Inflation of a Round
by Johnny Utah
A square canopy is an airfoil, a wing, a glider. The pressurized wing generates lift because it is an airfoil shape. In this article, round canopies are the topic, so....
A round canopy is a drag device. The canopy captures air as it is pulled in a direction. This capturing of air pressurizes the canopy with static pressure holding the round canopy in its inflated shape. The shape of the round canopy creates drag, which is the opposing force to whatever is pulling the payload in the said direction (i.e. gravity). It is this drag that allows a person to land with a safe rate of decent under a round parachute.
Inflation of a Round
For a round parachute to be considered inflated, it needs to be pressurized with air. Air goes in through the opening at the skirt. Some air will exit through any hole at the apex. This is because the airflow is moving from the skirt to the apex of the canopy.
During a round deployment with some airspeed, the canopy starts out in a stretched out streamer type of shape. The skirt of the canopy is gathered together in the center and therefore the opening at the bottom is very small if any. Because there is some airspeed, the airflow across the canopy's outer surface creates a low pressure. Since there is a lower pressure on the outer surface of the canopy than on the inside of the canopy, the streamer shape begins to expand. The opening at the skirt begins to expand as well.
As this happens, air goes in there. The air keeps collecting inside there, and it is at the apex of the canopy where it collects for the most part. Once the opening at the skirt becomes large enough to let in enough air to completely expand open and pressurize the canopy, then that is what happens.
The low air pressure on the outside of the canopy during initial inflation helps the canopy change from a streamer shape to an expanded, more open shape.
If a round parachute were deployed with its skirt completely opened up (full diameter), then it would inflate (pressurize) immediately and the need for an external low-pressure to help expand the streamer and open the skirt up (as during initial inflation) would not be needed. The static pressure created by the canopy capturing air is greater than the external pressure and therefore the canopy stays open.
When doing a high airspeed deployment, there is a low pressure created on the outside of the PC that helps it to initially expand. It seems to work very well and that is why I use a regular mushroom for those types of jumps.
Considering the reality of the occasional PC hesitation during a nil to low airspeed deployment; my point of view is that if you can get the mesh/rip-stop seam of the PC opening quicker (and reliably) on its own, then air will go right in there and pressurize the PC that much quicker. That is why I thought of the Super Mushroom idea years ago. That is its purpose.
If you pack the PC so that it is dependent on airspeed to begin to open, then you need sufficient airspeed at deployment time or enough altitude to get away with a hesitation.
With the regular mushroom, sometimes you can do a go and throw and the PC opens immediately with nil to very little airspeed. I believe this works because enough air successfully flows through the opening at the mesh/rip-stop seam and successfully inflates the PC. However, sometimes (with the regular mushroom) the airflow is not so successful to get in and inflate the PC at low airspeeds. The result is an occasional hesitation. I have seen PC hesitations that looked like they probably hesitated until an external low pressure did happen and helped get the PC to open. Not ideal in my opinion.