OK, back to the flaps. First thing is to further reinforce the cutout flap bay in the wings using some "standoff-scale" ribs. I chose to make six ribs from aircraft ply, spaced about three inches apart. I used my template from earlier steps to help make sure these ribs wouldn't run into the ribs I'll add on the actual flaps. This way I can make them deeper to get maximum support.
After cutting these out with a scroll saw and sanding them to fit perfectly up against the tri-stock, I glued them in securely. Be sure to feather them off at the trailing edge such that they will allow the flaps to recess down into the bay properly.
Once those were all done, I cut thicker ribs from balsa stock and glued them in at each end of the two flap bays. This adds stability and strength to the bays, but also gives the ends of the flaps a "ledge" of sorts to make sure they don't recess "too" far into the bays, especially should you pick the wing up by the trailing edge and pinch the flaps in.
Leading edge flap spar
Now for the flaps themselves. First step is to add a substantial spar at the leading edge of each flap. This not only provides the torsional strength that will distribute the deflection forces equally, but also is the "meat" that your hinges will need to bite into.
To start, I cut two spars from 3/8" x 1/2" balsa stick. I glued them to the LE of the lite ply flaps (supplied) with some Titebond glue. To get a good glue joint and keep the flaps as flat as possible, I clamped them down to a perfectly flat surface. The flattest surfaces I have for such a task are from a couple different materials.
On one flap, I clamped to a scrap piece of the heavy composite board that is used for countertops. This stuff is dead flat, very hard and works perfect for this. Be sure to put another piece of wood on top of the spar when clamping so you don't dent the spar with the clamps.
For the other flap, I used the same technique, but used a piece of 1/4" plate glass as my flat surface. Again, perfectly flat, strong and stable for clamping. Once the glue dried on both, I marked the position of the flap bay ribs along each flap spar. I then used a rat tail file to make a groove the each spar so the ribs would not obstruct the spar from completely closing down in the bay.
Hinge and horn blocks
While the hinge points will get a pretty good bite in the thick LE spar, I felt a little more wouldn't hurt, and, I didn't want the bare hinge points sticking out in the open area of the flap. So, I cut some balsa blocks up and glued them in position, centering them over each hinge position. Butt these up tightly against the spar and shape them such that they too will allow the flap to close all the way.
For the control horns, I have decided that I want to use some giant scale nylon control horns. So, instead of a hard dowel for a mounting point, I used a large balsa block. This will more evenly distribute the stress and leverage of the control horn across a broader area, and is a little bit lighter. I also prefer conventional horns over bolts whenever possible.
On the top side of the flap, the horn itself will have a hard, glassed lite ply surface to clamp down to. On the bottom/inside of the flap, I'll need more than just the balsa surface of the block to put the "sandwich plate" of the horn on. Therefore, I cut a small plate from some phenolic "G-10" type of material that I picked up in Toledo. This gives a good hard surface and "boxes in" the soft balsa block with a thick hard "shell" now. The horn will mount with 3 screws running through the block and sandwiching the two pieces of the control horn together.
Setting up Robart hinge points
Moving on to actually setting up the Robart hinge points, the first step will be to sand a bevel on the top edge of the wing and the flap, right where they meet, such that at least 45 degrees of deflection can be obtained. Then, I mark each of the four hinge positions, both on the flaps and on the wings.
Now, I need to be sure that I know what the proper position and angle for each hinge point needs to be so that I get a good flush fit and clear deflection. So, I tested this out by making a little mock-up from scrap balsa, hinging the two together as I expected it to work out on my actual wing. First try worked out great, so now I'm ready to move on to the real deal.
Remember, your hinge points will be entering the wing and flap right at the top edge/surface, so you'll want to angle them down into the wing and flap blocks to get the most material around each hinge as possible. You've got a lot of depth in the wing to angle down into (remember the hinge blocks built inside the wing), but the flap bay is shallow... so you'll need a shallow angle going into the flaps themselves.
To start adding the hinge holes, I file a groove at each hinge point, both in the wing and on the flaps. This gives me a flat angle that I can set my drill bits into. I start the drilling with a 1/16" pilot hole to set the hole center and pilot the angle. Then, I move up to a slightly larger bit and repeat the process. Then, I move to the final bit that will give the hinge point a good snug fit.
The Robart hinge points flare out from a cylindrical barrel to a nearly square box shape at the pivot point. Therefore, you'll need to manually go into each hole with a knife and/or file to shape the entry of the hole properly, allowing the hinge to recess in deep enough. In the photos here, you can see that the pivot point on mine is at the bottom of the bevel, and recesses halfway into the spars.
Finished
As you can see here, I have a pretty good fit with the flaps deployed and closed. I have a good 1/32" to 1/16" gap around the TE and ends of the flap to allow some tolerance and clearance after glassing.
Note that throughout this entire process, I have been referring back to thetemplatesthat I made back when I was originally setting up the wing... before sheeting. Good planning, note taking, and making templates can save a lot of headaches when you get to tasks like this that require you to know where things are that can't be seen anymore!
I'll cover the gluing and permanent installation of the flaps hinges and control horns later, after glassing the plane.
