pchapman

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Everything posted by pchapman

  1. That's the way it works at the C-182 DZ I frequent. (With a ceiling check exception that's already been mentioned, if the DZO OK's the flight.) It's easy for the jumpers to discuss in the air whether to ask the pilot to try for more altitude, and together take the risk that they'll pay for high but jump low. But what typically happens for a Twin Otter, where it's harder to have a meeting and arrive at a consensus? On the loads I've been on, it seems to have been the pilot's decision and you trust he'll try for a good judgement. Or is there an agreed plan before leaving the ground, that isn't changed in the air? (As mr2mk1g mentioned.)
  2. One study showing current draw for a Cypres is at http://www.pcprg.com/cyprespc.htm It doesn't sum up the current draw over time, but one can easily do that mentally. (Eg, when the LED is on, the current may be high, but only for a few seconds.) If the data is accurate, it suggests turning the Cypres off at the end of the day is worth it -- both for a flatland jumper and especially for the person driving home in the mountains.
  3. Years back I made some weights myself using steel shot from a hunting store. It had the advantage of avoiding lead dust, but because of the lower density, I ended up with pouches of less than the standard 1 pound per bag. The steel wasn't perfectly clean either, as it was rusty coming out of the bag. I put the shot in small plastic bags before it went into the pouches, but that also increased bulk. There are weights made with about 1 inch square plates of lead, avoiding much of the lead-on-lead grinding problem. I don't know the source of the lead. About four are sewn in a row into a pouch, making a beautifully flat weight, which is very comfortable in a vest. (Bulk isn't as critical for the often superior weight belt.) The flat weights I saw were built & sold by John Moore of Alberta.
  4. A friend had a compatibility issue when using a Vigil AAD in a Mirage container. The Vigil cutter (unlike the Cypres cutter) has a plastic sheath that, I am told, protects the loop where it passes through the cutter. After having been installed for just one packing cycle, the Vigil cutter's sheath was found to be cracked and broken when the rig was opened for a repack. This was on a very small, tight rig. The Vigil company is sending the jumper a free cutter, but didn't propose any long term solution. I presume they are looking at the issue. The apparent cause: On a Mirage rig (with the cutter moved due to the PSB1204 service bulletin), the cutter is pressed between the hard cap of the pilot chute, and the various hard flaps & grommets stacked above the cutter. On many other rigs, the cutter is not squeezed between two hard surfaces (E.g, it might be between a flap and the soft deployment bag). I don't know to what degree the Mirage's optional new concave topped pilot chute might be an improvement. I haven't seen the cutter yet, and don't know with what frequency this problem has happened.
  5. Just food for thought: A DZ I jump at just moved to a location where the land around the DZ is 200' higher in some directions, particularly towards the most common exit spot. The DZO is suggesting Cypres' be set for a landing offset of 200', but is leaving it at the jumpers' discretion. (Student reserve FXC's will be set to 1200' rather than 1000' as before.) Just curious what other DZ's have done, that have similar or even higher terrain near the DZ. There are obvious arguments both ways. (Plus some less obvious ones about altitude offsets that require careful understanding of one's AAD manual.) With 200' difference, realistically I expect most jumpers won't bother to set an offset. (It's a piston Cessna DZ too, so we don't have as much altitude to play with to begin with.)
  6. Anyone have an electronic copy of the manual for the Westway Innovator I container? I know there's an "Innovator.pdf" file out there somewhere. I've got the Innovator II manual but discovered that for some reason I didn't have the Innovator I manual.
  7. To the end of '05, 501 repacks 18 known saves The first 2 saves were when I made a poor dock on a buddy during a CRW dive. At least it cost us nothing to get packed up again. I have been fortunate to receive a few good bottles from customers.
  8. It seems we're talking about that last aft bit of stabilizer, between the D lines and the trailing edge. Where I jump, it is known as the "J", from its shape when students are taught to flake it when flat packing their mains. I don't know if anyone else uses the terminology. It has always been logical to me to flake it to the outside like everything else.
  9. The Aussie document is about taking wheelchair dependent people up as tandem passengers, leaving the chair on the ground. I understood that the RWS document is the same type of thing.
  10. Re - tandem skydiving with 'wheelchair dependent persons' Yeah I tried contacting RWS a couple times about it last year. (Starting before the whole falling-out-of-the-harness accident thing.) They said they had misplaced the paper copies during a move of one of their departments, were planning to get it updated or something, but I never heard back. One document that is floating around somewhere is the 1995 "Tandem Skydiving with Wheelchair Dependent Persons" by Paul Murphy from Australia. It's a 128k Word file that has some recommendations plus results from a survey of DZ's for their policies and procedures. I have a copy which I might have found on dropzone.com during the long discussions on that accident last year. It has some useful info for those of us who aren't familiar with all of the complications of spinal cord injury etc.. As for procedures to use, the document discusses a few different techniques, but still leaves it open to judgement what is best or sufficient in any particular situation.
  11. To address one of the topics in this thread, let's have a look at the issue of scaling people up and down, and how that affects wing loading and stresses on the jumper. It is useful because the effect of scaling laws are non-linear, and as someone said, we don't naturally think very well in non-linear terms. While this stuff should help to understand the effect of scaling alone, there are of course other at least as significant factors that have already been mentioned, like jumper skill, body position, and body shape. People probably don't scale up and down perfectly evenly (such as in the percent of muscle mass, or proportion of torso to limb length, or whatever). The proportion of skinny or more solidly built people may also vary depending on their height. ========= The basic physics of scaling are that volume (and mass) increases with the cube of the length, and area increases with the square of the length. As an example, consider a 6 ft tall jumper vs. 5 ft tall jumper, OF THE SAME BODY SHAPE: The taller one is 20% taller, would have a 44% larger wing area, would have a 73% greater mass, and therefore would have about at 20% higher wing loading. WING LOADING EFFECTS: This square & cube law thing is the primary reason why ants hardly notice a long fall, a mouse may brush itself off uninjured, a cat might limp away injured, while a human goes splat. The 20% extra wing loading translates into more speed at a similar glide ratio. Basic aerodynamics then says the glide ratio doesn't change. But additional factors might change things. (It isn't quite the same situation as for a jumper under a parachute, where the same person under a smaller, higher wing load canopy still has the same body drag area, same line thickness, etc. as under a larger canopy, like tso-d_chris wrote about.) Who knows, there might be some small tendency that at the same wing loading, a larger wingsuiter would have a better glide ratio. This is like swooping with weights and a larger canopy at the same loading. Crinkles in the fabric may not tend to scale up much, harness webbing stays the same size, etc., so their disturbance will be proportionately lower for a larger wingsuiter. MUSCLE STRENGTH EFFECTS: As for the comment about small people being able to outdo the tall & skinny, by better being able to hold a good body position, one can keep on playing with the effects of scaling. The taller person's arm is longer, the load is greater, but the shoulder size and muscle size are greater too. In the end, there's both a 73% increased muscle mass and 73% more force needed. But that doesn't even everything out, as the cross sectional area of the muscle has only gone up 44%. So the stress per unit cross sectional area is 20% higher on the taller person's muscles. Even without knowing the biological details, I expect that that is important to what force the jumper can apply. It goes right back to the idea of volume being cubed, but area squared. (I did run basic engineering calculations on this to confirm the numbers. The jumper's arm can be considered a cantilevered beam supporting a spanwise distributed load, with a muscle applying an inwards counteracting force at the bottom of the root of the beam.) So the scaling laws suggest the bigger person person is at a strength disadvantage, if the person is of the same body shape and proportions as the smaller person.
  12. As a comparison to the Vigil manual quote, the current Cypres manual does not specifically mention pressures inside the potential firing zone, but just says: "When using a pressurized aircraft, make sure that the cabin remains open when the turbines are started up. Leave a window, a door, or the ramp open a bit until after liftoff. This will ensure that cabin pressure cannot build up above the air pressure on the ground." That's the only direct reference to pressurization. It also states: "An aircraft must never descend to altitudes below the elevation of the airfield of departure." (There are a couple other statements about descending below the landing elevation, if the Cypres has a landing altitude offset entered.)
  13. Yeah, same as when backing up under a big square canopy in high winds. A short, sharp flare just before touchdown reduces the vertical speed without making one accelerate much backwards. (The dynamic swing-forward during a flare also helps in that regard.)
  14. Comments on all of this thread: -- Brian wrote: Excellent idea. This both gets the speed up AND keeps the angle of attack up*. (See footnotes for the asterisked items.) When flying faster rather than slower, the effect on the wing's angle of attack by a given sized gust is reduced. Having a higher angle of attack is beneficial when worrying about a collapse resulting from the front of the wing being pressed down rather than lifted up. One caveat for others would be that the entry into the diving turn should be smooth. I wouldn't want to either toggle whip or front riser too suddenly into the turn, which could for a moment reduce the angle of attack of the canopy to a low level, making it especially vulnerable to turbulence induced collapse at that time. -- Brian wrote: Also a good idea. This matches the idea in paragliding of "flying actively" -- keeping the wing over one's head by small, quick inputs, rather than letting it bounce around in turbulence, doing nothing until there's an actual collapse. -- Someone else wrote: Note that this is untrue for a parachute, which is in a steady state descending glide. It only works for a powered airplane that is flying level. When the flight vehicle is descending, the Weight is exactly countered by the vector sum of Lift and Drag. Lift is defined as acting perpendicular to the direction of flight. So if the parachute is descending at an angle 20 degrees below the horizon, Lift is tilted 20 degrees forward from the vertical, and drag is elevated 20 degrees from straight back horizontally. With gliding parachutes, we're actually held up in the air by a combination of Lift and Drag. -- Brian agreed with someone's comment that "in straight flight your line tensions must add up to your suspended weight". In a general sense, yeah, they'll typically add up pretty close. But the statement is wrong, even according to the most basic balance of forces calculations, because 'straight flight' for a parachute is 'descending straight flight' not 'level straight flight'. See the Weight = Lift + Drag explanation above. -- Brian wrote: and later I'll have to argue that that's simply wrong from a physics point of view. Whether the canopy is big or small, if one is descending at a constant rate (not accelerating) then one experiences exactly 1 g of force. But I think what Brian was talking about was not that we experence less than 1 g, but that there's less than 1 g of line tension. Here's my explanation of what I think he means: (Correct me if I misunderstood, Brian.) One can feel slighty lighter in the harness with a steeply descending parachute: The worse the glide ratio of a canopy, the more that drag contributes to carrying the weight, rather than the lift. This goes back to the Lift + Drag = Weight in vector terms. An extreme example would be someone flying a 25 sq. foot canopy, as used to fly formation with a wingsuit flyer. The guy under the small canopy would be in a steep dive (compared to under a regular parachute), going very fast, with a lot of wind pressure on him. As both Drag and Lift are counteracting Weight, the Lift is less than the suspended Weight, so there's less line tension. The jumper doesn't have nearly his full weight supported by his leg straps, but is also supported by the air pressure on his body. This is an effect of the angle that the canopy is pulling on the jumper, relative to the vertical, rather than an effect of canopy size in itself. -- Brian wrote how turbulence effects on a canopy depend on: It's a good point, that reduced time of exposure to turbulent air will reduce the total risk (As long as increasing speed does not increase the risk from a given turbulent gust. Which it doesn't according to the arguments in this thread.) Brian had a good response on the difference between an airplane and a parachute, where we under parachutes are concerned about too low an angle of attack, collapsing the parachute, but are not concerned about overstressing our flight vehicle. Brian had been responding to: That's a large part of the issue of why flying faster can help. A given sudden change in what the air is doing, will have less of an effect on a wing flying faster than one flying slower.** It's good to remember that turbulence can change the motion of the air in any direction. So we could have the sudden change in air motion be horizontal, or vertical. Looking at the case of a vertical gust: When flying 25 mph forward, the effect of a 5 mph down gust has a larger effect on the angle that the airflow is hitting the canopy, than if the canopy were flying 50 mph and encountered the same 5 mph down gust. The effect is about halved. (A change of 5.7 degrees angle of attack, vs. 11.3 degrees) A major complicating factor is what the angle of attack of the wing started out as, before the gust. If the wing starts at a low angle of attack, it takes only a few degrees reduction in that angle before the canopy collapses from a negative angle of attack.*** If the wing was flying at a higher angle to start with, there's a greater reduction of angle possible, before the canopy collapses. So if comparing the slow and fast canopy, it would be more dangerous to go fast, if the angle of attack of the fast canopy started out a lot lower, despite the change in angle of attack from the gust being reduced due to speed. Figuring out that trade off requires a more full understanding of the way the whole parachute system flies -- lift, drag, trim angle, line lengths, pitch coefficients, etc. Without trying to crunch numbers, I'm guessing that parachutes are usually flying at a reasonably high angle of attack, even in normal flight, brakes off. Here I don't have any simple answer from theory. We also could consider two different slow vs. fast situations here, that shouldn't be confused: 1) Should one fly faster versus slower on a given canopy (by using no brakes or partial brakes); or 2) should one fly under a big slow canopy, versus under a small fast canopy. These are four different angle of attack situations. I don't have proof or clear evidence even for myself, but my feelings are that for #1, the faster speed should have a lower angle of attack, as would be true for a regular aircraft. For #2, both the large and small canopy could well be designed to fly at the same angle of attack in normal flight. Then that really is in favour of going fast by using a higher loaded canopy. Whatever the situation, if one starts with a reasonably high angle of attack, then that would fit well with what others believe to have observed: Then it is beneficial to stay fast to reduce the effects of a gust. One example of where the trade off likely isn't worth it, is front risering on approach. While that also involves the complicating effects of a massive distortion of the airfoil, it may significantly reduce the canopy's angle of attack. The gain in speed isn't worth the reduction in angle of attack.**** I am still vaguely uneasy about the tradeoffs for normal canopy flight. Old canopies seemed to be trimmed to fly fairly slow, with a high angle of attack, while some fast modern ones must be trimmed fairly nose low to make them "ground hungry", to give them extra speed for the swoop and flare. (E.g., one early example was the floaty Stiletto vs. the divier Jedei. Brian could probably comment here...) The ground hungry trim would imply a lower angle of attack in normal flight. How much, I don't know. A lower angle of attack in normal flight should make collapses more likely. In practice, it doesn't seem to be a problem, but where's the limit? Is this something canopy designers have had to deal with? Flying fast becomes better when one adds in the idea of flying at above 1 g in a descending turn on approach, because increased g loading requires a higher angle of attack. -- A point of my own: There's the whole discussion of whether to hold a little brakes in turbulence or not, which has come up in other threads over the years. While Brian's line of thought suggests not to use brakes, there is one very particular case where "pulling a little brake" can be beneficial. That's when one is only removing slack from the brakes. So one is "pulling brakes" from the point of view of the control toggles, but not from the point of view of actually deflecting the tail of the canopy. Taking the slack out of the system allows two things: 1) It removes the slack that would make quick, precise control applications more difficult. 2) It allows the pilot to feel the pressure on the brake lines, which can be a guide to how well pressurized the canopy is, providing better feedback to the pilot. -- DuckDodger wrote: Yeah, the standard answer is inertia. A given gust will exert a given force on the wings of the plane, but if the plane is loaded up to a higher weight, the acceleration produced on the aircraft will be lower. (Acceleration = Force divided by Mass) So the g loading on the plane is less at higher weight. (And it is common, but not universal, for the allowed g loading to be the same for the aircraft at any allowed weight.) -- A follow on issue to Collapses & Turbulence is how to best react to a collapse. Brian touched on it, although it could be a topic for another thread later. One type of collapse is where one side of the canopy folds under, another is general deflations (whether due to a stall at one end or a negative angle of attack at the other). Brian obviously has plenty of practical parachute flight and design experience, but I sometimes think the physics of his technical explanations occasionally aren't quite correct, or at least aren't quite clear. But I'm sure others agree that clarity, comprehension, and correctness are also problems I struggle with when tackling these kind of issues! Footnotes: * The angle of attack is the angle at which the airfoil meets the oncoming airflow. It is sometimes incorrectly thought of as some form of trim angle (angle of the airfoil vs. the suspended jumper) or some sort of glide angle relative to the horizon (where someone says, "that ground hungry parachute has a really steep angle of attack"). Usually one can just talk about angle of attack without getting into the detail of what exactly zero angle would be. That can either be defined aerodynamically (where the airfoil has zero lift) or geometrically (such as by conveniently saying the canopy would be at zero angle of attack if the airflow is approaching parallel to the bottom skin of the canopy). Lift is pretty much proportional to angle of attack. Double the angle of attack, double the lift. This works up to the point where the canopy begins to get close to stalling. ** Regarding the way that a wing reacts to a sudden change in airflow: There are also inertia effects, and stability issues, which affect how quickly a flight vehicle can react to the change in airflow. Sometimes more inertia is better, sometimes less is better. Sometimes you want the vehicle to 'punch through', sometimes you want it to 'adjust faster' to the air. ***I'm using "negative angle of attack" to signify that the nose of the parachute is being 'pushed down' and the nose of the canopy collapses. That's a useful simplification, although technically with a ram air parachute, one has to consider more than just overall lift on the airfoil, but also pressure coefficients relative to the openings on the nose of the canopy, the airflow stagnation point, etc. Canopy collapse does not need to happen exactly at zero angle of attack. **** Supporting this argument is that paragling pilots are well known to be much more susceptible to canopy collapses from low angle of attack, when using their speedbar, which is a sort of efficient front risering technique.
  15. I'll expand on that one -- HTC 9014.20.8080 - AERONAUTICAL NAVIGATION APPLIANCE is the one that SSK uses for Cypreses. The standard way to ship a Cypres from Canada to the US (& back) for maintenance is to do it UPS, using SSK's own account. They then bill you along with the maintenance at the end. Details at www.cypres-usa.com under the headings: "Freight Collect" Shipping to SSK from Canada and Good News for Our Canadian Customers! When going into a shipping office, you have to say right away that it is going on the recipient's account, otherwise they have you fill out all the wrong forms, trying to set up your own UPS account, which isn't neeed. You still need to know what to choose to insure the Cypres for. And it still takes a lot of time to fill out all the paperwork. I use that method since SSK recommends it (as a good compromise of security and cost), but I haven't compared it to other ways.
  16. Good advice so far. I've used the postal services between the US and Canada quite successfully. But it's best for lower value items, due to insurance limits: Canada Post insures to $1000 Cdn only, and USPS to $675 US only (for Parcel Post which includes Global Airmail.) The value for insurance purposes can of course be different than the value for customs, which may be $0 if "returned to owner after repair" etc. (Although technically even after a repair, I think one should be paying tax on the value of the repair services?)
  17. Maverick 200 or Titan 265 Both old F-111's that fit into spare rigs. Boring, but at least less important to pull up high.
  18. Finally an all purpose scary-story thread! Great for wasting time, writing or reading. Regarding the kid who grabbed the wrong rig: Even though one may think "the kids today...", it reminds me of a story from the 80s, from the rigger I apprenticed with: This rigger does some work on a customer's gear, repacks the reserve, and leaves the gear at the DZ for pickup. The customer later comes by, gets the rig, and goes jumping. Maybe a week or two later the customer sees the rigger again and says, "I want to pay for the work, but I didn't see a bill when I got the rig back. How much do I owe you?" The rigger replies, "I left the bill under the reserve cover flap."
  19. I think more emphasis should be on the issue of the burble, than the simple linear distance between main and reserve bridles. I like the idea of going a little shoulder low when deploying a reserve past a PC in tow, to angle the reserve PC launch outwards. The idea is not so much to get the reserve PC pushing out into the airflow to get away from the main bridle. I don't think the PC will get very far to the side before it gets swept away by the strong relative wind from below, especially if the firing angle is still close to vertical. Instead, the idea is to get a consistent launch, placing the PC into cleaner air, instead of accepting the randomness that sometimes happens when a PC is deployed in a burble. (Of course things one theorizes at the computer aren't always practical when altitude is disappearing fast. But I could image going a little left shoulder low when my left hand came in to the reserve handle. After a good launch, I'd theoretically try to get my shoulders level again for a more symmetrical reserve inflation.) While many, many times reserves have fired past PC's or other junk in tow, there are enough cases where they haven't. One hears of one bridle 'wrapping around' another, something that might have started with a PC whipping around while making its way up through the turbulent, churning air of the burble. I'm not sure to what degree spring loaded pilot chute hesitations (due to the burble) have lessened using modern high strength pilot chute springs. I've seen some nasty AFF videos with a PC bouncing all over the student (although its a big burble with 3 people). To appeal to higher authority, I'll throw in a quote from the Great Bearded One (billbooth): It makes me think!
  20. What stuck in my mind from the manufacturer's video on the Z-Po was how carefully it was packed. While I don't recall the specifics, it may have shown the nose being carefully rolled, the slider carefully positioned, and the tail carefully and tightly rolled. All that seemed to be a big hint about how the thing might be expected to open! (Not unexpected really, for a Sabre-like canopy from the early Sabre era.)
  21. What hasn't been discussed much in this thread has been the degree to which the downsizing 'guidelines' were intended to be MANDATORY, ADVISORY, or something in between. I'd feel really sad at the loss of opportunities to newer jumpers if any downsizing chart became MANDATORY. If it were ADVISORY only nobody could be stopped from doing anything, but the chart would still be a useful teaching & learning tool if it is well thought out and supported by the industry. Some instructors (like myself) who started some years ago, never did go through a proper downsizing progression, making it more difficult to make specific recommendations for newer jumpers. Some industry guidelines would be handy to 'back up' anything one tells a newer skydiver. A recommendation to a newer jumper would be less likely to be seen as a personal attack. If rules were mandatory, I'd want to see exceptions allowed, perhaps using sign-offs by senior instructors, perhaps only for limited periods of time in order to 'try out' a canopy. Abuse of the system would be a potential problem. Jumping a canopy 'a few times' is different than jumping it 'all the time'. A jumper may be more conservative and not complacent with a canopy they are trying out for a short while, which helps keep the risk from rising too much. Even if you argue that the risk per jump is high, the total added risk may be relatively low because of the limited number of exposures to that risk. My own experiences are probably not much different than that of others who started in the sport more than 10 years ago: For example, at 200 jumps, with only 11 jumps loaded over 1:1, I got the chance to borrow and jump a Jonathon 92 loaded at 1.8, on a hot, no wind day. It was great fun to do a few jumps, although my run-outs weren't the prettiest. An ideal situation? No. Fun, exciting, and a good learning experience? Yes. Had downloading rules been in effect, I could never have made such jumps. I'd feel guilty, sad, and hypocritical if I automatically denied others from having that kind of experience! These are personal feelings only, biased by my own experience. They may not necessarily be appropriate these days, but I hope someone else out there has similar thoughts.
  22. Sometimes any additional qualifications or achievements one might have in skydiving can be spun in a much more positive way than just being a weirdo skydiver. For example, instructor ratings, a rigger rating, or success at competition. A specific achievement sounds better than general skills, as the acheivement is better condensed into a few words on a resume. (E.g., it's hard to explain how ones good qualities resulted in great sport skills given X number of jumps. It's easier to say that one competed in the Open category at the National Championships, whether or not that really meant much.)
  23. Good point - I didn't realize there were different versions. Just doing quick Google or Google Images searches brings up a few photos, that show a 3-lobe version of the original Barish wing being jumped as a parachute and a 5-lobe version paraglided. (Eg, at http://www.llab.se/kgp/dvdoc/barish/barish.html)