Net Design

(From Free and Captive Balloons by Ralph Upson)

    The net is made of horizontal rows of diamond-shaped meshes, and all meshes in the same ring being of equal size.  The top of the net is terminated in a ring, preferably of coiled wire cable, of a size to fit around the valve.  The lower end of the net connects through bridles (crows feet) with the footropes wich attach to the load ring.  The footropes are usually 12 or 16 in number, and the number of meshes in each ring are therefore, multiples of this number.  It is common practice to arrange the footropes in pairs each of which is in reality on piece seized together in the middle to form an eye which fits over it’s toggle on the load ring.

             The net is usually carried about 30 degrees to 40 degrees below the equator before attaching the crow’s feet.  The latter should extend 4 to 6 feet below the bottom of the balloon.  The length of the footropes is such that their angle with the vertical center line of the balloon is about 35 degrees.  If great refinement is sought, the stretch of the ropes and deformation of the balloon must be taken into account.

The two most generally used types of net are the similar-diamond and the equal dimension.

In the similar-diamond type the meshes are kept approximately the same shape; the diagonals, vertically and horizontally across the mesh having the ration of 2 to 1, and the size of the meshes being reduced from the equator towards the top.

In the equal-dimension type the ratio of the vertical to horizontal diagonal of the meshes is 2 to 1 at the equator, but increases towards the top, the dimensions between knots being kept constant.  This type is the most advantageous being easier to make, having less knots, and slightly less weight.  It must be laid out for inflation a little more carefully, however, so the meshes may be evenly distributed.

The knot used in net construction is the “single sheet bend” (see figure 4).  For joining the various rows of crow’s feet together, thimbles are generally used, thereby insuring an even distribution of the load.  However, it is also proven good practice to use knots alone.

The materials used in making nets are silk, linen, cotton and hemp.  Silk is the lightest, but it is expensive and hardly every used.  Linen is apt to cut at the knots.  Hemp nets are light but care must be take to dry them thoroughly as mildew otherwise will destroy the rope strength.  They are also likely to cut at the knots.  Cotton nets are rather heavy, but softer and more reliable and do not cut at the knots.   Braided sash cord is to weak for use in nets.

            In nets for larger balloons it is possible to reduce the strength gradually above the equator, until at the valve ring the strength of the cordage need be only about two-thirds of that at the equator and below it.  At the same time it is well to make the extreme bottom mesh about 25% stronger to take up the extra wear due to sandbag hooks and crows feet.  It is sometimes considered an object to have the net strong enough for climbing on it to examine the valve.  This can usually be attained by keeping down the number of meshes, providing the dimension between knots do not exceed 2 to 3 feet.  The higher the fabric factor of safety, the larger the net meshes that can be allowed.