Attach Wingtips with Hinges
From Matronics
Hate platenuts? So does Pappawhiskey (the original author of this article). Gonna put stuff inside your wingtips? (strobe supplies, wing leveler servo, antennae, aileron pushtube etc.) Pappawhiskey did. Wanna easy way to get to this stuff and still have your wingtips looking nice with no screws or paint damage from removal? That's what Pappawhiskey wanted and here's how he did it- It is highly recommended that you read through the whole procedure and make sure you understand what you're getting into before you start. It took Pappawhiskey four days at about 4-5 hours/day to do this, but that included a whole lot of head scratchin, YMMV ....
- Order four 6' long pieces of AN257-P3 (MS20257-P3) hinge. The actual length of the hinge pieces will be around 42", but the pins will need to be about 8" longer (you'll see why). If it's cheaper to buy 5' pieces, that will work.
- Make 1/2" wide spacer strips out of .040 aluminum to offset the hinges from the wing skins- this along with the thickness of the hinges approximates the thickness of the fiberglass in the tips.
- The actual length of the strips and hinges is not set in stone- they need to extend from the very rear edge of the upper and lower skins as far forward as you like. The lower ones can be longer than the tops even though they start farther forward because the bottom of the wing is flatter toward the front. My lower hinges are about 1.5" longer, but they could easily all be the same length. Just don't make them extend all the way into the sharply curved section of the front of the tip. 41-42" long should be about right. DO NOT shorten the pins at this time!
- Lay out and pre-drill #40 holes in the wing skin for the hinges- if you use the same spacing as the outer rib it will look very professional! A fan-spacer is excellent for this (you pre-pre-punch folks know what that means!)
Hinge on Wing
- Clamp one side of the hinge to the wing skin overhang with the spacer strip between the hinge and the skin. Lots of cleco clamps are very handy here. It is a really good idea to set these up so the REARMOST EYE on both upper & lower hinge is on the WING and not the tip. (This makes it easy to install the tip by yourself by having the upper pin "staged" inside the rear eye before you load the tip. Once the tip is positioned to the wing, the tip is held by one hand and the other can begin feeding the pin into the hinge. Once the pin is a foot or so in, the tip will safely stay in place by itself.) Note in the foto above this wasn't done on the lower hinge! Drill the holes, clecoing each hole as you go. You can deburr, dimple the skin, countersink the spacer, prime and rivet now, or wait, whatever works for you. Easy to rivet with a squeezer.
- Trim the factory mounting flanges off of the tips except for around the very nose of the tip where the hinges do not interfere- this helps hold the front of the tip in position on the wing and makes loading the tip easy. You will also need to sand down the trimmed edges to about a .065 thickness to match the thickness of the wing-side spacer and skin. Note that if your wingtips are significantly thicker here, you might wish to use a thicker spacer, but you definitely need good virgin glass and epoxy here to help bond the hinges on later.
- Make one Styrofoam "rib" for each tip (I used the blue Dow board stuff) by holding a piece of foam up against the end of the wing and tracing the outline- you won't need the front 8" or so where there is no hinge. Cut this out slightly smaller than the actual rib. This "rib" is then set into the tip about 2" deep to hold the flimsy thing in shape when you drill the holes for the outer half of the hinge. Tape it securely in place. Later, this will be glassed inside the tip to help hold its shape.
- Using a #42 or #43 drill (#40 would probably work OK, but smaller is better here) to pre-drill the rivet holes in the outer hinge- the spacing is not critical, as they don't really need to match anything since they will be invisible when painted. One and a half inches would be fine. We're going to make flox "rivets" between these later.
- Pin the outer halves of the hinges to the inner halves. The curvature of the hinges will cause them to stick straight out. Get a roll of duct tape and tear off some foot long pieces and set them on the wing skin so you can grab and stick 'em on while holding the tip in position with one hand.
- Load the tip to the wing and tape it in rough position. This will take some trial & error to get a nice fit- you will likely have to do some fine-trimming along the edges of the tip at the wing skin, but go easy here- too much off is hard to replace! Use a long board with sandpaper on it to keep the edge as straight as possible. The aileron was also installed at this point and the airfoil template was holding it in the correct in-trail position to help align the rear of the tip. A cargo strap was also used around the rear of the tip (not shown in foto) and the outer front end of the wing skin (taped in place) to put some forward tension on the tip, nesting it snugly into the front of the wing skin. Keep playing with the arrangement, adding more tape as necessary until you are satisfied with the overall fit of the tip.
Tip Clamped for Drilling
- Here comes the interesting part: The only way to drill the holes for the outer hinge is from the outside. You will need to rig some kind of strong light source INSIDE the tip to shine straight up/down through the rivet holes in the hinge. These little spots of light will be your drill targets, and you will need to be able to move the light every couple of holes to minimize parallax error. A regular shop light (trouble light) with a 100-watt bulb was used. Fish the cord in through the gap at the rear of the tip next to the aileron- you can attach a coat hangar wire to the handle of the light to help move it around while drilling. If your tips are heavily gel-coated like many are, you will also have to sand most of the gel-coat off the edges so they are translucent enough for the light to shine through.
Lighted for Drilling
- Re-install the tips with the light inside and tape it all into final position. A couple of pieces of thin plywood and a clamp were used to hold the rear edge of the tip in alignment with the aileron (template still in place- see previous foto) Position the light near the spar shining straight up. Using as little pressure as possible, drill down through the fiberglass into the hole in the hinge and install a cleco. This cleco will hold the hinge against the tip skin for the next hole. Repeat this, working your way fore/aft being careful to watch all alignments, and removing tape as you go. Repeat for the bottom and other wing. Also, go back and re-drill each hole to #40 for riveting, pulling & re-installing each cleco as you go- this will get all holes in good alignment.
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Hinge Clecoed to TipNow it's time to attach the hinges to the glass. Somebody offered this basic procedure on the RV-list a few years ago and it seems reliable, so here goes: Drill a 1/4" hole between each rivet hole in the hinge. Using very coarse sandpaper or wire brush, roughen up the side of the hinge facing the glass. Mix up some epoxy and brush onto the mating surfaces of the tip and hinge, then add flox to the mix that is still in the cup to make it almost as thick as peanut butter. Smear a thin coat of this on the tip and cleco the hinge in place.
Flox "Rivets"NOTE: Try to keep the squeeze-out on the eye side of the hinge to an absolute minimum, as this is the main reason the pins can be hard to install later- anything left between the eyes will have to be removed to allow the eyes of the two hinges to align properly.
Read the last sentence again, and repeat until memorized! Rivet the hinge on using soft rivets and your squeezer, being careful not to crush the fiberglass. Add a small dab of flox into each of the 1/4" holes to form a "head" of flox, just like a rivet head. Allow the epoxy to cure. The hinge shown in the foto had not been riveted yet, but it's not as messy riveting first.
- Once both hinges are riveted and bonded on, trim the foam "rib" to final shape- you may need to remove much of the front of it to clear stuff you may have mounted (or will mount) to the end wing rib- my Navaid wing leveler is out on the right tip rib, for example. You can tell where the flimsy part of the tip is anyway and this will show you where you need the extra support of the foam rib. Tape it in place, install the tip to the wing, and adjust as necessary. Then glass the rib in place. The ribs for the initial attempt at using this technique were about 2" inside the tip. An extra strip of glass was also laid along the hinge flanges at this time as a bit more assurance they never let go!
- Grind and polish a nice "bullet-nose" to one end of the pins, polish them with a Scotchbrite pad and slip them into the tips and wings separately. If there is ANY difficulty or binding you MUST find out why and correct it. Check each eye for alignment, as they are very easy to bend. Grind the rear of one pin to a slightly angled flat (sharp edges around the circumference), chuck it up in you drill and run this through the eyes to remove anything left inside the eyes. Once your pins will easily slide through either hinge half separately, install the tips and try feeding the pins through, starting with the upper one. Any binding here means there is something between one or more of the eyes keeping them out of alignment with each other. Remember the note in step 13? Hunt down and kill each of these obstructions! When done, your pins will slide in and out with ease. After paint, a light coat of wax or them will help keep the rust off and allow for even easier insertion/removal.
Hinge Pins Secured
- Now you can fit and install the close-out rib at the rear of the tip. Before you rivet this in, install a platenut for a 10-32 screw that will hold the pin security block in position. In the initial attempt at this technique Pappawhiskey hadn't figured out how to do this yet and had to pop-rivet a small plate with the platenut already installed. Now you can make the block. Any tough plastic will do, delrin, nylon UHMW type stuff- Drill and chamfer oversized holes for the pins to go through, but make the hole for the attach screw a bit smaller than the screw so that you have to literally "thread" the screw into the block. This not only keeps the screw from backing out, but will also keep you from losing it!
Wingtip Attached!
- That's about it- have fun! Any questions, contact tech support (the original author) Pappawhiskey.
-- Pappawhiskey 11:57, 22 Mar 2005 (MST)
