Drag wire tensioning

[Phillip Palmer]

Kevin,

 

I have installed the dragwires on one side of the wings (upper).  I have is "jigged" so that I can tweak in the 7 degree sweep. I started each at finger tight.  I started to tighten each corner block about 1/4 turn, starting with the inner most bays.  I was able to get the 7 degree sweep in and was able to keep the front and read spars straight (based on a taught string).

 

I don't know how taught the wires are supposed to be.  At present, they are crossing and in contact with each other. (As indicated in the threads).

 

When I pull a wire with a single finger I get about ~1/8" of displacement.  They seem like "bike spokes".  They "ring" when I pluck them.  They all "ring" at a similar frequency.  I noticed the outer most bay had a bit more slack and it was a little lower in frequency, until I tightened them. Now they are the same tension and freeqency.

 

So... do I need to measure the tension on the wires with a tool of any kind or do I just "wing it", pun intended?

 

Phil

 

Phillip Palmer, PhD

Pitts Model 12 builder

http://www.m12builder.com/

m12bu...@yahoo.com

603-321-1051

[Kevin Kimball]

Phil,

The wire tension is a tough thing to explain.  Basically, for both upper and lower wings, I wrap my 4 fingers around the wire on top at the cross point, adjacent to that point such that I can get my thumb on the wire under it.  I try to spread the wires apart with thumb pressure.  Perfect is when I can just get the tip of my finger between the 2 wires while holding them apart.

Sincerely,

Kevin Kimball, President

Jim Kimball Enterprises, Inc.

PO Box 849

5354 Cemetery Road

Zellwood, FL 32798-0849

407.889.3451 phone

407.889.7168 fax

www.pittsmodel12.com

www.jimkimballenterprises.com

Email:  Ke...@jimkimballenterprises.com

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[Andrew Budek-Schmeisser]

For what it's worth, Curtis Pitts once told me to use 250-350 lbs tension for the 3/16" wires in my S-1

[Phillip Palmer]

Kevin,

 

Thanks for the tip.  I'll head to the hangar tomorrow and see how they measure up.  As for the 250-250 lbs of tension...I like the idea of measuring the tension on the wires.  Problem is  I have no idea how to do that.  Anybody have a suggestion and recommendation of tools?

 

Phil

 

Phillip Palmer, PhD

Pitts Model 12 builder

http://www.m12builder.com/

m12bu...@yahoo.com

603-321-1051

From: Kevin Kimball <kjki...@me.com>
To: pit...@googlegroups.com
Sent: Sunday, August 18, 2013 8:10 PM
Subject: Re: [Pitts12Google] Drag wire tensioning

Phil,

The wire tension is a tough thing to explain.  Basically, for both upper and lower wings, I wrap my 4 fingers around the wire on top at the cross point, adjacent to that point such that I can get my thumb on the wire under it.  I try to spread the wires apart with thumb pressure.  Perfect is when I can just get the tip of my finger between the 2 wires while holding them apart.

Sincerely,

Kevin Kimball, President

Jim Kimball Enterprises, Inc.

PO Box 849

5354 Cemetery Road

Zellwood, FL 32798-0849

407.889.3451 phone

407.889.7168 fax

http://www.pittsmodel12.com/

http://www.jimkimballenterprises.com/

Email:  Ke...@jimkimballenterprises.com

Jim Kimball Enterprises http://www.pittsmodel12.com/
http://www.2wings.com/

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[Andrew Budek-Schmeisser]

I borrowed one, but ACS has some that should work. They're not cheap - and you do have to be careful with them, because calibration is everything.

It's not a bad idea to have one available to periodically check flying wire tension, especially if you do aerobatics, and especially for checks during the testing period. I've found that some biplanes tend to 'bed in' after the first few high-g excursions, and the wire tensions change. If you have a number of builder/flyers in your area, why not go in one one together?

[Kevin Kimball]

A flying wire tensiometer is designed for measuring the tension in streamline tierods.  They are made for specific ranges of wire size.  For example, the most common unit is for 1/4" to 1/2" wires.  We have one.  We also have one for 3/16" and smaller wires.  Neither of these is designed for round wires.  I suppose they could be used for round wires if you came up with a table of offset readings for the wires on a test rig.  Cable tensiometers are common but are made for braided control cables at low tensions and will not work on drag wires.  Some people try to use a fish scale and read some number of load for a given deflection.  That technique is only good for a given wire material , length, tension, etc.

If you read the wing assembly instructions with the HP kit, you will find the EXACT same verbiage as in the kit version of the S1E and S2E instructions.  It speaks of certain numbers of turns etc.  A pain in the butt and not that accurate.  But, Curtis told me, the FAA had to have something there instead of it 'feels right' as he really wanted to write.  The way I wrote to set the tension is what he did and is what we do here on the wings we build.  FYI, your feel is better than you might think.  I can get the wing wires within a few # of each other by feel and tune.  

Sincerely,

Kevin Kimball, President

Jim Kimball Enterprises, Inc.

PO Box 849

5354 Cemetery Road

Zellwood, FL 32798-0849

407.889.3451 phone

407.889.7168 fax

www.pittsmodel12.com

www.jimkimballenterprises.com

Email:  Ke...@jimkimballenterprises.com

[Andrew Budek-Schmeisser]

I did have a table of offsets. Should have included that - sorry.

There is a flying wire tensionometer at ACS that - it is claimed - can be used on streamline or round, 1/4 to 1/2.

The fish scale method is a decent one for getting all the wires in the same 'ballpark' - which is important. 

[Kevin Kimball]

The $248 unit they sell is not very accurate at all.  We bought one and tested it on a stearman comparing it with our genuine wire tensiometer.  We could not get consistent results with that low price tool.  We now have it hanging on the shop wall as a decoration.  The true tensiometers do not use a torque wrench.  They have the clamping device built in and allow the operator to let go and not influence the readings.  These units go for $2000-3000 each if you can find one.

As I wrote, the fish scale method will work OK for wires that are the same length.  In a Model 12, as with most wings, there are several lengths of drag wire in the structure so you would need a table of deflection/tension values for each wire length in order to set them at some value that is approximately the same tension.  

The method we instruct the builders to use and as I described earlier in this thread is does a great job in getting the wires at a proper and consistent tension.

[Darin Bishop]

How about somehow putting a numerical value to Kevin's method?

 

Since most rib bays are the same dimensions, would it possible to come up with a "fish scale" technique that specifies XX lbs of pull from the middle of the rod to get YY inches gap between the two wires?

 

For EXAMPLE ONLY (the following numbers are intentionally WRONG) for upper wing drag wires 75lbs of pull to get 1/2" gap between the wires, anti-drag wires 85lbs to get the 1/2" since they're shorter.

 

Darin

[Kevin Kimball]

Darin

Sure it is possible to do that but it may be overkill compared to how wings are typically trammed with drag wire tension. In some wings, the ideal tension just cannot be met without deformation of the structure or without leaving the wires too loose. "Feels right" plays a big roll in most wing tramming and drag wire tensioning exercises. I haven't met anyone who uses a wire tensiometer to set drag wire tensions. Maybe simply because a true tensiometer will not fit on the wire between the ribs of most wings. I don't know. 

Sent from my iPhone

Kevin

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[Darin Bishop]

So how did you apply and more impoartantly measure/verify you had acheived that specification?

[Darin Bishop]

Disregard - archived draft erroneously sent instead of deleted

----- Original Message -----

From: Darin Bishop

To: pit...@googlegroups.com

Sent: Tuesday, December 10, 2013 5:50 PM

Subject: Re: [Pitts12Google] Drag wire tensioning

So how did you apply and more impoartantly measure/verify you had acheived that specification?

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