Fig. 9.—Rib.
They are fastened to the horizontal members one foot apart, flush with the front and projecting one foot in the rear. One or two small wire nails are used to fasten the front ends and then a clamp placed over them and screwed down with two No. 5 round headed wood screws, one half an inch long. A small brad awl should be used to make a hole before starting the screw and so avoid any possibility of starting a split in the wood.
The clamps are bent out of sheet copper strips, 2 1/4 inches long and 5/8 of an inch wide. The ends are rounded and a hole bored through which the screws may pass.
The surfaces of the planes are curved to give them an increased carrying capacity and add to the gliding power.
Fig. 10.—Rib clamp.
The best method is to steam the ribs and then bend them so that when they dry they will retain their curve and not tend to push the horizontal beams apart. Only a very slight curve should be given and the amount of curvature should be the same for all the ribs.
Some designers construct gliders having flat planes, intending that the pressure of the air underneath the fabric shall produce a natural curve but such a method is exceedingly poor practice and results in a very inefficient machine.
The ribs must be perfectly rigid and the frame of the whole machine strongly trussed so that it cannot possibly be distorted by the air pressure. The following extract from the report of the Smithsonian Institute well illustrates this point.
"This new launching piece did its work effectively and subsequent disaster was, at any rate, not due to it. But now a new series of failures took place, which could not be attributed to any defect of the launching apparatus, but to a cause which was at first obscure; for sometimes the aerodrome, when successfully launched would dash down forward and into the water, and sometimes (under apparently identical conditions) would sweep almost vertically upward into the air, and fall back although the circumstances of flight seemed to be the same. The cause of this class of failures was finally found in the fact that as soon as the whole machine was up-borne by the air, the wings yielded under the pressure which supported them, and were momentarily distorted from the form designed and which they appeared to possess. "Momentarily," but enough to cause the wind to catch the top, directing the flight downward, or under them, directing the flight upward, and to wreck the experiment. When the cause of the difficulty was found the cure was not easy, for it was necessary to make this great sustaining surfaces rigid, so that they could not bend.”