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.

Jerry Jackson
7ECA
8T8

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.

Fuel sending units

The sender is grounded to the tank by the mounting screws but the tank needs to be grounded to the fuselage by a wire that goes from a tab on the tank to a screw on the fuselage root rib. Check for this ground between the tank and any good ground on the fuselage with an Ohm-meter. If the sensor is bad (eg. 2-4 ohms empty, 24-30 ohms full) you will need to drain the tank. It often takes a bit of worrying to get the sender, gasket, and float in place through a misplaced inspection hole. Been there, done that!

The Stinson fuel gauge is fairly easy to test: an open circuit to the sender shows full ++, a short to ground shows empty, a 15 ohm resistance to ground is 1/2 full.

The fuel level measurement system is basically identical to what General Motors used in automobiles from the ’30s until digital and computerized autos came along.  It consists of a gage which has a bimetal strip with some wires wrapped around it connected through a direct current source to a variable resistor that is in the fuel tank with an arm and a cork float to change resistance in proportion to the amount of fuel in the tank and the change in resistance results in a change in current through the circuit and through the wires wrapped around the bi-metal strip and the bi-metal strip is heated in proportion to the current through the wires and the bimetal strip which bends as a function of temperature and is attached to a lever or gear system that moves the visible pointer in proportion to the amount of current through the gage which is in proportion to the resistance of the variable resistor in the tank which changes according to the level of the float.
Unless someone has connected the two terminals on the gage directly across a battery and melted the wires inside, the gage itself rarely fails.  Like someone else mentioned the wiring from the gage to the resistor in the tank goes through a connector at the wing root and then is under a terminal on the sending unit in the bottom of the fuel tank.  The circuit is completed through grounding of the wing to the fuselage.  There was originally a grounding strap in the upper wing root accessible under the metal strip between the top of the fuselage and the wing.  
But, the most common failure is the sending unit itself.  Stinson uses a single gage switched by a switch above the fuel selector valve handle to determing which sending unit in which wing it is connected to.  Page 107 of the General Service Manual shows the wiring diagram for the fuel measurement system.
It is all pretty simple and a cheap volt-ohmmeter from Harbor freight is all that is needed for trouble shooting.   
Larry Wheelock  `

  

I am not Larry, but I am sitting here with an original fuel gauge in my hand. It is so original that it still has a small cardboard note on the left terminal (as viewed from the back of the gauge). The note reads: “Connect sender wire to this terminal. Warning- If hot wire touches, sender will be damaged.” The right hand terminal has a 1/2inch diameter device connected to it with a second connection at the end of the device. I believe this device is a resistor to allow a 6 volt gauge to be used with a 12 volt system. The 12 volt supply connection would be made to the end of this pellet.