Wing Rigging..Turnbuckles

When installing the drag and anti-drag wires in the wings we were unsure what tension was required. We spent ages researching 1930s & 1940s aircraft maintenance books for guidance, but to no avail. The most descriptive written information we found was contained in the plans for the Evans VP-2. In the builders notes supplied with these Bud Evans states, "Next, install all drag wires and with the aid of a lage carpenters square, even up the spar tips and tighten up all turnbuckles until the drag wires are drum tight, or finger tight plus 2 or 3 added turns with a spike through the barrel" (Note - VP-2 drag & anti-drag wires are 1/8 inch diameter rather than 3/32). We tried this but it seemed to make the wires far too tight, although this may have been because we had lubricated the turnbuckle threads (more on this later). Keith Matcham gave us some good advice and suggested around 40 lbf, possibly a bit more. We also noticed that cable tensionmeters sold by ACS seemed to have a range for 3/32 cable up to either 70 lbf or 100 lbf depending on the model. Since we wanted to get this absolutely right we spoke to PFA Engineeering, but they couldn't give a definitive figure either. However, after describing the research we had carried out and the snippets of info. gathered, they agreed that a tension of around 50 lbf would probably be about right. The next snag was that this was well outside the range of our cable tensionmeter which we bought at a Popham aerojumble years ago. However it was easily modified, and we then calibrated it on 3/32 cable by suspending 50 lbs of bricks in a bag from a spare cable. The drag/anti-drag wires were then quickly set up and all appears well. One thing we did notice however was that the subjective feel of the cable tension varies noticeably as you move from root to tip. The centre bay has longer wires than the root bay, and the tip bay has longer wires than the centre bay. If you set them all by equal feel this will result in uneven tensions between the three pairs of wires. However during our experimentation our tensions have been all over the place and it made no noticeable difference to the fit of the wings to the centre-section. We kept checking this as we were concerned about possible distortion.

Finally, we read time and time again not to lubricate turnbuckles. However the quality of the ones we bought (brand new AN130-16S from the States) seemed a bit variable and some of the threads would occasionally bind and become very difficult to turn. Some Copperslip made them beautifully smooth. However we found that this also makes it impossible to hold the setting of the turnbuckles when you try to wire lock them in the confines of the wing. Also, the wire locking method (even a double spiral wrap) cannot immobilise all possible movement of the turnbuckle, so they can creep off adjustment. So we had to degrease ours and start again. In the end we actually used some Loctite 222 as well as wire locking. This is good for mixed metals, lubricates on assembly (the original objective!), holds perfect adjustment during wire locking and still allows for easy dismantling or adjustment in the future. We hope that this may be of interest/use to other builders.

Best Wishes,

Chris and Fran