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ACRO E-mail Archive Thread: [Acro] Re: Flying wire tension adjustment with ...


Thread: [Acro] Re: Flying wire tension adjustment with ...

Message: [Acro] Re: Flying wire tension adjustment with pull scale

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From: DSowder at

Date: Thu, 22 Apr 2004 03:59:27 UTC


In a message dated 4/21/2004 10:44:40 AM Pacific Standard Time, 
pstambau at writes:
What is the corelation between the deflection on the
flying wire and the amount of pull (F) on the fish
The relationship between tension and deflection is not linear. Here's a 
formula you can use if you're of an analytical "bent", assuming that the fish scale 
is in the middle of the wire:

Which is equivalent to:


Where P = force on fish scale, T = Tension in wire, and Theta = deflection 
angle. The latter is the angle that you would measure if, after pulling with 
force P in the center of the wire, you were to lay a straightedge on the wire on 
one side of the load point such that the wire on the other side of the load 
point diverges from the straightedge. In other words, if you pull down on the 
wire, lay the straightedge on the bottom of the wire. T and P must be consistent 
units of force (e.g. pounds, Newtons, etc.).

Of course, you never actually measure Theta. Rather, you calculate it based 
on the length of the wire and the desired tension, and from that calculate a 
linear deflection from the chord at center span of the wire. Attach a tight 
string from one end of the wire to the other, bypassing the fish scale, to act as 
the chord from which to measure the deflection.

You must keep in mind that the Pitts uses wires in pairs, essentially 
connected at coincident endpoints. So testing one wire unloads the other...not to 
mention the fact that the wire being tested will be under higher tension when 
you've pulled in the middle with a fish scale than it normally is. Well, I say 
"must", but nobody keeps this in mind...

All that being said, I really like my Christen tensiometer. Holloway 
Engineering also makes a nice (and less expensive) tensiometer that you use with a 
torque wrench; see:
Both of these devices probably measure tension more accurately because they 
don't deflect the wire as much as the fish scale method. The required tension 
for flying and landing wires, along with a description of the fish-scale 
technique, can be found in the Aviat "erection manual", at least for the S-1S. I 
don't necessarily recall seeing the fish-scale technique in the S-2B manual. Or, 
you can hook up with one of the four IAC Tech. Tip manuals, it's in there 

Geez, I must be bored...

Doug Sowder
Attachement 1: part2.html


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