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Calibration and Use of Cable Tensiometers
Created 4/10/99
Updated 4/12/99
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Rigging of the control cables on the Stinson should be done using a cable tensiometer.  Here is information on using these tools and tips on the particular issues involved in rigging the Stinson 108.

Text and Images by Rich Tweedie

Cable Tensiometers come in three basic types (there is another type used for flying wires that will not be discussed here). The three types are:

    1. Direct reading rotating indicator (the Burroughs is an example of this one).
    2. Indirect reading (from a calibration chart) such as the Pacific Scientific and Tensitron.
    3. Direct reading dial type, such as a C8 or C9.
Below are examples of the various types:

Pacific Scientific           Burroughs                     C8

The best is the very expensive C8/C9 type, which also has the best range, and paradoxically, is the most likely to be out of calibration. The C type calibration errors are (I believe) related to people storing them with the handle locked in. This puts the internal coil springs at high compression, although the lower leaf spring is unaffected, and puts a "set" on the coil springs similar to what happens on click stop type torque wrenches that are stored without lowering the setting to zero. The included calibration bar is NOT evidence that the C type is calibrated over its entire range. When I first purchased mine (surplus) the calibration bar showed accurately at 192 pounds, but a 70 lb weight showed up as less than 30 pounds, and a 30 lb showed as about 10 lb.

Use of these meters is not as straightforward as it would seem. For example, the Burroughs unit instructions say to clamp onto the cable, let go of the Knob, and read from the proper pointer for that cable size. Actual measurements show that this results in inconsistent and inaccurate readings, such as showing over 40 pounds tension for an actual 30 pounds. To get accurate and repeatable results, after letting go of the knob, pull it OUT a ways and slowly let it go back. This technique was proven on three different Burroughs units (my brother Jeff Tweedie came up with this). One problem with Stinsons occurs with the Burroughs meter, the elevator cables require a range of 100 to 110 pounds tension, and the Burroughs has a Maximum of 70 pounds on the 1/8 inch scale. If the Burroughs is all you have, all is not lost. It is possible to hand calibrate it for those loads if a C8/C9 type is unavailable. This will be covered in the calibration section. For the Pacific Scientific, some will cover this range, and some will not quite. Fairly simple to use, take the reading and get the true tension from the calibration chart that comes with the meter (these are NOT direct reading, the 0 to 100 scale is used to get the real tension from the calibration chart). The C type requires you to zero the dial indicator for that cable size, lock onto the cable, then read the tension from the scale appropriate to that cable size.

To calibrate the tensiometers, it is only necessary to hang a variable weight on the cable size in question, and read the results. I use a steel bucket with heavy weights for various tension levels. Include the weight of the bucket. See below:

If you make a calibration setup like that above, you can put marks on the Burroughs dial at 100 lb and 110 lb (they will be close together as it is a non-linear scale, but gets you pretty close to the mark).

Some caveats on rigging Stinsons are in order. First, the elevator presents some problems, as a measurement will show a massive difference between the two cables if the elevator is not supported in approximately level flight position. This is due to the weight of the elevator behind the hinge line pulling on the top cable with extra force. Next, although the book says that you must have between 100 and 110 pounds tension, you cannot get it, even with the supported elevator. The cause of this is the elevator horn, which has one cable at 3 inches from the center, and the other 3 ½ inches. Archimedes shows us that this is a difference in leverage of about 17%, and sure enough, with the 3 ½ inch cable at 100 pounds, the 3 inch one shows 116 pounds. This tension is higher than usual in most planes of its size, but is required to get the full up elevator travel that is necessary. My suggestion is to make one of them in the 100 to 110 lb area, and accept what you get for the other one, preferably as close to the margins as you can get them (such as 96 lb and 112). Another weirdness is the published flap setting of 45 lb when up. There is almost no tension when the flaps are up, 45 lb would pull them right down. I believe that it is simply a misprint and that it should read 45 lb in the down position.

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