DESIGNING AIRFORMS OF DIFFERENT SHAPES
Here is an easy way to design and make airforms for any shaped dome: hemisphere, catenary, onion, mushroom, anything.
At any convenient scale, say 1 " = 1' draw a vertical building section through the apex. For example a hemispherical dome would be a semi-circle springing up from a horizontal base line.
On one side of the semi-circle (from the base to the apex), starting at the base and continuing up to the apex at the very top, make a dot on the curved line at every foot.
Strike lines parallel to the base through each of these dots and extend them horizontally through the entire building, or, alternately, to a vertical centerline passing through the apex (double this for the full width).
Each of these horizontal lines represents the edge view of a horizontal plane cut through the building at every foot along the edge line. It is a circle in plan view.
Find the circumference of each of these circles (3.14 16 X its diameter In inches).
To find the width of the gore at each foot along the gore pattern centerline divide the circumference of each circle (previous step) by the number of panels needed to make the airform. The number of panels is found by dividing the maximum width of your fabric in inches (your choice) into the circumference (inches) at the base. Generally, the more panels, the smoother the shape. For small domes (15' -20 ' ) use a panel width of between 3 or 4 feet at the base.
With the above information, draw a full size single gore on a long piece of butcher paper. Place this on top of a stack of Tyvek or polyethylene sheets (all of the gores you will need for the airform). Tack these down with small brads (right on the line if you intend to sew the gores together, 1 " out from the line if you intend to tape the gores together). Cut all the gores out all out at once with a sharp utility knife. Cut 1/4" out from the line if you are going to sew the gores together, on the line if you intend to tape the gores together. The nails should be randomly spaced (1/2" to 1-1/2" apart) if you elect to sew the gores together. The random spaced nail holes become a code to aid in registering adjacent gores in the sewing machine.
A 2 ' or 3 ' circular "eye" at the top will help the problem of so many gores converging at that point. Leave a little extra at the base for anchoring.
THE TAPING JIG
purple marker dome
Here's an easy way of making airforms by using tape to join the gores together.
Make a neat stack of all of the rectangular panels of Tyvek or other thin, strong fabric. Lay the gore pattern on the top. Using shingle nails, nail through the pattern and the entire stack every 2' or so and an inch or so outside of the pattern line. Cut through the pattern and all the panels right on the pattern line using a utility knife. .
I've done many airforms on the same piece of carpet for years without leaving any scars. You need a soft top surface like carpet to improve the cutting action of a thin sharp blade and you need wood underneath to nail into. Don't worry about ruining the carpet. Just don't use a too heavy touch.
Make "Tick marks" (little notches or marks) located at random every couple of feet along the cut gore edges to help align adjacent gore edges on the taping jig. Do this while they are still stacked after being cut from the rectangular stack of Tyvek panels. The tick marks will be used as a "code" and should be unique to each side.
Leave a little extra at the base (say 6") for anchoring the airform. Mark the "BASE" line on every gore.
Starting at one end of a gore, stick its edge down one side of this bender board line. Press an adjacent gore edge down on the other side(with this curved setup the gore edges will stay on the line and not wander away). Reverse every second gore so that the coded tick marks will be matched. Press a single long strip of package wrapping tape or duct tape over the seam. Peel off the now taped together gore panels and move to the next. Easy.
A 2 or 3 foot flat circular "eye" of Tyvek solves the problem of so many gores coming together at the apex. Tape this eye in place while the otherwise completed airhouse is draped over a small circular table.
Usually the more gores the smoother the inflated shape.
Refresh the adhesive from time to time.