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ACRO E-mail Archive Thread: [Acro] Re: Fw: Flying Wire Tensiometer (long d ...


Thread: [Acro] Re: Fw: Flying Wire Tensiometer (long d ...

Message: [Acro] Re: Fw: Flying Wire Tensiometer (long dissertation).

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From: DSowder at

Date: Tue, 26 Mar 2002 00:07:53 UTC


In a message dated 3/25/02 6:51:05 AM Pacific Standard Time, 
autotech at writes:

> I did some tests and comparisons a while back using high-priced wire
> tensionmeters and the fish-scale method.  The fish scale method came out
> as the most reliable.  I'm sure I did not compare every known
> high-priced tensionmenter, but I checked 3 other types that were
> available.
> The "unreliability" appeared to be due to the bending resistance of the
> wire.  On the aircraft I was rigging, wires up to 1/2" were being used,
> with others at various sizes down to 3/16" (and a few odd metric
> sizes).  On the smaller wires the high-priced meters worked fine as the
> small diameter wire did not contribute a bending resistance over the
> short distance of the meter test-points.  On the larger wires we kept
> getting a rather substantial variatioin that we could only explain due
> the bending resistance of the wire.
Don is right, of course. The usual wire tensiometer assumes that the wire is 
a pure tension member with no resistance to bending. For those with a 
computational bent, the formula is T=PS/4D, where T is wire tension (lbs.), P 
is fish scale pull at center of wire, (lbs.), S is the length of wire from 
pin-to-pin, (inches), and D is the distance in inches that the center of the 
wire is displaced laterally from the chord. The chord would be a taught 
string streched from pin to pin, and NOT hooked by the fish scale. There are 
a couple of simplifying assumptions in this formula: for small deflections of 
less than a couple of percent of the wire's length, tangent and sine of 
deflection angle are virtually equal. And, it assumes that the aircraft 
structure has some compliance, so that the wire tension doesn't increase 
appreciably as a result of measuring said tension. This wouldn't be true if 
both ends of the wire were anchored to the foundation of the universe. Other 
units of measure can be used as long as they're consistent.

That being said, the Christen tensiometer, an example of which I've owned for 
about 10 years, has calibrated scales for each of the sizes of streamlined 
wires used on the various types of Pitts and most other light biplanes. Note 
that streamline wire is assumed, and it's assumed that you're bending it the 
"easy" way. This calibration is intended to negate the effect of the wire's 
bending strength.

That furthermore being said, tensioning still isn't entirely straightforward. 
On the Pitts, The landing wires will always show significantly higher tension 
than will the flying wires for two reasons. Firstly, they intersect the wing 
at a more acute angle, so the component of force that they apply 
perpendicular to the wing is a lesser part of wire tension. More of the 
tension is "wasted" compressing the spar longitudinally. Secondly, the 
landing wires support the weight of the wings and I-struts. Net result is 
that with landing wires at near max. spec'd. tension, flying wires will be 
near minimum spec'd. tension. It's impossible to get 'em identical. Of 
course, since the members of a pair of, say, flying wires are only a couple 
of inches apart, testing one detensions the other, decreasing the accuracy of 
measurement. And, don't you wish you could turn a wire 1/4 turn instead of a 
full half turn? Of course, you can always back off completely and remove and 
rotate just one end clevis by a half turn. And my wife wonders what I'm doing 
at the airport... (This on the S-2B; the S-1's have a double-nutted arrang
ement at the outboard end that can be more accurately adjusted once you've 
bent a couple of special wrenches for the job).

In my opinion, the tail wires are too short to measure tension with any 
degree of accuracy. Not to mention the fact that trying to measure the 
tension in one stretches the other three, unless you think that the rudder 
post can actually resist any part of the wire tension.

Well, I said it was long. Sorry to bore those that know all this stuff; some 
might find it interesting. And that's just tensioning the wires....rigging is 
a whole 'nuther story.

Doug Sowder
Flies hands off!!!
Attachement 1: part2.html


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