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    Flight Path : Stable Descent : Fully Inflated Canopy  
MOVIE 1:   0 minutes 22 seconds

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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