pchapman

Oh this has indeed become a mess. It takes time for people to get their head around some of the basic concepts of canopy flight combined with basic physics. Once one has in incorrect framing of the whole issue it can be tough to get into a new more correct framework. Unfortunately we haven't got there yet. It's like every time someone posts, you now say "I understand now!"... but still come up with a wrong conclusion.

How accurate do you really need it?? Looking for an approximation of air mass? It can be a few pounds I remember calculating for a typical canopy but more for a student canopy. (With the volume rapidly going up, by the power 3/2 vs area.) Actually MEASURING the air volume is tough; calculating roughly what it would be is easier. I can't offer much beyond the obvious calculations: For a rectangular canopy the volume as constructed would be just span * chord * height * a factor for the area of a rib compared to the rectangle it fits into. (Somewhere between .33 and .5 lets say). If one doesn't have access to any fancy techniques to integrate the area inside the curve, just estimating with a drawing over graph paper (physically or on the computer) will work. One can either trace a rib of a canopy or find the occasional (sometimes rare) drawing of a canopy rib out there to get a typical shape. Thickness to chord ratios vary but one could figure out estimates for different styles of canopies if one doesn't have a canopy to measure. For tapered canopies, one can easily make linear adjustments as most canopies don't have a complex geometry like a paraglider. If you want to estimate the change due to distortion when inflated, with shortened span and increased bulge between ribs, that'll take a little guesswork. Guess the volume would increase slightly as the shape becomes rounder. I have seen Icarus publish estimates on % span reduction but not on thickness changes.

I see the 'outside RSL ring' issue from time to time. I don't recall any rigger or anyone objecting and forcing the jumper to buy new risers. But it is a visual distraction ('Is that right? Is that routed right?') and the jumper has to be very careful about the routing of the RSL webbing.

I'm curious how rare it is for DZ's to load their Sigma tandem rights right up around the 500lb maximum. (And is it the same for Strong?) --- and how rare it is to take tandem passengers up to 300 lb. I know some DZ's might cap a student at say 230 lbs. But with some instructors, one might take up a 300 lb student -- for example, 300 + 150 instructor + about 50 for gear, if one isn't being really exacting with measuring every pound of equipment including the student harness.* I had sort of fallen into the role at my local DZ as being one of the couple TI's who would occasionally take up big students, originally seeing it as a normal challenge of the work. (Originally out of widebody C-182s for no extra pay; now from a Caravan with extra pay.) Others now have pointed out to me that it isn't exactly normal. I can see the point. Plenty of equipment in skydiving has often gotten overloaded a little -- C-182 gross weights, heavy student rigs with 254 lb certification limits being used with supposedly 230 lb students, that sort of thing. However, pushing the maximum on tandem gear might be less forgiving. What about situations with big instructors: Do DZ's ever send up a 220lb instructor with a 230lb student? That again works right up at the gear's certified limits, but doesn't sound as surprising as when someone says they just jumped with a 290 lb student. (Like a student of mine last weekend named Geronimo. Seriously.) * Edit: although I'd like to re-weigh the tandem gear, one UPT document online that mentions the increase of certified weight from 450 to 500 lbs years ago does state, "The new approved gross weight limit is 500 lbs. This translates to approximately 50 lbs. of gear weight and 450 lbs. of personnel weight"

You are talking about FULL magnetic bags, with no elastics or tabs, at all, so that the bag closure is also done with magnets? Are they really that popular? And you have ones that work well? But if you are talking about bags using magnets for line stows only, that's nice but not relevant here. (I made one myself over a decade ago, and agree, they are nice.)

Quite the thread resurrection from 2010. The sub-topic is still interesting: To what degree the tail of a canopy might flip up when rear riser flaring, when one or more brake lines are detached, compared to the normal situation. Plenty of people clearly have landed canopies on rears with a broken or lost brake line or two, yet sometimes there have been problems with the tail flipping up from lack of support. Whatever the arguments back and forth in this thread, at least when you are faced with the problem on a skydive, you can test it out yourself with a few hard rear riser flares! Then you can make the decision to chop or not. I wonder if a large, slower speed canopy with less inflation pressure might actually be more susceptible to the unrestrained tail starting to 'flip up'. And the trailing edge on a large canopy will be further behind the D lines. (I've only done it once, on a Parafoil 260 that blew a brake line on opening. Rear riser flare was fine but sensitive -- collapsed it in a stall just as I touched down. Perfect.) Also, on smaller skydiving canopies one may sometimes have a lot of slack in the brake lines -- such as for front risering)... so the tail is already nearly unsupported... yet one can rear riser flare without the tail trying to flip up. (Although someone could argue whether just having just normal drag on the brake lines is enough to add a little tension to the tail to reduce flip up chances.)

A good point to make. After all, terminal BASE jumps work too. But we're talking F-111 style material in those cases. Probably not good to freepack (or BASE pack with a tail pocket) a modern higher performance canopy where slider placement and size tends to be crucial. (Did you restrain your slider with an elastic or leave it free?) And it does seem to be a premature, as the OP stated. I don't see evidence of a super quick pilot chute pull, in the moments his right hand was mostly out of view on the video. To combine what others have basically said, although a) the bag didn't cause the premature, and b) the slider happened to stay up until at least the very beginning of line stretch (but soon went on an angle and did not allow for a controlled, well staged opening) ... a bag that fails to contain lines and parachute at a sitfly speed is still probably considered a bad thing.

Softer openings indeed don't have to result in longer time or distance... but they MAY. Which allows for MEL to put forward the case that the Optimums do take a longer distance.

Thanks! Actually got that one already, that's from the International Planetary Probe Workshop #3.There are some other fun parachute related papers in the same folder, although nothing to do with sport parachutes! (One NASA page has the files too. Other sources for interesting parachute aerodynamics are of course T Knacke, G Peek & J Potvin, and JS Lingard.)

I'd just add that depending on the flap configuration, there's likely also some lever effect adding to the resistance. Eg, a flap with magnets over on one side (and a stitched seam on the other) has a 2:1 ratio for a canopy evenly loaded on the flap. So magnets good for 10 lbs would hold 20. But the overall idea still holds as you put it. The incident was a good example of something that works in normal conditions but got dangerous more quickly than someone expected in an unusual situation.

Nice use of basic physics. To expand a little for others, the slope of the line is the acceleration (deceleration) since it is the rate of change of speed. You point out one scenario where you can have the same distance and time, but harder or softer openings. Still, there are scenarios that are mathematically correct but might not be very feasible for an actual parachute. Eg, lets say an Optimum has a smoother steadier deceleration over its 3 sec opening. If a 'conventional' reserve started to open harder and faster at the start, it would be difficult for it to take as much distance as the Optimum. The conventional reserve's speed would already be low, and the canopy spread wide, so it couldn't just "hold off" and pretty much stop decelerating long enough for it to keep adding enough distance to equal the Optimum at the 3 sec point. The sketch shows some scenarios. The second one down is a situation of a quicker initial opening that mathematically gives the same total distance. But as I noted above, the more likely scenario is the 4th one down, where the quicker initial opening leads to less distance used. [inline "deceleration.jpg"]

My exact thoughts. Don't come to my dz. [This is only a side topic for this thread] Guess you guys' DZ's doesn't care about decent horizontal separation. How can he make things worse than normal? If he pulls low, he's safely away from others. If he pulls high, well plenty of noobs pull way high anyway. If flies up jump run, well, he could do something dumb like that at 'normal' altitudes. If the next jumpers are closing the gap horizontally, then a higher pull reduces the time available for them to do that, so that's a benefit. Heck, if he pulls 10 seconds out the door, its almost impossible anyone else on the load could conflict with him (by removing any horizontal separation so quickly), so a really high pull would be safer. Anyway, maybe you can think of some scenario I wasn't thinking of. But jumping without an alti once doesn't seem like a huge crime.

Nope. Not unless you are bailing out of an aircraft moving horizontally, which is how the tests are done. Unfortunately it is a big myth in skydiving, thinking the 300 ft thing is vertical, something that many people were taught at some point -- me too. The vertical distance is needed doesn't have to be a lot more than 300 ft necessarily -- as long as there's no pilot chute hesitation -- but there's no requirement for it to be under 300 ft in a vertical drop. In any case, the Optimum will meet the regs. Some PDR's are of course C23d, while the older ones are C23c. (Some C23d ones are allowed slightly over 3s to open, as there is an allowance for higher gross weights, but its only a trivial amount except for the PD281.) For both TSO's, most of the opening time tests are only done with a 170 lb person or dummy, and only up to 110 kts. It is only the strength tests that are done at max certified speed time 1.2 test factor -- and there are only 3 of them. The opening time rules apply there as well. Or for C23d, opening altitude can be measured instead, 300 ft max (again with slight additions for high gross weight canopies). From what I've heard over the years, the suggestion is that companies have a tough time getting canopies to open fast enough at the slow speed tests, without opening too hard and fast in the high speed tests. I suppose the place where differences can occur between the two models is if the Optimum has a short snivel (acknowledged in the Optimum flight characteristics document by PD), while the regular old reserve probably has no real snivel, like many typical old school F-111 canopies. Both might be open within the required 3 sec or so from pack opening, but with a snivel there might be more altitude loss and less shock on the jumper when it fully opens. ... But what this all means in terms of actual vertical opening distance, for the two canopies, I don't know. [For those getting picky and into the details: In AS8015B, which gives the C23d specs, section 4.3.6 is about opening time or alternately distance. There's nothing about having the test dummy travelling vertically during tests. I believe the part about optionally measuring distance instead of time, where one is 'measuring along a vertical trajectory' is still about the measurement direction, not the actual direction of launch. OK?]

What the heck I'll kill some time. The usual caveat is talk to your instructors about any issue; don't just use something you read about on the web that could be misinterpreted or not properly weighted in relation to other factors or not in line with the teaching system at your DZ. Licenced jumpers might have a mal every 800 or 1000 jumps, either due to the fault of the canopy or sometimes themselves (packing), or some mix of the two. But that's just an average so will vary a lot. Knowing when to cut away? Usually it is pretty obvious that you don't have any sort of flyable parachute. But there are also more subtle, grey area cases where opinions differ, where a student might cut away and a more experienced jumper might not - but both decisions might be right for the skills & experience of the jumper. It is a challenge for students to learn the subtleties beyond the basics told to them as students -- but usually the basics are enough for a newbie. Sounds like you could use a good review of the procedures. Sometimes students learn them early on but there isn't enough review throughout the student process. While an altimeter helps one understand typical turn heights in a circuit pattern, where you are in the sky matters more than an arbitrary number on an alti. So you want to be at the right angle to the landing area when you turn on final, whatever that angle might be depending on the speed of your canopy vs. the wind speed. While not turning low is a good rule, at some point you need to learn what 'too low' is. If you just need a 90 deg turn to avoid an obstacle, a moderate speed turn can work from 200 ft or whatever. So a small turn to avoid an obstacle can still be quite easily done when already on final. Over time students get a feel for how fast a turn can happen and how much extra altitude they lose in a tight vs shallow turn. But some of that needs to be done through experience with doing turns in the circuit, lower to the ground. (Although digital altimeters can help.) Learning to be good at adjusting one's flight path under canopy and landing in the right area is one of the big challenges of being a student. Tons more could be said on any of these topics...

For one PC I put into a somewhat modern piggyback container, I used regular wide risers with 3 rings. Unfolded the tops of the risers so they would accept L-bars. Lots of bulk with all those lines, especially with the knotting & sewing system they used back then to make the line loops at the links. So I stayed with L-bars. Tacked the L-bars in place in the riser ends to prevent them getting pulled out of place (eg, end-on loading). Might be better to actually sew the riser loop down smaller. Control lines just went to the usual place on the back of the risers, but since they go up into the centre of the canopy, they wrap around the inner edges of the risers in use. I didn't worry about reinforcing the edge of the riser, as a few jumps a year isn't going to cause a lot of wear, and there's no tension on them normally either. Used a pretty standard Dbag and a big non-collapsible PC. Whatever sizes are appropriate for students or accuracy canopies should do for the latter. Then added line stow loops inside the top of the bag. Crown lines stow in there. (Since I had a regular Mk I PC, bulkier that what you have, I built an extra large main container that could be velcroed on overtop of the normal main container.) That's all just what I did; I don't know if others have better ideas.

What was the source? I'd like to check out the details further. Sorry if I missed it somewhere, even doing a quick search on the thread. Even if Canada isn't a haven of peace and harmony, we're not quite so worried about whether the Danish are better than us for gun violence (I'm sure they are; good for them). We're more worried about the mess to the south of us; as whatever happens in the US has some influence on us. Probably also a major source of illegal guns in Canada. No easy answers when a country is that awash in guns, whether used legally or not.

Sounds promotional. There are plenty of "Remove before flight" articles, from keychain tags to women's t-shirts, available for sale to pilots and other aviators. Based on the various red Remove Before Flight tags on pitot covers, engine inlet covers, etc for aircraft.

A DZ I've been at has a mix -- there's an old military backpack with a C-9 or something, there's an older pilot rig of some type, and there are a couple old retired '80s skydiver rigs with pillows in the main compartment and a round reserve. Basically whatever was available. In recent years they even get packed reasonably regularly!

"His exit abruptly shifted the weight aft, rendering the aircraft uncontrollable" What B.S. Either you would have to show that the aircraft was already very poorly loaded with an aft CofG -- or else every jump operation with a C-182 would crash (or front door 205/206). And the nose had already dropped. So either the plane was already starting to stall, or the pilot mishandled the plane and didn't get the nose down enough when the engine died. And what are you arguing about seat belts? Do we even know if they had belts on or not during the climb out? So why is the crash being used for saying that seatbelts prevent movement in flight? The description never mentions the aircraft pitching up, so even if they were unbelted, what does the crash have to do with your idea of having seatbelts keep jumpers from moving about? As for "Sacred duty" of the seatbelt to keep people from bouncing around during evasive maneuvers? Hardly. It's a very rare occurrence. If it were important, then you would have to argue that we should keep seatbelts on at all times until jump run, or at least above the 1000' or 1500' typically used now, completely contrary to current practice. You could well argue that, but didn't. So which is it? How often do pilots really do "top gun shit" when flying jumpers? Besides, you've been in skydiving planes -- You could bounce around a whole heck of a lot with a typical belt, though at least in a large cabin aircraft not everyone would end up at one end or other. "Dive to the right"? Nothing like this is ever taught. A pilot is more likely to apply sudden positive G during a rapid turn. I await your expanding on the "au contraire" thoughts about seatbelts in flight. Maybe you have examples. While you have undoubted skills in different areas, and have the ability to write, your flippant "know it all" writing style has become annoying. That's more personal preference -- Others may like it. It sometimes makes it look like there are simple easy answers to everything, and you're here to enlighten us all to the one true way. But when facts and interpretations are tricky, slow down that writing of yours -- you might whip out a nice sounding paragraph really quickly -- but saddle it with some debatable advice -- so think through it all more. Mind you, Part 1 of your series seemed quite reasonable, with a decent review of the history of the seatbelt issue!

I find it curious that news sources mentioning the newer video from the bystander tend to put in their headlines the bit about the wife repeatedly saying "Don't shoot him!" to the cops, but make no comment on her also repeatedly saying to her husband, "Keith! Don't do it!" Now that doesn't say what he might have been doing or what was in his hands, or anything else about the situation. But even from my left leaning point of view, it seems very unfair and biased to focus only on the one but not the other.

Nice find. I can't comment on the Lazy Bag in general, but there could be some "what did they expect" criticism here. If someone's bag is only closed by magnets, there will be some rate of acceleration of the bag that is too much for it to handle...and that jumper found it. Not at normal belly terminal, but only in a failure situation of a premature at sitfly speeds. Conventional elastics might hold in a canopy at a higher pilot chute acceleration or freefall speed, Tube Stoes might go even faster, and high speed gear might even use other methods like bungees. That bag was a little too lazy to protect the jumper at that speed...

Yes good point! As with the subterminal case, that long 10 seconds thing, and no speed data in the report we see, makes things messy. So both "subterminal" and "terminal" aren't necessarily what we might expect them to be throughout each test. And who knows how much body position changed as well. Just like in the case of someone with a bag lock, body position can affect the result a lot. Edit: I expect the people involved in doing the tests weren't exactly dummies, but the info that got passed down to us is a little limited in telling us what really happened...

I thought it interesting how much lower the Sustained loads were compared to the Peak loads at terminal velocity. And even the Sustained loads are low for terminal velocity compared to figures published for pilot chutes only. The key difference is that these tests are with the pilot chute stretched out on a reserve bridle behind a jumper, with all of the burble effect. It takes time to dig up numbers but here are a few: -- A Jumpshack publication on calculated drag of some pilot chutes , based on tests: At "terminal velocity" forces were 145-180 lbs for Racer PC's (big & small), the MA-1, and Vector I PC. (And Vector II more like 75). The numbers are of course just from one company with a particular viewpoint but better than nothing. [Source: I think the jpg I had of these values is from one of John Sherman's videos from about 2011 ?? when he was quite active in the pilot chute debate] -- An Australian Parachute Federation study from the early 1980s ?? tested pilot chutes with a scale outside of a testing vehicle at up to 120mph. Loads of 98 to 146 lbs were seen for some spring loaded pilot chutes of the era (except with the Vector II at 84 lbs). In contrast, the new PIA Sustained values 85 to 124 lbs for Average Peak force at an unknown terminal velocity. That's a good chunk less than those tests mentioned above. And that's after the pilot chute hits the end of the bridle with a bunch of momentum. The Average Sustained at terminal was just 34 to 57. On a rig by rig basis, the sustained values were thus often just 45% to 55% (very roughly) of the Peak forces. Sometimes the Average Sustained forces were not much higher than in the subterminal cases. Although since they were averaging over 10 seconds... the "subterminal" case isn't all that subterminal any more after that long. I'd want to see some speed info to judge whether the small gap between terminal and subterminal forces were really a surprise or not. The 34 to 57 is far lower than the typically 100 to 150 or more values in the other tests. That's a huge loss. Since the data is from completely different tests, the reasons for the difference aren't clear. But it does suggest that the burble effect is larger than many might expect. I suspect the average dynamic pressure far back in the burble isn't hugely less, but if a pilot chute is being batted around in turbulence it could be spilling air and not working efficiently. Who knows. And that's not just 5 or 6 ft away, but at the end of a perhaps 12 or 15 ft long reserve bridle. But the forces should still be enough. Lets say there's a tight pack and it does take 18 lbs to extract the freebag from the rig (a number at the top end of what John Sherman suggested as being acceptable). Forces of 85+ peak forces will extract that. And even a sustained force of 34+ lbs would then accelerate a say 7 lb medium sized reserve away at 34/7 = 4.8g acceleration or more. Anyway, all this is just some playing with rough numbers and quick speculation. Any additional data on spring loaded pilot chute drag values would be welcome.

Which is good. Still, the way we are trained falls far short of all of the subtleties of the problems that can occur, and there's often no perfect answer on whether to stay with or chop some problems. That's the problem that the whole thread grapples with. The canopy sounds like it was squarish, flew and flared ok. Fair enough he said it did pull a little to the left, and I don't know what his training said. Sometimes students are warned that canopied don't fly perfectly straight even in normal conditions. I figure doing a flare tends to weed out canopies that are 'barely stable'. And looking for a canopy that is still 'square' (rectangular) tends to help avoid canopies that are too distorted to fly well at a reasonable descent rate, although that can be a little harder for a newbie to interpret. But anything more than a closed end cell can start to be serious as a canopy gets smaller. I can understand that skydivers seeing a mess of lines being wary of it. It is unusual that others could still see the mess on the ground -- tension type entanglements tend to disappear at that point. So I could see that it might be a situation where a lot of jumpers (but not all) would chop when there's a clear tension knot entanglement of some type, that interferes with both the slider and the placement of the risers -- even if a control line check made it seem OK. Still, a student might conclude from their training (which could differ from place to place) that it was still acceptable.

Ahem. Your request is reasonable but the other things you wrote can attract some attention. I believe the USPA (for their members) upped the minimum pack opening altitude from 2000' to 2500' a year or two back. (I'm personally flexible with these things but anyway.) And I hope the Swift is at least a Swift Plus; those are old but still decent canopies at moderate loading.