My reason for searching out this topic is to follow up on a phone call I just got from David Stone of Steen Aero (purveyers of finest of Brunton flying wires). He had recieved a phone inquiry from a fellow Hatz builder about the tension required on the flying wires of the Hatz. David responded with the information he had of Skybolt builders using a fishing scale to calibrate the tension to 30 lbs. of pull at a deflection of 1 1/2". The tail wires are 20 lbs. of pull at 1 1/2" deflection. And, as it's told, someone eventually used a real life, honest to goodness tensiometer to double check the tension on wires tensioned by the fishing scale method and found the readings to be close (Caasi, close means a little less than perfect ) It seems to me that the Skybolt is similar in dimension, right? Or not?

So heres for the discussion...

Mark T
Hatz #493

Mister Timely Response, that's me!

I'm going to speculate that the reason that you've found little published about specific flying wire tension is that for the Hatz, as for most biplanes, within broad limits it doesn't really matter.

The flying wires' purpose of course is just to hold everything snugly together so the geometry of the rig doesn't change. (Don't want wings flying off, or dihedral changing, or whatever). Whatever value it takes to do that is tight enough, and that sets the minimums. We also don't want them so tight that they deform the structure under greatest load (max rated G's), so whatever top number is just below that deformation point is the maximum value.

There's probably some w-i-d-e differences between those min and max numbers on any airframe, and anywhere in that range is okay. Indeed, whatever value you finally settle on is gonna change as the weather and flight load changes anyway.

{As an aside to illustrate the point: I once asked homebuilt designer Mike Fisher how tight he set the flying wires on his designs. He told me that because he trailered his aircraft to various fly-ins and shows as a marketing thing it was a pain to keep tightening and loosening things -- so he left the main flying wires so loose that he could remove the attachment bolts by hand without ever unsafetying the wires' turnbuckles! Not that I'm advocating the same for us, but I guess it emphasizes the wide limits I'm describing.}

But while the exact amount of tension may not be critical, it is important that whatever amount you select be consistent so that there are no unnecessary asymmetric loads anywhere. The purpose of the oldtimers setting their wires to hum B-flat (or whatever silly note they chose) wasn't to set a specific tension -- it was to set a similar tension. If similar wires all hummed the same note, they were all at the same tension. I think that's what we want to strive for.

Presume there's nothing Hatz-specific that makes any of this invalid, but if there is I sure hope somebody'll straighten me out!

regards as ever,
Terry