Skyhook reserve deployment

[billvon 2,406]

>(1) why is it dangerous for the poster to state that his opening
> seemed like 50ft? if anyone....NO. If ANY skydiver chops at 100ft,
> then they shouldn't be skydiving to begin with.

It can mislead people into thinking that a Skyhook really does open in 50ft, and so while 100ft may be too low, 200ft may be OK.

[pchapman 262]

To add a little bit to all the time & altitude stuff, particularly what 980 and billvon wrote about, I timed some of the openings in two RWS Skyhook videos, just using a stopwatch, not frame by frame analysis.

Time from cutaway to fully inflated canopy:
3 sec typically, from a straight flying canopy

As we know, that's not the same as having a canopy flying stable. In one case on video, the canopy had a big swing on opening so it was 2 more seconds until the canopy was flying close to normally. (The RWS video with the slider down "100 ft" cutaways shows another example of the big difference between just 'open' and 'close to flying properly'.)

From a spinning canopy, time to full inflation:
2 sec typically in the best cases
2.5 & 3 sec seen in cases where the reserve snivelled for a moment

Time to extract the canopy from the freebag:
1 sec on the videos from flying straight
1/2 sec by billbooth's frame analysis, cutting away from a spinning canopy

To go from cutaway to canopy extraction one needs maybe 8.5 ft for lines (PD-126 A-lines), at least a foot for the risers, a foot for the bag, maybe a couple feet for the stretched out canopy if you want to include that, and 7 ft for the bridle (Billbooth once posted 7 ft to bag to skyhook, plus 5 more to PC). Total 17.5 to 19.5 ft.

In 1 second (ignoring minimal air resistance) one accelerates downwards 16 feet. If one starts with initial downwards speed, one has to add that on, e.g., 31 ft total vertical distance if starting at 15 fps down. (I'll leave aside just what the main canopy is doing, as one needs enough vector distance [Vector distance?
In videos a cutaway may start at the camera flyer's level, and end up with an open reserve not far below, but of course that's relative to the camera flyer who is also descending.


So for a slow speed cutaway, the Skyhook does about as well as could be expected from the simple physics. There's significant vertical distance just to get the canopy extracted, maybe 17, 20, even 30 feet, which is going to take a full second or more. But the reach, peel & pull beforehand, and the canopy inflation afterwards, are going to take more time and distance than the part that's under billbooth's control!

[billvon 2,406]

>To go from cutaway to canopy extraction one needs maybe 8.5 ft for
> lines (PD-126 A-lines), at least a foot for the risers, a foot for the
> bag, maybe a couple feet for the stretched out canopy if you want to
>include that, and 7 ft for the bridle (Billbooth once posted 7 ft to bag
> to skyhook, plus 5 more to PC). Total 17.5 to 19.5 ft.

Plus whatever distance the main has traveled in that time. For a fully inflated canopy flying normally, it's probably about a few feet. A malfunctioning canopy will almost always see a longer distance.

[tso-d_chris 0]

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A simple trip to the freefall tables (found in many log books) says than you fall 62 feet in the first two seconds. So, if we assume an initial vertical velocity of 20 feet per second under the malfunction, and a two second reserve opening time, the distance fallen between breakaway and reserve opening is 20+20+ 62, which equals 102 feet. (If I got that wrong, some math student please correct me.) Since Skyhook openings almost always take less than 2 seconds, I believe that the 100 foot reserve opening number can easily be defended. (In fact, if you plug in 20 fps initial vertical rate, with a one and a half second opening, you get under 75 feet.)

Let's try some more realistic numbers.

I isn't realistic to use the low descent rates of a properly functioning ram air canopy as an initial speed. Most people don't cut away from properly functioning canopies.

A student CYPRES fires at 1000 feet if it detects a descent rate greater than 29 m/s, sensing a malfunctioning canopy. Let's use that number, since it is more realistic, even if it is still rather arbitrary.

29 m/s = ~95 ft/s.

95 ft/s X 1.5 s = ~140 feet

So we have 140 feet before we even consider the acceleration due to gravity.

Vf = Vi + a*t

where Vf = final velocity (unknown at this time), Vi = initial velocity (95 ft/s), a = 32 ft/s^2, and t = time (1.5 s)

Vf works out to be 143 ft/s

d = .5 * (Vi + Vf) * t

where d = distance, which works out to be about 180 feet.

Granted, I have disregarded the wind resistance, but I have also disregarded the fact that the jettisoned canopy starts out with the same descent rate as the jumper underneath, and I have modeled it as a fixed point. One assumption causes an error in one direction. The other assumption causes an error in the other direction.

The 100 feet deployment distance claim is off by about 40% or more. Claiming under 75 feet is laughable at best.

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[Vertifly 0]

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It can mislead people into thinking that a Skyhook really does open in 50ft, and so while 100ft may be too low, 200ft may be OK.

dude...exactly why i said...

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If you've made it that low...with a problem that will cause you to cutaway...then, it is my opinion that you deserve to be rolling the dice anyway. Because you've already taken fate and left it to chance by waiting too long.

point being...i see little need to chop that low (you say dangerous below 200ft i say up to about 400ft maybe more) unless you know the risk is friggin huge. if there is such a thing as a hard deck, then that should be it. below that, for me, it would have to be to not consider ANY other options but to chop and pray.

[pilotdave 0]

29 mph for a student cypres, not 29 m/s. That's 13 m/s.

That 140 feet in your first calculation becomes 63 feet.

In the end when you do the math all out (as you did it), d = ~100 feet.

Almost like that 100 foot figure wasn't so arbitrary.

Dave

[billvon 2,406]

>i see little need to chop that low . . . unless you know the risk is friggin huge.

Right. A recent Incidents thread had people wondering whether it would be a good idea to cut away from a canopy collision at 100 feet. It was discussed in at least two threads. So while I agree it's a bad idea to cut away that low, some people _have_ been considering it. If the Skyhook really opens a reserve in 50 feet? Maybe it might be a better idea than landing with a collapsed canopy.

There's nothing wrong with trying things and posting the results, or your interpretation of the results. But sometimes a bad interpretation can lead to people taking it seriously. That's the only reason I posted - so that people know it doesn't _really_ take 50 feet.

[tso-d_chris 0]

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29 mph for a student cypres, not 29 m/s. That's 13 m/s.

That 140 feet in your first calculation becomes 63 feet.
In the end when you do the math all out (as you did it), d = ~100 feet.

Almost like that 100 foot figure wasn't so arbitrary.

You're right. I've eliminated the incorrect calculation.

Bear in mind that the 1.5 seconds represents the freebag separating from the canopy, not reserve inflation.

In other words, the 100 feet reserve deployment is still not an accurate claim.

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[sammyc 0]

My friend had a skyhook assisted cutaway from a step through mal at emporia in Feb. From the video footage we timed the reserve deployment as 1.8 seconds from cutaway to fully open reserve.
We opened our mains at the same height and he lost very little altitude relative to me due to his cutaway.

[billvon 2,406]

>We opened our mains at the same height and he lost very little
>altitude relative to me due to his cutaway.

So the total height he lost was the amount of height that you saw him lose compared to you, plus the height you lost during the deployment. Making a few assumptions that would put it in the 70-100 foot range.

[sammyc 0]

Yes, i would say this was correct.

[billbooth 10]

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29 mph for a student cypres, not 29 m/s. That's 13 m/s.

That 140 feet in your first calculation becomes 63 feet.
In the end when you do the math all out (as you did it), d = ~100 feet.

Almost like that 100 foot figure wasn't so arbitrary.

You're right. I've eliminated the incorrect calculation.

Bear in mind that the 1.5 seconds represents the freebag separating from the canopy, not reserve inflation.

In other words, the 100 feet reserve deployment is still not an accurate claim.

Nope. The 1.5 seconds represents the time from breakaway to a "fully open" reserve canopy. 0.5 seconds is the time from breakaway to reserve canopy out of the bag. It's kind of amazing the first time you do a Skyhook breakaway. By the time you finish pulling your breakaway pillow to arms length, your reserve is at line stretch. I can see some people thinking they're fully open in 50 feet, even though it isn't true.

[mjosparky 3]

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I can see some people thinking they're fully open in 50 feet, even though it isn't true.

That should bring this discussion to an end.

My idea of a fair fight is clubbing baby seals

[BillLockwood 0]

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I am well aware of the major and minor contributions this manufacturer has made to skydiving.

That is nice, so again, what are your qualifications and what testing have you done to dispute the manufacturer?

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Were you aware that the 100 foot cutaways that were made used slider down/off reserves? There was no altitude to spare on those deployments. That is not a realistic deployment scenario by any stretch of the imagination.

Yes, I am aware of it and have seen it many times, but what does that have to do with what I said since I never referenced it? If “there was no altitude to spare on those deployments” as you claim then how come it was eight seconds between opening and landing?

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I didn't just make the claim with no evidence to support it. I challenge you to find a single video of a Skyhook deployment with a reserve packed to manufacturers specifications (slider up) that can be reliably measured and shown to occur in less than 100 feet.[

Challenging me for the known videos that clearly show sub 100ft. openings has nothing to do with the fact that you have failed to show a single piece of evidence in the form of test, data etc. to dispute it.

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As for what I have, I can only rely on Newtonian Physics, and his kinematic equations, and the knowledge of the slider down deployments. Are you saying that the force of gravity changes during a Skyhook reserve deployment?

Like I asked before, your “knowledge” is based on what? Show us the videos of the test that you have done to dispute the factory results.

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BTW, slander is spoken, not written. The word you are looking for is libel. However, the statements must be false to be libel (and against an individual, not a group of people according to my handy dictionary). A good example of libel would be you accusing me of slander.

You would be correct if I had not personally heard you say it at the drop zone!!! BTW, that would therefore not be libel and that is according to case law and not your all knowing dictionary.

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While a few people have argued passionately about the perceived greatness of the Skyhook, I have yet to see ANY evidence supporting the manufacturer's marketing claims of < 100ft deployments. If you know where such evidence can be obtained, feel free to post a link.

"perceived greatness of the Skyhook"? Clearly you have now shown your prejudice about the Skyhook. Just because you have not seen it does not in any way make it less credible, it just weakens your case for not having reviewed all of the data prior to making unfounded assumptions. I would recommend that you go over to RWS and let them show you the videos before you comment further.

So that you understand, I do not condone ever deviating from emergency procedures including altitude. The discussion that I am having here is only what the equipment is capable of.

[MB38 0]

Here's an image I tossed together from exporting frames of a Skyhook deployment video. It appears sloppy from compression artifacts and the camera's exposure changing, but I assure you that I lined up each selected image with the previous. What that means is that the sky background plate is as it existed on the day it was shot. That said, consider the dramatic shift in perspective as the jumper drops to the camera's altitude and below.

I chose the first frame as the moment before cutaway [the last frame before the jumper began to fall] and I chose the last frame as the first frame that the slider stopped sliding. If you want to see just how much the reserve is "flying" at this point, feel free to check out the video on RWS's site [in the Skyhook section].

If somebody wants to get all technical and measure the distance the jumper fell from cutaway to flying reserve based on the length of the lines as the perspective changes over the course of the video, knock yourself out. Either way, it'll give a decent idea of the "actual" [read: this particular instance's] cutaway to flying reserve altitude.

I really don't know what I'm talking about.

[980 0]

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Challenging me for the known videos that clearly show sub 100ft. openings

Are you talking about the RWS Skyhook video that is available online too?

If you are (and I think you are), I call bullshit.

None of the Skyhook videos I have seen CLEARLY show a sub 100ft opening.

and that includes the slider down deployments too

I am also curious what precautions were taken with the slider down reserve jumps to avoid a high airspeed deployment of the slider down reserve?

Were the jumpers wearing tertiary reserves to use in the event of a main baglock or a similar malfunction that would have allowed the jumper to build up a dangerous amount of airspeed for a slider down deployment?

Or were the mains deployed in a way that made this impossible?

I will once again say I think the Skyhook is a great and useful device, but I stand by my statements that it cannot deploy a standard sport skydiving reserve (packed as per manufacturer's instructions, which included a slider last time I checked, so you can survive a high speed main malfunction) from a standard Vector 3 after a cutaway from a standard skydiving main, in 100ft or less.

If you have any REAL EVIDENCE to support your claims that it can, please bring it forward and I do not mean SUBJECTIVE ESTIMATES based on videos, or altitude loss calculated with over simplified formulas.

Thank you and please try to not take this personally.

Cya
Sam

PS – 8 seconds between opening and landing on a highly loaded skydiving reserve is likely just enough time to see what it is you’re going to hit on landing. I am not surprised that the RWS video only shows the one landing from the 2 slider down deployments, the other one looks like it was going to be ugly before it goes out of view.

[980 0]

You realize that what you did is largely useless because the camera is mounted to a person under a descending parachute also?

So unless you want to get the data regarding that parachute’s flight and do some really fancy math you cannot just use that picture as evidence for altitude loss on a Skyhook reserve deployment.

[980 0]

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That should bring this discussion to an end.

well yes, it should have, but apparently the poster directly below your post still wants to argue about it....

[tso-d_chris 0]

Not according to the video on your site.

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[tso-d_chris 0]

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You would be correct if I had not personally heard you say it at the drop zone!!

You heard no such thing, because as a rule I don't discuss skydiving gear when I'm not at work, and I don't work on a DZ. Care to share which DZ you heard this at and when you heard it? Your claim is 100% untrue.

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[MB38 0]

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You realize that what you did is largely useless because the camera is mounted to a person under a descending parachute also?

So unless you want to get the data regarding that parachute’s flight and do some really fancy math you cannot just use that picture as evidence for altitude loss on a Skyhook reserve deployment.

Fully understood. I forgot to comment on that. If somebody really wants to get technical about this perceived issue, they would have to take into account the glide and altitude loss of the camera flyer. While we don't know that exact figure, measuring altitude based on line length is pretty inaccurate as well.

I really don't know what I'm talking about.

[fly_girl 0]

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>So while I agree it's a bad idea to cut away that low, some people _have_ been considering it. If the Skyhook really opens a reserve in 50 feet? Maybe it might be a better idea than landing with a collapsed canopy.

Whilst i don't have a skyhook myself, and therefore don't know much apart from it's basics...
if your canopy has collapsed then surely it won't provide the force necessary for the skyhook to open the reserve any quicker than the reserve pilot chute??

[billvon 2,406]

>if your canopy has collapsed then surely it won't provide the force
>necessary for the skyhook to open the reserve . . .

It can be difficult for new packers to get a main parachute down to a size that's smaller than their pilot chute. It is EXTREMELY unlikely that a collision (or turbulence) will, by chance, result in a packed main parachute. What is more likely is:

1) the parachute will get hung up on the other jumper - in which case the skyhook is effectively using the drag of the other person's parachute.

2) the parachute will be physically damaged by the collision, and not be landable. It will still be generating a lot of drag.

3) the parachute will be spun up by the collision, and not be recoverable before impact. It will still be generating a lot of drag.

[fly_girl 0]

Thank you for clearing that up in my head

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[980 0]

I guess you didn’t like my questions either, judging by your silence now?