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Old Nov 29, 2014, 09:08 PM
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United States, MI, Grand Ledge
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How do I make functional wing struts

I'd like to put functional struts on a plane I'm putting together, but have never done a set. I've used them, but never built them. Someone guide me through the procedure, including attachment points? I surely would appreciate the help.

Barry
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Old Nov 29, 2014, 10:26 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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Size of model? That makes a HUGE difference. Also the struts should always anchor on the main spar. If you want to use "V" struts or dual struts then the wing should have two spars so the spars and struts work together. And that's the point. The spar and struts on the real things along with the cabin structure form a locked triangle. So it all has to work together.

The good news is that if you're using load bearing struts you can greatly simplify the wing joint at the cabin or center cabane section. The struts take all the bending load so all you need is a locating joint.

Note that you want carry through compression struts in the top of the cabin or cabane section. The spars cause the wings to push inward strongly. You exchange bending loads for inward compressive loads.

Similarly on the lower side of the cabin (I've been assuming all through this that it's a high wing cabin model) the struts will be trying to pull the lower area apart with a lot of tension. So there should be a strong'ish cross member. And all joints to brackets or plates in this cross member, strut ends and wing attachment should be pinned through and not rely just on glue. The shallow angle of the struts means that the wing load is multiplied by a factor of the strut angle. So there's a bigger tensile stress on the joints, lower cross member and the strut itself.
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Old Nov 30, 2014, 05:43 AM
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I follow you Bruce. The plane is about a 6' high wing design, I like the idea of the dual or "v" strut as it should reduce the twist on the wing panels. The wing is a very old one that was delivered to me cut in half. I am repairing the spars, following far 4314 best i can. I am extending the ws during this procedure by adding in a center section that encloses the top of the cabin. In essense, then, I have 2 joints that are new. Likewise, I do the same to the rear spar, te and le. This wing was built very light as they did back then, no stress webbing, and that's the basic reason for the struts.

I have full width light ply that has been added to the bottom of the fuse, and can do the same along the top, though I considered using 4 bolts to hold the center section in place and allow the center section to pick up the compression, as the wing is being put together as a one piece.

Where I go to next is looking for a good way to join the 2 strut sections together at the apex of the "v", how to reinforce that joint, and what folks do for easy, but reliable, attachment points. What is a good strut material? I see aluminum and hardwood airfoil shapes, but could form my own I suppose.
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Old Nov 30, 2014, 06:49 AM
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UK, Greater London, Uxbridge
Joined Mar 2001
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Music (piano) wire is good for functional struts - allows you to join easily at the 'V' and to solder attachment fittings. If you don't like the 'bare wire' look, dress them up with balsa or hardwood fairings.
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Old Nov 30, 2014, 10:13 AM
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San Diego, California
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"dress them up with balsa or hardwood fairings. "

Or put them inside of streamlined aluminum tubing.

Les
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Old Nov 30, 2014, 10:26 AM
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United States, NY, Baldwinsville
Joined Mar 2009
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I used K & S aluminum airfoil tubing. I inserted a V shaped piece of plastic into the joining end, pinned and epoxied. The strut is attached to the fuse with a screw and a T-nut in the cabin floor.
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Old Nov 30, 2014, 03:30 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Ah, the broken wing...... So you don't trust the repair? Or you have concern over the wood itself? In either case struts becomes a viable option.

Since this will be a one piece wing you really only need helper struts. Although if the wing has front and back spars then "V" struts becomes a natural option.

Just about anything can be used for the actual material. Myself I'd likely go with spruce or pine that I cut a groove into to allow for running 1/16 wire down the grooves which would be filled in with epoxy. The wire would have formed eyes on each end and run screws into screw blocks glued to the spars at the right locations.

Do NOT drill directly into the spars. You'll just make weak points. Instead glue plywood screw blocks to the side of the spars. And make them long enough that you have a good load bearing glue joint. The blocks should cover one whole rib bay and be tapered on the ends to either side of the screw holes.

At the fuselage I'd likely fit a plywood screw plate across the bottom so it takes the tension in the plywood. This plate could be either internal or external depending on the model and how much work you want to do. The ends of the struts would extend onto the bottom of the fuselage so that the screws used to secure them go into the plywood at least 3/16 back from the edge so they won't try to rip out. Also by making the end eyes wrap underneath instead of onto the side the loads on the screw is in shear instead of in tension.

If using music wire it will be enough of an eye to make the wire bend into a tight C shape on the end. If using wire like 1/16 welding rod I'd suggest you form the wire so it comes back on itself and then bind and solder over about 1/4 inch to ensure the eye doesn't open up.

The V struts will strongly lock in any washout angle of the wing panel once secured. So it's worth leaving the drilling and securing of the rear strut to last. And if that rear eye is made SLIGHTLY larger than needed it opens up the option of using small cresent shaped shims to alter the rigging angle to push the rear of the wing up or pull it down. Once set the shims can be epoxied or soldered in place. Just shim it so they can't move.

There are certainly other methods. But this is one which I like and which I would personally use if I were faced with this same problem.

I know that some folks have used things like metal quick links for struts. But I just don't trust them for a load bearing job like this. Mind you that was back before the metal quick links with 4-40 thread ends and 1/16 wire pins. If you use the jumbo hardware option then I'd guess that they could take the loads for a model of this size quite well. Of course then you need to use attachment plates on the outside that are screwed into the same sort of screw plate that is glued to the sides of the spars internally.
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Old Nov 30, 2014, 05:25 PM
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Bruce..... I think the repair will be fine. Wing is straight and flat, even! No, I'm going to be asking more from this wing than the designer had in mind. Old plane, no ailerons. Ultra low weight wood, no stress webbing. I added in 7 1/2" of center section. I will be adding in aileron, flap and le slats to put together a high lift STOL. The fuse is an Eagle 2 (Goldberg) that I broadened and canted the rear fuse section up for a larger rotation angle, still on trike (yuck) gear. Interestingly enough, after adding in the center section to the wing, the span came out to be 63"! The original span to the Eagle. No, total fluke. Appears that the AUW will be somewhere between 4 1/2 to 5 #. Will run with electric. I fella from Texas sent me a cast off fiberglas cowl which I cut up and converted to an opposed cylinder style like a Super Cub or similar would have.

Thanks for the strut guidance. Once I put in the 8 new ribs, center section and le sheeting, I'll mount the wing and start on these struts.

If the original builder ever caught wind of what happened to this wonderfully built wing he'd freak, I'm sure.


Barry
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Old Nov 30, 2014, 10:04 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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This should be an interesting project. You ARE going to do a build thread I trust.....
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Old Dec 01, 2014, 06:34 AM
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Well, Bruce, not sure about a build thread. Honestly, hadn't thought about it, not sure where it would go, even. I don't have pics from previous work, though I'm not that far along, I did do a lot of juggling to get to this point. You think a "kit bash" would be worthwhile?
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Old Dec 01, 2014, 10:50 AM
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Yes, a bash is always worthwhile.

For your size a plane just about anything will work for wing struts. The main load willbe in tension so your main focus is on how you make the joints at the ends of the struts.

If you want them adjustable then use threaded rods and clevises at the wing end. At the fuse end join the wires (solder) and bend a 90 deg to stick into a hole in the fuse. Like above, I'd use a 1/8 ply cross piece in the fuse and attach to that. If you want the wing removable, then don't glue the wire into the fuse. Use a saddle clamp to hold it in place (or something similar). If you want them faired then either slip over some streamlined Al tubing (my favorite) or build up with balsa.

For non adjustable then I'd just use the streamlined tubing with ends epoxied in place. I use Robart hinge points for this kind of thing and JB Weld. I also cross pin the hinges in the tubing. I have seen JB Weld 'let go' of Al tubing after many heat and cold cycles. I used thins method on several planes, both lighter and heavier than yours.

FYI, you can make the Robart hinge points removable as well. Drill out the hinge pin and replace with a #2 screw.

You can also use electrical 'eye' connections at the wire ends. Rough up the ends of the wire well. Assemble on the plane with the correct dihedral and washout. Crimp the ends in place. Then solder them. If you use larger dia wire you can also add cross pins for added security.

There are many variations on strut mounts. Just choose one that suits you.

charlie
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Old Dec 01, 2014, 11:28 AM
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Charlie, thanks for the tips. I'm saving off these pieces of wisdom so I can refer back without having to do a search.

Yikes, another positive for a build? This is just a cobble, not a scale anything, more of a "coulda-been-scale, but...". That's not to say it doesn't have its challenges.

Charlie, Week or so ago I ran a query about finishing suggestions. I believe you contributed to that thread. This is the plane that started the discussion.

Barry
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Old Dec 01, 2014, 01:11 PM
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Canada
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IF it's a cobble together model?
Then simple Carbonne rods will do very well, as Ca sticks really well to the material.
As noted above it's the anchoring and the attendant structure that the struts attach to /distribute the loads that matters.
Struts .. typically in tension .. have an easy life, most anything that will resist your test pulling on the ends will do.
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Old Dec 01, 2014, 01:19 PM
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Barry,

Thanks, and do post it. We all like to see a model 'hacked'.

charlie
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Old Dec 01, 2014, 01:42 PM
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Well, lets see how I do putting a build together. Where should it go???? (Be nice!) Will folks ask for better pics or whatever if I am not clear enough?
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