base736

I've seen some HD stuff that was pretty impressive, all right, but for my money I'll take three regular chips over one HD chip anyday. Once you've seen 3CCD, single-chip looks like badly colourized black and white... Though I have to admit I'm curious about the colour on the unit you cite, which claims to be an improvement on the usual single-chip quality. I've been jumping Panasonic's first consumer 3CCD camera (the PV-GS70, now superceded I think by the PV-GS65). The colour can only be described as amazing, though admittedly the low-light performance suffers a little.

Woke up early this morning with the intention of heading out to the mountains. Weather sucked. Anyway, I was inspired to run the experiment I suggested -- talk is cheap, right? So I built a 20" pilot chute of the type I described above and did some drop tests -- five each, "normal" and inverted. I've uploaded a video of the whole thing to http://www.skydivingmovies.com; it should show up soon as "pctests.mov" in the miscellaneous section. You can check it out and make your own conclusions. Mine go something like this: First, I rock at building pilot chutes. Second, and not at all surprising to me, the pilot chute was most efficient (ie, gave the longest descent time) when it was dropped open side down. It was also the most stable in this configuration, and inflated most quickly. Third, and perhaps most interesting, although it was unstable, the pilot chute did repeatedly fully inflate when it was dropped inverted (frame-by-frame will confirm this; sorry about the interlacing). The Big Conclusion: Lateral lift forces are, in fact, important in inflating a pilot chute! It is entirely possible that they are dominant in that respect. Direct inflation pressure is also important, but perhaps only in providing a "reserve" of pressure to keep the pilot chute stably inflated. Alternatively: The pilot chute was most stable when the red side was up. It's therefore also possible that pilot chutes with red topskins are the most stable, while those with purple topskins are black death. Edit to add: Couple of frame grabs here (no interlacing), one inverted and one normal.

I don't see how that would isolate lift forces from inflation forces... Perhaps you could elaborate?

Very cool. And I second the call for video.

I think this runs the risk of rapidly degenerating into a debate along the lines of whether a dam is technically "earth" or "building". The solution, IMHO, being that it's neither. When you inflate a balloon, is the force that opens it up lift or drag? The answer, again, is "neither". Direct inflation pressure is neither lift nor drag, in any meaningful sense. Anyway, in the spirit of being more productive... How can we tell whether inflation pressure or lift dominates in "inflating" the PC? May I suggest the following experiment. Take an all-fabric PC a la RW and hook it up the usual way, open end down. So that inflation pressure is allowed. Make a few jumps (or, if you prefer, measure the pull force from a moving vehicle or something). Now, hook it up the opposite way, open end up. Make a few more jumps. Lift will operate equally in both cases with respect to opening the pilot chute, since it's about the bulk of the fabric (and not about the hole, except as a way for air to get in passively). Inflation forces, however, will act to force air into the PC in the first case, and out of it in the second case. SO... If lift dominates, the PC will work equally well both ways. If inflation pressure dominates, you'll be glad you didn't jump the second configuration (or very, very upset that you did). If lift is at all important, the second configuration will still be "okay", even if it's not quite as good as the first. It's an easy and decisive experiment. The force that a PC pulls back with is certainly drag almost to the exclusion of any other type of force. But that wasn't the topic of the original "lift" post.

Umm, false. Lift, by definition, is a force perpendicular to the undisturbed airflow (http://scienceworld.wolfram.com/physics/Lift.html). Drag, by definition, is a force into the undisturbed airflow(http://scienceworld.wolfram.com/physics/Drag.html). Therefore: (1) Drag never coincides with lift; and (2) Lift can not be responsible for pulling a pilotchute away from the jumper (though it can be responsible for pilotchute oscillations). Thanks for playing.

I voted "Yes, after I had jumped a solid object". I didn't give much thought to my DBS after my first couple of jumps on my canopy, in which I had an ugly backward surge on a jump using the DBS (vented canopy). I did the next 80 jumps in shallow brakes before I decided to try deep brakes again at the Potato bridge. Although I didn't actually adjust them, the reason for that is that I found that the factory setting was (I'm quite certain) optimal -- ie, zero forward speed but without opening issues. Edit to add: Deep brakes are now my default setting for slider-down jumps.

I totally agree. I guess what I want to emphasize is that it's not a black and white thing. Some people might be absolutely compelled by brain chemistry to jump, or to gamble. These people are no healthier (I think) than the folks who are compelled by brain chemistry to, as you say, start fires. Some people feel no compulsion at all to get near the edge of a cliff. In fact, I know a lot of people for whom that holds absolutely no reward. They feel the fear, but not (for instance) that tingling that says "get closer...". Me, I get a kick out of standing near the edge even when conditions are such that I'm not going to be jumping. I guess my thought is that many BASE jumpers who are really drawn to the sport are probably predisposed toward it, probably at a level so basic that it's biological. I choose to be in the sport, but I probably wouldn't have made the same choice if I didn't feel the basic draw...

Dude, you almost made sense there for a moment. It was awe-inspiring, and a little scary.

I hope I never become a "real" BASE jumper. Too many rules and regulations, if what I've read around here is any indication, and the us vs. them thing does nothing for me. If you're looking for me, I'll be over here making fake BASE jumps. Edit to add: What if the prescription made what was previously enjoyable and frightening just plain frightening? If a person quits jumping after a bad injury, would you actually tell them to their face that they were peripheral to the sport to begin with? I suspect not...

So that's a no on "the idea of a correlation interests you"? I wonder why you took the time to post...

That may well be. But I'm willing to bet that, while 100% of the people on the "jumpers" side of the line are about to huck themselves off of a bridge, less than 50% of the people on the other side would take the "new business" risk if given the chance. I certainly wouldn't claim that 100% of BASE jumpers share some defining trait. However, I have read reports of research (and I wish I had a more concrete reference than that; perhaps nicknitro71 can help out here) indicating that (if I recall correctly) the genetics of the prison population code, on average, for statistically fewer saratonin reuptake inhibitors than one finds in the bulk population -- for, so they hypothesize, reasons similar to those quoted in the article crwper started this off with. Does that mean that everybody in prison shares this trait? No. Does that mean that everybody with this trait winds up in prison? No. But genetic predisposition is one important (and, I think, interesting) factor in determining behaviour. Anyway, if the idea of a correlation interests you, then cool. If not, then it's really not that important.

[sarcasm] I'm with you, man. My wife, who's a paramedic, claims to have seen people whose behaviour was altered by -- get this -- high levels of blood alcohol, or low levels of blood sugar. As if molecules can be closely correlated with particular behaviours... [/sarcasm] Seriously, though... While I respect that you may have a basis in mind for your statement, what you've said is clearly so broad as to be completely unsupported by the facts. If you have references, though, I'd love to read them.

That, in my opinion, would be the weakness of beeps. Beeps are great if you're trying to optimize your glide angle on this dive. But if you land, repack, and head up again, are you really going to remember how fast the thing was beeping last time? A glide ratio of 2.2:1 will always be better than a glide ratio of 2.1:1. But is "beeping like crazy" better or worse than "beeping super-fast"? Increasing the volume, I think, would be worth a shot, and ultimately the more versatile solution.

With a title of "Southern British Columbia"? Hehehe... You are from the lower mainland.

Every time somebody brings up the old "driving to the object is more dangerous than the jump" things erupt into a melee of "well, per hour..." vs. "well, per kilometer..." vs. "well, per outing...". If you're going to use the word "higher" or "greater" then you'd best quantify the things you're putting on either side of it. If I might ask, though... I think you'll find the excercise neither entertaining nor productive. What are you hoping to get out of determining whether BASE is more risky than gardening, or climbing, or bull riding?

Here's a block of four buildings with a wind coming in ten degrees off of one street. Synopsis, I think, goes like this... (1) Winds are funneled down the streets, the effect being larger the better the wind is lined up with the street. (2) Crosswind streets don't see much wind at all, but what is there might well be turbulent. (3) Downwind, even by ten degrees, is bad news -- a world of swirling vortices and smaller turbulence. I'll let you draw your own conclusions about crosswind options.

I can't guarantee the 9-10 feet, and clearly the circle I've got here is smaller than a human body, but as Jaap says, one shouldn't take these things too seriously anyway. But here's something... The vortices, of course, aren't right where they were in the unperturbed case, but they're there and they're a lot like they were before... Edit: Oops. Vented arc. I'll get on that.

We mean different things by "settled out". The simulations do, in fact, include turbulence. They are not steady-state; if you watch the full movie from which the above frames were grabbed (which I haven't posted), you can see vortices developing, being shed, etc.. The simulation begins with a set of initial conditions which are entirely unrealistic, however (uniform velocity), and it takes time for the system to develop to a more realistic (but still dynamic) state. A pilot chute oscillates so slowly relative to the speed of the airstream that I'd be surprised if its lateral motion made much difference.

I think that's a great thought. One could also treat the pilot chute as a pendulum (with the force of gravity replaced by the total force on the PC) and find out what its resonant frequency is at each speed. Pilot chute oscillations will only be important near the speed(s?) at which that frequency is near that of the vortex street.

If this is an honest question, then the answer is a big shrug. I've tried to choose the parameters (but may have failed) such that they're appropriate to something maybe a meter in size moving at something on the order of ten meters per second in air. Anyway, it looks good, and isn't that what really matters? If, however, you're following up on Jaap's success yesterday, I'll only point out that the Reynold's number only applies after nondimensionalizing the Navier-Stokes equations.

Not quite that bad. I'm using a band-diagonal solver, so that for a 10x10 grid (for instance), the thing is a float[10][100]. I'll look into your suggestion.

I'll work on that. I'm cheating a little on the sparse-matrix stuff right now, though, and as a result increasing the size by an order of magnitude would increase the memory requirements by a factor of 1,000 and slow the thing down by about the same. As an appeal to all the geeks on here, if you're aware of a sparse linear solver for nice, banded systems which doesn't require the installation of a monster library, I'd love to hear from you...

Inspired by Jaap's post some time ago (http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=1577452;search_string=kayaking;#1577452) referring to some guy's fluid dynamics solver, I decided to replicate that work and see what it had to say about the kind of stuff I like to look at. Anyway, as a result I've built a virtual wind tunnel. If you own a Mac running OS X, email me and I can fire it off your way. If you own anything else and would like to take a crack at compiling it (requires OpenGL, LibTiff, and Lapack), email me and I'll send you the source. ANYWAY... I've attached a series of simulations (each is a frame grab from an animation, taken when things had settled out) that came up in a discussion Jaap and I had last night. The first is a simple circle in the wind tunnel. I've hand-drawn a flow line (and pointed out some vortices) to point out the presence of a Karman street (see http://www.galleryoffluidmechanics.com/vortex/karman.htm for a good explanation and links to a bunch of excellent animations). When something (like a pilot chute) is dragged through the air, it develops a low-pressure region behind it. This region is unstable, and collapses into a Karman street, in which the object sluffs off vortices first from one side, then the other. Here's the important thing for BASE... It's not the "spilling" of air that causes a PC to oscillate or a round to pendulum. It's the presence of this Karman street, which places alternating sides of the round (or a location on the PC which circles around the perimeter) at low pressure. The spilling of air is a result, and is directly responsible for the higher descent rate on a penduluming round. The next two attachments give the same simulation for a semicircle and an arc in the tunnel. The Karman street is still there. I half expected these to turn out about the same, since each should generate a dead space in front of itself, but there do seem to be important differences. I'm working on quantifying the force on the object (which I think is the really significant difference here) -- I'll get back to you all with that. Finally, we have a "vented" arc. Notice that letting a little air through the apex of the arc results in the Karman street being pushed back, so that it no longer affects the object directly. It also has an effect on the dead space inside of the arc, of course, and I assume the pull force will drop a bit accordingly... I've done some simulations on buildings, wings, cliffs... All with results a lot like one might expect. Let me know if you're interested in seeing any of this stuff, or if you've got any requests. What fun!

You bugger. I spent a half an hour trying to figure out what you thought you were talking about...