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Drag wires

Progress on the airplane has been very limited as of late. It has, however, been made.
After a long episode of researching and calculating I have found drag wires.
  Not the drag wires the plans call for, but still. The plans want 1050 alloy steel wire to be threaded with a #6-40 thread and retained with what are essentially spoke-nipples from motorcycles wheels. The aircraft rated nipples are about $7 each and I need 24 of them. Spoke nipples are 20 to 50 cents each. We'll, I wasn't going to spend $170 on bolts, but $12 seemed reasonable, so I ordered a bunch of spoke nipples. I had intended to match the thread of the nipples with my drag wire stock, but they are just irregular enough that I couldn't find a thread that matched and it seemed like sending wires out to have the correct thread rolled on wouldn't be worth the effort. So, despite having bought my spoke nipples, I abandoned the idea of using them.
Enter the Baby Great Lakes. I researched drag wires and alternative ways to attach them while still using the brackets I have to attach them to the spars. In my search I came across the Baby Great Lakes, or Baby Lakes for short, a single seat, aerobatic biplane. It uses aluminum rod threaded at both ends and held in place with simple AN nuts inside of the mounting brackets instead of a nipple with some jam nuts to allow the wire to be turned when tightening.

Baby Lakes drag wire end. Needs a washer under the nylock.
 He has a great post explaining how it's done over on his blog which this picture is taken from: buildingthesuperbabygreatlakes.blogspot.com/2015/03/finishing-drag-wire-assemblies.html
 I decided to forgo my weirdly sized and problematic spoke nipples for this much simpler and proven solution. "But wait," I hear you cry, "what about the wires themselves, surely you're not trusting your wing to ALUMINUM drag wires?!" You're right, I'm not. Even though they work perfectly well in the Baby Lakes, the structure of my wing calls for 9/64" diameter 1050 steel wire and I don't know how aluminum would change the way it takes loads. Thus, I had to find some steel rod with a yield strength equivalent to that of 1050 steel. Through much searching I found out that either 416T stainless, or  a cold-finished 304 or 316 stainless steel has almost exactly the same yield as a 1050 alloy steel.
It was somewhere in here that I also decided using a #6 thread wasn't going to be practical because any stock would have to be turned down in diameter to be able to be threaded correctly. If I had a machinist's lathe that would have been easy, but then so would custom threading something to fit my spoke nipples.
  I opted instead to up the diameter of the drag wires to 3/16" which can be easily threaded with a #10 die. All of a sudden this opened up a much larger variety of steels as I was now trying to surpass the yield strength of a 9/64" rod (.107" minor diameter) with a 3/16" rod (.151" minor diameter). Now 4130 steel could be used, or any of the stainless alloys mentioned before. I opted for the 304 stainless rod as it's supposed to be easier to tap and ordered material for all 12 drag wires. The pieces are a little longer than needed, so I have lots of little bits to practice threading. 
This rabbit hole kept me busy for a long time, but I feel that the solution it led to is simpler, stronger, and more repeatable for anyone else looking to build an Ace. The only downside I see is that the larger wire adds about 2.3 pounds to the wings. I'll try to make that up somewhere else if I can.

Comments

  1. Andy McBNovember 5, 2018 at 2:05 PM

    You talk about yield strength... wouldn't the main concern be stiffness (i.e. Young's modulus of Elasticity)? Or doesn't that matter?

    ReplyDelete
    Replies
    1. Brian McBNovember 6, 2018 at 3:12 AM

      Stiffness is certainly a concern, but all the materials I looked at were as stiff or stiffer than the 1050 originally called for.

      Delete

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