Reserve Deployment Thoughts

[Hooknswoop 19]

I started thinking about how the largest and the smallest containers of a particular model have the smae size reserve PC.
Theoretical:
Take two identical jumpers, same speed, altitude, weight, etc, deploy same type of reserve at smae altitude, except one is 100 sg ft and one is 200 sq ft. they fire the reserve at the same time. Because the PC's are the same size, the smaller reserve will come out of the D-bag sooner. assuming they inflate at the same rate(sq ft / sec), they would both inflate to 100 sg ft at the same time. the difference being the 200 sq ft canopy would be only 1/2 open.
So which reserve would open faster? this isn't a quiz, i don't have the answer, but we have some pretty high speed people on this forum that could provide some great insite. So how about it?
Hook

[PhreeZone 15]

Well... the launch of the smaller reserve would be a touch faster since it has to lift less weight at the start.
This is my theory....
They don't inflate at the same rate.... The larger one starts out with more inflated then the smaller one does at the start right? i.e. More material in the wind. The rate in which the canopy catches air will be at the same rate, but inflation speed will not be. They both are having the same amount of air particles flow over them, but the larger canopy can use more of them then the smaller canopy can at any given point in time. (the mouth of the larger one can scoop more then the smaller)
The amount of intake to inflate is the difference. Now the question is... does the larger reserve scoop more or less volume of air over the same peroid as the smaller reserve? If its more... then it will inflate sooner, if its less it will be slower.
My best guess..... both are about the same for intake but since the smaller reserve launched faster.... it'll be open a little higher then the larger reserve (I'm guessing less then 25 feet diff)
Of course packing style blows this whole thing out of the water right? Anyone who sees any holes in my thinking let me know.....
Cause I don't wanna come back down from this cloud... ~ Bush

[rhino 0]

I'm thinking OVERALL the small reserve would completely open quicker but the larger reserve would slow you down faster.
When all is said and done they should be flying steady at the same altitude. It would just take the bigger canopy more time to get there??
I tried :)
Blue Skies and Smooth Rides!!

[Hooknswoop 19]

I can compare the launch times of big and small reserves w/ the same size pilot chute and it is a significant difference. I had 6 cutaway on an XRS w/ a 109 and recently deployed a Furry 220 out of a J4. Big difference in the time to get the canopy out of the d-bag at pretty equal speeds. Even worse for an Eclipse tandem reserve deployment- took forever for that heavy reserve.
I think most of the time, inflation doesn't play a big role in canoy deployment. I have been wanting to take the topskin and ribs off a canopy and compare the deployments, before and after.
Hook
Hook

[PhreeZone 15]

Inflation plays a huge roll in the opening of a canopy. Look at the difference inbetween the inlets of a reserve compared to those of a main. The Reserves are way more radical and have a larger over hang then the mains. I don't have my books here at work but I was thinking it was like a 7 inch difference in size on a reserve and only like 5 on a PD main. I'm probally wrong on the numbers but....the over hang on the reserve is designed to open faster then a main.
Also on taking the top skin off... There would be no cells to inflate to create pressure to force the slider down.... If you use another type of reefing system, it might work....
Cause I don't wanna come back down from this cloud... ~ Bush

[Hooknswoop 19]

The 5 in & 7 in measurements, where are you measuring? And are you measuring a 7 cell and a 9 cell or a 7 cell main and a 7 cell reserve from PD?
I have watched my Safire open with the top skin laying on the bottom skin and the ribs completely slack in the between the top and bottom skins then it pressurizes and off we go.
Hook

[PhreeZone 15]

I can't find the info any more.... I think I heard it from my rigger now that I think about it.... But what it was talking about was at full presureation, if you drew a line straight up from the leading edge of the bottom to the top skin, that overhang. And he might have been talking about the difference between a Raven and a PD. Ignore my earlier comments on the measurements until i can find conctete answaers to my questions
Cause I don't wanna come back down from this cloud... ~ Bush

[Hooknswoop 19]

Remember- a 7 cell will have a thicker airfoil section than a 9 cell, all other things being equal.
I've got an old clipper 195. I'll jump it as is, then chop off the ribs and top skin and compare the openings. Cool another test jump!
Hook

[Beachbum 0]

Hook ... I'm starting to think you just like to tinker!

[Geoff 0]

I've never heard a smaller BASE canopy recommended because it will inflate faster. and I believe the BASE industry understands inflation speed much better than the skydiving industry.
Just a thought -
Geoff

[BPO 0]

Quote

I've never heard a smaller BASE canopy recommended because it will inflate faster. and I believe the BASE industry understands inflation speed much better than the skydiving industry.

Good point!
But.. that might also have to do with the fact that BASE jumpers usually have to land their parachutes in a small area, hence they need a 'sinkable' parachute?
Just another thought..

[Hooknswoop 19]

In my original post, I was assuming identical inflation speeds, the big point was different size and therfore weights reserves being deployed by the same size PC. The total difference in deployment altitude can be affected by the longer time it takes for the canopy to come out of the bag.
And why not larger PC's for larger reserves?
Are the small containers have too big a reserve Pc or do the large containers have too small a reserve PC?
Hook

[rhino 0]

This is kind of entertaining actually.. lol
Blue Skies and Smooth Rides!!

[PhreeZone 15]

Quote

And why not larger PC's for larger reserves?

Why not? This is totally a guess on my part, but I'm willing to bet not too many companies want to have to pay a the fee to get a TSO for each size container and Pilot chute combo that they could have. Anything reguarding the reserve/reserve container I was under the impression that it had to be tested and certified.... If one size fits all and it will work, it would be cheaper to make them too....
Cause I don't wanna come back down from this cloud... ~ Bush

[riggerrob 558]

First of all, the smaller reserve will lift out of the container slightly faster and since it requires fewer air molecules to fill, will inflate a fraction of a second faster.
The difference in inflation times will be insignificant.
As for the notion of different sized reserve pilotchutes for different sized reserve canopies .... it really only makes a difference when you get into tandem sized reserve canopies.
And remember that most reserve pilotchutes are over-sized to begin with (36 to 42 inch diameter). They produce hard openings if used on mains.

[Hooknswoop 19]

"And remember that most reserve pilotchutes are over-sized to begin with (36 to 42 inch diameter). They produce hard openings if used on mains."
I have always wondered about this one, and have a few spirited discussions about it. If the size of the PC (assume it is a kill line) only affects the amount of time between pull and the canopy coming out of the bag, why would a canopy open harder w/ a larger PC. I can see faster total deployment because it comes out of the bag sooner, but the canopy shouldn't inflate faster........??
I agree that manufactures use one size reserve PC because it is cheaper, one TSO, one size for manufacturing, etc, but which size are they made for? I would guess the middle sizes and the the smaller containers the reserve PC is over-kill and the larger containers, the PC should be bigger. I know that on my 2 tandem reserve rides, waiting for the reserve to come out of the bag took a while. And I know the fury 220 takes longer to come out of the free-bad than my 109 does. Seems like the same distance for inflation between the two after the canopy is out of the D-bag though.
Hook

[PhreeZone 15]

The reason larger PC's cause harder opening on smaller canopies is that they slow the canopy down faster then the canopy slows the jumper down. The larger the pilot chute, the faster the bag gets stopped while the jumper continues at terminal. A small PC might take .5 seconds to lift the bag and pop the line stows and extract the canopy from the bag, while a large PC might only take .2.
The difference is like running into a brick wall with your car at 100 mph while braking or at 25 mph while in the middle of braking. The slower you hit it, the softer the impact. The faster the bag comes off your back, the less distance/time it takes to open. The less time it takes means a faster/harder opening.
Cause I don't wanna come back down from this cloud... ~ Bush

[Hooknswoop 19]

I disagree. If a canopy takes 1000 ft to come for the last stow to come undone or 500 ft, you are travelling the same speed and the canopy will open the same. The time the d-bag spends getting to line stretch has no impact on how fast the canopy will inflate. Once it is out of the bag, and the lines are taunt, the canopy will inflate at the same rate regardless if it takes 1 second or 30 seconds for that to happen. the overall opening altitude, from PC toss to full canopy will be shorter on a larger PC, but the opening should be exactly the same (for a kill-line PC).
Once the canopy is out of the bag, the PC is collapsed and has no effect on inflation of the canopy, so big or small, smae opening.
Hook

[PhreeZone 15]

The person that could really explain what I'm thinking about is Dave Brownell. He once explained on here or rec. why he used a small PC. It had to do with a Large PC with a small canopy results in a super high G force transfer to the jumper.
Cause I don't wanna come back down from this cloud... ~ Bush

[KellyF 16]

Openings are broken down into three stages- snatch force, snivel, and inflation. With a larger PC, snatch force will increase, since you just presented the canopy to the airstream at a lower airspeed than you would have with a smaller PC. The jumper will then re-accelerate the canopy to his own speed. The greater the difference in speed, the higher the snatch force. Imagine a rope 15 feet long tied around your waist, and tied to a wall on the other end. When you walk away from the wall and reach the end of the rope, it will tug at you and make you stop. Now imagine running away from the wall at full speed. When you reach the end of your rope, you will be jerked quite violently (and painfully!). This is snatch force.
A higher snatch force could affect the snivel also, by putting higher initial loads on the canopy, and disturbing the packjob. Actual inflation speeds should be about the same, but could be different because of the shock put on the canopy with the higher snatch force.

[Hooknswoop 19]

Thanks- that makes a lot of sense.
How much then does the size of the slider affect snatch force? Following the same logic, bigger slider means bigger snatch force, all other things being equal.
Hook

[KellyF 16]

Correct.

[PhreeZone 15]

KellyF explained exactly what I was trying to.... only better. The Larger slider would not be more snatch force since the canopy would already be clear of the bag. A larger slider would present more snivel since it takes more force to push it down the lines, the canopy will not inflate as fast. Does a pocket slider cause harder openings because it presents more surface to the airstream?
Cause I don't wanna come back down from this cloud... ~ Bush

[riggerrob 558]

I didn't really want to get into too much detail about main pilotchutes, but. ...
Reserve pilotchutes are "over-sized" to begin with, typically 36" to 44". They would produce hard openings if used as main pilotchutes.
For example, if you look at last year's fashion in AFF student rigs, with main ripcords and and spring-loaded main pilotchutes, you will see that most use massive springs to launch their main pilotchutes clear of the huge, 3-way burble. Most manufacturers equate massive pilotchute springs with massive pilotchute canopies. This is great for initially snatching the d-bag out of the container, but massive main pilotchutes become a liability by the time you get to line stretch. By line stretch, the pilotchute and d-bag are going slowly, even if the jumper is still falling at the same rate.
The key point is that main pilotchutes remain tied to the canopy. So when you reach line stretch, the main piltochute exerts an extra "spike" in the deceleration curve. This spike is transmitted directly down the lines to the harness. This spike often results in tears near the bridle attachment point.
Hint, I just overhauled 6 Mantas and 2 Raiders that were used for AFF.
I am so current on canopy patches and repairing bridle attachment points!

[riggerrob 558]

To quote George Gallowy (Precision Aerodynamics), a larger slider means that the canopy starts spreading earlier, but it takes longer for the slider to come down the lines.
The end result is a softer opening.
Videographers have been known to install tandem sliders on their Sabres to soften openings!