Wing Brace or drag wires and trammeling
Control cable tensions are set with a relatively inexpensive device. Drag wire, anti-drag, and flying wire tensions are set with a similar device.
Drag wire tensions are NOT set by measuring the torque on the end nut.
Here is an explanation of the tension-measuring process from the Yahoo! group.
From: Jerry Jackson
Sent: Wednesday, November 09, 2005 11:48 AM
Subject: Re: Tail brace wire tension
When the wire diameter is much, much smaller than its length it is easy o
calculate the tension from measurement of the deflection caused by a known force. The relationship is T=(L*F) / (4*x) where:
L=length of wire between supports
F=force applied at midpoint of wire
x=deflection of wire due to the force
For example, consider a 30 inch horizontal wire that has a one pound weight suspended at the midpoint and suppose the dial indicator shows a deflection of 0.05 inches. Then the
tension is : T=(30*1)/(4*.05)=150 pounds
That relationship will be accurate so long as:
(a) the force and deflection is applied and measured at right angles to the wire
(b) the ratio of length to wire diameter is so large ( say 100 or more) that the wire stiffness does not affect the amount of deflection.
(c) the force is small compared to the tension (no larger than a few
percent) so that the amount of deflection is much, much smaller than the wire length.
The problem with this procedure on a Stinson is that the wire stiffness DOES affect the amount of deflection. To do it right requires the correct tool. A Holloway Flying Wire Tensiometer http://www.radialengine.com/flywire/flywire2.htm ) is available from Aircraft Spruce (part number 12-12500 http://www.aircraftspruce.com/catalog/topages/flywire.php ) for $226.00. You can read about the use of this tool at http://www.radialengine.com/flywire/index.html .
Do it right. One pilot of a Fly Baby died because of improper drag wire tensioning, according to the NTSB report.
This brings up a very good point. I am nearly to the point in my restoration where I need to tension my anti drag wires. I have talked to Mr Holloway about his tool, and he has assured me that the lower tensions required on the Stinson wings can be measured with his tool, even though the scale he publishes doesn’t go down far enough. He says to just extrapolate the tensions needed by the Stinson. Fine, whatever. My question is: how did Stinson build them? Where are the tools the old timers used for this? How do home builders set their wings up? Sometimes it feels like I’m the only guy in the world who ever wanted to tension my wires “by the book”. I’ve asked the old timers, and they say just twang em, which I suppose is fine if you’re just checking them, but in cases like mine where I’ve had to replace the false spars, I have nothing to start from. I would think that if the old timers used the method you specify, it would be in 43-13 or CAR 3. What am I missing here? From an engineer’s standpoint, it seems more important to have the spars square to each other, with at least a certain minimum threshhold of compression on the compression members, than to have the tensions right on the book figure. Obviously, too much tension would be detrimental, even if the spar s were still trammelled. Insight, please!
It is just a guess on my part, but the Stinson’s contemporaries were Stearmans, Wacos, and Ryans that had lots of flying wires and tail bracing wires. I would expect that every self respectiong mechanic had a heavy duty tension meter that he purchased at a war surplus store for a buck or two. I was there at the airport when the Stinson was new but I was too young and dumb to have paid close attention.
I’ve used the Holloway tool on two sets of wings, and it works well. Here is the website: radialengine.com/flywire/index.html To use it for a Stinson, you’ll need a pretty small inch/pound torque wrench.