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"Credit Card" Fins
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Following is a way to make fins from plastic credit cards (gift cards, phone cards, etc.) and rubber bands or cable ties.
The plastic cards have a series of holes made with a paper hole puncher--each card is identical to the next. Note that if you use rubber bands, slits may have to be cut between the edge of the card and the outer holes to make it possible to slide on the rubber bands.
Three overlapping holes punched close together allow the cards to grab and lock onto the flange on the neck of the bottle--as this part of soda bottles is standardized regardless of the volume or shape of the bottle these fins can fit any bottle. This type of fin arrangement will not work with external releases that hold onto the bottle flange, but will work with my Sport Cap release as well as other cap releases and internal releases such as those made by Anti Gravity Research and designs by others featured in my "links."
If you are using a bottle that has a long thin neck (like a wine bottle or beer bottle shape) you can eliminate the use of the three holes altogether. Instead, just assemble the fins around the neck above (below?) the flange. Note that this will move the CP forward and so may necessitate adding more nose weight. However, if your goal is to make a side-glider this fin arrangement works quite well, even with short rockets.
This animation explains the process and the use of cable ties
Fins made from plastic credit cards and rubber bands
Fins mounted on a bottle
Bottom view
