The correct size parachute for you is packed in one of 3 different style containers. The correct style for you depends on how you fit in the cockpit, and where you have room to spare. Sometimes two different parachute styles could be worn in the same cockpit, depending on the size, length and bulk distribution of the user, and where the seat cushions are placed. In the case where more than one partner will be flying the aircraft usually one parachute sized for the heaviest pilot works fine.
The back style is the most common and has the parachute container all behind your back. Some backs are constructed with a wedge shape (thicker at the bottom) for some extra lumbar support.
The seat style is all under your butt, and are most often worn in the deep seat pans of many warbirds, but they are used in some smaller aerobatic type aircraft as well. The area behind your back is usually just thin padding over the harness webbing located inside. Some seat packs have a slight wedge shape so it's a little thinner where your tailbone would rest, and a little thicker under your knees.... this can be helpful where there are torso height restrictions.
The chair style is split between your back and under your butt, and are most commonly worn when you have very little room to spare behind you or under you.
We can help you determine which is the correct size and style for your best fit in the cockpit.
A Softie "Mini" backpack with 240 lb. max suspended weight canopy installed.
Picking the right parachute system for you
National 360 seat parachute with quick ejectors on legstraps and chest. Phantom 24' canopy.
National 360 chair parachute with B-12 snaps on legs and chest. Phantom 24' canopy.
Military 28' seat parachute with quick ejectors on legstraps and chest. Not an overly comfortable harness but authentic for the warbirds. Note the extra bulk of this canopy, which makes the container about 6" thick. It completely fills the seat pans of many vintage aircraft. Bulk is due to size of the canopy and heavier construction, making for a very strong canopy.
Basically the right parachute for you will depend on your weight, density altitude, and potential opening speed. Heavier pilots need larger parachutes for softer landings -- wing loading again. If one does the math, a 24' diameter flat circular parachute has a surface area of about 450 sq.ft, a 26' has about 530 sq.ft, and a 28' sports about 615 sq.ft. Larger canopies require more lines and longer lines, plus a lot more fabric, so this equals higher pack volumes and therefore a heavier system.
With higher altitude operations (eg. gliding over the mountains) if one had to bail out they could be looking at landing at 6-8000 ft. ASL, and even where the field elevation might only be 3000 ft. a hot August afternoon can produce similar density altitude conditions that could contribute to very hard landings.
As with everything else in aviation, the drag increases in square to a speed increase, and the drag of an opening parachute is called opening shock. For higher speed operations there are some special parachutes made that are either strong enough to withstand a high speed opening, or have a sequencing system which allows the parachute to deploy slightly slower, which reduces the peak loads on the canopy. The newer parachutes incorporate meshed modifications that allow some pressurized air inside the canopy to leak out at a controlled rate, giving reduced oscillations plus some forward speed and steering capability. Some of the older parachutes incorporated a line release system that allowed the same capability. While the skydivers converted over to the ram air wing style parachutes (squares) in the 80's, most pilot emergency systems are still 'round' parachutes. They are less expensive to build, work fine and need less skill to obtain a good landing.