Wing Construction Continued:
The next thing to do is to make the top skins. Because of the curved path along the top of the wing, the top skins will be a bit longer than the bottom. Glue two 3″ sheets together, then add your leftover strip from making the bottom skins. Place the top skin on top of the wing and align the top and bottom skins at the trailing edge.
Make marks on each end about 1/8″ in front of the leading edge of the rib. There’s a reason why you should mark it on the underside of the sheet. If you mark from the top side of the sheet, and you mark it very close to the correct dimension, when you flatten it out and cut it there is a chance that it could be a bit too short. This is a lot less likely if you mark it on the underside.
Cut the sheet to size, and then make the second one using the first one as a guide.
You may recall way back at the beginning I said that if your sheets aren’t all the same length you should use the short ones on the bottom and the longer ones on top. If you did it the other way around, your top skins won’t completely cover the wing. But if you did it the right way it shouldn’t be a problem.
The next step is potentially the most difficult in the entire wing construction, so you’ll have to pay close attention to get it exactly right. First, turn the top skin the correct direction. It should have two full widths of 3″ wide balsa, plus one about an inch wide. Turn it so there is a 3″ sheet at the leading edge.
Practice the next steps dry, with no glue, so you know what you are doing before you reach the point of no return. When you are ready, here is the entire procedure, with glue. You can also perform this step with Titebond or white glue, which will allow you to reposition parts and eliminate risk of gluing it in the wrong place, but you will need to weigh or pin the structure to a flat building board while it is drying.
Place medium CA glue on the top edge of all ribs, plus a line of glue on the beveled trailing edge.
(I hope you can see the glue in that photo.)
Very carefully align one corner of the top skin at the trailing edge.
Keep your thumb on it, and carefully put the other corner of the trailing edge in place. Don’t lower it in the wrong position or it will stick there. Bring it down in the correct position only.
Now start pressing the trailing edge to set the glue.
Continue along the trailing edge until it’s all stuck.
Now slide your hands forward and stick the top skin to all of the ribs in the mid-wing area. Move from side to side to make sure everything is stuck. Meanwhile you are holding the wing flat to the table so it will stick in a position where the wing is straight and unwarped.
Once again, slide your hands sideways to stick all of the ribs. This should take only a few seconds.
Now move your hands to the leading edge of the wing and stick the ribs in the middle,
and the edges.
Note that you have to press the skin down at the front so it follows the rib contour.
When you pick the wing panel up and look at it, you may notice that one or more ribs didn’t get stuck to the skin at the leading edge.
This is easily fixed with thin CA glue. Fix all ribs that have this problem, so they are all stuck all the way to the leading edge.
Check the trailing edge for spots that may not be stuck also.
In the event that your trailing edge made a crooked landing and your top and bottom skins do not line up, place medium CA glue on the offending area and use a leftover strip of 1/16″ sheet to fill the gap. Trim the excess off and sand it lightly until it looks right. It isn’t the end of the world.
The next step is to sand the excess balsa sheeting off the front so we can add the balsa leading edge sticks. Use a sanding block with 120 grit paper. Any finer than that and you will be doing too much pressing and not enough cutting. On the other hand, if your sand paper is too coarse you’ll tear the balsa up.
I apologize for the terrible photo, but you want your sheeting to be flush with the fronts of the ribs, without sanding dips into the sheeting. Make a nice straight line.
Now it’s time to attach a 1/4 x 5/8 balsa stick to the leading edge. This can be purchased as a stick, or it can be stripped off the edge of a 1/4″ balsa sheet. You can save a few bucks by making your own sticks, but either way is acceptable.
There are two ways to attach the leading edge. One is to apply medium CA glue to all of the ribs and the top and bottom sheeting, then press the stick into place. The other way is to apply white glue or Titebond and hold the stick into place with at least a half dozen pieces of masking tape.
Before you glue the stick on, place the wing on a flat table and see if it has any warps. If it is warped or twisted, hold it flat to the table as you attach the stick. If no warps are present, you can safely work with the wing in hand, but do not introduce twist or warp while attaching the leading edge because after this piece is attached the wing will no longer twist (or untwist).
I placed my wing panels on my desk to check for warp, but unfortunately it is a brown desk that is terrible for photography. I found that both panels were a bit low in the middle and high on the ends. So I applied the glue and then held the wing down with my forearm while attaching the leading edge stick. I’ll demonstrate this in a photo taken in better light, but the leading edge is already in place. If you have to do this, hold the wing down firmly on a flat table, with the leading edge protruding a quarter inch beyond the edge of the table.
If your wing was warp free to start with, and you decided to live dangerously and glue the leading edge on while holding the wing in the air instead of on a flat table top, check it again for warps. If it’s warped, cut the leading edge free on the top or the bottom, then put the wing on a flat table, and re-glue it.
The wing tips are made of 1″ triangle stock. Sand your wing tips flat. Remember, the wing roots have an extra rib, and the root rib is leaning significantly. The other end is the tip.
I made my tips from a piece of 1″ x 1.25″ balsa, which I split with a table saw, and then I sanded the saw tracks off. Glue the tip to the end of the wing as shown in the photo below.
This type of tip is known as a Hoerner tip. I chose it because it enhances lift and is easy to sand and cover. The easiest way to finish a wing tip would be to glue a piece of 1/8″ sheet flat to the tip of the wing, then sand the edges flush. It looks good and it’s easy. If you prefer a different kind of wing tip, go ahead and get creative.
Use a utility knife to carve the tip roughly to shape. Make sure your knife doesn’t slope off when you are carving. You want the top of the wing to be a continuous line from the wing to the wing tip.
Here’s the wing tip after carving.
The wing is finished, except for the sanding and joining the two halves. This will be covered in step 3, finishing and assembly.
For the next step in the process, go to the menu at the top and find the fuselage instructions.