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Drawing an ellipse - 1/17/2012 1:02 PM   
abufletcher



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From: Zentsuji, JAPAN
Status: offline 		I have a need to draw a just slightly elliptical circle. Short of trail and error, what is a mechanical method for making an ellipse with a certain width? I vaguely remember twin nodes and a string but don't know how that would work in practice.

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RE: Drawing an ellipse - 1/17/2012 1:10 PM   
AmishWarlord



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RE: Drawing an ellipse - 1/17/2012 3:57 PM   
crash bandicoute


 

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Status: offline 		i pulled this out of a model masters series book. hope the pics are good enough to see the lines.

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RE: Drawing an ellipse - 1/17/2012 4:05 PM   
AmishWarlord



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RE: Drawing an ellipse - 1/17/2012 4:22 PM   
crash bandicoute


 

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Status: offline 		i have 4 different books actually. they were like bibles when i first started building. that article for the ellipse was out of the building techniques one. i picked them up at the LHS a few years ago, and now whenever i see them. there's so much cool stuff in them.

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RE: Drawing an ellipse - 1/17/2012 4:35 PM   
AmishWarlord



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Status: offline 		I have that book. I though it looked familiar.

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RE: Drawing an ellipse - 1/17/2012 5:36 PM   
Chad Veich



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Status: offline 		The method demonstrated below is very easy to do in a CAD program but can be done by hand pretty easily as well. Let's say we want an elliptical former to fit our new Sea Fury design at the station shown below in red.



First we must determine the distances necessary for our finished ellipse as determined from the side and top views. The blue line on the side view represents the datum upon which all the sections are based and in the top view the blue line is the centerline of the fuselage.



Next we make up a bounding box based upon those dimensions.



Divide distance A, B, and C into an EQUAL number of segments, twelve in this case. The more segments you choose the more mathematically perfect your ellipse will be but, at some point, you run into diminishing returns for the effort involved. Just make sure to use the same number of segments for every former that you create.



Starting with the top-left corner connect each of the segments with a line. You should end up in the bottom-right corner at the datum as shown. This will be an exercise in futility if you failed to divide A, B, and C into an equal number of segments!



Every point where the lines cross (shown circled in red) indicates a point on the ellipse.



Simply connect the dots! Very easy in a CAD program but also simple with a French curve if you're doing it the old fashioned way.



Repeat the above steps for the lower half of the former...



...and mirror the image to complete the section.



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RE: Drawing an ellipse - 1/17/2012 5:42 PM   
AmishWarlord



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Status: offline 		Thanks Chad I'll give that one a try.

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RE: Drawing an ellipse - 1/17/2012 5:48 PM   
Lnewqban



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Status: offline 		String and pencil.

The smaller the slack of the loop, the closer to a circle shape.

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RE: Drawing an ellipse - 1/17/2012 8:08 PM   
allanflowers



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Status: offline 		I just have to say that the last one (Lnewqban) is geometrically good and gives a true ellipse.
The first youtube method is really really tedious and requires extreme accuracy to get it right. The second method, which has one true radius at the ends and another true radius for the sides is simply a horrible approximation of an ordinarily elegant curve. The tangent method is interesting but I don't think produces a true ellipse, just a smooth curve.

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RE: Drawing an ellipse - 1/17/2012 9:39 PM   
Chad Veich



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quote:

ORIGINAL: allanflowers
The tangent method is interesting but I don't think produces a true ellipse, just a smooth curve.


It will produce a true ellipse if segments A and B are equal length. I have found very few fuselages where that is the case however.

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RE: Drawing an ellipse - 1/18/2012 12:10 AM   
R8893


 

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Status: offline 		Chad, can you post a drawing of an elipse from your CAD program superimposed on the curve you constructed above?
Chuck

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RE: Drawing an ellipse - 1/18/2012 1:39 AM   
abufletcher



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From: Zentsuji, JAPAN
Status: offline 		Lots of good stuff here, gentlemen! Thanks. Maybe the easiest approach for my "not quite round" purposes is just to draw a circle in photoshop and then "squish" it.

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