Ways to figure out dimentsions for a transition between 2 body tube sizes

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Aug 22, 2020
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Sorry if I'm in the wrong forum but I tried some searches and came up empty. I need to make a transition between 2 body tubes and looking for
how to come up with the measurements to make it out of cardstock with some Ctr rings out of balsa or plywood?

I have yet to find a transition template program where it's straight on one side... an eccentric conical transition.

Eccentric Cone.jpg
I have yet to find a transition template program where it's straight on one side... an eccentric conical transition.

View attachment 647433

I have a book, "Metric Drafting", that has several chapters on pattern development for various manifolds like this. Sorry, the book is in storage, otherwise I'd post the equations.

I've never found equivalent coverage in another drafting book. Maybe there's a pattern development book somewhere.
I have yet to find a transition template program where it's straight on one side... an eccentric conical transition.
Over on the Paper Modelers forum, prolific plans maker scissorsandplanes shared formulas for making a pattern for an offset truncated cone. But the formulas only find the corners and midpoints of the pattern. I'm not sure how he gets the curves. He says he uses an Excel file and plots points on curves as a "radar plot" (polar coordinates).
Many moons ago I whipped up a tool for work to create concentric and eccentric reducers - generally from stainless steel sheet. It creates a DXF template. Because it was an inhouse thing, I have no idea if it'll work (install correctly) on external computers. So, treat it as a last resort (possible) option.


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This is an old but tried and true simple method... You need a ruler and a compass... Works for basically any tube sizes. Note does not work that well if the transition is extremely short.. I.E. if this example was only 1/4" tall...errors build up quickly...
Top Horizontal line = diameter of bigger tube.
Bottom line = smaller tube.
Vertical distance is your transition height.
Connect the ends of each line to fond the center point. (Where the circle is...)
Use compass to draw circles at the midpoint of the horizontal lines.
Cut out the curved piece longer than needed, don't forget room for a glue tab... Eyeball it for approx. length...
This example is a 1.5" tall adapter for BT-50 to BT-60... Length along curve about 4.77" Cut out about 5 1/2" along curve, you can trim back to include enough for the glue tab.

Mount smaller tube into larger one with centering rings, making sure the ring where the larger tube is juts out enough to have a spot to glue the transition.
Pre-curve the paper, cardstock or whatever and dry fit it before gluing.

There may be some slight error, try a plain paper one first. Wrap it around and look to see how the ends line up, if square, your golden! If the large diameter part overlaps more than the smaller, then the center point you used needs to move down a little bit. (This changes the ratio of the two curve lengths)


Or... 3D Print it...
I was going to build a tail cone for a scratch built downscale of the Estes Sprint so I spent a little time deriving the equations. I've seen similar equations on the internet but in slightly different form. Basically the equations tall you the angle and the 2 different radii. You map it out on a piece of paper and cut it.

A few years ago I did a scratch build with 3 side pods around a central core, the side pods were slit so they fit partly onto the central tube. I wanted offset cones on the side pods. I figured out the geometry using equally spaced points around the perimeter of the pods and then calculating the hypotenuse to the offset point of the cone. I mapped this out on a piece of paper, cut it out and did a test fit. Then I made a slight change to the next couple of patterns until it fit the way I wanted and then cut the final cones out of cardstock. Something like this might be possible for the offset cone that someone asked about earlier.

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