OK. You've just finished 'bagging a new set of wings. You've trimmed the edges, applied the leading edge, and now you're ready to hinge the surfaces. Nothing says you have to do anything special when hinging a 'bagged wing. Conventional hinges or even plain old tape will work. However, a vacuum bagged wing is an ideal candidate for a tubular "knuckle" hinge. The thin, hard skin surface makes it possible to get a perfect fit around the hinge tube. You can put these steps to work with a veneered wing, but it is more difficult to get a clean fit around the tube.

A tubular knuckle hinge (when done right) makes a clean, smooth hinge line, with no gaps, regardless of surface movement. It is better suited for push rod control rather than torque rods (you hate torque rods anyway, right?), but if you must, torque rods are possible. Essentially, the surface hinging is done with Robart Hinge Point hinges, positioned such that their pivot pins are centered within the tube. The tube makes the leading edge of the control surface, and keys into the recessed trailing edge of the wing. As the surface moves, the tube rotates around the hinges. There is no gap to open or close as with conventional hinging methods!

A couple of points to make first. Since the hinge tube is just that, a tube, it is of constant diameter---I use 1/4" or 3/8" K&S aluminium tubes. This presents a problem for tapering wings that are thinner at the tip, as most wings are. The tube needs to fit into the wing at the point where the wing thickness is equal to the diameter of the tube. This generally dictates that the position of the tube (hinge line) will be further forward (chord wise) at the tip than at the root. The result is a surface that has more chord at the tip than the root. This is not necessarily a bad thing, in fact, I find it desirable for slope aerobatics.

Next, since the tube thickness dictates where the hinge line will be, the size of your control surface will be decided by available tubing size. Most hobby shops stock K&S tubing, tho not all have the aluminium tubes over 1/4" OD. Any reasonable size slope ship is probably going to require a 3/8" OD tube, so plan ahead in case you have to special order the tubing. STRAIGHT tubes may be hard to find too!. Also remember that most tubing is available in 36" length only. You can use two pieces butted together if your hinge line needs to be longer than 36".

Finally, knowing the the tubing is of constant thickness, and the hinge line must be straight, the wing core itself needs to be fairly accurate. Poor foam cutting techniques can result in varying wing thickness--usually a thinning trend around mid span. This will give you fits locating the hinge line. Plan to use knuckle hinges on only your best cores! ;-)

Enough talking, lets do some hinging! With all of the above noted, and your 'bagged wing all ready to go, you need to start by locating the hinge line. Set a caliper to the diameter of the tubing. Move the trailing edge of the wing into the caliper, until the ends of the caliper touch the wing skins on both sides. Mark this position on both sides at the root, tip, and several places along the span. You are essentially marking the location where the wing is the same thickness as your tube. When you sight along these marks, they SHOULD all be in a relatively straight line. This will be your hinge line. If you core cutting was poor, the position where the calipers touch the wing surface will vary along the span, resulting in a curved line when sighting along the marks. Do this same step with the other wing half, praying all there time that the lines will fall in about the same place on both wings halves! Unless you have managed to cut the world's first pair of perfectly matching wing cores, the position of your hinge line (as determined by the calipers) will differ slightly--up to 1/4" is probably OK. You do want the halves to match, so fudge the final lines accordingly to make them match. Mark this final cut line with a straightedge and a Sharpie.

Cut out the surfaces along the marked hinge line. A good jig saw will work if you can cut in from the tip--support the wing in its original foam blank. If your hinge line stops short of the tip, you're stuck to use a razor saw or blade. Cut along a straightedge if possible. Once the surfaces are off, block sand the edges straight. Don't remove much, just enough to smooth the edges of the 'glass skins. Next, take a short piece of the tubing (same diameter as the hinge tube and about 4 to 6" long) and fold a piece of sandpaper around it (don't roll it). 80 grit is OK to start, but you'll want to move up to 120 before you get to the 'glass skin. Beginning with the control surface, gently sand the foam away from between the skins on the leading edge of the control surface. You are sanding a smooth, "U" shaped groove to accept the hinge tube. Once the sand paper is down to the 'glass skins, switch to 120 grit and continue until the tube fits exactly half way into the groove. Repeat the process, sanding a groove into the trailing edge of the wing until you can place the tube in the groove AND place the control surface back into position on the wing. The grooves should accommodate the tube along the entire hinge length.

Cut your hinge tube to the length of the control surface. Use 5 min epoxy and glue the tube into the groove on the control surface only! DO NOT glue the tube onto the trailing edge of the wing! Position the tube such that it forms a smooth semicircular leading edge of the control surface. There should be no gap or "bump" where the 'glass skin of the surface meets the tube--fill and sand if there is. Repeat for the other control surface.

Once the tube has cured, trial fit the control surface into the trailing edge. You will have to relieve the wing skins a bit to allow for the the tube and control surface rotation. When the surface is pressed snuggly into place, it should be able to smoothly rotate enough to allow full surface throw, without conflicting with the wing skins. When the control surface is in position at neutral, there will be about 1/8" of the tube exposed. Once satisfied with the fit, position the surface in the trailing edge groove at full down deflection. With a Sharpie, mark hinge locations on both the surface and wing---I place a hinge about every 6" to 8".

Using a round "rat-tail" file, cut a vertical slot though the leading edge of the hinge tube at each hinge location. The slot should be wide enough for a hinge to fit through without restriction, and long enough to allow the necessary rotation/control throw. Deburr the edges of the slots. Carefully drill a hole for the hinge through the back of the tube, opposite each slot. This hole MUST be centered top to bottom within the surface thickness. Drill a hole into the trailing edge of the wing at each hinge location. Again, the hole must be centered top to bottom within the wing thickness. Push your Robart Hinge Points into the control surface, through the slots in the tube. Position them such that their pivot point is centered within the tube. Trial fit the surface into the trailing edge. Make necessary adjustments to the hinge holes, the slots, or whatever is needed to allow the surface to move to its full extent while snuggly fitting into the trailing edge. There should be no more gap than the thickness of a sheet of paper. Run your 120 grit paper back and forth with the control surface in place to take down any binding areas.

Once everything fits and moves properly, use 5 min epoxy to glue the hinges in place. Start by gluing them into the control surface, taking care to center each Hinge Point's pivot pin within the tube. Once they have cured, apply epoxy into the hinge holes in the wing trailing edge. DO NOT allow epoxy to get onto the inside surface of the groove. You don't want to glue your tube to the trailing edge too--just the hinge points!

Attach your control horns however you like. I usually make my own from Bob Violett CF flex plate and epoxy them into a slot in the control surface. Off the shelf plastic horns are fine too. Just remember the pivot point is in the center of the tube, so your horns will have to be tall enough to make a solid linkage. You can see how torque rods would be difficult, but not impossible. You would need to cut a slot through the front of the tube, and a hole through the back of the tube for the torque rod to fit through when positioning the surface. Also, the torque rod must be centered within the tube just as the hinges are.


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Copyright © 1996, Chris Fouquet.
Revised: August 01, 1996.
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