Tuesday, March 25, 2014

Internal Wing and Fuselage Brace Wires

The wings, tail surfaces and landing gear are held in position with what are called non-flexible steel cables.  The cable for this external bracing is made with 1 bundle of 19 strands (1x19) of steel wire.  We'll talk about it more when I get to making those cables.

To stiffen the wings and fuselage there are wires inside the structures.
In the wings there are drag and anti-drag wires which stiffen the wings fore and aft.  These wires are made form 1/8" (8 gauge) music (piano) wire.
The fuselage is braced with a mix of  1x19 cable and music wire.  Cable stretches more than (hard) music wire so, bracing with cable isn't as rigid as using music wire.  While music wire makes a more rigid structure it can't absorb vibration as well as cable.  The firewall station and the forward fuselage bays are braced with 5/32" cable to absorb the vibration of the engine and the landing gear.  WACO originally used 1/8" music wire for all of the bracing in the cockpit section of the fuselage.  They changed to cable in some of these places because the wire tended to fatigue and break.  The cable is larger diameter to provide comparable stiffness to the fuselage.
In the aft section of the fuselage all the wires are 0.081" (12 gauge) music wire.  In the forward fuselage the wires are single lengths of wire from corner to corner in each bay.  In the aft fuselage they are loops around a piece of bent tubing in each of the corners of the fuselage tubes. The story is, they figured out how to make the welded tube fuselage when they were invited to look at some Fokker D-VIIs being tested at McCook field, just a few miles away.  I haven't investigated the D-VII to see if it matches the WACO construction.

To make up all these wires I started with a list of the wires needed so I could figure out what  to buy.  So where do you buy the correct wire.

First I needed to understand what was this wire since no one sells Aircraft Hard Wire anymore.   I took 2 of my best old references and created spread sheets so I could understand the kind of wire needed.  In 1916 NACA (National Advisory Committee for Aeronautics) published Report #3, titled "Report Covering Investigations of Aviation Wires and Cables, Their Fastenings and Terminal Connections."  The report was done by John A. Roebling's Sons Company.  Their business was stranded cables so they at least didn't over rate the hard wires they tested.  The other reference is an old Nicholas-Beazley, Aeronautical Parts & Supplies, catalog "K".  There is also an International Aircraft Standard which says wires were made from published in Flight magazine in 1918 which said wires were made from No. 1065, No. 1070 , or No. 1080 steel wire.  Based on the strength of wires listed in the other 2 references the steel to use is ASTM A228 (SAE 1080).  The minimum yield strength for A228 wire is 261.000 psi for .125" wire and 281,000 psi for the .081" wire. I calculated the breaking strength for the listed wire sizes in psi to compare.  The yield strength is where the wire starts to permanently deform which is a lower strength than where it actually breaks.

With 1x19 Cable we have a variety of ways we can terminate the ends of the cable to attach it to a fitting, turnbuckle, etc.  We can use Swaged Terminals, Nicopress Sleeves, and a soldered wire wrap method as they did in 1926.
With hard wire, a loop is made in the end of the wire and held secure with a Ferrule wound from the same wire.  Just like the wire none of the aircraft supply houses sell the ferrules anymore.  These look easy, just 8 or 10 winds of wire in a tight oval.  How hard can this be?

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