Details

Category: Build Alongs

Created on Monday, 09 January 2012 16:42

Written by Ron Peterka

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Well, I agreed to do an upgrade on the Waco airframe from a 'sport' model to a 'competitive' level and the job is close to being finished. The last big hurdle of making all the cable and aerodynamic flying, landing, and bracing wires is complete.  You can see from the the detail photo that it gets pretty crowded between the wings.  Four 'flying' wires for each side and two 'landing wires for each wing. two wire braces on each inter-plane strut.  That comes to a total of twenty four manufactured ends and eight clevis ends plus almost twenty five feet of 1/8 inch wide flat steel material and a couple of feet of steel fishing leader for the strut braces.

Flying wires are the cables or rolled aerodynamic steel supports that run to the bottom of the wing(s) that take up the flight loads lifting the wings.  Landing wires are generally the same materials leading to the top of the wing to absorb downward loads during landing and inverted flight loads.  Between the two sets of 'wires' the wings are very rigidly supported and can be rigged to balance wing lift from side to side,or set wing dihedral.  In some old-timers there are paired wires side by side to make a stronger brace.  ON full scale aircraft the wire ends are threaded one end 'right hand' and the other end 'left handed' so the 'wire' can be installed slightly loose and then rotated to tighten or loosen the rigging.  On models finding left handed threaded dies or taps is not a readily available option so we have to make do with adjustable clevis ends and install the clevis to the airframe as tight as possible.

Each flying wire end was made from a 1 3/4" length of 1/8" dia. welding rod.  A 1/2" section is threaded by using a 5-40 die, and then going over that with a 4-40 die to finish the threads.  A section of about one inch was ground to create a flat section on the end opposite the threaded end to about the center of the rod.  A DuBro #335 steel 4-40 threaded end clevis that uses a 2-56 bolt to attach the end to the airframe.  This model required designing and manufacturing a large number of brass or aluminum fittings and brackets.

With the wings attached and supported close to their final position the two ends of each wire are bolted into place and the length of the wire is set by the distance overlapping the two ground flats.  After removing the ends, the 1/8" wide spring steel material, available from Proctor models in 25 ft coils, is then silver soldered.  Silver solder is a mix of about 2% silver and tin.  It is much stronger than common lead free solder and requires an acid flux hta must be removed after soldering to prevent severe rusting.

Most of the flying and landing wires are attached only to the wings themselves, unlike on the full scale airframe where the wires would attach to the fuselage or cabane structure.  On the model you can remove one wing set by removing two flying wires at the fuselage and four screws at the wing root.  A thick foam filler can be inserted between the wing roots for transport.  Assembly at the field should take minimal time and effort.  The DuBro clevis ends look more 'scale' than the pin type control clevis.  The flying wires make a very rigid assembly resisting flight and landing loads efficiently.

This challenge has been an interesting and satisfying effort that makes modeling such a joy to me.  At this point it is impossible to say how the model will actually fly, but the original sport scale model is said to fly nicely.  I'll try and return with a progress after the model is covered and test flown.  It is intended to be flown in the U.S.  Scalemasters Open Class which allows the pilot to enter an aircraft exempt from the "builder of the model" rule where it would be qualified.