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# Rate of deceleration

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I was wondering how quickly we actually slow down after deployment. For example, if a 175 lb person is under a 175 sq ft. canopy, what is the velocity 1 second after full inflation, 2 seconds, 3 seconds, etc.

Does anyone know?

Thanks

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You go from about 170 fps in freefall to 17 fps under canopy. And that's about the same from the moment the canopy opens and afterward. Some canopies are faster than others. And jumper input can either speed up or slow down the descent rate. How many G's you pull *during* opening depends upon how fast the canopy opens, which is highly variable, from one canopy to another, even with the same canopy, and from different packing techniques.

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So, essentially, if the canopy were fully inflated 10 ft above the ground, your speed when you reached the ground would be the same as if your canopy fully inflated at 1000 feet?

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So, essentially, if the canopy were fully inflated 10 ft above the ground, your speed when you reached the ground would be the same as if your canopy fully inflated at 1000 feet?

For the most part, yes. Keep in mind that this is the speed of the canopy in the deployment configuration (as-in with the brakes set on most canopies).

In the case of slower openings, the bulk of your freefall airsped is bled off by the time the slider comes down to the stops. In the case of a faster opening, there may be some residual energy left over due to the lack of time for decelleration, so maybe the fully opened canopy might descend faster than normal for a period of time (a very short period).

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>For example, if a 175 lb person is under a 175 sq ft. canopy, what is
>the velocity 1 second after full inflation, 2 seconds, 3 seconds, etc.

Most canopies take 2-5 seconds to open. 1G is 20 mph per second, so going from 110 to 10mph in 5 seconds would be 1G deceleration. (Which plus normal gravity would result in feeling 2G's of deceleration.) A 1 second opening would mean 6G's total which would be painful.

Most canopies descend at about 10mph, which you reach after the opening is complete. That's affected by brake settings etc so often you slow down to 10mph then speed up slightly when you release the brakes.

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1G is 20 mph per second, so going from 110 to 10mph in 5 seconds would be 1G deceleration.

Bill - explain that, if you would. I don't understand what that means.
Chuck Akers
D-10855
Houston, TX

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>Bill - explain that, if you would. I don't understand what that means.

1 gravity of acceleration is 32 feet per second, per second. Or 20 miles per hour, per second. So if you were in a car that went 20mph after the first second, then 40mph after the next second, you'd feel 1G of acceleration pushing you backwards. (Plus the normal 1 gravity you feel from earth's gravity pulling you down of course.)

So if you accelerate or decelerate by 20 mph per second you feel 1 gravity of acceleration/deceleration. If it's 60mph per second you feel 3 G's.

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Most canopies take 2-5 seconds to open. 1G is 20 mph per second, so going from 110 to 10mph in 5 seconds would be 1G deceleration.

That assumes that the deceleration is at a constant linear rate over those five seconds, or an average over the five seconds, which my not be true. Some openings could have very little deceleration at first, followed by a very violent opening shock at the end, giving a peak that would equal far greater G's.

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That assumes that the deceleration is at a constant linear rate over those five seconds, or an average over the five seconds, which my not be true. Some openings could have very little deceleration at first, followed by a very violent opening shock at the end, giving a peak that would equal far greater G's.

A couple of brutal openings I had testing a new reefing system for a Delta 2 parawing would suggest that IS definitely the case. Bruises the size of dinner plates on the inside of the thighs took a couple of weeks to disappear.

Opening shock followed by instant pain was the go.

Needless to say, that bright idea went straight in the bin!!!.
My computer beat me at chess, It was no match for me at kickboxing....

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I was wondering how quickly we actually slow down after deployment. For example, if a 175 lb person is under a 175 sq ft. canopy, what is the velocity 1 second after full inflation, 2 seconds, 3 seconds, etc.

Does anyone know?

Jean Potvin and Gary Peek of the Parks College Parachute Research Group studied opening forces in the late 1990s using the rectangular parachutes which were prevalent at the time.

http://www.pcprg.com/s01out.htm

Here's a graph from their PIA presentation showing opening forces (in G) on Sabre 120, 150, and 230 canopies with a 205 pound suspended weight.

http://www.pcprg.com/pia01f2.gif

where peak forces are 4.5 - 5.5g with the quicker openings pretty much done in 3 seconds (back then parachutes opened faster and harder and kids these days would complain about that).

You could integrate to get speed if you wanted (true airspeed started at 120 MPH) or just find some contemporary altimeter recordings if you want speed.

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Here's a graph from their PIA presentation showing opening forces (in G) on Sabre 120, 150, and 230 canopies with a 205 pound suspended weight.

http://www.pcprg.com/pia01f2.gif

where peak forces are 4.5 - 5.5g with the quicker openings pretty much done in 3 seconds (back then parachutes opened faster and harder and kids these days would complain about that).

A jumper of our club recorded some of his jumps with external equipment for a programmable calculator. He found similar readings, but also that landing can result in short peaks of 5+ g and the same for jogging on the pavement.
The sky is not the limit. The ground is.

The Society of Skydiving Ducks

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Very nice work .... oh and a great way to get your sky dives paid for .......

Note to self ...... Do some skydive 'research' (N.B I'm self employed ... wonder if my accountant will go for that one ... don;t see why not aye?)

(.)Y(.)
Chivalry is not dead; it only sleeps for want of work to do. - Jerome K Jerome

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I use the K.I.S.S. principle: Hard/Soft.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

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There are 2 things going on during canopy deployment.

First is “snatch force”.
The force generated during deployment when the bagged canopy reaches line extension and the pilot chute is accelerated back to the speed of the jumper. The greater the speed is between the P/C and the jumper at line extension the greater the felt force

Second is “opening shock”.
The force generated when the load is slowed by the opening canopy. Happens just after snatch force and depending on the mix of components it feels like snatch force and opening shock accrue at the same time. The amount of felt force during opening shock has a lot to do with the canopy design and how well it is reefed.

Sparky
My idea of a fair fight is clubbing baby seals

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Thank you, good people.

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Simple physics

Speed at time of deployment (after main is out of the bag) = Vo = 110 mph = 160 ft/s

Speed after deployment = Vf = 10 mph = 15 ft/s

Y = distance traveled = 0.5*a*t^2 +Vo*t+C

t^2 = t squared

C is a constant that is zero in this case (trust me)

a = acceleration to stop from Vo to Vf

t = time interval from Vo to Vf (read snivel time)

Take the first derivative of y=0.5at^2+Vo*t+C

dy/dt = a*t+Vo

dy/dt = speed after the reaction = Vf

Vf = a*t+Vo

Both Vo and Vf act downward and “a” acts upward against the speed (that’s why you stop falling)

-15*ft/s = a*t-160*ft/s

145*ft/s = a*t

a = rate of acceleration during inflation = (145/t)*ft/s/s

g = normal gravity = 32.2*ft/s/s

G’s pulled = 1+a/g

Table of Values
t (secs) a (ft/s/s) G’s
1 145 5.5
5 29 1.9
8 18.125 1.56
10 14.5 1.45

At that’s why a line dump smacks you.

(fixed the table)

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At that’s why a line dump smacks you.

Nope. Line dump doesn't smack you.

Bag strip might though.

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At that’s why a line dump smacks you.

Nope. Line dump doesn't smack you.

Bag strip might though.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

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At that’s why a line dump smacks you.

Nope. Line dump doesn't smack you.

Bag strip might though.

Line dump is when the lines come out of their stows before line-stretch. This may cause knots but since the stows do not appreciably slow the opening it will not be harder. You could free coil your lines in the bottom of the container and have perfectly acceptable openings (I am not recommending this).

Bag strip is when the canopy comes out of the bag before linestretch. In this scenario the canopy is exposed to the wind and may start to open up and/or the slider may come down the lines. In this scenario, a hard opening is very likely at line-stretch.

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The amount of felt force during opening shock has a lot to do with the canopy design and how well it is reefed.

terminology question: what is reefing? With regards to a parachute.

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Reefing means to hold back the inflation of the parachute for a short time while the parachute slows down. The slider is a reefing mechanism.
Elastic suspension lines would soften the opening shock, but would not be a reefing mechanism (just for example).
You don't have to outrun the bear.

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Reefing means to hold back the inflation of the parachute for a short time while the parachute slows down. The slider is a reefing mechanism.
Elastic suspension lines would soften the opening shock, but would not be a reefing mechanism (just for example).

What he said. If you deploy a ram air canopy at termina without a slider you would lose you shoes.

Sparky
My idea of a fair fight is clubbing baby seals

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[replyWhat he said. If you deploy a ram air canopy at termina without a slider you would lose you shoes.

Sparky

Somewhere in NE Georgia there is a Walmart Special (black, 8.5) hiding in the woods. If found, please return it to Skydive Monroe.
My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

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When Katanas first came out, I borrowed a 107 from a buddy just to see what they were all about. He jokingly told me to be careful because his girlfriend, who was learning to pack, was the last one to pack it.

Sure enough, it popped me, and then went into a pretty good spin (not a brake fire). The first thing I noticed during the spin was that one of my feet was colder than the other one, and once I got the canopy under control, I looked down and sure I enough I was missing a shoe. Vans, size 11, black, if anyone finds one in the trailer park down the road from the DZ.

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The slider is a reefing mechanism.

thanks!

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