1/48 MiG-15 engineering question....

[Pierre Sacha]

Good day guys,

 

Can someone please help me with a calculation here? I want to create a cone shaped ring of plastic to add to the inside of a 1/48 Mig-15 intake in order to smoothen out that abruptly ending lip just aft of the intake mouth, a problem plaguing all Mig-15/17 models apparently. 

The smaller and larger circumference of the cone is easy using 2πr, right? But since this cone is created using a flat curved strip of plastic, how do I calculate the radiuses and corresponding angle between the two radiuses in order to create a curved strip that, when joined together, will result in the correct cone ring? I hope I explained it well enough 😔 

I know the two circumferences of the cone become arcs of two bigger circles and that the angle that results in the two arcs must be the same. The two arcs (circumferences of the ring) are  47mm and 57mm repectively and the strip must be 4mm wide which tells me the difference in the two radiuses must also be 4mm. From here I'm lost howerever😄.

 

Thanks for any help,

 

Regards

Pierre

[martin_sam_2000]

off the top of my head..I can't come up with a mathematical system, but I would use some paper to make a template.. Draw out the shape in a 2D form (diameter of the two ends and how long you want it. ..like a top down view) That will give you the the length the two sides. now extend those to side limes up to form a full triangle and extend it down slightly longer than you need, cut a rectangle out of the paper that matches this larger triangle, roll it into a cone  with you measured lines on the outside and then cut the cone at the proper lengths of the shape you are trying to make. that *SHOULD* give you what you are looking for.

 

 sean

 

[Pierre Sacha]

Hi Sean,

 

That is an excellent idea, thanks kindly sir!

 

Pierre

[habu2]

I have derived this in the past when replacing a fairing on a rocket model. I will look for it tonite. 

[Rob de Bie]

22 hours ago, Pierre Sacha said:

Can someone please help me with a calculation here? I want to create a cone shaped ring of plastic to add to the inside of a 1/48 Mig-15 intake in order to smoothen out that abruptly ending lip just aft of the intake mouth, a problem plaguing all Mig-15/17 models apparently. 

The smaller and larger circumference of the cone is easy using 2πr, right? But since this cone is created using a flat curved strip of plastic, how do I calculate the radiuses and corresponding angle between the two radiuses in order to create a curved strip that, when joined together, will result in the correct cone ring? I hope I explained it well enough 😔 

I know the two circumferences of the cone become arcs of two bigger circles and that the angle that results in the two arcs must be the same. The two arcs (circumferences of the ring) are  47mm and 57mm repectively and the strip must be 4mm wide which tells me the difference in the two radiuses must also be 4mm. From here I'm lost howerever😄.

 

Maybe there's an easier solution? You can glue a steel or brass wire of the right diameter inside the intake, and fill the rest with epoxy putty? Solvent putty could work too, but it will shrink, and you will have to build it up slowly to avoid melting the plastic. Like this:

 

 

Rob

[Pierre Sacha]

Thanks guys your help is appreciated!

 

Pierre

 

[habu2]

OK this may be difficult to follow because I don't have the resources to post drawings, so - math. 😬

 

You have two circumferences, 47 mm & 57 mm.  Rounding slightly, that equates to two diameters, (47/pi) = 15 mm & (57/pi) = 18 mm, or two radii 7.5 mm & 9 mm

 

Looking at your intake from the side, this coned ring looks like a trapezoid, 15 mm across the top, 18 mm across the bottom, with ends 4 mm long.  If you extended these 4 mm sides upward to where they intersect you would have what looks like an isosceles triangle (a cone viewed from the side).  It may help to draw this out to help you visualize it.

 

Now we need to know the length of the line you drew if you extended those side edges to where they intersect.  This will be the outer radius R of the arc you need for your coned ring.  Using similar triangles, (R/9) = (4/1.5) or R = 9*(4/1.5) = 24 mm

 

So your outer circumference of 57 mm is now the length of an arc with a 24 mm radius.  Solve for the arc angle, which is (57/24) = 2.375.  That's in radians, so convert to degrees (2.375 * 57.3) = 136 degrees.

 

Now draw a 136 degree arc with a radius of 24 mm.  Draw a second arc inside this one with a radius of (24-4) = 20 mm.  That should be your template for a coned ring.  This pattern will assume you are butt-joining the two ends of the strip.  If you want some overlap then extend the arc angle a bit to make the curved strip a little longer.

 

Note this does not account for the thickness of the plastic sheet stock you would use.  That's a whole 'nuther dimension of complexity...☹️

 

I used this method to create the conic adapter sections of a model rocket.

Edited May 9, 2020 by habu2

[crackerjazz]

Hi Pierre, I've done lots of these I can help you.   Given the circumferences you mentioned I came up with this.  I mean, the software did : )      What thickness styrene will you be using?

 

 

 

 

Then I flattened the cone:

Flattened cone (print on paper, glue to 0.25 styrene and cut).PDF

 

Try to print that on paper and glue onto styrene sheet.  I use a Pritt stick as it leaves no residue).   Use a pair of scissors to cut and peel the paper off.   The ends of this flattened cone don't overlap -- you can add a little tab if necessary.    I just saved the flat pattern in pdf -- try to see if it prints in 43.83 width -- that's how wide it should be.  

 

Edited May 8, 2020 by crackerjazz

[Pierre Sacha]

On 5/8/2020 at 7:41 PM, habu2 said:

OK this may be difficult to follow because I don't have the resources to post drawings, so - math. 😬

 

You have two circumferences, 47 mm & 57 mm.  Rounding slightly, that equates to two diameters, (47/pi) = 15 mm & (57/pi) = 18 mm, or two radii 7.5 mm & 9 mm

 

Looking at your intake from the side, this coned ring looks like a trapezoid, 15 mm across the top, 18 mm across the bottom, with ends 4 mm long.  If you extended these 4 mm sides upward to where they intersect you would have what looks like an isosceles triangle (a cone viewed from the side).  It may help to draw this out to help you visualize it.

 

Now we need to know the length of the line you drew if you extended those side edges to where they intersect.  This will be the outer radius R of the arc you need for your coned ring.  Using similar triangles, (R/9) = (4/1.5) or R = 9*(4/1.5) = 24 mm

 

So your outer circumference of 57 mm is now the length of an arc with a 24 mm radius.  Solve for the arc angle, which is (57/24) = 2.375.  That's in radians, so convert to degrees (2.375 * 57.3) = 136 degrees.

 

Now draw a 136 degree arc with a radius of 24 mm.  Draw a second arc inside this one with a radius of (24-4) = 20 mm.  That should be your template for a coned ring.  This pattern will assume you are butt-joining the two ends of the strip.  If you want some overlap then extend the arc angle a bit to make the curved strip a little longer.

 

Note this does not account for the thickness of the plastic sheet stock you would use.  That's a whole 'nuther dimension of complexity...☹️

 

I used this method to create the conic adapter sections of a model rocket.

 

Thanks habu2, sorry for the late reply. I love these math problems, lol. Will have a go at it now. Thanks for taking the time!

 

Pierre