Inflation of a Round
by Johnny Utah
Some
basics:
Square
Canopy
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....
Round
Canopy
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.
Pilot
Chutes
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.
Super
Mushroom PC Packing Technique