Envelope Design

(The following information is taken directly from Part 1 of Free and Captive Balloons by Ralph H. Upson. The Roland Press Company c 1926. ) pgs 19-23

          In determining patterns, the usual assumption is that the fabric can be curved in only one direction at a time.  Both of the two most common patterning systems depend on the same general assumption.

          The cylindrical gore system consists of a series of gores curved from top to bottom of the balloon, all the same and of a horizontal width commonly governed by the width of the fabric.  The width may really be anything however, provided the number of gores be not less than 10-12. There may be any number of transverse seams running in any direction whatever, as long as the sides of the finished gore maintain the proper curvature when laid out flat.  The panels may be staggered, brick fashion if desired, but in this case they have circular horizontal edges.

          In the conical ring system the straight elements of the panels run vertically instead of horizontally.  Thus the balloon is built up of a series of horizontal conical frustums, each of which must be calculated separately.  In this case the shape of the finished balloon depends on the curvature of the horizontal edges, so that theoretically the vertical seams may occur anywhere.  Practically, however, there is too much waste of fabric (due to the deep curvature of the horizontal edges) if the number of gores is reduced much below 12 to 16, depending on the size of the balloon.  Therefore, unless the panels are to be symmetrically staggered there is no real excuse for the conical ring system with it complicated patterning and large waste of fabric.

          It should be noted that the above mentioned pattern system have nothing to do necessarily with the overall dimension or general arrangement of panels.  As far as appearance is concerned, it is often impossible to tell from inspection of the finished balloon what system has been used.  The term gore when used in connection with the conical ring system simply means the series of panels bounded by two adjacent vertical meridian seams.

           Procedure.  The author recommends the following adaptation of the cylindrical gore system:

 where  V is the volume.

2.     Take as the minimum number of gores                             

 but never less than 10.

3.    Lay out carefully a quarter of a full gore to a scale not  smaller than one-then, using the ordinates figured from the diagram of Figure 3.

4.    Beginning at the equator, by straight lines parallel to it, cut off a series of panels of the maximum fabric width obtainable, allowing for seams.

5.    If the curvature at the end of the panel (side of gore) is such that it’s height in the center is less than 1/200 of the semi-gore width, without serious error it may be made straight instead of curved.

6.    For the actual gross pattern dimensions add the width of the seam on two sides, or half of the width of the seam all around.

7.    If the method of sewing requires the panels to be staggered, make the stagger as little as possible, about 1 inch., and reverse it in the next panel of the same gore. In this way curved horizontal seams are avoided.

The above procedure applies to the standard type of balloon using practically any kind of fabric.  If 2-ply bias fabric is used it is unnecessary to alter it, right and left hand, as in the case with airship envelopes.

The proper valve and appendix diameters may be obtained from the safety code.  For a balloon which is to be frequently used, it will be found and economy to have an over-size valve, as this reduces the number of rip-panel landings.

The appendix length is mainly governed by practical considerations.  With the modern “close coupled” balloon it is usually limited to about four times the diameter of the appendix opening.  The present preference, especially in rubberized balloons, is to do without the old “appendix-ring” by attaching the bridle cords permanently to the bottom of the appendix. The only disadvantage of this is that the 2-ply fabric and connection patches, required for strength, introduce to much stiffness and weight for the appendix to close tightly when the balloon is descending.  It is, therefore, recommended that an extra single-ply sleeve be attached, inside the main appendix which is free to function as a check valve.  This sleeve should have a length at least equal to its diameter.

The valve-cord may be brought down through the appendix, but the rip-cord must have a separate sleeve, made to be tight at least when the balloon descends.

The rip-panel is commonly made in the form of a narrow slit with a length equal to about one-fifth of the balloon’s circumference.  A much better design with a more positive discharge area and minimum length of seam for reinsertion is the triangular or quadrilateral type as using in the A.D.C.* balloons.

 * A.D.C. - Aircraft Development Corporation