Rate of deceleration

[erdnarob 1]

Hello Marco,

First the title. Using the expression "Rate of deceleration" means that the deceleration (or negative acceleration) is changing or not constant. If this is what you have in mind, it's OK. I would rather use "deceleration" alone.

That deceleration and consequently, the time of canopy inflation depends on :

1) the type of canopy you have
2) the size of the canopy
3) the type of packing you have done

* Shorter is the time of inflation, greater is the deceleration (hard opening) and shorter is the vertical distance needed ;
* Greater the time of inflation and smaller is the deceleration (softer opening)and longer is the vertical distance needed.

Canopy manufacturers have done a lot recently to slow down the canopy openings. Some of them have done so "well" that the vertical distance lost is just too much (1000 ft) !!

DECELERATION EXEMPLE 1

My Sabre 2-170 used to open on an average vertical distance of 340 ft. Putting that in the appropriate formula yields to these results :
vertical speed before opening = 120 mph (176 ft/s)
vertical speed after opening = 10 mph (15 ft/s)
results are :

a) average deceleration = 45.2 ft/s(square)
b) time for inflation = 3.56 seconds
c) vertical distance (given) = 340 ft

DECELERATION EXAMPLE 2

My Katana 170 opens on an average vertical distance of 600 ft
Vertical speed before opening = 120 mph (176 ft/s)
vertical speed after opening = 10 mph (15 ft/s)
results are :

a) average deceleration = 25.6 ft/s(square)
b) time for inflation = 6.3 seconds
c) vertical distance (given) = 600 ft

Note : These results have been obtained using the graphs downloaded from my Vigil AAD (using the Vigil interface and software). For each canopy I have evaluated 10 jumps graphs. Those graphs described the real distances, times and speeds as recorded in the AAD. This the most accurate method I have found.
With my Sabre 2 (it just happens once) my hardest opening was on a opening vertical distance of 180 ft which makes a deceleration time of 1.88 seconds (a slammer).

precautions to avoid a slammer :
1) make sure your slider grommets are against the grommet stoppers (check again at the end of the canopy bagging)
2) have the right rubber bands and stow the lines correctly
3) just after having launched your pilot chute, resume your box position (you can do a reverse arch) and look at the horizon
4) have magnetic riser covers like those on Vector. Magnets give at always the same force providing you with a symmetrical riser launch. Tuck tabs are not constant and may
lead to one riser going before the other one. That can create a slammer.

As you see, it is more complex that you have thought. I hope that those two exemples will give you some answer.

Have fun and be careful.

Learn from others mistakes, you will never live long enough to make them all.

[DubJump 0]

Nice information. It's all very interesting as a new jumper. Quick question to add on though. What are grommet stoppers?

Voodoo Symphony in the M-I-N-D. Wav files rub on your brain files constantly.