The design of the shape of the skirt panels so that when they are sewn together they take up the desired 3D shape is not immediately intuitive. However, it's not too difficult to generate the shape needed, as long as you limit yourself to a skirt made up of intersecting cylinders or parts of cylinders. This page describes how to do it.
Please note, however, I'm not offering to do it for you. I've made this explanation as straightforward and step-by-step as I can. If you can't follow it, I don't think I can make it any easier, sorry.
For more complex geometry, you can extrapolate this method to conic surfaces, but it's probably best to track down a technical drawing book for more details.
All the pictures on this page are for an imaginary skirt geometry, and are at the scale of 1 pixel=1mm. I used CAD, which makes it easier to delete things and move things around, but the method works even if you're using pencil and paper.
|Draw a vertical cross-section through the proposed skirt. This is a bag skirt, the outer visible bit being 50mm radius, and the inner face is 100mm radius. I've drawn it attached to the corner of a box hull, but that really doesn't matter.|
|Rotate it 90 degrees (of course, you can actually draw it like this in the first place).|
|Starting at one end of the skirt, divide it into equal length
portions. I've chosen 20mm, and here I'm part-way round, using a
circle to find the next 20mm on around the skirt.
To be precise, you'd divide it up so the distance round the arc from
one mark to the next is the chosen length. The construction here makes
the straight line from one mark to the next 20mm, but the arc length
then is 20.136mm, an error of 0.68%. However, that's close enough (and
if you made the spacing 10mm, the error would drop to 10.017mm =
|The skirt fully divided up. You could imagine this as a cross-section through a skirt made from cloth with lines running along the length of the skirt.|
|Now draw a plan view on the piece of skirt, carefully lined up
with the cross-section. I've decided that I'll have two skirt pieces
joined at right angles (90 degrees), so the intersection between them
will be a plane at 45 degrees to the axis of the cylinder. In plan, it
will just look like a line. Include the 'hidden' bottom part of the
skirt (I've drawn it in a dashed line).
If you had pieces joined at 45 degrees, the line at the end of the piece would be at 22.5 degrees from vertical; joined at 60 degrees the line would be 30 degrees from vertical; etc.
|Draw the corner of your fabric piece above the plan of the skirt. This is where you'll create the shape of the fabric piece laid out flat. The edge needs to be exactly parallel to the axis of the cylinder making up the skirt.|
|Now draw in the first set of construction lines.
From the end of the skirt in the cross-section, draw horizontally 'till reaching the line that's the cut end of the skirt piece in the plan, then draw vertically until you get to the edge of the fabric piece, and put a mark there.
|Now the process that gets repeated all the way round the skirt. Start by drawing a horizontal construction line 20mm (or whatever the interval was on the cross-section - I just chose 20mm) away from the last line on the laid-out piece. Next, from the next mark round the section, draw horizontally 'till you get to the cut end on the plan, then draw vertically 'till you get to the new construction line, and put a mark there.|
|Carry on around the skirt. I've done the next three points here.|
|Carry on to the end of the skirt.
I was sneaky here - my skirt neatly divides into 20mm intervals. If you end up with a part step, you do exactly the same but when you do the construction line on the laid out piece do it closer to the previous line.
For example, suppose my skirt end was 14mm from the last mark I had when dividing it up, instead of putting my last laid-out construction line 20mm from the previous one, I'd put it 14mm and otherwise do everything the same.
|Join the dots.
The closer your interval was (the 20mm I chose), the smoother and more accurate your curve will be, but it will be more work to get to this point, so it's a trade-off.
|The bit that joins onto this bit will be a mirror-image, if the mirror is placed on the line that was the cut face at the end of the segment of skirt. This is very easy on CAD, somewhat more work by hand. However, you don't actually need to draw it again - you could just cut out the piece you've done above and turn it over.|
The shape drawn is the shape of the piece with no allowance for joins. Since you need a bit extra for seams to connect the two pieces, and to join the skirt to the hull, you need to cut out the fabric pieces 5 or 10mm bigger all round (depending how good your sewing and sticking is). The pieces join up along the drawn lines.
The easiest way to understand the process of the construction is to imagine you're drawing a skirt with lines along it. On the cross-section, you just see the ends of the lines (the marks made at 20mm intervals), on the plan, you know where the lines end (at the cut face), and when you lay the piece of fabric out, obviously the lines are 20mm apart.
Once you've got your head around how this works, the process is relatively easy to refine. For example, at the back corners of the Mk2 I have set in a partial piece at 45 degrees, to round the corner. This is purely cosmetic, so there's not a sharp corner on the skirt when you look down on it in plan.
As an isometric, the view on the left is the simple 90 degree join,
and that on the right is with a 45 degree insert:
The construction of the back corner of the skirt on the Mark 2 therefore looks something like this sequence:
|The section and plan laid out.
Note that the plan has a 45 degree cut line for the inside of the skirt (where the two pieces join at 90 degrees) and a 22.5 degree cutline for the outer face, where there will be the extra piece set in at 45 degrees.
|The construction lines and points all laid out.|
|Now I mirrored the whole lot in the 45 degree mirror line to get the other main piece, and mirrored the first part of the curve in the 22.5 degree line to get one edge of the inset piece, then mirrored that in the 45 degree mirror line to get the other edge of the inset piece.|
Made up, the corner of the skirt looks pretty much as expected:
The templates for all the pieces on the Mk2 skirt therefore end up
looking like this. This drawing has my 10mm seam / joining allowance
added on all round each piece. (If you click the picture, you'll get a
scale extract at 2pixels=1mm):
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