RichM

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Yes, density-altitude is based upon the International Standard Atmosphere.

You might want to have a peek at this article.
http://futurecam.com/densityAltitude.html

If you need exact density-altitude calculations, then there are calculators available for that (and other) purposes, but the rule of thumb calculations given in the article will work pretty well for everything from bush flying to skydiving.

Thanks Quade, will absorb that this week and program my calc to carry around with me. I expect to see d.a. and perception tie up quite nicely.

Rich
Skydiving nerd ;P

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Temp. measures the average speed of the surrounding molecules. As it gets hotter, the molecules of air are not as dense because they moving faster. So the canopy will fly like you are at a higher altitude where the air is thinner. That is to say, faster.

Thanks for the explanation that even I can understand

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Additionally, as relative humidity increases molecules of air are being replaced with molecules of water. So the air becomes even less dense on a hot and humid day.

Can I just confirm you do mean relative humidity?

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A Crossfire has a negative recovery arc so it will stay diving after you let the risers up.

My Crossfire 119 loaded at 1.5 (recalled and line trim done) has a positive recovery arc, and I sometimes can't hold double fronts down to surf height as they have loaded too much for my skinny arms. Hehe

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You should be able to avoid digging out of the corner if you let up on the risers a bit higher. let the canopy fly to the surf window instead of driving it there. If that makes any sense.

Thanks, it does makes sense and my aim is to finish the natural recovery arc too high and so need double fronts to reach surf height to give me that larger margin for error.

Thanks to all who have offered advice, it seems clear that this was likely caused by density altitude. Is there any way of measuring density altitude on an absolute scale?

Rich

I spent yesterday at Langar, UK on a stinking hot day with nil winds and found that my normal swoop initiation height was a shade too low and I'd be interested in understanding why. My normal DZ is Headcorn, UK.

This DZ is about the same altitude as my home DZ. The grass landing area though is in the centre of a triangular triple runway setup with large cornfields all around. There were powerful thermals from the runways.

I can think of 3 possibilities:

1) Perception. I judge initiation height by eyeball only so the slightly smaller landing area may have fooled my depth perception. I have virtually discarded this as I was definitely going much faster than normal at ground level and slotted a significantly longer surf than I normally do.

2) Thermal lift from the surrounding areas and runways may have made a large and steady downward airstream above and onto the landing area. I'm tending toward this one.

3) Density altitude giving a much thinner and dryer atmosphere. It was about 10C hotter on this day than is normal. Barometric pressure unknown I'm afraid. I kinda grasp the d.a. theory but don't understand how it affects canopy flight.

Any suggestions on why would be greatly appreciated as I found on my screaming swoop I had to just dab the brakes at about 10 feet to dig it out of the corner whereas I normally square the canopy with double fronts and ride them down for the last 5-10 feet.

But that was a damn fast and long swoop and I loved it apart from the dig :-D

Cheers, Rich

I don't agree with the suggestion of a certain number of jumps as being a good guide to canopy skill. Someone who has done 300 jumps (to use your example) all in the last year should be a lot better at canopy control than another who has done 300 jumps in 30 years. Additionally you can kill yourself almost as easily under a heavily loaded square irrelevant of jumps numbers.

I think the main problem is that there is no official or recognised method of teaching advanced canopy skills (I mean swooping not CF).

I have been on the receiving end of significant peer and senior pressure to not begin swooping and then again to not continue with it because it "kills people". our local "ban skydiving" club uses that argument and yet many skydivers do not see the irony nor the danger of using and so giving credibility to the same argument. But I had to teach myself as no-one would teach me, I had a little help, but mostly had to research information (a heck of a lot of it came from here, thanks guys and gals) and carefully analyse the risks and build an approach to minimise and mitigate them.

I had to do it myself but as in all things, structured education is the key to making it safer. Kibi at Eloy delivers (maybe devised?) a teaching approach to skysurfing which is a level progression system much like AFF and RAPS are, and you have to pass each level before moving on, and so demonstrating the required level of skill before progressing.

So I think a similar approach would significantly reduce the risks of learning the skill of swooping. This is probably nothing more than any decent informal swoop coach is doing anyway. I might even start an approach and post it here for thoughts and improvement, anyone think it's a good idea?

Have fun, stay safe
Rich M

"No shit, there I was..." except it's not really a scary story. I post it to allow me and others to extract any learnings from it.

I jump a Crossfire which takes nearly 1000ft to open. I pull at 3200 to allow for this. I had done the previous 10 jumps with a Sabre 120, opens in about 500ft.

I chuck the pilot chute out at 3200 and 4-5 seconds later I'm still facing mother earth. That enough for me and before I'm really concious of what I'm doing I have both handles at arms length and the reserve blooms out above me. Alti tells me I'm at 2000ft
Mental note to (a) at least check behind me next time, it might be a pc in tow (b) more practise checking my alti before panicking and pulling handles.

I'm fundamentally happy that I dealt quickly with a high speed "mal" albeit there is no evidence that it was a mal. I kept hold of the small bits and picked up the big bits, so only my rigger left to placate as he hates packing this stupid racer with its far too small pop top. The bottle of Jack D should help there.

And everyone got beers off me of course. I cancelled the planned loan of the cutaway rig for #500, no further need

Still, if you have any suggestions on improving my actions please respond.

Rich

Thats bad news, I wish Drefus a speedy and full recovery. I jump a Crossfire 119 at about 1.5:1, it's one of the USA built ones, that has been recalled and the trim done. I set up a reasonable surf on a calm day a few weeks ago and half way through the surf the front left riser very suddenly went slack and then immediately taught again, it kinda "jumped".

The canopy flew straight and true, the surf ended without drama, I was looking forward not up at the time so didn't see what happened and no-one else was looking at me iether, so I don't know what it was. But I do know it wasn't right.

I'm gonna get me a Samurai instead.

Take care out there

There is one relevent point to turbulence - I guess that the amount of turbulence near the ground will increase as the wind increases. This would lead to different overall flight characteristics for a canopy as it lands in turbulent air, resulting in truly different lift characteristics on windy days, although with no difference up/downwind.
Also, is it possible that on windy days there are reasonably different wind speeds at 0ft and 20ft agl, so the mass and inertia of the skydiver at 0ft might cause the canopy at 20ft to fly through more air and so more turbulence when flown into wind than when flown downwind, resulting in truly different landings.
All the above is pure guess work. I'm just trying to think why so many people have beliefs that are not supported by the simple fluid dynamic model. Any thoughts?
Rich

Mine is 5000 breakoff, 3200 deploy, 1700 harddeck. People that have the flatline set to "Pull now!" worry me, as there is nothing after that to indicate harddeck, and under a malfunctioning canopy it is easy to loose altitude awareness while fighting it. If I hear my flatline I stop thinking and pull all the other handles, irrelevant of how close I am to sorting out the mess above.
Please think about it. I have met several people who have spent some time listening to their higher altitude flatline while fighting a snivelling main. All have lived so far, but they do reluctantly admit that after the audible went flatline they had no real idea how low they were until the canopy opened and they looked at their alti. All have changed their flatline to harddeck now.
Rich

In addition to nacmacfeegle's summary, I feel I would also benefit from a better understanding of how layers of winds in different directions to ground wind would affect jump run and possibly exit order.
An excellent thread here, thanks. Try to be nice :)
Rich M

I agree, but I would add to the above that if the speed difference was that much then you went for a too radical swoop too early. There are many steps between straight in flare, and a 180 degree snap toggle turn with outrageously fast landing speed. You should get each one right before progressing or you will get hurt. Find yourself a canopy skills coach.

Thanks for all your input ladies and gents, very much appreciated. I shall start assembling some paperwork now and see wher I can get :)
Blue skies
Rich

My apologies for starting what I'm sure is an old and tired thread, but I was unable to find anything by searching.
My DZ mostly puts freeflyers out first unless I'm the jumpmaster. I've been talking to people for a while and have probably 50% agreeing with my reasoning, but they won't overtly support me and if they jumpmaster they will revert unless I'm on the load. There is a small hardcore of people who just won't listen.
It is my aim to write a paper for my CCI explaining the reasoning. My goal is to get the CCI to give overt guidance and set dz policy.
My reasoning essentially consists of 2 arguments:
1) the well known one about wind drift affecting freeflyers less because they spend less time in freefall.
2) my other which I haven't yet seen anywhere else is that by the same process of wind resistance, freeflyers present less surface area to the relative wind on exit and so take longer to lose the aircraft speed and travel further fforward during that time. I realized this when I did a flatish exit to film a 2 way headdown (was never going to work, I know now, but taught me a good lesson).
I have also read Brian Burke and David Brownell's papers on the subject, and will include those as whole items in my "thesis".
I would like my paper to be complete, so I would grateful for any further reasoning or input. It would also help me if you could identify your "authority" to post opinions, but I don't mind if you'd rather not (e.g. dzo, dz manager, years and jump numbers, ff experience, what have you). Being only 450 jumps experienced, some of these people seemingly refuse to listen on the basis that they have so much more experience than me. "Yes You Get That" I guess :)
Many thanks for all input.
Rich M

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Similarly, if there's a problem with collisions in freefall, I would think that step 1 would be to avoid such collisions by choosing loads where they are unlikely to occur. Changing gear to prevent deployments during collisions seems a little backwards. While there are cases it might make sense to do, I think those cases are far in the minority.
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I don't actively set out to have a collision, I think I already choose loads where I don't think a collision going to happen, but I know that it does happen and so it might happen to me at some point. But doesn't changing both, sum the risk reduction from each?
Rich M

In my short 5 years in the sport I've seen lot's and been a major facor in some of those. From each of these I have survived (so far) and learned. I was talking to a friend of mine yesterday who has 100 jumps, he has just gone through the UK B licence, so he has been breifed, tested and cleared for spotting, jump mastering and flight line checking. He commented on how much he had learned about the complex decisions that are made on every lift, yet prior to this had little idea that it was happening. The only comment I could think of that was vaguely sensible was the more experienced skydivers look out for the less experienced, and if you see something you think is ikky don't be frightened to question it.
It seems lacking as an answer.
Rich

As you correctly say headdown and headup have been around a long time. I am under the impression that rigs designed have moved to accomodate some of the problems associated with the increased speed and the fact that the rig is now in exposed directly to that wind rather than sitting in the burble.
BOC is the predominate deployment type, and is used almost exclusively for freeflying. Looking at older rigs I find design changes in modern rigs in the way the bridle and risers are packed to minimize the possibility of premature deployment or riser escape. Would you think it's fair to say that non flat flying has been influential in some of these design changes?
Sorry, it turns out I didn't even read riggerrobs post right. Having reread it I agree with it. I lost the context in which it was made. I had a bad night. Sorry.
Rich

"If a system is proven and has worked well for them for many years, why change?"
Because skydiving is changing?
Rich

I do about the same as you freeflir29. In addition I have started my group of regular FFers doing a "no more work" wave at 4500ft as normal, and then just hanging there for a couple of seconds and checking where everyone is and looking up. So we're all sure we know the air is clear before we go flat and cork up.
Rich

A quick question. Lifting your legs under canopy reduces your surface area presented to the relative and so allows you to achieve a higher speed through the air, allowing an increased chance of getting back from long spots.
Is this technique used in swoop meets for distance?
Rich M

I started skydiving because I'm an adrenaline junkie, and I found I could go straight to freefall with this thing called AFF. I was on an AFF course 6 weeks later.
I continue because I still get a buzz from how surreal it is too sit in the open door of an aircraft and watch the world in the same sort of way that any other "approaching" middle aged man might sit in his comfy chair and read a paper. I get a buzz from flying with my friends and being free of gravity for a while (I know that's technically incorrect but we know what I mean :)) I get a buzz closer to my old motorbike racing days when I execute a really dialled in surf on no wind days.
I love the attitude of skydivers. The sport transcends class, wealth, race and gender, and nearly everyone in it is really cool. I love the social ("hic") life :)
I'm happy freeflying and I am very pleased the sport has developed this way. I'm not so sure I would have stayed in the old FS only days. A bit too regimental for me, but then it was a sport full of ex-squaddies - no offence intended here guys and gals, in many cases you are now the people running the dz's and making this sport available to all.
I love the continuing development and introduction of new ideas, although I tend to wait for a quite a few people to try them out before I go near them, but that's my choice.
I love that choice, and the almost frontier nature of the sport.
I love being around first time jumpers, their excuitement is infectious :)
In the future I hope the innovation continues, and it would be great if we could break down the unreasonabble fears associated to skydiving and introduce many more people to the sport.
There's probably tons I missed but I wrap it there.

Erm, I'd hardly call myself an expert, but I don't get your logic there. I understand that you are flying your canopy in a body of air. The canopy will fly through that body of air the same irrelevant of whether the wind is blowing left or right, doing 1 knot or a million knots, becuase you are being blown with it, so the relative speed is zero. The only difference will be the relative speed of the ground, which makes no difference until you need to touch it, i.e. land. That is if the winds are blowing parallel to the ground. Have I misunderstood anything here? (Sorry if this is a tired argument, but my simple understanding of fluid dynamics has just been drained out of it's recepticle:))

Over here in UK, if they refuse twice they aren't allowed up again, as they pose a danger to themselves and everyone else in the event of an aircraft emergency when exit is an option. Not sure if this a UK rule, or just my DZ. Must go and read the Ops Manual again.

I think you are referring to a gentle toggle turn used by all students and many many experienced jumpers to change direction.
There is a much more severe version that is used to gain landing speed by initially losing canopy speed and making your body act as a pendulum. This kind of toggle turn has to be done exactly right every time, as there are very little options to correct a misjudgement. The front riser turn increases landing speed but does so by increasing canopy speed meaning your canopy is much more responsive to corrections.
I hope that helps. Rich M

Hehe, I'm still a newbie here, and I'm looking forward to Quades response :) I have lots to learn and these sorts of points keep that education flowing. "Let's get it on!"
Rich M