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highspeeddirt

why 750Ft firing altitude?

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Reading the new article on AADs (on this site), it noted that the burble on a flat flyer would add about 260ft to activation. I was wondering about the same and assume they took this into account.

That said, seeing a cut-away from the ground makes you think "damn that is low"
"Pain is the best instructor, but no one wants to attend his classes"

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highspeeddirt

any one have info on why the cypres was designed with a 750 ft firing altitude, instead of what used to considered the "norm" of 1000 ft?



Generally it is because many people are not careful enough to specify the conditions when quoting such an altitude. Those quoting 750 feet are referring to (whether they know it or not) flat and stable, belly to earth. For most other body orientations it would be about 1000 feet. When a person is flat and stable belly to earth, there is a low pressure area where an AAD is located (in the "burble"), and makes the AAD think it is higher in altitude. There are some variations to this, but in general this is what is going on.

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To add to what Peek said, old AAD's typically had sensors outside the pack and not as susceptible to the burble effect between belly to earth and other attitudes. So for them, presumably the variation in firing altitudes wouldn't have been as much. (Although one can always come up with better or worse body positions.)

FXC sensed from the front of shoulder; for the Sentinel, the box was often placed around the hip / lateral junction I think?

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>any one have info on why the cypres was designed with a 750 ft firing
>altitude, instead of what used to considered the "norm" of 1000 ft?

A cypres with a 750 foot firing altitude will fire at about 1000 feet if the jumper is not belly to earth. The lower pressure around the sensor results in the 750 foot altitude; if the jumper is in another orientation, the sensor reads a higher (lower) pressure and fires sooner.

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Try saying that again Bill. On a belly flyer a cypres fires at about 750' agl, already taking into account the burble and the sensor reading a pressure altitude of about 1000'. On your back it fires at a pressure altitude of a out 1000' which equals the no burble agl 1000'.
I'm old for my age.
Terry Urban
D-8631
FAA DPRE

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skydiverek

***A cypres with a 750 foot firing altitude will fire at about 1000 feet if it is on a belly flyer.



It will fire at 750 feet, if the person falls belly to earth. At headdown, it will fire at 1000 feet.

Also incorrect. So much incorrect from people with so many jumps :(

Open Vigil Manual. Page 9, I think. The 260 ft offset is from specifically belly to earth to back to earth (sensor in high pressure zone).

Also this is all Vigil only. My understanding is Cypres post-processes their altitudes ("our AAD is 'smarter'") so the difference between the two is theoretically smaller.

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highspeeddirt

any one have info on why the cypres was designed with a 750 ft firing altitude, instead of what used to considered the "norm" of 1000 ft?



I was at the PIA Symposium when Helmut Cloth introduced the Cypress AAD. He stated his reason for the 750 foot activation altitude but I don't remember what it was. You will have to ask him

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lyosha

Quote


It will fire at 750 feet, if the person falls belly to earth. At headdown, it will fire at 1000 feet.



Also incorrect. So much incorrect from people with so many jumps :(



This is a very interesting point. To which there is no quick answer. I believe you are incorrect lyosha, but for totally understandable reasons. You are totally right based on the Cypres manual alone, but there's more. As for what's true or not, this is what I've been able to figure out:


The idea of the burble affecting a Cypres has been around a long long time. (eg, at least 2003 on dz.com. I haven't checked rec.skydiving archives though.) And it seems to be largely accepted.

However, perhaps you are thinking of the manual: The Cypres manuals (Cypres 1 or 2) say nothing at all about any burble. All they mention for the regular Cypres is 750 ft. That's it. Nothing about body position or burble etc. (Correct me if someone finds something.)

So where did that thinking about the Cypres come from?

Cypres & SSK never really tried to dissuade people from thinking the burble would cause a higher activation in other body positions, nor that their algorithm was somehow superior in that regard specifically.

There is a Cypres Testing Report out there, for PIA's TS-120 AAD testing standard. There they mention how their algorithm takes care of pressure fluctuations.There is also a chart showing recorded pressure (with spikes up and down as the jumper does maneuvers, along with a smooth 'derived altitude'.)

Nevertheless, the impression one gets is that they are care of short term body position related changes, and it is unclear how the system really works or what the 'regular' body position of the jumper was.

An instructive document is one that used to be on the SSK page but is now gone from their new site; I had to retrieve it from archive.org:

https://web.archive.org/web/20050911063002/http://www.sskinc.com/cyp13.htm

The relevant bit:

EMPHASIS IS MINE.

Quote


The following is a copy of the response from Airtec (the manufacturer of CYPRES) to some questions and comments fielded from the "rec.skydiving newsgroup" on Internet, concerning CYPRES accuracy and "misfire rumors". It is reprinted here to provide additional information for all CYPRES owners and riggers.

In regard to a recent Internet letter we want to clarify some issues by first quoting an article which has been published in the September 1992 issue of the U.S. "Skydiving" magazine:

- - - - - - - - -

"...what seems like 750 ft. to a CYPRES (or any AAD that measures altitude by sensing air pressure) may not really be 750 ft.

An altimeter doesn't actually measure altitude, it measures air pressure. It reads air pressure and converts that reading to altitude.

AADs have a tough job because the air pressure around a falling jumper varies from one location on his body to the next. If he's falling face to earth, air pressure is higher on his chest and lower on his back."

Helmut Cloth took this into account when he engineered the CYPRES. It is designed to be mounted inside a piggyback reserve container. In freefall, the air pressure here is about 300 ft. "higher" than ambient. (An altimeter mounted there would read 1300 ft. as the jumper fell through 1000 ft.) The CYPRES compensates for this when it monitors a jumper's descent.

The situation is complicated by the fact jumpers don't always fall in a stable face-to-earth position. If the jumper makes rolls, loops or tumbles uncontrollably, the lower pressure situation on his back can change to the true pressure situation - that corresponding to his altitude - or even to the higher pressure situation on his chest.

The CYPRES' computational powers play a vital role when a jumper is tumbling or rolling. Its software compensates for fluctuating readings to help prevent the device from firing too high or too low.

There are two ways a jumper can outfox the software and make a CYPRES activate high.

The first way is to fall a long time in a stable back-to-earth position. "This will definitely cause a higher activation altitude than 750 ft.," Helmut said. In this situation, the device is fooled into thinking the jumper is lower than he actually is because it is sensing higher pressure than normal.

The second way to trick a CYPRES is to deploy the main at a low altitude while simultaneously rotating to a head-high or head-low position.

This maneuver will cause the CYPRES to think altitude is decreasing rapidly because the low-pressure area on the jumper's back has been cleared.

The CYPRES uses its microprocessor to study the situation and try to determine if the sudden increase in pressure is the result of an attitude change or decreasing altitude.

If it has any doubt as to the jumper's altitude - remember, the CYPRES already knows the jumper is low - it fires the cutter to open the reserve container.

Even when combined with the device's operating accuracy, a "premature" opening created by these circumstances (a low pull while rotating vertically) would still be at about 1050 ft.

All pressure-sensing AADs are affected by where they are mounted and the position of the jumper in freefall. Short of equipping a jumper with some sort of vertical-seeking radar altimeter, there is no solution to the problem..."

- - - - - - - - - -



Very very interesting, and not a document many will have read lately.

So they are both saying that the Cypres accounts for the burble with good algorithms.... but also admits that it can still be fooled sometimes!

The difference suggested in the 1992 document is up to 300 ft.(Cypres and Airtec just have different estimates of what a normal burble is; without real data I wouldn't say either is better or worse for estimating 260 vs. 300 ft.)

But in typical cards-close-to-the-chest, we-know-what's-best-for-you Airtec fashion, nothing about burbles or any error in activation height is mentioned in the manuals, whether 20 years back or today!

My Conclusion:
According to the manual, a regular Cypres fires only at 750 ft (maximum). However Helmut Cloth admitted in 1992 that other body positions may make it fire higher, probably up to 300'. There is no indication that I know of that the Cypres 2 is any different than the Cypres 1 in that regard.

Thus the burble and body position effect does exist for the Cypres, relative to the normal 750 ft for 'normal' belly to earth (no matter how good its algorithm may be at filtering out short term fluctuations and jumps in pressure, that do not correspond to sudden altitude changes) Both the Cypres and Vigil firing altitudes are affected by burbles in a similar way.

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RMK

Reading the new article on AADs (on this site), it noted that the burble on a flat flyer would add about 260ft to activation. I was wondering about the same and assume they took this into account.

That said, seeing a cut-away from the ground makes you think "damn that is low"



Seeing one from the air makes you think that too. I had one last Saturday and cut away at 2500 feet. The witnesses in the air thought I was much lower. The guys I was jumping with on my first one also thought I was much lower than I was -- that one was well over 2 grand, too.

750 doesn't seem like it gives you a lot of getting-your-shit-together time, but maybe they assume you're out to lunch and won't be getting your shit together anyway. Reserve is supposed to be fully deployed in what, 400 feet?
I'm trying to teach myself how to set things on fire with my mind. Hey... is it hot in here?

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councilman24

Try saying that again Bill. On a belly flyer a cypres fires at about 750' agl, already taking into account the burble and the sensor reading a pressure altitude of about 1000'. On your back it fires at a pressure altitude of a out 1000' which equals the no burble agl 1000'.











Terry ....... I just hope none of us ever find ourselves on our bellies at terminal velocity at 750' or on our back at 1000' ...... unless we are knocked out, there is no reason we should be. Jump Safe. :|
Life is short ... jump often.

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I have no strong brand loyalty, having packed reserves (and mains) containing: Argus, Cypres, FXC 8000, FXC 12000, FXC Astra, Sentinel, Vigil, etc) I have also packed PEPs containing even more obscure, military-surplus AADs.

When FXC was developing their Astra AAD, they experimented with a dozen different air pressure (altitude) sensor locations. FXC concluded that any sensor located around the collar/yoke produced consistent pressure readings.
Meanwhile, pressure sensors hidden inside the reserve container experienced errors up to 300 feet.
That is why FXC AADs have external sensors.
Most other AADs install air pressure (altitude) sensors in their battery boxes, which are packed deep inside the reserve container.

Also consider Airtec's logic when they were designing to Cypres 1 25 years ago. Belly-flying was the norm with only a handful of silly Frenchmen experimenting with "chute assis" (sit-flying). No one predicted that anything other than belly-flying would continue to be fashionable (date we say "manly"?) for experienced skydivers.

Airtec also went to great pains to conceal the Cypres because back then only students and .... er .... "People not manly enough to pull their own reserves" wore AADs. Many jump-planes only climbed to 7,000 feet AGL and experienced skydivers routinely "sucked it down" to extract the 30th second from their precious freefall. Fashionable main canopies were still in the 220 square foot range and spun much slower when they malfunctioned.

Debate aside, anyone who scares an (Argus, Cypres, FXC, KAP-3, Marrs, Sentinel, etc.) AAD is a scary distance below normal opening altitude and deserves no sympathy.
"That will cost $80 for a reserve repack and twice that for a new cutter."

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Hi Rob,

Quote

Also consider Airtec's logic when they were designing to Cypres 1 25 years ago.



I won't profess to completely know their logic. However, one other thing comes to mind. Those '25 years ago' the standard reserve was a round canopy.

I think we all have seen the video of the guy who cutaway on a Wings, went onto his back and spent some time watching his reserve pilot chute & bridle dance around above him and not getting the reserve bag/canopy out.

Unless it is actually hungup on something, a round canopy will snake out and get open in almost any body position. With a square reserve, it is 'somewhat' an all or nothing type of situation.

Attached is a photo of a round canopy 'snaking' out.

Jerry Baumchen

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JerryBaumchen

Hi Rob,

Quote

Also consider Airtec's logic when they were designing to Cypres 1 25 years ago.



I won't profess to completely know their logic. However, one other thing comes to mind. Those '25 years ago' the standard reserve was a round canopy.



My rigging log book shows I first packed a ParaFlite Safety-Flyer on March 21, 1979, (36 years ago). So there were square reserves around for 10 years before CYPRES hit the market. I have no idea what percentage they had by 1990.
"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

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just a note of interest..... the first square reserve was Para Flite's "safety flyer''. we shipped the first one the friday prior to labor day weekend 1978. by the way it was hand delivered by Fang Fenimore to Jim D'Allario at the Herd boogie. He ended up using it that weekend and actually became the first Square reserve "save"

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highspeeddirt

just a note of interest..... the first square reserve was Para Flite's "safety flyer''. we shipped the first one the friday prior to labor day weekend 1978. by the way it was hand delivered by Fang Fenimore to Jim D'Allario at the Herd boogie. He ended up using it that weekend and actually became the first Square reserve "save"



PF had a mfgr certification required to pack the S-Flyer.

One Saturday morning, I met a master rigger who PF had certified to qualify riggers, and packed it up, and got my certification. Then I went out to the DZ that afternoon and made my first jump with it in my container.

Mr Murphy was apparently watching...as I rode it down on the first jump.>:(
:D
"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

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ryoder

So there were square reserves around for 10 years before CYPRES hit the market. I have no idea what percentage they had by 1990.


This is turning into an old time rigging stories diversion.

But going along with that, when I started rigging in 1991, with 31 jumps, of my first reserve packs in that year (excluding pilot rigs), 15 were round, and 4 were square (Safety Flyer, Cricket, Raven, X-210).

(Caveat: Ratios also depend on how many people spend money on nearly new gear vs. having old stuff. This was in Ontario, while in a big market like Florida perhaps more newer gear got sold.)

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FastRon

I'm still confused here.
Q. In the quote from Helmut (6th... paragraph?) If the air pressure is higher inside the pack- wouldn't that make an altimeter read a lower or true altitude?



He said the pressure is 300 ft "higher" in quotes, implying higher in altitude, not numerical magnitude.
"Confusing?" :)

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