This is about PC hesitations on short delays when going stowed. The problem with PC’s that have been packed in the BOC for a longer period of time should be well known, so this isn’t really what I was going to write about (although it plays a role in the hesitation described below)… but just in short-, the material (especially ZP) remembers its shape after some time, do not underestimated this effect, as it will slow down the opening of the PC drastically. Repack your PC prior jumping!
But even with a freshly repacked PC hesitations can happen, like those here and here. (although the second PC was packed for quite some time afaik)
The inflation process of a PC when load is applied, (bridle reached stretch) divided into three stages:
1. Mushroom, skirt closed.
Has the same shape as the mushroom when packing the PC. (The skirt is the seam around the PC that holds upper and lower part of the PC, ripstop and mesh, together.)
2. Inflating, skirt open.
Air enters through the opened skirt and inflates the PC starting at the top (same as inflation of a round canopy). This stage can be rather short, since it does not take a lot of air to fill the PC, and the PC practically being a apex-pull-down round canopy (it has a center line), so the skirt can move freely, and therefore can spread very fast.
All PC’s in the video below have been packed just minutes before the jump, thanks to weaselman for the last clip, original video can be found here.
The mushroom shape in stage 1, can stay stable while being dragged through the air for quite some time, like in the last part of the video above. The opening sequence just stops here because the skirt is closed off tight and only little or no air enters the PC. Some factors have to be met to make this happen, certain airspeed, little deformation of the PC prior reaching bridle stretch (this is where the shape memory problem comes into the equation), etc, so this is really hard to reproduce. The main cause for this type of hesitation, is closing the PC’s skirt too tight when packing it.
This form stays stable until the skirt is opened, which will happen by chance as airspeed increases and flow around and behind the PC is getting more turbulent as it is accelerated, as well as the field of decreasing pressure around the PC is pulling its fabric outwards. In the video above one can see that at first only one panel of the skirt got pulled aside immediately letting air in so the rest of the skirt is opened as well.
If the PC made it to stage 2, therefore the skirt opening reached a certain size and air is entering the PC, no hesitations are going to happen anymore (except there is some mis-rigging issue, damage, etc). As stated above and seen in the video the PC inflates very fast from here on, depending on air speed, maybe literally “pop” open the PC. The figure aside (Fig c1 and c2) shows that horizontal expansion is not linear, but the bigger the skirt opening, the faster the further skirt expansion. (both graphs show expansion within 0.1s time, taken from “Parachute inflation, a problem in aeroelasticity” by Stein and Benney)
To reduce the risk of hesitations:
- Leave some mesh to the sides of the mushroom when packing to not close the skirt all the way. Be careful that the little slack of centerline can not entangle on an internal handle or similar.
- After folding the bridle, carefully fold the mesh over it, without pulling more mesh from the bottom, or rolling the excess mesh.
- Repack your PC before jumping.
- Go handheld if in doubt.
This kind of hesitation can happen with vented and unvented PC’s. I have the idea that a vent will make this more likely to happen, if the little air that enters a small skirt opening leaves the PC through the apex vent it could possibly delay inflation, but I don’t have sufficient data to do any comparison.
“Parachute inflation, a problem in aeroelasticity” by Stein and Benney
“The fluid dynamics of parachute inflation” by Peterson, Strickland and Higuchi
“Inflation of a round” by Johnny Utah
Thanks to Mahle for pointing this out to me in the first place, and to Johnny Utah for sharing his experience. Comments are appreciated!