DaVinciflies

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Everything posted by DaVinciflies

  1. Deployment Wind Can you expand a little on this, or confirm that you are joking? Thanks.
  2. I found the link. Maybe Peek would like to chime in here! www.pcprg.com/sliderdragcancel.pdf
  3. Dave - I believe I remember reading some work where the investigators theorized that the air pressure between the slider and the bottom skin of the canopy was lower than the force on the underside of the canopy so the slider was experiencing lift in a similar way to a wing. They proposed that anything which causes a premature "leak" between the slider and bottom skin allowing ingress of high pressure air would disrupt that equilibrium and cause the slider to descend faster. This would seem to explain the effectiveness of domed sliders. Does that sound familiar to anyone?
  4. Sorry to disagree, but the way I meant it, bag strip is when the canopy is partially or completely out of the bag before line-stretch. What you have described, to me, would be line dump. Or, in the case of a reserve or semi-stowless bag, totally normal!
  5. A few things that spring to mind: 1. Slider position (and ensuring it is uncollapsed!) 2. Avoiding bag-strip 3. Canopy choice - certain models have a reputation for slammers. 4. Modifications (e.g changing the slider for an RDS) could change the opening characteristics. 5. Skydiver airspeed at deployment (e.g premature openings)
  6. How much paper are you taking that you have to use a barrow to get it home?
  7. I read this as "maintain outwards pressure on the riser groups without pulling down on them". Is that a fair interpretation? [This is what I do during my openings.]
  8. I may be missing something here, but here is my understanding of what happened - please feel free to add your interpretations if different: 1. Bridle wrapped main PC reducing drag enough to fail to pull main pin. 2. Reserve ripcord pulled 3. Reserve PC wrapped main PC 4. Combined drag of main and reserve PC pulled main pin 5. Main deployed faster than reserve resulting in reserve bag being hung up in the lines of the main. Where I was going with the MARD idea was that if the jumper had cutaway the main it MIGHT have pulled the reserve PC/bridle/freebag assembly clear of the reserve which could then have deployed. On the other hand the main lines could have remained wrapped around the reserve in the bag and then the situation would have been much worse.
  9. Just to play devils advocate, those same variables could contribute to a higher terminal velocity at pull altitude. Might that affect the openings? (Not that I am suggesting that this is the answer, merely offering a counter-point to your assumptions). If you want to get really speculatory, how about: The electrostatic charge in the pre-storm atmosphere coated the canopy in electrons which all repelled each other when the canopy came out of the bag!??!
  10. The physics behind it is centrifugal force: Fc = m*v^2/r Fc= centrifugal force (can be used to figure Gs) m = mass (kgs) v = velocity (m/s) -- need to convert from RPMs to m/s r = radius (m) Cheat calculator at : http://www.calctool.org/CALC/phys/newtonian/centrifugal Shorter lines say, 1m, 1 RPM 60kg diver Fc = 60 kg * (6.28 m/s)^2 / 1m Fc = 2368 Newtons Gs = 4.02 Longer lines, say 4m, 1 RPM 60kg diver Fc = 60 kg * (25.12 m/s)^2 / 4 m Fc = 9474 Newtons Gs = 16.1 The short of it is, take the difference in the lines and multiply by 4 and that will be how many extra Gs the longer set will have, given same RPMs. You have calculated your velocities at 60 RPM (1 rev/second), not 1 RPM as you stated in the example. At 1 RPM the forces are negligible!
  11. He might have been better of as the reserve might not have entangled his main, and the departing main entangled with the reserve pc could have acted like a MARD/Skyhook. [might/maybe]
  12. Not everyone agrees with this statement. http://www.youtube.com/watch?v=4Nsca5add8g&feature=player_embedded
  13. I believe that Mike Gruwell at Chutingstar has an escrow service for this kind of transaction. And if you can't trust him, then you can't trust anyone!
  14. Are they doing this in response to TMs reports of hard openings? I see a lot of people who shove the nose into the packjob and don't really have a good reason for why they are doing it.
  15. Thanks for providing the facts. I am glad both jumpers are OK. That was scary as f**k.
  16. Glad you get it - and well done for manning up and saying so in public. No harm done!
  17. Not a great example, particularly this year. There is probably considerably less canopy traffic in downtown San Francisco than Perris.
  18. You are so far out of your depth. The example above has nothing to do with what we are talking about as far as I can see, but you are still wrong. The valve stem momentarily has a zero vertical vector to its velocity, but it continues to have a positive velocity due to it's horizontal component, which coincidentally is maximum right at the point where you say it is zero. Not that this has anything to do with the discussion. As for the Top Gun Goldfish - NFI (no fuckin' idea).
  19. Maybe if you make a jump and experience a flat turn or spiral you will understand. Your description of what would happen - just does not happen! You keep relating things back to the ground which, until you come back into contact with it, has NO BEARING on how objects in flight behave. (ETA: for the sake of completeness with the exception of things like object-induced turbulence and thermals) A given canopy/jumper combination in full flight will have the same airspeed whether you are going upwind/downwind or crosswind. When you lose contact with the planet's surface, "wind" ceases to exist for all practical purposes and all that matters is relative wind. ie. the movement of air relative to the wing. If I took a fish in a tank in a 747 traveling at 500mph over the ground, would the fish have to swim at 500mph to keep up? Would it get "whipped round" if it tried to do a 180? That fish's water environment is exactly analogous to the air-mass we move in under canopy. Out of interest - have you ever tried your flat-turn experiment?
  20. Your grocery store makes $2 bills??? Is that entirely legal?
  21. You seem to be very confused between airspeed, groundspeed and "physical speed" what ever that really means. An aircraft flying at 40kts into the wind then doing a 180 will continue to have an airspeed of 40 kts. Its ground track will change, but the wings will not "know" that it has gone from flying upwind to downwind. Wind as we use the word is air movement vs the ground. An aircraft in flight no longer has reference to the ground as it is moving in the airmass. You might as well be saying that I am moving at 490,000 miles per hour as I sit here. Because relative to Galactic Central Point I am (more or less) but its a meaningless comparison.
  22. I will leave the discussions as to what you should have done to others more qualified, but you might want to take a look at this video by Brian Germain from a prevention point-of-view: http://www.youtube.com/watch?v=axCeYlY_6io
  23. It allows the risers to spread apart further (assuming you have brought your slider down to the base of your risers). This allows the canopy to spread further and assume a flatter shape which is more efficient at producing useful lift. This extra useful lift can help in many situations including (but not limited to) swooping and getting back from long spots. Imagine a canopy with the slider stuck near the top of the lines - it will not be flying as well as one with the slider all the way down the lines on the links. This is just an extension of that really. Bringing the slider down and loosening the chest strap is by no means something you have to do. It is a preference for some jumpers. ETA - beaten to it by mchamp whose answer is much more complete and in-depth than mine.
  24. Ask yourself this question: When you are flying your canopy with the wind (ie in the same direction as the wind) what direction is the relative wind coming from? Do you feel it on your face or on your back? Think that through and you will have your answer!
  25. I find it a little less convenient to loosen my cheststrap after I release my brakes, but the potential safety gains are worth it in my opinion. I dread to think how hard it could be to find my handles with my 36" cheststrap completely loose. Please consider changing your procedures if you can to loosening your cheststrap last. As has been mentioned up thread, this has been a real problem for some and is not just theoretical.