Suppose you’re building from plans that show the wing held on with rubber bands, but you want your plane to have a bolt-on wing instead. This is not hard to do, even if it’s not shown on the plans.
The plane in the photos is a 40 size Telemaster from Hobby Lobby plans by Joe Bridi. The plans show wing hold-down dowels through the fuselage, and the wings are held on with rubber bands. In the first photo the wing center section is built pretty much according to the plans, with balsa trailing edge stock and glass cloth.
I wanted to attach my wings with bolts, but if I put wing bolts through this trailing edge stock, the wings would break off as soon as I open up the throttle and jerk the elevator a little bit. (Ask me how I know this…..) What this plane needs is a piece of 1/16″ plywood to spread the load to the structural members of the wing when the bolts are clamped down on the trailing edge stock. Trim the plywood to a shape that looks nice. I like to put a nice curve on the front, and then sand a taper into the curved part to make the Monokote blend nicely.
I made this curve with a compass after finding the point on the center line that would reach both sides and the front on the same curve. In other words, if the plywood needs to be 3 inches wide and 4 inches long, the radius will be 1.5 inches. So draw the center line and place the compass on that line 1.5 inches from the front to draw the curve. Also, make sure that the grain runs from front to back to allow the plywood to bend with the dihedral angle.
After this piece is ready, check for proper fit and alignment, then get ready to glue it in place with medium CA. After you do this on a couple of planes and get better at it, you might find it helpful to put some accelerator on the wing surface to make it stick instantly.
You can see in the photo that I added a little bit of balsa on either side to help the plywood reach all the way to the edges of the trailing edge stock. Here’s what it looks like after it’s been glued on.
Remember when you start the wing installation your windshield should not be in place yet. Leave it open so you have a clear shot at the wings through the front bulkhead with a long drill bit. In the case of a low-winger, make sure you don’t put the chin block on until the wings are installed.
The next thing we need to do is put some hard wood in the center section to attach the front dowel. Here’s one way to do it that involves digging a little hole in the root ribs with an XActo knife and a utility knife to gain access to the back side of the hardwood spars. Once I finished that, I used epoxy to glue a piece of 1/8″ plywood to both spars, as well as another piece on the back of the leading edge sticks. Note that the piece of plywood at the spars will have to be a couple of inches long to reach the top spar and leave enough sticking out the bottom to anchor a quarter inch dowel.
Once you have these tabs in place, it’s time to get a long drill bit and drill a hole all the way through for the dowel. Notice that I have installed a 1/8″ plywood plate on the front of the balsa fuselage bulkhead.
There is another plate just like it on the inside surface of the bulkhead as well, so the dowel will be anchored in two pieces of plywood on the bulkhead. When drilling a long hole such as this one, it is a good idea to drill pilot holes first with a long 1/8″ bit, then move to the 1/4″ bit for a 1/4″ dowel, because a your drill will wander more if you start with the bigger bit. Hold everything firmly in place and go slow.
After you have the hole all the way through, put in your dowel. I like to chamfer both ends slightly with a pencil sharpener or sanding block so they’ll go through the holes more easily. As you can see in the photos my drill bit didn’t go straight. It never does… The important thing is to hold the wings in place on the center line and then it doesn’t matter if the drill wanders a little bit to one side.
After you get the dowel glued in place, you can try it on for size and see if everything still lines up. (Never mind that ink squiggle. I was just trying to make my pen write for the centerline marking.)
Once you have the front dowel all done you can go ahead and put on the windshield. Now it’s time to move to the back of the plane. I dug around in my scrap box and came up with a piece of 3/16″ plywood about an inch wide that would make a nice rear wing bolt anchor. I trimmed it to length and stuck it in there with a little bit of corner bracing under each end. (Notice the grain direction on the corner bracing. This is much stronger than using standard triangle stock.)
We want the bolts to go through the middle of this anchor rather than at the front or back of it, so I drew a centerline on mine. The line is 5/8″ from where the trailing edge of the wings will be, and the two cross lines are simply an inch in from each outside edge, to mark bolt locations.
Once you have these dimensions figured out, transfer them to the wings.
Now we need two drill bits. One of them should be the diameter of the shaft of the bolt, not counting the threads. The other is a larger bit, the size of the outer diameter of the bolt including the threads.
Using the smaller drill bit, and holding the wings firmly in place with your other hand, drill ONE HOLE, but not the other. Drill all the way through the wing and through the anchor plate. Be sure that the drill bit is perpendicular to the surface you are drilling. In other words, if the wing surface has a slant caused by dihedral and the sloping airfoil, don’t drill straight down, drill squarely into the plywood, which will cause the bolt to slant into the anchor plate a little bit. If you don’t drill straight into the plywood wing plate, your bolt heads won’t lie flat on the surface. Note that this is not an issue on a low-winger with a flat bottom airfoil.
Remove the wings from the plane and harden the hole through the anchor plate with thin CA. Give it a shot of accelerator to make sure you don’t foul your tap with super glue.
Now tap the hole for threads. Press firmly, and be sure to follow the path of the hole. Usually wing bolt holes will be drilled at funny angles because of the slant of the airfoil, so don’t push your tap in straight down if the hole slants to one side.
Remove the wing from the plane, then drill out the hole in the wing trailing edge to the larger size for the bolt to pass through. Remember these two important points. This hole should be large enough to pass the bolt through fairly easily, but not big enough to be sloppy-loose. You don’t want your wing to be loose, so don’t make the hole terribly oversized. The other point is to remove the wing, and THEN drill out the hole. If you just leave it in place while you drill the hole to the larger size, I can guarantee that the drill bit will end up in the hole you just threaded in the bolt plate, and if that happens it will draw itself in and destroy your work. Ask me how I know that……
Trim the bolt to a reasonable length (enough to thread through the anchor plate, plus about 1/8″), and chamfer the end with a pencil sharpener.
Bolt the wing in place with your first bolt, and THEN drill the second hole.
Remove the wings, drill out the second hole to fit the bolt, thread the second hole in the anchor plate, etc.
There is a good reason why the entire drill-tap-bolt sequence is done one bolt at a time. You’ll find that the drill bit likes to wander when it hits a piece of plywood. If you drill your first hole and then drill the next hole without tapping and securing the first one, the holes won’t all be lined up because something will move regardless of how tightly you’re trying to hold it in place. So just take the time to switch back and forth from one bit to the other a couple of times and do one hole completely, then the other.
Here’s the finished installation.
The procedure is pretty much the same for a low wing plane, only upside down. If the fuselage bulkhead covers the leading edge of your wings you’ll want to think ahead and install plywood anchor plates inside your wings at the leading edge and at the spars. Then drill the dowel hole right through them instead of having plywood tabs sticking out of the center section. You certainly can build it with protruding tabs, but try to plan ahead and make the installation as tidy as possible. You can adapt this basic method to just about any plane you build.
Here’s a photo of the finished Telemaster.