The surfaces of a motor driven aeroplane are usually made of some material which is practically air tight. The Herring-Curtiss Co., use Baldwin’s rubberized silk, while most of the foreign aviators prefer a balloon cloth known under the name of "continental."
Ordinarily the surfaces of a glider are not covered with any preparation to make them air tight and is not necessary, but since it will considerably increase their efficiency it is offered as a suggestion to those who are able or care to undergo the expense.
Aero varnishes for this purpose are obtainable in the market and may be applied with an ordinary brush or by immersing the fabric. One gallon will cover approximately 100 square feet of ordinary Cambric, although much depends upon the weave. The more open or coarser the goods, the more varnish it will require, while fine fabrics take the least amount.
Varnish is expensive and is not considered in the estimate of cost made at the beginning of the book.
The surfaces are formed of cambric or muslin stretched tightly over the ribs. Thirty yards of material, one yard wide will be sufficient to cover the machine, including the rudders.
Seven strips 4 feet 6 1/2 inches long are cut and sewed together along the selvages so that a surface 4 feet 6 1/2 inches wide and a little over 20 feet long is formed. Twenty one strips, 4 feet 6 1/2 inches long and 1 1/2 inches wide are cut and sewed to the surface at right angles to the long edges and one foot apart, between their centre lines. The edges of these strips are turned under 1/4 of an inch on each side so that they form a reinforcement 1 inch wide which will come directly above each rib.
Fig. 20.—Method of hemming up edge of cloth.
Reinforcing. The long edges of the surface are then doubled back and hemmed, turning under 1/4 of an inch and forming a 3 inch hem as illustrated in the upper part of Fig. 20. This 3 inch hem is then doubled back one inch and sewed again so that the result is a two inch hem, composed of two thicknesses of cloth save for one inch back from the edge where it is made up of four thicknesses.
This reinforcing is necessary to avoid ripping and tearing the cloth out from under the tack heads when it is under pressure during a flight.