Flying wire tension

Restoring a 1935 Aeronca C-3. The rigging instructions to set the tension on the flying wires something like this. Pin center lengths of 125-1/8 inches equals 800/1000 pound load on the wire. Using a formula I was given (T=PS/4D) were T=wire tension in pounds, P=scale pull at the center or the wire and S=length of the wire, i'm getting 15 lbs. of pull on my scale. This seems to low to me.

How are you measuring that 15 pounds of pull? Or are you calculating it based on the formula?

I've got one of these:

https://www.aircraftspruce.com/catal...?clickkey=7594

My Fly Baby's bracing system is *roughly* the same as the C-3s, in that there isn't "cross bracing" like a biplane. With a biplane, two opposing turnbuckles oppose each other, and you can alternately adjust them to get the pair of bracing wires very tight. In contrast, on the Fly Baby, like the C-3, tightening turnbuckles pull the wings tighter to the fuselage; there's no direct opposition from the wires on the other side.
Attachment 9376
I haven't checked the actual tension on my Fly Baby's flying wires for quite a while. Last time I did, each of the forward flying wires were showing 40-50 pounds.

The other fact is that on the ground, the flying wires (the ones below the wing) are not under flight loading. It makes it difficult to tighten them to get a major preload.

Ron Wanttaja

What are you using for "D"?

F = 4DT/L where T is tension, D is deflection, and L is cable length.

So for 1" deflection at 1000# cable tension, you would need 32# force to get 1" deflection. 15# would be 469# tension.

If you're using a different value for deflection, of course the numbers are different. 15# with a 1/2" deflection means 938# tension.

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Originally Posted by rwanttaja

My Fly Baby's bracing system is *roughly* the same as the C-3s, in that there isn't "cross bracing" like a biplane. With a biplane, two opposing turnbuckles oppose each other, and you can alternately adjust them to get the pair of bracing wires very tight. In contrast, on the Fly Baby, like the C-3, tightening turnbuckles pull the wings tighter to the fuselage; there's no direct opposition from the wires on the other side.

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That's not quite correct. Despite not being crossed, the vertical component of tension from the upper and lower wires still oppose each other. Monoplane or biplane, the horizontal component of tension pulls the wings tighter to the fuselage.

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Originally Posted by Dana

Quote:

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Originally Posted by Wanttaja

My Fly Baby's bracing system is *roughly* the same as the C-3s, in that there isn't "cross bracing" like a biplane. With a biplane, two opposing turnbuckles oppose each other, and you can alternately adjust them to get the pair of bracing wires very tight. In contrast, on the Fly Baby, like the C-3, tightening turnbuckles pull the wings tighter to the fuselage; there's no direct opposition from the wires on the other side.

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That's not quite correct. Despite not being crossed, the vertical component of tension from the upper and lower wires still oppose each other. Monoplane or biplane, the horizontal component of tension pulls the wings tighter to the fuselage.

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But on the biplane, the vertical tension is also pulling the wings AWAY from the fuselage, toward the interplane struts.
Attachment 9377

The biplane bracing is a classic box-type diagonal bracing, while the Fly Baby has only half of that. The C-3 is like a Fly Baby that way....
Attachment 9378

Ron Wanttaja

The tension in the wires is still pulling the wings inward from the outer ends, nothing's pulling them outwards. The inner ends are attached to the fuselage and cabane struts, no different from the C-3's fuselage and kingpost attachment.

Either way, there are 4 wire runs (not counting multiples fore and aft). Flying wires on both sides, and landing wires on both sides. The only difference is that they cross on the biplane, and don't on the monoplane.