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Fully Inflated Canopy
One design rule-of-thumb students come up with early in
their parachute design work is that bigger is better:
adding more surface area increases the drag. Certainly,
a canopy with the diameter of a penny and holding a load
of two washers will fall faster than a chute with a single
coffee filter. Such reasoning focuses on one featured
benefit while ignoring associated trade-offs. It misses
a second rule-of-thumb students also propose: that heavier
chutes fall faster than lighter ones. More coffee filters
mean more mass, which increases gravity's force on the
system.
Students typically fail to notice a key criteria to good
parachute performance: how fully inflated the canopy
is during descent. Large-canopied chutes may potentially
have more drag, but they do not fully fill up with air.
The chutes do not fall fast enough for there to be enough
air flow and pressure to fill up the heavier canopy.
Use this and other movies to help students focus on
a chute's moment-to-moment descent. Show the movie frame-by-frame,
and ask your students what they notice about the canopy
as it descends. Try to note the forces at work that
cause the parachute to lose its fully inflated shape.
Then apply this "focused" way of viewing the
falling parachute to the drops that students do in their
own tests.
Adding more washers can help solve this problem, and can help. In fact,
if your class is going to do its final test outdoors, the
problem of crosswinds, which can collapse a chute, is formidable
and can be solved by increasing the load attached to the
canopy. Increasing the load gets the parachute carries to experience enough air flow to keep its shape fully inflated,
even in the face of a brisk crosswind.
DESIGN HINT: Students should add coffee filters until they
see that the canopy does not fully inflate and then go back
to the largest number of filters that stayed inflated throughout its descent.
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