Learning to fly in the U.S. Army

by E. N. Fales

In dealing with metals like steel, it should be remembered that they are subject to crystallization and fatigue.

Repeated jarring may cause a bar of steel to break easily at a particular point, when the metal is said to have crystallized there.

Fatigue of a metal may be defined as loss of springiness which may come from repeated bending and which lessens the strength of metal. Above all, however, corrosion of steel must be guarded against.

The above points should be clear, as in airplane work you are dealing with a structure which is safe with perfect materials and workmanship. The factor of safety, however, is not great enough to permit carelessness, or defective material.

Linen.—The almost universal wing covering is fine, unbleached Irish linen, stretched rather loosely on the wing frames and then treated with dope.

The linen used weighs 3¾ to 4¾ oz. per square yard, and should have a strength with the length of the cloth or “warp” of at least 60 lb. per inch of width. The strength in this direction is slightly greater than that taken crosswise of the cloth or on the filler or weft. There is a gain of strength and tautness by varnishing or “doping.”

In general, it is desirable to have wing material which will not sag easily and have the fabric yield rather than break. This often reduces stress and saves complete failure.

Dope.—The linen must be coated with a more or less waterproof dope. Some form of cellulose acetate or nitrate with more or less softening material is used and to these some suitable solvent as acetone is added.

The cellulose acetate or nitrate in the dope acts as a waterproof sizing, shrinks the cloth tight, and prevents it from changing in tightness due to moisture. Spar varnish protects this layer from peeling and makes the wing more waterproof. In service, varnish or dope must be applied every few weeks.

The U. S. Army practice calls for four coats of cellulose nitrate dope followed by two coats of spar varnish to prevent inflammability. Cellulose nitrate is more elastic and durable than the acetate but is also more inflammable.