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.