Bottle प्रोफिलेस
What are they for?
When you build a water rocket, you n
eed to attach fins to the bottle near the neck which is almost always. For stability, you need fins and as a rule of thumb, you need them as far back as you can easily and practically get them. This means swept back fins that extend beyond the neck of the bottle where the bottle lid was. This also means unless you are using extension spars, you need to attach them to the portion of the bottle that is tapering down to the neck from the main part. This area is seldom straight so cutting fins to mate to that curving surface is usually a trial and error, trim-fit-trim-fit, process and that process is rarely quick and easy nor does it usually result in a nice, full-contact attach area.
With the profile of the bottle laid out o
n a grid, you can easily design your fins so that when you cut them out, they match the bottle profile perfectly the first time with no trimming unless you didn't follow the cutout lines closely.
You can also cut out the contour of the bottle and use the outside of what you cut to check bottle contours to see if they match a bottle not listed before making fins. There seems to be one very common profile for 2-liter bottles because Pepsi, Squirt, 7-Up, and Safeway are all the same bottle profile. Other times, profiles will be very close but not identical.
How are bottle profiles obt
ained?
People have tried all kinds of ways to get profiles such as using right triangles and a flat surface, taking circumference measurements at intervals, dividing them bi pi, and plotting them on a grid, etc. I have seen it suggested broadly to just use a light and trace the shadow. This is almost right except that you must use the sun as your light source unless you have a powerful light a long w
ays away or can focus a light to parallel beams (no easy method).
Light sources within reasonable distances of your bottle will fan out in all directions from the filament of the incandescent bulb or the tube or bulb surface from florescent and similar light sources causing distortion of the projected image. The sun is so far away that the light rays reaching earth are parallel for all practical purposes and so project an accurate image as देमोंस्त्रतेद
It didn't work very well because the bottle was not secured enough and would move around. Also, grass is not a good place to try to prop up the fixture, and getting a profile with the label still on does not give a very accurate profile of that area.
Second Fixture
So okay, maybe just a little more effort was in आर्डर
I used some scraps on a board--a block with some foam to hold the mouth of the bottle, and a screw through a piece of angle iron with a taped hole on the other end. I used blocks of various sizes between the screw and the bottom of the bottle, 1) so the screw wouldn't cut into the bottle and 2) so I could used different blocks for different lengths (heights) of bottles. This held the bottle a lot more securely--enough to easily take the profile without it moving. I taped the paper down this time instead of using push pins. Bricks were used to prop up the fixture so it could be adjusted perpendicular to the sun and this was done on concrete. The blanket in the picture was because this was done in the summer and the concrete was both hot and hard on bare knees (while wearing short pants). This time the fixture and process worked very well and profiles were obtained for all my bottles. Note the label was properly removed this time for all profiles.
The sun moves across the sky and so your shadow will move with it. It doesn't move so fast that you can't trace a bottle outline without realigning your fixture but it does have to be realigned after a few minutes. Also, it is easiest to trace when the sun is directly overhead plus the shadow doesn't change nearly as fast as when at low elevations. So the best time is closer to noon standard time and if you really want to get particular, do it on the fall or spring equinox at noon. Just kidding, you can do it at any time but the middle part of the day is better. Guess what, this doesn't work if the sun is hiding behind a cloud either. Even thin clouds diffuse the light and make a fuzzy shadow.
Final Contour Processing
During the process I was also able to determine which bottles had the same profiles because I could line them up perfectly with previously taken profiles.
I added a line exactly down the center of the raw profile with marks at exactly 6" or 8" depending on the bottle size. This is so the final contour could be scaled in a pdf file properly.
The raw contour was scanned directly into Microsoft Image Composer which was bundled with FrontPage 2000 (unfortunately not later versions) at double normal size for more accurate curve plotting.
Using a point plotting function (shapes-polygon), I plotted points around the image so that it would create a nice dark "smooth" curve accurately depicting the raw image. I created lines for the scale lines on the scanned image. The curves and lines were separate from the scanned image and the original image was not used after this point.
The profile was reduced back to near scale size, saved as a full bottle profile in gif format, then cropped and saved as a side profile in gif format.
A quarter inch grid was created in MS Word (for easy conversion to a pdf file) and saved as a template.
A generated profile was inserted into the grid document, resized to the final accurate scale to match the grid, and saved as a pdf file.
Technically, you only need to take a profile from one side of the bottle but to obtain a centerline, I did both sides. The centerline is used to align the bottle sides with the grid so fins can have parallel and perpendicular sides and then was also saved in case someone wants the full profile.
Bottle Contours
Here is what the 2-liter coke bottle profile with grid looks like (not to scale) with a couple of sketch fins:
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