Scale "hidden" hinging tutorial
 

I must say I am really impressed by Jeremy’s painting how-to. I enjoy looking at how craftsman do their work.

Although requiring nowhere near the talent that Jeremy obviously has, I am offering a little write up I did on so-called “hidden” hinging technique. I did it for my club’s newsletter, thought I would post it here.


It's big so don't open this thread if you are not interested!

Here it goes
 

I am hinging a Yellow Aircraft Stingray wing. This is slightly unusual, as the elevons are already partially removed and feature a precut gap. Normally the gap is carefully measured and cut, in this case I kind of had to make it up as I went along. Anyway, since I use Robart large hinge points, I need full depth penetration of the hinge to get maximum shear strength. So first I route out some holes in the foam and insert hard balsa plugs, sanded to fit against the skins as best as I can without warping the skins. I glue these in with white glue or epoxy. I put matching plugs into the elevons as well.

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I have not yet trimmed the plugs flush, but I will and then fit a 1/8th hard balsa cap over the trailing edge. This is not really as thick a cap as I would like, but again I am trying to work within the confines of the precut gap. Glue the cap on with white glue (I use Pica Gluit exclusively, it sands very easily). Again, the elevon is not shown here but is capped in an identical manner. It does not matter how wide the cap is as we plane the excess off later.

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OK now the glue has dried, and I have carefully planed down the caps. I have marked where the plugs are as you can see. (good idea to do this before you glue the caps on!!)

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I now mark the centerline of the trailing edge cap and the elevon with this tool from Great Planes. This is an important step so get the line as accurate as you can.

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Now you need to mark corresponding hinge point lines across the trailing edge of the wing and the leading edge of the elevon. I use a square and try to get these accurate. If you have a little mismatch you can fix it later, but it is easier if you try to keep the error out as you go along.

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Now I drill the hinge point holes with the Robart Hinge Point drill jig (not shown but really necessary to get this right). After I drill these holes I look at how accurate (or inaccurate) they are and transfer the holes to the elevon to match. That inside hole is a real treat, I leave it to the reader to figure out how to drill that one.

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This picture shows a couple of steps. First, I use a BVM Permagrit round file (excellent tools BTW, I have every tool he makes) to file a trough into the leading edge of the elevon. It is easier to do this now, before the hinges are installed. The trough needs to be deep enough for ½ of a yellow inner nyrod to lie into. Then I square up the hole, this time with a square permagrit file, to let the hinge drop in for enough.

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Trial fit ½ of the hinge (you can actually buy the hinge points from Robart without the pins installed, or just cut them with a pair of small dykes). This shows how deep the hinges will need to go in. The hinge line itself is coincident with the edge of the elevon, BTW.

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Now dry fit all of the hinges. It is a good idea to set the orientation of the “tangs” in some consistent way so you will not get it mixed up later. Anyway, with a piece of .055 music wire, line up the hinges. Almost no matter how careful you were while drilling, you will need to use those files to get the holes tweaked so that this pin is straight.

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I use a cheap set of digital calipers (a must-have in the shop) to check the alignment of this wire, all along its length. It should be even, and the wire itself should lay ½ way into the trough you dug out earlier.

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This might look like overkill, but trust me it is key to making this system work. The errors tend to accumulate as you get further into it, and after a point you can't get the errors out. Dig those hinge holes out as much as needed until you can get this to line up easily.

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Now I glue the hinges in with BVM Aeropoxy. I carefully clean any glue that got into the hinge “tangs”, then I insert the pin, triple check the alignment and set it aside to dry undisturbed.

After it is dry you can go ahead and CA in the guide tubes as shown here.

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Now I insert the other side of each hinge point, and start trying to work it into the hinge holes in the trailing edge of the wing. This step can be tricky, so have some patience. It is also hard to do with a highly swept trailing edge. The Stingray is not too bad in the respect. I use a short hinge wire for this step, just long enough to engage all of the hinge pins.

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Now I rig a hole for the hinge wire to go through the wing root (in some cases the pin will need to go through the tip as a torque rod will be in the way on the root). Now I check the alignment of the elevon to the wing all around the perimeter, again opening up the trailing edge hinge holes as needed to get things lined up. Once it looks good I slob in the Aeropoxy, plenty of it. Some oozes out but that is good as it helps stabilize the exposed part of the trailing edge side of the hinge. This is a VERY critical step, so I usually line up the entire perimeter top and bottom with alignment sticks (little bamboo cocktail forks). I put tape on both sides of the gap and then CA the sticks to the tape.

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After the glue dries you get a chance to see the kind of extreme throws that you can get with this technique. It is good now to establish how much throw you need to you can check your clearances as the process continues.

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Now its time to make the elevon leading edge cap. Get some oversized medium balsa, and groove it, either with hand tools, or with a router.

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I use one of these from Dremel. Can never have enough tools!

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Cut sections of the routed leading edge cap to fit between the hinges. Install some extra hinge points with the hinge pin in order to check clearances. Glue them on with Gluit and tape them down.

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After the glue dries use the razor plane to bring the edges down even with the elevon. I use a little black plastic planer, Model Master I think. Maybe BV sells it now?

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Now I mark the semi circle that is defined at the center by the hinge pin and the outer edges of the elevon trailing edge. Try to get this accurate. Sometimes I make a little template out of polyply, but here I just drew them on using a paper template.

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Now I clamp a long Permagrit sanding block to the bench (after clearing enough junk) and start cutting down the radius of the leading edge. Take your time and check the end template marks often. This step does involve some pretty handy craftsmanship so take your time.

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When you are done it should slide back into the hinge points, and move freely over the whole range of motion. You will probably have to keep sanding a bit to get it to move freely, but be careful, the wood is hard to put back. The more sweep the trailing edge the harder it will be to get the surface in and out, so be patient.

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Now for the cuffs. I have used .014 carbon fiber plate for this before, but this time I tried plain old 1/64 ply wood. Cut pieces ½ inch wide, about 1 inch longer than the span of the elevon.

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Lay the 1/64 ply cap on the hinge gap, and move the elevon up and down to get an idea of what kind of cap clearance you are going to need . I usually end up about 3/32 back from the hinge line. Mark a line all around the cap where is touches the wing.