obadz

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Posts posted by obadz

  4. Quote

    PEPPERELL -- The History Channel (THC) was at the Pepperell Skydiving Center in June, shooting footage for a new series called "The Works."

    The upcoming episode will feature a tandem skydive, wingsuit flocking and wind tunnel flight.

    To capture the show's footage, host Daniel Wilson flew as an observer on a Twin Otter, catching footage and calling play by play as wingsuiters exited over Skydive Pepperell. He later made a tandem jump.

    Wilson, who has a PhD in robotics, has written five books and is a columnist for Popular Mechanics magazine. Running the gamut, the new series will have episodes on garbage, beer, motorcyucles and tattoos, among aother subjects.

    According to the THC Web site, the show will begin airing Sept. 19 at 8 p.m. on The History Channel (Charter channel 54).

    In Pepperell, the crew filmed a segment about the wing-suit jumpers (aka Birdmen), and Wilson did his first skydive. Pepperell skydiver and photographer Phil Roberson shot the freefall footage, which will be used on the program.



    http://www.nashobapublishing.com/ci_10445906

  5. Quote

    >just out of curiosity, what didn't you like about the Crossfire?

    I have a Crossfire2 109 and I like it, but honestly the Safire2 109 I've jumped flew about the same, even had slightly less riser pressure, a bit longer arc and more consistent openings. If I had to do it over again I would have gotten the Safire2 and saved the $100.



    I'm going to have to disagree here :)
    I've jumped a safire2 149 and a crossfire2 149 at loadings that are close to ozzy13's. I didn't do that many jumps on either and I don't have billvon's experience but I found the riser pressure to be high on both and the recovery arc to be short on both (compared to a samurai of same size), but on a relative basis, these properties were much more pronounced on the safire than on the crossfire.

    Care to get your hands on a safire other than that demo from square1 and verify that what your observation isn't just specific to that particular canopy? ;)

  8. Quote

    I know what we do is dangerous and in the end we will have incidents that result in death. But why not try to make it as safe as possible and in doing so we just might save someone's life? Who knows, could be yours or mine that gets saved.



    Hey don't look at me like that: I jump an RSL ;) I'm not arguing against it, just against the regulation aspect of your post.

    That doesn't mean I want to force that onto anyone else. If some guy is going to jump with his RSL disconnected on every jump, who am I to decide he should have some piece of gear flapping around his mudflap?

    If some swooper is going to jump with his AAD off, who am I to decide he should carry some piece of expensive electronics that'll add to his weight and pack volume?

    Regarding the people on the ground: my incentives are aligned with theirs: I don't want to fall onto them any more than they want me to. So now the question is, if there is a piece of equipment that would help reduce the probability of that happening, should there be a rule making that equipment mandatory for every skydiver? You could argue that. And you could probably make decent case for mandatory AADs (which many countries have done). But if so many highly experienced people disagree on the merits of the RSL, I don't believe it's such a clear-cut case.

    Has there ever been a case of a skydiver hitting someone on the ground in freefall? If so, would an RSL have made any difference? I think the "reasonable extent" qualification I used applies here.

  9. Quote

    [...] But there has to be some regulation in everything we do. [...]



    I disagree. It's very easy to determine when regulation is required: "Your freedom ends where my nose begins."

    In my humble opinion, as long as you're not affecting others, there's no need for what you do to be regulated. And my wearing an RSL (or not) really does not affect anyone but me (to any reasonable extent).

    Shooting rockets up evidently affects pilots which is why it needs to be regulated.

  10. Quote

    Quote


    I was told it was mandatory (from the manufacturer)to have an RSL install on Javelins unless it was waived by a master rigger.



    [...]

    It's time that there is some requirements in this country other than you have to have your reserve repacked by a master rigger. There are other countries that actually require an AAD. Why not RSLs as well? [...]


    Oh yeah, regulation is definitely the answer to that one :S

  12. Quote

    There's certain minimum L/D required for canopy to naturally planeout after a dive.



    You again ;)

    I guess we should discuss this at the DZ because I don't immediately see how L/D affects recovery arc length and are you talking about L/D of the canopy alone or including the jumper? I would think the main factor would be drag on canopy vs. drag on jumper, how far forward the center of lift is on the wing at high speeds and line length.

    In any case, I returned the canopy in question and I'm demoing canopies with longer recovery arcs in hope of buying one before the end of the season. This weekend, it will be a Samurai 150.

  13. Quote

    My point here is that every canopy has a natural tendency to recover.....to it's natural full flight, or even a little less steep angle...until it gets back to it's natural full flight when the speed slows down. But none of my canopies has leveled out to level flight even for that short period of time.



    I've had canopies level out after a turn with no input or even climb a bit.. I have to say I found the latter most annoying.

  14. (Note to Ian: I'm replying in that thread to keep the posts together. I'm assuming you will move us into a different thread as you see fit)

    Quote

    [...] (since the speed changes approximately proportionally to the square root of AoA inversed). [...]



    I'm getting to similar results. I will work on this a bit more and try to plug some numbers to see if I can get anything realistic.

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    [...] The lower this difference, the more chance that some rotor will have pressure gradients higher than this difference and cancel it out, making the nose fold under.



    Agreed.

    Quote

    Does our genius theory mean that making a front riser turn on a lightly loaded canopy in turbulence is not a good idea?



    Hmmm.. I don't see a direct link as everything we discussed was mostly steady state, full flight, level wing. Front riser turns are much harder to analyze. Besides, during the recovery of a front riser turn, 1) glide ratio is flattened, 2) airspeed is increased, 3) front riser pressure is increased. All these are good things in your model. Now during the initiation of the turn, you might be vulnerable..

  15. Quote

    Here you go - diagram compliments of the USPA sim



    Ian,

    If you want to discuss the fine points of no-wind flaring - lets do it in a different thread B|

    Jokes aside, yoink said

    Quote

    It is certainly possible to come to a zero horizontal speed under a Sabre 135 on a nil wind day.



    I'm curious to hear how you explain the magic ability of the canopy to produce lift at zero airspeed? In no wind conditions, I'm pretty sure I've always had to take a few steps, slide on my feet, or even run a bit ;)

  16. Quote

    Quote

    if the Katana has a steeper trim built-in (i.e. a "permanent double fronts" without the distortion), does that make it more vulnerable in turbulence?



    [...] Suppose we have 2 canopies of the same area and same airfoil trimmed to have angle of attack (measured from the line of zero lift) A1=10 degrees and A2=5 degrees in full flight, respectively. Also, the canopy 1 has the total speed of 26.9mph (25mph horizontally, 10mph vertically), canopy 2 flies at total speed of 33.5mph (30mph horizontally, 15mph vertically) due to more aggressive trim. [...]


    Interesting set of assumptions. Neat little model :)
    Here's what I worked out as the "critical downdraft speed" (defined as the speed that would trigger a collapse in your model):

    critical downdraft speed = airspeed x sin[ AoA ] / cos[ AoI ]

    where AoI is angle of incidence (effectively your G-A). Of course, one would want the largest possible critical downdraft speed for one's canopy.

    Now, AoA and airspeed both depend on AoI, so we should rewrite as:

    critical downdraft speed( AoI ) = airspeed( AoI ) x sin[ AoA( AoI ) ] / cos[ AoI ]

    By assuming airspeed/glide ratios, you effectively arbitrarily defined these functions in a way that may or may not be reasonable (double the assumed airspeed on your steeply trimmed canopy, suddenly there's no problem and your conclusion flips)

    Clearly, an increase in AoI will always lead to an increase in airspeed which on the margin makes my critical downdraft speed higher (speed is good).

    At the same time, that would likely make the AoA smaller which would have an offsetting effect.

    Moreover, cos[ AoI ] would get smaller which would contribute positively to downdraft resilience.

    Because these effects are offsetting each other, one would have to actually work out the equilibrium AoA and airspeeds as a function of AoI. I'm sure you're up for it ;)

    I think that how a canopy recovers once the downdraft is gone is more important than its critical downdraft speed. If the canopy loads mostly the rears as it regains lift (i.e. light front riser pressure), it would stand to reason than the nose could fold under (collapse) more promptly than if the fronts are heavily loaded.

    Thoughts?

  17. Quote

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    I'll clarify my question further: with the front riser pressure being so light, the line tension on As & Bs must be relatively low, it would stand to reason than the nose is then that much easier to pull down and fold under.



    Interesting question. I'm not sure that your assumption about A&B line tension being low is the reason the front riser pressure is so light though. Hmm, interesting question...



    It is an interesting question, isn't it? Might deserve a thread of its own..

  18. Quote

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    The consensus used to be "add a bit of brakes in turbulence". Now the consensus seems to be, "fly through turbulence in full flight". I think most would agree double fronts in turbulence isn't a good idea. So my follow up question to all this: if the Katana has a steeper trim built-in (i.e. a "permanent double fronts" without the distortion), does that make it more vulnerable in turbulence?



    What do you think? What makes your canopy stable and flying? Air pressure in cells and tension in lines. Keep maintaining those! Speed keeps your canopy pressurized and line tension keeps it in shape and you in control. If you feel that your lines getting loose because some bump you'd rather apply some breaks to pull it back.



    Agreed, but you do realize my question was about the Katana's relative behavior in turbulence, not about what a pilot should do.

    I'll clarify my question further: with the front riser pressure being so light, the line tension on As & Bs must be relatively low, it would stand to reason than the nose is then that much easier to pull down and fold under. Am I missing something? Are there other design parameters that people can think of that would offset (or worsen) this tendency? Has anyone had first hand experience with Katana collapses?

  19. Quote

    >which one have better wind penetration . . .

    In general the Katana will, since it has a slightly steeper trim than the rest of them.



    I guess the answer would be dependent on what gives you best glide in those winds. If you find that hanging on the double fronts gets you further, then the Katana will probably have the best penetration. But in many cases depending on loading / wind speed, a bit of rears extend the glide. In those instances, you'd probably be better off with a canopy that has a flatter trim.. no?

    Anyhow, to me the more interesting question is: does the Katana's steep trim make it more prone to collapsing in turbulence? I've been wondering about this for a while now and have not found an answer that satisfies me.

    The consensus used to be "add a bit of brakes in turbulence". Now the consensus seems to be, "fly through turbulence in full flight". I think most would agree double fronts in turbulence isn't a good idea. So my follow up question to all this: if the Katana has a steeper trim built-in (i.e. a "permanent double fronts" without the distortion), does that make it more vulnerable in turbulence?

  21. Quote

    Let's face it, it's much safer starting on a Safire2 or Sabre2 at 1.4 than a Katana at 1.4.



    I disagree with putting the Safire2 and Sabre2 in the same basket when it comes to recovery arc. I found the Safire2's recovery arc to be much shorter than the Sabre2's.

    Just to quantify this a bit, in my opinion:
    - The recovery arc of a Safire2 @ 1.4 is shorter than that of a Sabre2 @ 1.25
    - The recovery arc of that Sabre2 @ 1.25 is shorter than that of a Katana @ 1.25
    - And: the magnitude of the differences is about the same.

    I'll add as a disclaimer that I'm relatively low experience (so take my opinion for what it's worth) and I only have one jump on the Katana although that shouldn't really matter as my statement is mostly a comparison of the Sabre2 and Safire2's recovery arcs.

  23. Quote

    Always release brakes last. Once you have toggles in hand, they should remain there



    The latter does not require the former.

    I usually do chest strap then slider then toggles. Sometimes I'll forget the chest strap and unstow my toggles right after the slider. In these instances I've simply put both toggles in my right hand and loosen my chest strap with my left (and the help of my upper arms)