tensioning drag/antidrag wires

[stearmoth]

Hi Hatz nuts!
We are now installing our homemade lightweight square drag/antidrag wires (tie rods) in the wings and are wondering about the correct tensioning load. Does anyone can provide some advice about how tight they should be set?
Best regards
Hans & Sam, H.C.#78, Switzerland

[wassbiplane]

I would set them at about half the max design load---this should keep the opposing wire in the same bay from
going slack during stressful maneouvers--of course you have to adjust both wires at the same time to keep from racking the bay out of square, and the whole wing panel, for that matter. Trammel your panels for squareness as you go.

Then get a short piece of pipe just big enough to slip over the end of the wire & bear on the nut,washer, & pull block--
Give it a tap or two to seat the washer into the block, then READJUST the tensions again

To determine the stress in the wires Pluck or Bow (as in violin bow) them and note the pitch while the wire is stressed with a known weight--you can gradually add weight to the system and make a graph which can be extrapolated if necessary.

A convenient way is to clear off a space on a table--attach one end of rod at one end of table--attach 1/8" or possibly 5/32"
cable to free end of rod--run cable over pulley at other end of table. Suspend platform, bucket or whatever you have to hold your known weights. ---put small hardwood block under each end of wire at approx the point where it will be constrained in
wing cell.-This creates a vibrating length ~=~ to the installed length--Play Maestro !!

[ha622]

Hello Hans and Sam

I do not quite agree with wassbiplane's statement that the drag/antidrag wires should be set to about half their design load. I would tension them as little as possible, basically just enough to have them are reasonably tight. You definitely don't want any slack in the wires in the unloaded case.

The drag/antidrag wires are there to carry the loads generated in the plane of the wings. These loads can be pointing to the rear (drag) but in high lift cases they can also be pointing forward. In fact, the highest load in the wires is encountered in the case of a maximum g pullup just below stall (for the experts: point A in the flight envelope) and is pointing forward.

Let's now see what happens to the wires when loads are applied. We will consider one wing bay consisting of two compression struts, the front and rear spar, and the two crossed wires. The ribs and the spars form a parallelogram. If we assume the inboard rib to be fixed and we imagine pulling the outboard rib to the rear (to simulate a drag) the parallelogram would deform, except for the wires which keep it in place. To do so one wire will be elongated (the one from the inboard front to the outboard rear) while the other will be shortened. A compressed wire will go loose and will not be able to carry any loads. Therefore, only the other wire will resist the deformation and carry the full load. This is not a problem since the wire is designed to carry this load.

If the wires are preloaded both wires will carry part of the load. The stress on the wire which is elongated will increase and the stress on the wire which is shorted will decrease. At some point, when the compression load is higher than the preload, the wire will go slack. We are back to the situation described before and the one wire under tension will carry all the load. Since this case can never be completely excluded design practice will always consider that the full load can be carried by the single wire under tension. Therefore nothing has changed for the wires.

Let's now take a look at the spars. The preload on the wires compresses the spars (and the compression struts, hence their name). The critical failure mode for the wood spars is in compression. Thus any additional compression load on the spars will reduce their safety margin.

An additional effect, although probably of less importance, is the stress concentration at the crossing point of the two wires. I assume that in most airplanes all the wires attach points are in the same plane (at least that's the case in mine). Therefore the wires will bow out slightly so that they can cross each other in the middle. As a consequence the wires will not be loaded in pure tension anymore but a bending moment will be superposed, increasing the stress in the wires. This effect is also reduced by decreasing the preload: if one wire goes slack, the other wire can align itself in the plane of the wing and will then be loaded in pure tension.

Sorry, for this lengthy discourse. In summary:
- I don't see any reason to preload the wires excessively.
- one just has to make sure that they are tight so that the one which will be under tension carries its load right away and that there isn't any slackness that has to be overcome first.

Best regards,

Juerg Mueller
Hatz S/N 622

[Bill Rusk]

Great post Juerg. Good to see you here.

Bill

[stearmoth]

Hi Juerg, wassbiplane and all,

Thank you both for your input! After thinking about the sugestions we decided to go Juerg's way. Here a couple of photos.
Hans & Sam, H.C.#78, Switzerland

P1050169-1.JPG (32.27 KiB) Viewed 11019 times

P1050171-1.JPG (69.33 KiB) Viewed 11019 times

[Jackal]

I like your wires, what would a set cost??

[stearmoth]

Hi Jackal,

Thank you for your comment! Yes, we would make such wires available. Please drop us an email to hans.u@aeroplaneworks.ch and I will send you a quote.
We are delighted with our drag/antidrag wires as they meet with ease the AN703 specs. (same strenght as Bruntons round drawn tie rods). We make them with a 1/8" square section and rolled UNF 10-32 threads (left- and righthand) from AISI 316 stainless steel. We test loaded each wire for security. Those set of wires weights 2.6 lbs. less than with ordinary round dia 3/16" tie rods.

Kind regards

Hans & Sam, H.C.#78, Switzerland

[Nick]

Juerg, that is a very helpful post. Thanks for taking the time.

Nick

[stearmoth]

Hi Jürg and Hatz buddies,

Upon Sam's first annual inspection he found that the drag (not antidrag) wires in the first bay (left and right to the tank) in the top wings are much less tight than they were when installed and tensionned before covering. According to your suggestion, we didn't tensioning them to much, we did it just by "feeling" and never checked this afterwards, also did not before first flight.
As we testloaded all wires up to the max. allowable stress, it seems not be possible, that this relief occured due to overstress in flight!
Might it be possible, that this comes from the flying wires tension?
What is your recommandation, should we retensionning those drag wires until they feel tight?
Anyway, last Saturday Sam made the first flight this year with 3 smart landings - all went well!

Kindest regards
Hans & Sam, H.C.#78, Switzerland

[kconner]

Hello Hans & Sam
After I completed the wing panels for my CB-1 I applied two coats of epoxy-varnish with a brush. The wings were stored hanging on a rack for approximately 5 years. When I began the covering process I first had to re-tension ALL of the drag and anti-drag rods. All of the rods had lost tension. I believe the wings did shrink even though they were thoroughly coated with varnish. This is why I would recommend completing the wings first when building a Hatz. Re-tighten the rods and I think they will hold tension until your next inspection.

Kevin