Team Wharton B
We designed a framed parachute in order to maximize the time the tennis ball spent in the air after we dropped it.
When we dropped it from the second story of Hicks, it had a drop time of 4.84 ± .02 seconds.
We started brainstorming ideas as soon as we got our materials. We first discussed making a triangular glider or kite, with the three dowels joined at the one end (braced by the plastic container and flared out at the other, then covered with the opened plastic bag). We hoped that a glider would look and act like a paper airplane, falling slowly by catching air currents as it moved forward instead of just dropping. We batted this idea around for a good long while, thinking that it would be more interesting than a parachute. We even tried looking into what previously designed free-fall gliders looked like, but never found any designs we thought were suitable.
Eventually, we realized that the glider would need horizontal motion to get started - we would have to throw it, not drop it. Without a horizontal impetus, it seemed likely to fall nose-down, very fast. We discussed compensating for the lack of initial forward motion by distributing the weight of the tennis ball further back, to pull the back of the glider down and keep it horizontal. However this solution, if it worked at all, would have required a lot of trial-and-error adjustments to distribute the weight correctly, which meant risking damage to our materials in the process.
Upon deciding that a glider was too risky to attempt, we settled on a parachute design. It would have a wooden frame to keep the parachute open as wide as possible and to hold the tennis ball. After considering several possible configurations the for the dowels, in the end we went with an H-shape, to maximize the surface area. We then decided to open the sides of the plastic bag and pleat the edges. Pleated and attached to the frame, the bag would poof out when dropped, allowing it to catch as much air as possible. The tennis ball would hang underneath the apparatus, providing stability. We thought about cutting the tennis ball up to distribute its weight farther out to the sides of the frame, but felt that it wouldn't really gain us anything. At this point, we intended to use the red drawstring from the plastic bag as string, and to supplement it with twisted tape. During the building process, however, we concluded that the drawstring would be better used reinforcing the frame and that it was simpler and more effective to fold the tape instead of twisting it.
Our first order of business was to cut open the plastic trash bag and reinforce it with tape in a grid pattern. A grid of tape would keep the plastic from sticking to itself, strengthen the parachute, and encourage it to open and fill rapidly and completely when dropped. At the same time, we cut the bottom piece of the plastic container in half, for use in reinforcing the frame's joints. We were worried that without enough rigidity, the frame might twist, making the parachute unbalanced. The plastic semi-circles would make it more structurally sound.
Next, we lashed the three wooden dowels into the H pattern, taping the joints as tightly as possible to keep them from moving. We removed the red drawstrings from the plastic bag and pleated the edges until it fit the dimensions of the frame at the sides. The greatest challenge here was to pleat the bag evenly, so that it wouldn't be bunched up on one side and stretched out on the other. If that happened, the parachute would catch air unevenly and tilt. We solved this problem by dividing each edge into sections and then pleating each section to the same length. We then fastened the pleats with two layers of tape, one on each side.
We modified the plastic braces to fit properly and attached them. We then proceeded to attach the red draw-strings from the trash bag to the wooden frame, giving it another two sides, as well as added stability and strength. Finally, we attached the pleated trash bag to the frame and suspended the tennis ball from the whole thing using folded over tape as string.
As we had the best of the free-fall times, it is hardly surprising that we are quite happy with how our design worked. The only real glitch was that our "string" (made of tape) had a tendency to unfold when the device was left alone for any significant amount of time, which meant that it needed some minor repair when we took it out again. It is possible that if we had gone with the original plan of twisting the tape this would not have been an issue.
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