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.
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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