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.
[Illustration: 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.
[Illustration: Fig. 21.—Section of cloth hemmed, and reinforcing strips
sewn on.]
*The bottom planes.* The cloth on the bottom planes is made up of two
sections, divided by the space in the centre of the lower plane which
the operator occupies. These sections are made and reinforced in exactly
the same manner as that for the top plane just described but are one
foot less than half as long.
*The cloth is tacked* over the front horizontal beam and then stretched
tightly over the curved ribs and fastened with tacks at the ends. Fasten
the corners of the cloth first and smooth it out before driving the
tacks in the ribs. Ordinary brass headed upholsterer’s nails are used
but they should not be long enough to pass all the way through the ribs.
A strip of felt 3/8 of an inch wide and four feet long is laid on the
cloth directly over each rib so that it comes between the head of the
tack and the cloth. This precaution may seem unnecessary to some, but it
greatly reduces the liability of having the cloth tear when under
pressure. The tacks along the ribs are spaced about 4 inches apart A
heavy weight held against the under side of the rib by an assistant,
when the tacks are driven in will provide a firm foundation to hammer
against.
A very good method of fastening the cloth to the ribs is to sew a pocket
on the under side of the surface and into which the ribs may be slipped.
The rear ends of the ribs may be fitted with metal tips by tapering the
end down until it is round and measures 1/2 inch in diameter. A 1/2 inch
brass ferrule such as that used on file handles is then forced on.
*The rudder planes* are covered on both sides. The fabric is stretched
tightly over the frame and then tacked along the edges. The edges should
be turned under before tacking so that there is no possibility of the
cloth tearing out.
The cloth at the ends of the planes should be securely fastened to the
struts by means of tacks. This will relieve the ribs of some of the
strain and correct a tendency for them to pull in towards the centre.