The fabric used for nearly all hot air balloons is made of Nylon material
that is woven in a "rip-stop" pattern. Hence the name "rip-stop Nylon".
For projects of this scale, fabric is always purchased in rolls. For this design, we assume that the rolls conform to the US standard of 60 inches in width. (Roll length may vary. However, the length is typically 100 yards per roll.) If the fabric you obtain is wider than 60 inches, then you must cut it down to 60 inches in width when you cut the panels. Fabric rolls narrower than 60 inches can not be used for this design.
What's in a "Yard"?
Pay attention! This is important!
There is a very confusing ambiguity in the use of the phrase "per yard" in reference to fabric. In casual use, "per yard" is sometimes used to mean "per square yard" and at other times used to mean "per running yard". Disambiguation is only possible by paying attention to the context.
A "square yard" is the usual 3 foot by 3 foot area. A "running yard" is an area of fabric that is 3 feet in length and the width of the roll being used. So for a typical 60 inch (5 foot) wide fabric roll, 1 running yard will contain 1 and two-thirds square yards.
The weight of fabric is almost always stated in terms of square yards. For example, "1.9 oz per yard" means 1.9 oz per square yard.
The price of fabric is almost always stated in terms of running yards. For example, "$7 per yard" means $7 per running yard.
This ambiguity is hopeless ingrained in the culture of textiles. Don't expect it to change any time soon.
One last point: Beware the "nautical" or "sailer's" yard. It is only 28 inches square. If you see fabric listed at a wonderfully good price and/or low weight be sure absolutely sure to check that it isn't being quoted in "sailer's yards".
The major portion of the envelope is comprised of a set of "gores". Each gore
is roughly analogous to a piece of banana peel. In this design, the envelope has 16 gores. All gores are the same width.
Each gore is comprised of 13 "horizontal" panels. The panels are called "horizontal" because given the way that they are cut from a roll of fabric, the "warp" (long direction of the roll) of the fabric will run horizontally on the finished envelope.
The panels are roughly trapazoidal in shape. Surprisingly, the only curved edges on the entire envelope are on the parachute top and the scallops in the skirt. Every other cut is a straight line.
The panels are numbered from 1 to 13 starting from the mouth at the bottom to the hole at the top.
Here is a
diagram that shows the numbering of the panes in a single gore.
Long Skinny Woven Materials
In addition to the rolls of fabric that make up the majority of the surface
area of the envelope, there are long skinny "tapes" or "webbing" used in various places.
The most important tapes are the "load tapes" that run vertically up the side and over the top of the balloon. The weight of the bottom end is suspended from these tapes. The bottom ends of the load tapes have "Delta" links that connect to the suspension cables that run to the basket. The tops of the load tapes have loops that are attached to a carabiner (or metal ring) at the very top of the envelope.
Other places you will find "tapes" are along the edges of the fabric including the mouth, top port opening, and along the edge of the parachute.
Envelope Color Planning
Most builders will plan the color scheme of their
envelope using a diagram such as
The first thing to make clear is that the process of building a balloon doesn't
have much of a resemblance to everyday sewing. Sewing is a very precise craft that
usually involves intricate corners and delicate constructs.
There is nothing delicate about balloon building.
The sturctures employed are designed and build for toughness.
And, there are no corners.
Everything is a straight line (or nearly so).
Further, the scale of the components will make most practitioners
of sewing flee in terror.
It would be more appropriate to call the process "industrial fabric bonding."
However, the sewing label is here to stay. Try not to be distracted by it.
The edge of a piece of fabric is called the "raw edge". If the pieces of an envelope are sewn
together correctly, all of these raw edges will be hidden.
This is done both for aesthetic reasons as well as to keep
the edges from fraying during use.
So a good rule of thumb is that if you have a raw edge showing, something is probably wrong.
Most of the fabric edges will be joined to the edge of some other piece.
A joint between two pieces of fabric is called a "seam". Just like the
hundreds of different types of knots used to join two pieces of rope, there are literally
hundreds of different types of seams. The difference between the
different types of seams generally relate to the way the fabric folds
within the seam.
For balloon envelope construction in general,
and for this design in particular,
all of the seams are the "French fell" type.
The French fell seam is used because it is both strong and it hides both
of the raw edges of the pieces being joined.
Sewing French fell seams requires a special type of sewing machine that has two needles (rather than the usual one)
which make two parallel lines of stitching in a single pass.
These are known as "double-needle" sewing machine.
Double-needle machines are more expensive and harder to find than
So, you may be tempted to use a single-needle machine and make
two passes to complete each seam.
In fact, people have built balloons using this two-pass method.
However, we discourage this approach because it is impossible to
keep two separately sewn lines of stitching precisely parallel.
Each small variation in the distance between the rows of stitching
will create stress concentrations in the fabric.
Also, it takes nearly twice as long to sew each seam.
So bite the bullet and buy a double-needle machine.
It's the right tool for the job.
French fell seams have a specified width. For this design,
the seams should all be 3/4" wide. The needles on the
double-needle machine should be set to 3/8" apart. To use
the technical term, the "guage" of the double-needle machine
should be 3/8". On most double-needle machines the guage is
set by switching a few components. So when you buy a machine,
be sure to make sure that it is either already set up for
3/8" guage or buy a "guage set" that has the appropriate parts.
For the edges of fabric that are not being joined to another piece
of fabric, the edge of the fabric is folded over and sewn down to form a "hem".
A piece of webbing is then sewn down over the hem to reinforce the
edge and to hide the raw edge.
In this design, webbing reinforced hems are used at the top rim, bottom rim,
and outer edge of the parachute.
In this design, the fabric is folded 3/4 of an inch when hemming.
All sewing should only be done with a "lock stitch" machine.
With a lock stitch, there are two separate threads.
There is a "top thread" which runs along the top of the seam
and a "bobbin thread" which will run along the underside of the seam.
There should be 6 to 8 stitches per inch for each line of stitching.
Sewing thread comes in different "weights" (i.e. diameter) and materials.
The thread used throughout this design should be #69 Polyester. The weight "#69" is also called "size E" thread.
All sewing should be done with #18 needles.
Most people use a contrasting thread color (e.g. white thread on black cloth) to make inspection of the stitching easier.
Many people use factory wound bobbins both for ease of operation and because they are packed tighter and thus make bobbin changes less frequent.
Top thread should always "bonded." Bobbin thread (in particular factory wound bobins) are typically not bonded. However there is no harm in using a bonded bobbin thread.
If you have to "break" a line of stitching in the middle of a seam (for instance if a bobbin runs out of thread.) then restart the new lines of stitching with at least 4 inches overlap of the lines.
When making a joint in load tapes, overlap the pieces being joined by 1 foot and use 10 lines of stitching to connet the two pieces.
Some Details about the Pieces
The dimensions of each fabric components has two parts:
1) its finished size and 2)
margins added to all of the edges that are folded into
the seam or hem as appropriate. The margins around the edges
are called the "seam allowance". These plans use 3/4" French
fell seams which require a 1 1/8" seam allowance. (The details
of the derivation of this rather
odd 1 1/8" number is too complicated to present here.
We will only point out that it corresponds to one-and-one-half times 3/4.)
Having margins around the edge of the seam is very handy during
the layout and sewing processes. In particular, it is useful
to add marks at regular intervals (called "registration marks"
or "index marks")
that allow one to keep track of the alignment of the pieces
being sewn together. In a perfect world pieces being sewn would
always be drawn through the machine at precisely the same rate.
However, in the real world, one piece or another will be drawn
at a slightly faster rate than the other. If unchecked, this
difference in rate will accumulate to a difference of several
inches over the length of a long seam. The person sewing will
need to apply different amounts of tension to the two pieces
in order to keep the pieces reasonably aligned. Registration
marks are key to being able to judge seam alignment.
We recommend putting registration marks at 2-foot intervals.
(If you have a curved edge, then 1-foot intervals are
more appropriate. That said, this particular design has no curved edges.)
For this set of plans, the hems are all 3/4". Consequently,
the "hem allowances" are all 3/4".
Layout and Cutting
Given that fabric comes on large rolls, it need to be cut to
the proper sized for the panels of the envelope. Layout and
cutting can be a surpisingly big job. For this design, expect
to spend 10 to 20 hours laying out and cutting the fabric.
Some folks do this work on the floor. Some use large tables.
The choice depends upon the flexibily of your spine and your budget.
Keep in mind that you will be spending a fair number of hours
doing the cutting. Both of the authors of these plans use tables.
There are a variety of cutting surfaces to choose from.
The two most popular are masonite and the
used by quilters.
Popular cutting tools include scissors (won't mark table),
rotary cutters (very sharp), and
wall paper knives (found at most hardware stores.)
It is important to keep the fabric from shifting on the
surface during layout and cutting. Options for holding
the fabric in place include:
- tape (lots required)
- pins or staples (in the outermost 1/2" only)
- shot bags
Bones of Contention
There are several optional methods used during sewing.
Different people will give equally empassioned arguments for
and against each of these. Clearly, it is possible to build
an envelope using any combination of the following. The best
advice we can give is to talk to several builders, give
them a try if practical, and form your own opinion.
Folder Attachments -- For French fell seams, the
fabric is folded over on both edges as they are fed into
the sewing machine. Once can purchase folder attachments that
bolt onto the base of the sewing machine to aid the process.
Some people swear by these attachments. Some swear at them.
Mechanical Pullers -- These are rollers that
fit onto the sewing machine and press down on the fabric
as it it leaves the needles. The rollers are linked to the
machine drive so that whenever the machine runs, the rollers
rotate and pull the fabric forward. There too, some builders
love pullers, some hate them.