pchapman

This may help: You say, "it's all about airspeed", strongly questioning the the accepted method (as billvon wrote in this thread) of "groundspeed plus wind at opening". That gets some people all aflutter, "OMG where's the airspeed? Surely it matters whether you are jumping a balloon or 727?" Follow this through to see that billvon's statement still takes into account airspeed. Let's call the billvon's calculated value the B value: B = Groundspeed + WindAtOpening We know Groundspeed = Airspeed - UpperWinds Substituting the two equations, B = Airspeed - UpperWinds + WindAtOpening which is the same as B = Airspeed - (UpperWinds - WindsAtOpening) When the uppers and lowers are the same, it is just Airspeed! Jumpers are all in the same airmass, so it is just Airspeed that gets them separated horizontally. Only with all winds all being the same, is Airspeed the sole answer. When the uppers are stronger, one decreases the value of the equation by the difference between upper and lower winds. So the speed value B is lower, and one needs more seconds between jumps to give whatever one's desired separation distance is. Voila! To see that this works, think of the classic example of the jump aircraft in a gale up high, doing 80 kts but zero groundspeed, yet calm air down low. B= 80 - (80 -0) = 0 This gives the correct answer that one would need to wait infinitely long for horizontal separation because once under canopy, everyone stays in the same place. (In reality of course parachutes descend so eventually one will get vertical separation.) Here's another way to wrap one's head around using "Groundspeed + WindAtOpening":* Ground speed is what moves the airplane across the landscape away from where each jumped exited. Then the wind at opening is what moves the jumper under canopy across the landscape away from where he just opened. The combination of the two gives separation.** Anyone unclear on the math, think through that one to understand it all! Footnotes: * This is for upwind jump runs. Use vector addition as necessary if this is not the case. ** All this is for "all jumpers equal", before taking into account belly vs freefly etc, where the distance blown by the wind in freefall will differ due to time of exposure etc.

But there are documeted and clarified by Airtec; cases of cypres2 units firing when they should not have. Hang on, I was referring to a direct quote about "not firing when conditions were met", not what you are talking about, "firing when they should not have". (eg, 2008 sensor issue, or Russia 2011 ground fire under investigation) Discussing these things can get messy, trying to defining exactly what one is talking about.

I agree that there haven't been proven cases. To add to the discussion, I'm guessing he's thinking of those cases where the Cypres fired but the guy bounced and people swear they didn't see a reserve until a couple hundred feet. (Like the cases in that conspiracy theory pdf document on zoho that keeps getting linked to, or the Brooke Baum accident for example. Some of the stuff in the document is crap but it at least it enumerates some AAD-fired-but-died cases.) What were the reasons: Tight container? Long loop? PC in burble? PC caught around tumbling jumper? Those were the traditional explanations but some ask whether low AAD activations could have been factors too. As much as I like Airtec, it would be interesting to see the pressure graphs from jumps like those, to see if they are consistent with their statements that the AAD fired on time. The problem always is that there's no independent sensor evidence to try to confirm what an AAD is sensing. The Cypres says it fired at a pressure equivalent to 750' over the ground, so Airtec says it fired at 750', and that's it. But there is a chance of teasing some additional data out of the pressure graphs, to see whether the interpreted speeds are consistent. If, say, an AAD did somehow get confused and fire at 300', thinking it was 750+, there could be some evidence in the pressure graphs (both raw & filtered) to show something odd going on, before, during, and after activation. All the AAD companies seem to agree that the normal burble difference is only 250-300' between front and back, and that's already accounted for, so it is hard to see what would fool the AAD to fire hundreds of feet lower. One might look at Cypres' more complex algorithms, that are less hair trigger than those for the Vigil. Still, it seems unlikely that it would take an extra 2+ seconds for a Cypres to make up its mind. We don't really have good answers for the fired-but-died cases, beyond vague accusations of AAD coverups and poor rigging and containers built too tight. So we continue to speculate.

Agreed. Germain's article is a reasonable one about how to make good use of a tool. But soon we'll hear on the DZ how the BEST way to fly the circuit, the most modern way, is with canopy altitude alerts. "But Brian said you have to..." So despite it being a reasonable article, some of us are going to be thinking, @%#$# Brian.

Traditionally the convention was time from exit to pull. (In the US, the USPA [edit: not the FAR's as I first wrote, I think] may refer to pack opening altitude, a little after you actually start pulling, but people think of the altitude they pulled at.) With electronic gadgets now, one just accepts whatever number they tell you -- to some point later in the opening when a large decrease in speed is apparent. What point that is, will depend on the gadget, the setting (eg, more sensitive for wingsuits), and the speed profile of your canopy opening. Doing it without electronics was a massive guess or extreme pain in the ass if you tried to calculate accurately. (eg, "That was a 12.5 jump... except we got closer to 13... first half was sitfly, and then it was flatfly...let's split the difference...that gives X seconds... oh, no, I'm a liar and a cheat! ... I pulled a little higher than usual because it was a long spot... I have me knock off a couple seconds...but I don't recall exactly how high I pulled...") It's not worth the time to calculate in detail. So if you still need freefall time logged for some license or something, you can work out an approximation for some typical jumps you do, and stick with that. (eg, 12.5ish, plus my typical opening alt, for mostly flatfly = X seconds)

Interesting to hear the current base thinking as Robin reported. Using a toggle and riser is not quite as bad as having to think to pull the riser, say, 1" for every 4.5" of toggle movement. It's a closed loop system, not an open loop one, so you use feedback from what's happening: Basically what you are doing is flaring with one toggle, and countering with the opposite riser to stay level. I've never tried that, but on a small crossbraced canopy have flared with one toggle and either a partially stuck toggle (wrapped around lines) or a lost toggle (grabbing brake line above the guide ring with one finger). It sort of worked, flaring with one side and doing whatever was needed with the other not to dive off sideways into the ground. Still, I'm personally still not convinced that I'd recommend riser plus toggle over both risers. Big base canopies can be more forgiving of errors so one smaller canopies, if you aren't really sure of being able to do riser plus toggle, staying with both rear risers might be safer, despite having less overall flare performance available. In any case, like Robin says, test your alternatives first.

Just my opinion: The D-line attachments do tend to move apart. I see it happening for two reasons. One is the wrapping/folding to reduce the stack width. But before that, just flaking the tail has an effect. While the tail may be neat at the trailing edge, the geometry gets messy towards the top of the canopy, so it tends to pull the D's apart. When the tail is stacked up it tends slide off to the side instead of stacking up perfectly from the centerline outwards. It is a different kind of flaking at the tail (D's to tail, then along the trailing edge) than one is doing between the A's, B's, and C's, where all the fabric is moved into one big fold outboard. The geometry of the folds in the area is inherently messy and complex, so it tends to be a messy part of the pack job. At least one can sort of see the D-line attachments, and feel them, through the top skin. So one can grab them through the skin to some degree, retensioning them and keeping them from migrating outwards too much. (And if really needed one can reach up under the trailing edge to grab them.) I accept a little movement and spreading due to the bulk issue, but don't want them migrating to the outer edge of the width-reduced stack or getting into that fold. The top D line is most likely to get pulled outboard.

Whoops, highest L over D is best glide angle (through the air mass). Lowest descent rate will be slightly slower on the drag polar. Although on a slow draggy vehicle the two points will be very close. As for the question by shah, the simple answer is that it just works out that the lowest descent rate is in fairly deep brakes. Curving the back of the air foil a lot, with brakes, allows it to create more lift at a slower speed, but also more drag. A wing is efficient at deflecting air down from its original path around the wing, creating lots of lift per unit drag. The equilibrium condition will be slower flight. Even if the angle of descent is a little steeper than at the most efficient case, you're still slow enough that the overall descent rate is lower than at any other time.

Yeah, I don't know the fine details of speed flying, but in paragliding, launches from a sharp edged cliff are dangerous -- have to watch for that rotor/turbulence at the edge. I'd want to make sure the canopy is stable and kited near the edge before accelerating off it. There just isn't much time for decision making if making a running launch off a sharp edged launch.

Yeah, that's where we have issues in teaching people because the terminology in skydiving isn't that clear. We have to emphasize that we are talking about timing between actual drops or jumps and not 'exit' as in starting to climb out.

I agree for practially all individual jumpers. I repack a few V 3's from the late 1990s and they're still fine. Stitching pulled but nothing or almost nothing broken. The rigs are still sort of grounded (on an advisory basis) until someone can take a minute to have a look. I was thinking more about student rigs, where it seemed every one of them that's a couple years old, had stitching ripping. In practice I think it'll be a judgment call, most work being deferred until repack time, and only the worst cases being resewn now. This also highlights one of those grey areas in rigging. As a rigger, I don't have to give a damn what happens to the gear once it leaves my hands. It is the owner's responsibility to ensure that they are up to date with their equipment. But in practice, it is good courtesy if as a rigger one can try to inform one's clients of new bulletins affecting their gear.

http://www.youtube.com/watch?feature=player_embedded&v=6IWVRlPHyY8 Apparently the low pull out is the one seen at 24:10 of the above video. I haven't confirmed that this is absolutely correct, but it seems reasonable based on what I've seen on the web. Edit: shorter version of the video http://www.youtube.com/watch?v=dlUWQlXQB20

Ok, I've only rigged up one MT-1XX system, but it sounds like that's pretty much the h/c for the MC-5 system. I'll admit they are functional, and bulky because they need to be strong and have adjustments and extra attachment points etc. But still ancient in design and some things would be done differently now. Needle fold staging loop? Ancient. (Tho' I wondered why it took so long for them to come back in sport para!) Two pin? Ancient. Grommet tabs on edge of PC for closing loops to go through? Ancient. (But it does locate the PC well.) Velcro all over the reserve flap? Ancient. Isn't the PC pretty wimpy too? Ancient. Deployment assist pockets on reserve bridle? Ancient. Stubby wide early 80s toggle design? Ancient. ParaFlite style line attachment sewing? Ancient. Snap fasteners for riser covers? Ancient. (Maybe still useful & functional though.) Type IIA closing loop? Ancient. (Not sure but I thought that's the original type.) And that lower reserve losing loop, isn't that a pain to pack, and a bizarre design with it snaking around the pack job and stowed lines, rather than taking a straight path from the backpad like on modern designs? All this from a company that couldn't cut it in the real world. ( = sport skydiving) 'Course, you might think it is the other way around, where the military is the real world, the serious stuff, not the fashion conscious sport side of things.

Thank you, it is "advisory", with users "strongly recommended" to inspect, and where parts are "in need of repair", riggers are "requested" to make the repair. So that does leave it up to personal choice, anywhere between (a) as often argued on dz.com, even if it were 'mandatory', maybe all SB's are only advisory unless the FAA issues an AAD, to (b) a student nearly got killed, it would be highly prudent for a DZ from a liability & safety point of view to inspect and repair rigs as requested.

A couple of quotes to show that this is about the stitching between the reserve container and harness diagonals, the area where the stitching is often ripping out: It does leave it up to the rigger to determine "in need of repair", although they show photos of damaged stitch patterns which are said to need repair. So one might interpret this anywhere from grounding a lot of UPT gear right now, to letting people say, after immediate inspection, 'it's not seriously damaged, we can work on it at the next repack'.

You have a point of course. Probably a hell of a lot cheaper to acquire than a Sigma rig. But I'll add that they are ancient in design and ugly from a rigging standpoint. Nothing that awkward and outdated in basic design is used in skydiving today. (Let's leave Strong Dual Hawk Tandems out of this. ) Good for early 80's nostalgia though, kind of like rigs such as the RTS Mirage or the ParaFlite Swift.

I'll email them to you, PD. Or tell your customers to come see me. :-)

So building the military stuff in Honduras is OK? As long as it is all American materials perhaps? But that's still taking sewing jobs from Americans? In other words, what's with that Berry Amendment thing? eg, There was some scare in the parachute industry a few years ago when it turned out some thread made with US nylon, had been spun in Canada, or some such thing, and then incorporated into military products. (That's not quite accurate, but I'm just recalling what I saw in Skydiving magazine.)

I don't myself have a 'perfect theory for what to do in turbulence', although I've tried to teach about it in a few small canopy related courses. Calvin 19's ideas look generally good to me too, although I'm not absolutely sure what the best brake position to use is. To expand on the idea of transit time from one airmass to another: If the transition between air masses is sudden, with a large shear in vertical air movements in a short distance, then a faster canopy will be better, because a given down draft will create a smaller angle of attack change relative to the faster forward speed of the canopy. But if the shear happens over a larger distance, a slower canopy will be better, as there will be more time for the canopy, despite its inertia, to change its flight path to restore a normal angle of attack with normal lift. In between those two cases, it will be a mix of those factors at work. So there is no one canopy or flight style that is inherently "the best" by some physics calculation. It depends on what kind of conditions exist out there, with what probabilities, and whether you are flying in those conditions. I recall one time when I was flying a 1.9 loaded crossbrace on a turbulent day. When I flew it to an accelerated landing, it felt ok although there were some sharp 'hits' from turbulence. When I then flew a conservative unaccelerated pattern on another jump, it bounced around a lot more, the wingtips getting softer at certain points, and it all felt scarier. But which was actually safer? High speed was probably further from any canopy collapse limits and safer in that way, yet if something had folded, flying at lower speed probably would have been better -- more time to react, less sudden canopy dive if one side folded, and a lower descent rate to begin with. Things will depend on the canopy too. A highly loaded canopy might like to be flown to an accelerate landing, while landing straight in leaves less flare power -- especially if hit by a down gust while about to plane out. So being a bit slow in a little brakes reduces safety at landing, whether or not is helps when up higher against turbulence. I like the idea of loading the canopy up a bit by a gradual turning descent, something that Germain describes. But that works best with a ground hungry canopy. It might be awkward to do in a pattern with a short recovery arc canopy, as one would have to start very low even to do a gradually descending 180 from downwind to landing. There's no one perfect answer for all conditions and all canopies.

If it acts anything like a helmet mounted ProTrack (and I'm not sure if it is), don't trust the MAX speed readouts. The devices can be fooled for a few seconds by body position changes. I can track off well from RW formations, by my ProTrack always shows a spike in speed up to 160 mph or something...

Hey Twardo, Yes, curving the airfoil more gives more lift (and drag) right away, but when you slow down because of that, then you are back to pretty much the same lift as before -- the airfoil is working harder (higher lift coefficient) but with less speed, keeping you in an equilibrium. With an airplane flying level, lift = weight (ignoring the sometimes downwards lift of the tail to maintain balance), while with gliders and skydivers under canopy, on a descending glide path, weight is counteracted by both lift (along the flight path) and drag (perpendicular). Talking aerodynamics can get messy because whenever one tries to say something brief, there are often a bunch of "fine print" conditions and explanations that go along with it. I only brought up the lift issue to show how John Sherman was being confusingly imprecise when trying to teach about lift. We all talk about "lift" and "drag" in a very non-scientific way at times, which is OK, but we have to be more careful when trying to explain things precisely. For example, say a jumper has a bag locked main canopy. Not much drag there!, we say. But technically that's wrong. If the jumper and gear is 200 lbs, then he's got 200 lbs of drag between him and the pilot chute, just that that equilibrium exists only with him plummeting at 110 mph. It's not that he needs more drag, because he'll have the same 200 lbs landing under a big round parachute. He just needs something that'll give him 200 lbs of drag at 10 mph or less descent rate. (Or an equivalent upwards combination of 200 lbs of lift & drag if he's got a square reserve.) So the guy under the bag lock needs "more drag" only in a casual, you-know-what-I-mean kind of way -- more drag per unit of speed than he can achieve with a bag lock.

The scene: Last Twin Otter load of a busy day, pushing things a bit, half hour plus after sunset. The guys with the Neptune altis on the load think they're smart because their altis have back lights. Problem is, in the dark during the climb they can't read the menus to figure out how to turn on the back light.

Sorry, but yuk! That's not a good document. I agree that we don't have an absolutely clear consensus on what the best practices are, even if we have concepts that are reasonable to follow. But Sherman's document that is quoted has serious errors. Some of the aerodynamics is correct, but things like the following are wrong: It is totally wrong to say that the air on top needs to be faster to meet the air below. Molecules next to each other at the nose of an airfoil don't meet up again at the tail. For a plane slowing towards the stall in level flight, the lift stays essentially the same. The lift coefficient, however, increases. Sherman misses that distinction, muddying his explanation. The pressure difference top to bottom on the airfoil has nothing directly to do with the strength of the boundary layer on top of the airfoil. That varies from nose to tail. And the boundary layer is in no way "strong" close to the stall -- that's where the airflow is about to separate from the top of the airfoil, decreasing lift, increasing drag, and creating a sharp nose down pitch moment -- you know, a stall. And it isn't the boundary layer itself that "blows off" or separates from the airfoil in a stall. It's the division of the airflow between the boundary layer and faster moving air that moves away from the airfoil. It's a very imprecise use of the term. So with his amateur understanding of aerodynamics the first section of the document isn't of much use. As for later parts of the document dealing with flying in turbulence: Overall, Sherman recommends flying near the brakes-set position in turbulence. I think that is too deep on typical canopies, where the brake setting might be 1/3rd brakes (Clearly brake settings vary a lot by canopy, from zero brakes to the edge of stall, but most are, what, maybe in the 1/4 to 1/3 brake range these days.) If there is debate, it should be between flying at say "1/4 brakes" and flying with "zero brakes except all slack taken out of the lines and just a little pressure applied to maintain a feel for the brakes". And there can be debates about how flying in one way might reduce the chance of a collapse (e.g., full speed, or in a turn with added g loading), but if there is a partial collapse, such flight may make it more difficult to recover in the altitude available. Tradeoffs! John Sherman's idea of brake set position may not be a bad one for actually recovering from a collapse or stall, but even then one could debate between that and a traditional stall recovery position with brakes not far above the stall point.

Wow, hadn't heard anyone mention that in years. History & Trivia! It really was a nice little RW training book, with all sorts of practice dives organized in sequences, by different levels of difficulty. The Sport Parachuting Council of Ontario organization basically disappeared in the mid '90s, and Paule has long been out of the sport (but did show up for a reunion a couple years ago). I don't think anyone is going to mind if one uses it now -- but Andre should be able to contact her. She'd probably be happy to know that someone remembers it!

So far it doesn't look like there's much understanding of what is meant by biomechanics. In a way, everyone learns and teaches biomechanics. Legs tucked up and arms out = backsliding due to the location of drag vs. center of mass. Or a little more advanced idea is that spreading the legs more makes it harder to arch at the hips. That may be biomechanics, but just isolated facts applicable to skydiving, not some broad understanding of how to apply sport biomechanics to skydiving. In my CSPA coaching course a decade plus back, we learned things like breaking down sports movements into sections by time and role (eg, things like initiation, force production, follow through) or by body part. Those sorts of things should qualify as applying biomechanics principles to the sport. It would be interesting to hear -- maybe after you collect more responses -- what you see as useful biomechanics for skydiving, or what kind of things you learned from the USPA on biomechanics.