. fortunately for us, surfaces respond primarily to the static pressure. wings, or canopies, have relatively high pressure on the lower surface and relatively lower pressure across the upper surface. lift is generated. period. describing it as "air sucking the wing up" or "air pushing the wing up" are both valid.
the inflating pilot chute does not contain a vacuum. to inflate, air molecules MUST find there way inside and stay there.
thus the air inside the pilot chute is constrained and impeded (i.e. captured). it can not flow readily. with less airflow, the dynamic pressure is lessened, and the static pressure is higher. the converse is true on the exterior of the pc. increased airflow results in increased dynamic pressure, lessening the static pressure.
pilot chutes inflate simply because the static pressure inside the pc is greater than the static pressure outside the pc. it doesn't matter if you say "the air inside pushes the fabric out", or "the air outside sucks the fabric out." both are correct.
Re: [wwarped] Can YOU see something wrong here??
I am confused. My understanding is essentially that the principle is a simle expression relating pressure of a fluid to its velocity, and roughly explains certain aspects of wings, etc. Is there a reference you can point me to confirm your thoughts or mine? My background is not in air flow theory but in atomic theory.
Re: [460] Can YOU see something wrong here??
Ok, when a guy with a Ph.D. in physics starts looking for references, I think we're officially out of the depth of the average BASE jumper.
Re: [TomAiello] Can YOU see something wrong here??
In reply to:
....average BASE jumper.....
HEY! Not happy with that comment
Re: [460] Can YOU see something wrong here??
Here are some things I posted on Blinc regarding this issue:
In reply to:
Re: Pc In Tow Video
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I have to agree with Johnny on this. If you take a PC and drop it in still air without a force being applied to the bridle it will probably not inflate. Rather it will tend to fly in the direction of least resistance or most mass. This would tend to cause the PC to fall to one side.
When you apply force to the bridle however, the PC acts as a scoop and catches air. This scooping action deccelerates the PC until it has obtained an optimal volume. At this point the PC flies as a solid object and because the PC is symmetrical and the force is centralized about the apex the PC acts as a brake.
IMO, the very minuscule vacuum created by a PC traveling through air could hardly counteract the force of gravity on the material, much less pull a canopy, weighing many times it's own weight, from a container. Maybe I misunderstood your original post???
It seems to me that the primary function of a round canopy or PC is to present an even, controllable surface in the direction of movement to resist the force of gravity. The air inside the parachute is still and the addition of vents is to increase the stability of the surface presented to the direction of motion or to create the ability to steer the canopy.
After further research I found this:
"Because of the system's rapid descent, air rushes in through the tube's opening and accumulates at the apex of the canopy to create a high pressure air "bubble". Steady inflow continues to build up internal pressure, thus allowing the bubble's volume to expand horizontally as well as vertically....Like for any blunt objects moving through air, wake turbulence generated on the downwind side of the parachute causes the external pressure to be lower than the internal pressure near the apex....On the other hand, rapid expansion generates a large external pressure which squeezes the bubble on its upwind side, thus slowing down the expansion." - 'Parachute Inflation' By Dr. Jean Potvin
More can be foundhttp://www.pcprg.com/inflate.htm]here
This seems to support my theories regarding inflation, but I'm still wrestling with the dynamics of a round canopy once it has inflated...
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Re: Pc In Tow Video
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It's not contradictory but it lends credibility to some of what you mentioned in your post. From the quote I included in my previous post, the external pressure being less than the internal pressure aids in filling the canopy, but the bridle pulling the mouth of the canopy through the air captures the greatest volume of air and therfore seems to be the primary contributor to the inflation. If the mouth was covered with F111 fabric, as you previously mentioned, rather than the mesh we see on most PC's the inflation time would seem to be much greater, probably proportional to the diameter of the hole in the fabric.
For instance, look at pulling a trash bag through a swimming pool. There is very lttle resistance to the motion of the trash bag until it is full of water and the action of pulling the bag through water does little to induce a change in pressure at the apex of the bag until the bag is full. Once the bag is full the resistance to movement of the bag through the water is proportional to the diameter of the mouth of the bag and has little to do with any "vacuum" created on the downwind side of the bag.
In air, the system is more dynamic in that air is compressible. Now the difference in external vs. internal pressure may cause "surging" as an equillibrium pressure is found. But, once the PC is inflated, I would think these small changes in pressure can be ignored and the PC would experience forces similar to the trash bag in water.
These opinions are based on my observations and a very little amount of research (two articles from one Google search). I am in no way saying this is how it works. I'm just expressing my thoughts. As you said, this is an interesting and eye-opening discussion and our lives depend on this phenomenon.
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Re: [JohnnyUtah] Can YOU see something wrong here??
I dont have a PhD, and I cant rattle off a bunch of scientific terminology or theories, but I try to maintain a simple understanding of this stuff. This conversation has really developed, so here are some more comments of the way I understand it all. I could be wrong or I could be right, or maybe a little of both. Bottom line, get a parachute open before hard deck.
Some basics:
Square canopy
A square canopy is an airfoil, a wing, a glider. The pressurized wing generates lift because it is an airfoil shape. In this thread, round canopies seem to be the topic, so…
Round canopy
A round canopy is a drag device. The canopy captures air as it is pulled in a direction. This capturing of air pressurizes the canopy with static pressure holding the round canopy in its inflated shape. The shape of the round canopy creates drag, which is the opposing force to whatever is pulling the payload in the said direction (i.e. gravity). It is this drag that allows a person to land with a safe rate of decent under a round parachute.
Inflation of a round
For a round parachute to be considered inflated, it needs to be pressurized with air. For air to get into the round parachute, it goes in through the opening at the skirt, not the apex. Air exits through any hole at the apex.
During a round deployment with some airspeed, the canopy starts out in a stretched out streamer type of shape. The skirt of the canopy is gathered together in the center and therefore the opening at the bottom is very small if any. Because there is some airspeed, the airflow across the canopys outer surface creates a low pressure. Since there is a lower pressure on the outer surface of the canopy than on the inside of the canopy, the streamer shape begins to expand. The opening at the skirt begins to expand as well.
As this happens, air does go in there.
The air keeps collecting inside there, and it is at the apex of the canopy where it collects for the most part. Once the opening at the skirt becomes large enough to let in enough air to completely expand open and pressurize the canopy, then that is what happens.
The low air pressure on the outside of the canopy during initial inflation, helps the canopy change from a streamer shape to an expanded, more open shape.
IMHO- If a round parachute were deployed with its skirt completely opened up (full diameter), then it would inflate (pressurize) immediately and the need for an external low-pressure to help expand the streamer and open the skirt up (as during initiating inflation), would not be needed. I agree that the static pressure created by the canopy capturing air is greater than the external pressure and therefore the canopy stays open.
Pilot Chutes
I have no doubt in my mind that when doing a high airspeed deployment, there is a low pressure created on the outside of the PC which helps it to initially expand. It seems to work very well and that is why I use a regular mushroom for those types of jumps.
In this thread we were talking about a very low airspeed PC hesitation. My point of view is that if you can get the mesh/rip-stop seam of the PC opened quicker (and reliably) on its own, then air will go right in there and pressurize the PC that much quicker. That is why I was mentioning the Super Mushroom. Thats its purpose.
If you pack the PC so that it is dependent on airspeed to begin to open, then you need sufficient airspeed at deployment time or enough altitude to get away with a hesitation (like we see in the video).
With the regular mushroom, sometimes you can do a throw and go and the PC opens immediately with nil to very little airspeed. I believe this works because enough air successfully flows through the opening of the seam and successfully inflates it. However, sometimes the airflow is not so successful to get in and inflate the PC at low airspeeds with the regular mushroom. The result is an occasional hesitation. In the video, it probably hesitated until an external low pressure did happen and helped get the PC to open. Not ideal in my opinion.
Re: [JohnnyUtah] Can YOU see something wrong here??
hey no problem. i think an intuitive understanding is generally the best understanding. i'll research the issue if anything dramatically differs from these common sense aerodynamics.
Re: [460] Can YOU see something wrong here??
In reply to:
In reply to:
I am confused. My understanding is essentially that the principle is a simle expression relating pressure of a fluid to its velocity, and roughly explains certain aspects of wings, etc. Is there a reference you can point me to confirm your thoughts or mine? My background is not in air flow theory but in atomic theory.
I pulled out the most basic book on aerodynamics on my shelf and created a response. Bernoulli's equation is fairly easily derived from Newton.
too bad I did not see an e-mail address listed in your profile, otherwise I'd PM you.
don't think the forum wants to see THAT much detail...
btw, my background is in Aeronautical/Astronautical engineering and I too have worked for NASA.
Re: [wwarped] Can YOU see something wrong here??
In reply to:
don't think the forum wants to see THAT much detail...
I'd like to see it. Either here, or via email or PM.
Thanks!
Re: [TomAiello] Can YOU see something wrong here??
In reply to:
I'd like to see it. Either here, or via email or PM.
it may be a bit off topic, but at the moderator's request...
I pulled out my first textbook regarding aerodynamics.
(Introduction to Flight by John D. Anderson, Jr. 1978, McGraw-Hill)
section 4.3 on pages 69-73 discuss momentum along a streamline. (a stream line is the path along which an infinitesimally volume of air flows.)
mass:
the volume of the piece of air is dx dy dz.
therefore it's mass = ro(dx dy dz) (ro is the density of air)
acceleration:
a = dV/dt = DV/dx dx/dt = DV/dx V (V is velocity, t is time)
Newton:
F=ma
-dp/dx (dx dy dz) = ro(dx dy dz)V dV/dx (p is pressure)
(pressure x area = F = ma)
dp = -ro V dv (Euler's equation)
or
dp + ro V dV = 0
integrating it along a streamline yields
p + 1/2 ro V^2 = constant (Bernoulli's equation)
(static pressure + dynamic pressure = total pressure = constant)
static pressure is the pressure as measured perpendicular to the airstream.
dynamic pressure is the pressure resulting from the motion of the air.
assumptions:
-gravity is insignificant
-frictionless air
-the formula is for any point on the same streamline
-incompressible flow (i.e. ro is a constant)
"if all the streamlines have the same value of p and V far upstream, then the constant in Bernoulli's equation is the same for all streamlines."
------
air cannot flow into a solid object.
at best it will flow parallel to the surface.
thus the pressures affecting a wing, and creating lift, are STATIC pressures.
freefalling bodies will feel TOTAL pressures.
otherwise the following statements conflict:
“the faster air over the top of the wing results in a lower pressure.”
“the faster you fall the ‘harder’ the air becomes.”
we would be better served if we prefaced “pressure” properly…
(side note: aircraft use pitot tubes to determine airspeed. these devices compare the pressure parallel to the airstream with that perpendicular to the airstream.)
-----
as far as pilot chutes go...
it is a very complicated situation.
at the risk of committing over-simplification,
obviously, the airflow within the pilot chute is less than the flow outside.
Bernoulli thus states that the static pressure within the pilot chute is greater than that without.
but, an uninflated pilot chute can not support a load. any hint of pressure differences will cause the fabric to move.
therefore, the phrases “the air inside an inflating pilot chute pushes the fabric out” or “the air flowing by the inflating pilot chute pulls the pilot chute out” are equally valid.
Re: [wwarped] Can YOU see something wrong here??
In reply to:
mass:
the volume of the piece of air is dx dy dz.
therefore it's mass = ro(dx dy dz) (ro is the density of air)
acceleration:
a = dV/dt = DV/dx dx/dt = DV/dx V (V is velocity, t is time)
Newton:
F=ma
-dp/dx (dx dy dz) = ro(dx dy dz)V dV/dx (p is pressure)
(pressure x area = F = ma)
dp = -ro V dv (Euler's equation)
or
dp + ro V dV = 0
integrating it along a streamline yields
p + 1/2 ro V^2 = constant (Bernoulli's equation)
way to kill a perfectly good thread.
In reply to:
assumptions:
-gravity is insignificant
Oh well...
pope