Piper Cub wing struts - RC Groups
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Oct 24, 2017, 10:54 PM
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FlybyDan's Avatar
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Piper Cub wing struts


Any ideas where to get or how to construct some decent wing struts for my H9 Cub 40. It is the older 1 piece wing version 80 inch WS. Everything I tried so far has been crap.
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Oct 25, 2017, 12:12 AM
B for Bruce
BMatthews's Avatar
These are just for decoration I assume?

If you have access to a table saw make some from pine strips ripped from a clear plank. Cut at the right angle and glue together. Thread wraps and glue to reinforce the joint. Add sheet metal screw plates at the ends to suit the existing holes.

That's pretty standard stuff. Pine, spruce or very hard balsa. What have you used so far?
Oct 25, 2017, 07:17 AM
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They are functional and instructions say do not fly without them. I bought this 2nd hand and it came with hard balsa strips, the end were just threaded rod glued in with clevis screwed on. These things would just pull apart in flight.
Now I have I think #4 rod with a clevis soldered at one end, another clevis threaded at the other end. A couple of times these have worn at the threads and pulled right out of the clevis. These are better but not reliable. I'm not too concerned with scale appearance.
Oct 25, 2017, 07:59 AM
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I can picture wood with metal hardware anchored securely at the bottom. But what type of hardware and how to secure it at the top where the length needs to be adjustable
Oct 25, 2017, 08:26 AM
Registered User
The clevis-on a-pushrod does tend to wear out if you allow it to vibrate. So, to prevent that, first find the correct adjustments to get the rigging angles right. Then screw the clevis off, counting how many turns it takes. Clean the threads (both clevis and rod) with acetone. Apply slow epoxy to threads. Screw the clevis back on, counting the turns. Check the fit (adjust if necessary) and clean off exess epoxy before it sets. It is best to leave the plane rigged while the epoxy cures, so that the angular position of the clevis exactly matches the hole on the lug. If you need to adjust it later, burn off the epoxy with one of those small propane torches used in the kitchen.
Oct 25, 2017, 10:46 PM
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Thanks for the good info, that might work good. I am thinking a nice piece of bass wood could be drilled and taped for a threaded rod piece and then epoxied in and that should hold up. The hard balsa did not work, threads would pull out.
Oct 26, 2017, 12:21 AM
Scott
Pylonracr's Avatar
Why not make like a pushrod? Carbon fiber tube with a 4-40 threaded rod end and bolt on ball link on each end. Probably overkill, but it won't pull apart.

Scott
Oct 26, 2017, 01:24 AM
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I looked at all the carbon fiber stock at the LHS and it won't work or look good. They're too much flexible and connecting rods to the ends looks about impossible. Hardware is the key. I want like a quick connection pin at the bottom end and a adjustable or adjusted to correct length end at the top. I had metal rods and they flex and vibrate and fall apart.
Oct 26, 2017, 10:44 AM
Scott
Pylonracr's Avatar
These are pretty popular in racing circles. Scroll down and see if any of them might work for you.

http://www.darrolcady.com/Carbon_Fiber/carbon_fiber.htm

Scott
Oct 26, 2017, 01:33 PM
B for Bruce
BMatthews's Avatar
Wood is still an option but I think for a model of that size I'd opt for something stronger than balsa. Balsa would take the tensile strength OK but the fittings on the ends are the issue. You see the geometry of the strut setup and angles magnifies the flight loads on the wings that the struts need to hold. Not past what the wood itself can tolerate but it really puts a focus on the fittings on the end and how they need to be done really well.

Wood with more growth Rings Per Inch is what you want. Aircraft spruce is going to have at least 14 RPI when looking at the end grain. And often more like 18 RPI . That's the sort of wood you want to find to make new struts from if you make the option to go with wood. And the trick to getting a good solid connection to the rod end is to install the rod down into a groove and bend the end at 90 so it fits into a hole at the end of the groove to lock it in place. THEN bind in place with a close wrap of good sewing thread and glue.

If you don't want to use wood again I'd say that there are some good ideas on that page from Pylonracr. You'd want to use balsa fairings along with the carbon but what is shown looks pretty good.
Oct 26, 2017, 02:19 PM
Registered User
At 80" span those wing struts are sizable. They are also Airfoiled. Suggest 1/4' x 1/2" ...spruce... sanded to a decent aifoil and use metal screwed down fasteners at each end.
Also don't forget those wee Jury struts halfway along.. They aren't merely decorative.
Oct 26, 2017, 06:34 PM
Registered User
Quote:
Originally Posted by BMatthews
........if you make the option to go with wood. And the trick to getting a good solid connection to the rod end is to install the rod down into a groove and bend the end at 90 so it fits into a hole at the end of the groove to lock it in place. .........
Just building on Bruce's idea above........ use a built up strut of an inner 3/32" aircraft plywood flat plate with top and bottom balsa fairings..... lay the threaded rod into a groove - with an 90 degree anchoring notch - in the plywood inner core that is glued to one side of the balsa fairing and then glue the other balsa fairing on top. The rod won't strip out and the inner plywood core would provide strength to the strut. Flat carbon fiber strips or even fairly cheap carbon tow could be used for this center core of the strut.

Somebody somewhere (but where?) makes a small threaded metal insert - 4/40 size - that would provide a strong anchor into a one piece wood strut and its female threads, being metal would not strip....... or inlay/epoxy a dense wood plug in the end of strut, one that can be tapped..... thin CA is used to harden threaded wood blocks used to anchor nylon wing bolts.

Etc.

Michael in Ontario, Canada
Oct 27, 2017, 08:01 PM
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Those parts are called threaded inserts and Tower has them. They also have something called rod ends which have a slot and a hole. I plan on using a nickel steel strap on the fusilage and one of these rod ends goes around it and you install a bolt through it and the hole in the strap. I'd like to find a locking pin to connect these. Still not sure about the wing end, I tried to find a strut mount but no luck yet. I could maybe use the little strap parts it has now. I haven't yet began to figure out any jury struts, it didn't have them at all.
I want to use wood. I can get bass wood at Sig, they claim it's better than spruce because it has less pitch (sap). So glue will stick better. And the properties are very close to spruce. I have never found where to get any spruce. What about warpage? Does spruce start out and stay straight? Bass wood will stay straight or warped, whatever it is. I guess jury struts would eliminate any small warpage problem.
Last edited by FlybyDan; Oct 27, 2017 at 08:07 PM.
Oct 27, 2017, 08:41 PM
B for Bruce
BMatthews's Avatar
The issue with inserting an insert into the end of the struts is that the "threads" in the wood will all be short "end grain" threads. And that's the weakest possible way of using such an insert.

A little variation on what Micheal suggested would be to cross drill and glue in a 1/4" diameter maple or birch dowel in the strut near the end. Then drill down into the end grain of the strut and through the side grain of the dowel. Screw in a length of 4-40 threaded rod and seal it with some soaked in thin CA. That would give you some nice strong support. But that depends on the size of the struts. For a 1/4" dowel I would not want balsa struts to be anything less than 5/16 x 3/4". Or for pine or spruce 1/4 x 3/4". Otherwise the cross dowel does not leave enough spanwise wood.
Oct 27, 2017, 11:19 PM
Scott
Pylonracr's Avatar
If you want wood why not make it easy on yourself. Take a 4-40 rod full length and glue wood to it and shape to suit. No worry of ends pulling out from the wood.

Scott


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