Learning to fly in the U.S. Army

The materials of construction for airplanes should be of such material, size and form as to combine greatest strength and least weight. With metal parts in particular it may be necessary to substitute less strong material for the sake of getting non-corrosive qualities, ability to withstand bending, ductility or ease of bending, etc. With wood, absence of warping is important as well. The materials which are considered are the following: wood, steel, including wires; special metals as aluminum, brass, monel metal, copper, etc., and also linen and dope.

Strength of Materials.—It is important in a general way to understand the terms used in speaking of strength of materials. Thus we may have strength in tension, strength in compression, or strength in shearing, bending and torsion. Some material fitted to take tension will not take compression, as for example wire; some material, as bolts, are suited to take shear, etc.

In general all material for airplanes has been carefully tested and no excess material is used above that necessary to give the machine the necessary strength.

Tension.—This means the strength of a material which enables it to withstand a pull. Thus wires are used where strength of this kind is required.

Compression.—This refers to strength against a pressure. Wire has no strength for this purpose, and wood or sometimes steel is used.

Shearing.—Refers to strength against cutting off sideways. Thus the pull on an eyebolt tends to shear the eyebolt, or the side pull on any bolt or pin tends to shear the pin.

Bending.—In bending material the fibres on the outside tend to pull apart; those on the inside tend to go together. Thus on the outside we have tension, and on the inside compression. Along the center line there is neither tension or compression, it is the “neutral axis.”

Torsion.—Torsion is a twisting force, such as an engine propeller shaft receives.

Testing for Strength.—If a wire is an inch square in cross-section and breaks when a load of 150,000 lb. is hung on it, we say that the strength of the wire is 150,000 per square inch. Smaller wires equally strong have a strength of 150,000 lb. per square inch also, but they in themselves will not support a load of 150,000 lb. but only the fraction of that, according to the fraction of a square inch represented by their cross-section.

In the same way, a square inch of wood under a compressive load may break at 5000 lb. If, however, the piece of wood is long in proportion to its thickness, it will bend easily and support much less weight. For example, a perfectly straight walking cane could perhaps have a ton weight put on it without breaking but if the cane were not set squarely or if it started to bend it would immediately break under the load.

These cases illustrate the importance of having struts perfectly straight, not too spindling and evenly bedded in their sockets. Some training machines are built with a factor of safety of 12. That is to say, the breaking strength of any part is twelve times the ordinary load or stress under which the piece is placed. It should be remembered, however, that under any unusual condition in the air, such as banking, etc., extra strains are placed on the parts and the factor of safety is much less than 12. Factor of safety of 12 thus does not mean exactly what it does in other engineering work, where allowances are made for severe conditions. The so-called factor of safety of 12 in airplane work is probably no greater than a factor of safety of 2 or 3 in regular engineering work.

There are three all-important features in the flying machine construction, viz., lightness, strength and extreme rigidity. Spruce is the wood generally used for parts when lightness is desired more than strength, oak, ash, hickory and maple are all stronger, but they are also considerably heavier, and where the saving of weight is essential, the difference is largely in favor of the spruce. This will be seen in the following condensed table of U. S. Government Specifications.

A frequently asked question is: “Why is not aluminum or some similar metal, substituted for wood?” Wood, particularly spruce, is preferred because, weight considered, it is much stronger than aluminum, and this is the lightest of all metals. In this connection the following table will be of interest.

Wood.—Present practice in airplane construction is to use wood for practically all framing, in other words, for all parts which take pressure or compression. Although wood is not as strong for its size as steel and therefore offers more air resistance for the same strength yet the fact that frame parts must not be too spindling, in other words, that they must have a certain thickness in proportion to their unsupported length, has led to the use of wood in spite of the greater strength of steel. Some airplanes, however, as the Sturtevant, are constructed with practically a steel framing.

It should be borne in mind that any piece or kind of wood will not answer for framing, and more especially for repair parts. There is a tremendous difference in the strength and suitability among different woods for the work. For instance, a piece of wood of cross or irregular grain, one with knots, or even one which has been bored or cut or bruised on the outside, may have only half or less the strength of the original piece. Air drying doubles the strength of green wood, proper oven drying is better yet.

Notice how the ends of each piece are ferruled, usually with copper or tin. This is to prevent the bolt pulling out with the grain of the wood, and also prevents splitting and end checking and gives a uniform base on which the pressure comes.

It is generally advised not to paint wood as it tends to conceal defects from inspection. So varnish only.

Wrapping wooden members with linen or cord tightly and doping this, both to make waterproof and to still further tighten, increases the resistance to splitting. The absence of warping tendencies determine often what wood to choose.

The selection of lumber and detection of flaws is a matter of experience and should be cultivated. It is, however, nothing more than the extension of the knowledge that leads a man to pick out a good baseball bat.

Woods.—1. Spruce.—Should be clear, straight-grained, smooth and free from knot holes and sap pockets, and carefully kiln-dried or seasoned. It is about the lightest and for its weight the strongest wood used. It is ordinarily used as a material for spars, struts, landing gear, etc., as it has a proper combination of flexibility, lightness and strength.

2. White Pine.—A very light wood used for wing ribs, and small struts.

3. Ash.—Springy, strong in tension, hard and tough, but is considerably heavier than spruce. Used for longerons, rudder post, etc.

4. Maple.—Used for small wood details, as for blocks connecting rib pieces across a spar or for spacers in a built-up rib.

5. Hard Pine.—Tough and uniform and recommended for long pieces, such as the wooden braces in the wings.

6. Walnut, Mahogany, Quarter-sawed Oak.—The strength, uniformity, hardness and finishing qualities make these woods favorites for propeller construction.

7. Cedar Wood.—Is used occasionally for fuselage coverings or for hull planking in hydroplanes as it is light, uniform and easily worked. Veneers, or cross-glued thin layers of wood, are sometimes used for coverings.

Laminated or built-up wooden members have been much used for framing and for ribs and spars. The engine bearers are always of wood on account of vibration and are also laminated. In lamination the wooden strut is built up of several pieces of wood carefully glued together. The grains of the different layers run in different directions, consequently a stronger and more uniform stick often is secured. The objection to laminated pieces comes from the weather causing ungluing. Laminated pieces should be wrapped in linen or paper and freshened with paint or varnish from time to time.

Forms.—Attention should be called to the hollowed form of many of the wooden members. In any beam or strut, material at the center of the cross-section is of far less value in taking the load than the material away from the center. Therefore, to secure greatest strength with least weight, it is permissible to lighten wooden members if done understandingly.

Steel.—There is a tremendous difference in the strength, wearing and other desirable qualities among different steels and irons. For airplane work none but the best qualities are allowed. For this reason the use of ordinary iron bolts (as stove bolts) or metal fastenings or wire not standardized and of known qualities should not be permitted. The airplane is no stronger than its weakest fitting. This does not mean that the hardest and strongest steel must necessarily be used, as ease of working and freedom from brittleness may be just as important qualities, but the steel on all metal fittings should be of high-grade uniform stock. A ductile, not too easily bent, mild carbon steel is usually recommended for all steel plate, clips, sockets and other metal parts. If any parts are required to be tempered or hardened it must be remembered that they become brittle and can not afterward be bent without annealing or softening. Tool or drill steel is a name given to uniform or rather reliable grades of steel adapted to heat treatment as tempering or annealing. Often the bolts, clips, nuts, pins, devices and other fittings are of special heat-treated nickel steel which must not be heated locally for bending or for attachment. Such work seriously weakens the steel. The steel is often copper-or nickel-plated and enamelled to prevent rusting. Do not forget that the proper material may be twice as strong as other material which looks the same but which has not received special treatment.

Wires.—Only the highest grade of steel wire, strand and cord is allowable. Manufacturers, as Roebling of Trenton, N. J., manufacture special aviator wire and cord, which is given the highest possible combination of strength and toughness, combined with ability to withstand bending, etc. Steel wire ropes for airplane work are divided into three classes as follows:

1. The solid wire = 1 wire (as piano-wire grade) and known as aviation wire.

2. The strand stay, consisting either of 7 or 19 wires stranded together and known as “aviator strand.” Flying and landing wires on Curtiss.

3. Cord or Rope Stay.—Seven strands twisted together forming a rope, each strand being of 7 or 19 wires and known to trade as aviator cord. The wires are either tinned or galvanized as protection against rust, etc. Ordinarily galvanizing is used, but hard wires and very small wires are injured by the heat of galvanizing and they are therefore tinned.

No. 1. The single wire is the strongest for its weight. Single wires will not coil easily without kinking and are easily injured by a blow, therefore their use is confined to the protected parts of the machine such as brace wires in the fuselage and in the wings.

The strand stay (No. 2) of 7 or 19 wires is generally used for tension wires, as it is more elastic (can be bent around smaller curve) without injury, as the flying and landing wires on the Curtiss. The smaller strands usually have 7 wires, the larger ones 19 wires.

No. 3. The Tinned Aviator Cord.—The 7 by 19 cord is used for stays on foreign machines. It is 1¾ times as elastic as a solid wire of the same material. On the Curtiss it is used for control wires. For steering gear and controls extra flexible aviator cord is also recommended. This has a cotton center which gives extra flexibility and is used for steering gear and controls. It is 2¼ times as elastic as a single wire.

Although wire strands or cords are not quite as strong for the same size as a single wire they are preferred for general work, being easier to handle and because a single weak spot in one wire does not seriously injure the whole strand.

Especial care is necessary to avoid using common steel wires, or strands which have a frayed or broken wire, or wire that has been kinked and then straightened or wire that has been locally heated or wire that has been bruised. All these factors weaken steel rope much more than is supposed ordinarily.

Wire Fastening or Terminal Connections.—Wire terminals are of four classes:

1. Ferrule and dip in solder, then bend back the end. With or without thimble; used on single wires or on strand; 50 to 94 per cent. as strong as the wire.

2. Thimble and End Splicing.—The splice must be long and complete. Used on cable; 80 to 85 per cent. as strong as the strand; breaks at last tuck in the splice.

3. Socket.—Nearly 100 per cent. strong.

4. End Wrap and Solder.—Simple and serviceable; not used for hard wire.

Present practice is rather toward elimination of acid and solder, imperfect bends, flattening of cable on bends, and toward care in avoiding all injury as kinking to wire, strand and cord due to unskillful handling of material in the field.

Other Metals.—Other metals as aluminum, brass, bronze, copper, monel metal (copper and nickel) are used for certain airplane fittings for the reasons of lightness, non-corrosive qualities, or ease of bending, etc. The trouble with these metals is that they are not uniform and reliable in strength and in an important part the great strength combined with minimum weight given by steel is not equalled by any of these metals. Aluminum is used on the engine hood and also for control levers and for the backs of the seats. In other words, for parts and castings which require light metal construction, but which are under no particular stress. Tin and copper are used for ferrules of wire joints and for tankage. Copper or brass wire are used for safety wires. Special Tobin bronze is used for turnbuckles as the part must not only be strong but free from any tendency to rust. Monel metal (nickel 60 per cent., copper 35 per cent., iron 5 per cent.) is strong and has the special property of being acid- and rust-resisting. It has been used for metal fittings and even for wires and for the water jacket of the motor. Until more strength tests show greater uniformity of strength, it is to be recommended with caution.

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.

Commercial dopes with various desirable properties are: Cellon, Novavia, Emaillite, Cavaro, Titanine, etc.