Students frequently attempt to increase the surface area of their
parachute systems in two ways -- by spreading more filters out
(A) , and by stacking them up (B). Spreading out the coffee
filters to make a larger, single-canopy parachute is most common.
Others stack up filters vertically in a few ways.
1. One approach is to stack the coffee filters one inside another
(C). Having multiple filters fitted into one another may provide
the slight advantage of added stiffness to the canopy -- this would
help maximize the inflated area. Students can say they want to prevent
air from flowing through the paper, but this does not occur at such
slow rates of descent. Real parachutes move so quickly that air passes
through the material. Basically, stacking filters adds to the overall
mass of the parachute system without increasing the drag. This is
a lopsided trade-off, with little advantageof which to speak.
2. Another approach is to tie one filter to another with strings
whose length varies according to the designer. Students may get this
idea from seeing the drone parachute that helps deploy the main chute
in large parachute systems. This design has problems with poor
inflation because of air turbulence. The flow of air forms eddies
behind the lead canopy. These eddies interfere with the flow of air
into other layers, which do noit s not fill up fully with air. This
partial inflation lowers the parachute's efficiency, worsens
its performance, while adding more weight. Watch this page's movie
to see drops of multi-tiered chutes.
3. Some multi-tiered parachute that students have been found to work
quite well. Having a regular, cupped-shaped canopy, but with the weight
taped to a single coffee filter oriented with its open end up
(D). Essentially, the lower filter's drag lessens the overall
pull of the washers on the strings and canopy. The system is stable
since the parachute falls at such slow speeds that the turbulence
does not disturb the inflating of the upper canopy. A real parachute
that might encounter wind speeds of 6 m/sec would have a wake
that would interfere with a parachute three times its width, and separated
by as much distance as a few meters.