How are 7 cell parachutes more stable than 9 cell parachutes?

[Westerly 61]

It seems that whenever reliability is really needed--reserves, BASE mains, wingsuit mains--the go-to is a 7 cell parachute. Why? How do fewer cells yield more reliable openings? I always figured the planform shape (e.g. square vs fully elliptical) is a far more important factor in determining opening reliability than how many cells there are. Is that incorrect?

[DHemer 0]

My understanding is it is the aspect ratio (AR) that makes the biggest difference.

7 cell parachutes are usually around 2:1. Twice as wide as they are deep.
7 cells are typically also more "square"/"Less elliptical" which you have mentioned, but this does not have to be true.

The trade off is the lower the aspect ratio the less efficient a wing is. The larger cells compared to a 9 cell also result in a top skin with more deformation which further reduces the efficiency of the wing.

For comparison from Aerodynes website, they list aspect ratio and planform factor (PF -how tapered the wing is)

Smart (7 cell reserve): AR=2 PF=0
Pilot 7 (7 cell main): AR=2.3 PF=8.57
Pilot (9 cell main): AR=2.51 PF=6.6
Zulu (9 cell higher performance main): AR=2.75 PF=10

So the Pilot 7 is more "elliptical" than the Pilot when compared by Aerodynes formula.
There is so much going on in how a canopy flys and opens that the planform is not meaningful.

John Leblanc knows best: https://www.youtube.com/watch?v=f-mUyy1fhjE

[pchapman 261]

Yes, saying "you want a 7 cell" often actually means " you want a canopy which has been designed with the aspect ratio and nose inlets and thickness and other characteristics typical of historical 7 cell canopies that are built for conservative performance when it comes to on-heading performance and steering sensitivity"

After all, PD Velocity & Valkyrie are "7 cells" according to the naming conventions we use, even if they may have 3 chambers between line groups instead of the traditional 2 for a canopy.

I supposed fewer lines on a reserve might theoretically very slightly reduce the chance of tension knots, who knows. There have been reserves with 9 cells, but that's unpopular except for really big, heavy duty reserves where a few more lines could help structurally -- tandem reserves in particular.

Nice to see you "just ask a question" instead of starting off with some (mock?) accusation and outrage about how the way the skydiving world works is all wrong and you have the superior way to do things!

(Yes there are always things that aren't as simple as they seem or are confusing to the newbie due to how wording and naming conventions have evolved historically.)

[riggerrob 558]

Aspect ratio is the key determinant in opening on heading. The farther apart end cells are, the greater the risk of one completely inflating before the other catches any air.
Cross ports help reduce assymetrical openings.

Off-heading openings should not be a problem with skydiving reserves. If you bought a reserve that is dangerous with off-heading openings, then you bought too small a reserve!

As for stability ... the larger the end cells, the more stablity in roll. Seven-cells provide the best compromise.
In comparison, the last time I jumped a 5-cell reserve, it turned allay but was unstable in roll. I did the bare minimum of turner to land on the DZ.
OTOH, nine-cells have smaller ends cells = even less stable. Tapered nine-cells have even smaller end cells.

[Alexg3265 0]

Less parts, less stuff to fail, dirtier airfoil means more drag(more ballooning/less ribs) which translates roughly to stability... a square 5 cell at 1-1 ratio would open amazingly on heading and be super reliable until it comes to producing the lift to land. the 2:1 A/r 7 cell seems to be the best middle of the road right now as far as pack volume/ acceptable landing performance/ and reliability.

I was that kid jumping out if his tree house with a bed sheet. My dad wouldn't let me use the ladder to try the roof...