Vent openings can be located at the top or apex of a cup-shaped
canopy, or near the canopy's skirt or perimeter. They can
help in meeting two design criteria: stable flight
and a full inflation of the canopy. Since most parachutes tend
to oscillate, having
a method for lessening or dampening that behavior is important.
Vents work by allowing a portion of column of air that a parachutes displace to pass
through the canopy, which reduces swinging.
But are there tradeoffs associated with this benefit?
Many student designers focus on the positive effects of
a change in a design, while neglecting to consider drawbacks.
Most students would predict that vents holes reduce drag
by reducing the surface area of the chute. Actual tests
show that for vent holes up to 15% of the area of the chute,
there is no reduction on the overall drag force for a chute
using vents compared to one without. Thus, you get the benefits
of greater stability, but not the drawbacks of loss of drag
force, when you use vents up to 25% of the total canopy
You might want to review the movie to hear again about how air flows
through vents to give the benefits of more stable flight without loss
of total drag force.