Installing spars and repairing a bent wing

The first set of pictures shows how to use the wing jackets to hold the wing panels straight and true while the spars are curing.

Check out this easy to make spar groove sander.

The second set is another way of installing spars on a new wing after the panels are glued together. You can use either method on any of our planes but the first one is the best.

the third group of pictures shows how to repair a wing that has been bent.

Note: our kits now come with the groove cut in the wing and 1/8 x 3/16 spruce spars. you may have to sand the groove a bit - usually toward the wing tip - to make the spar fit flush with the surface of the wing. Also, since we can't get full length spars into our shipping box, you will need to do one "scarf joint" in the center of the wing center section. Similar to figure number 3 below but with one joint instead of two. We still highly recommend using Poly-urethane glue for the strongest joints and spars possible.  

 
The first thing to do is cut the wing jacket in two. Just slice off the bottom just before the leading edge gets thick. Sand the ridges off - these are the bumps that came out of the spar pocket cut from the wing. Cut a strip of waxed paper and tack it to each of the jacket halves - top and bottom - use spray glue or water based contact cement. You can see the sand paper and the waxed paper - don't know why the sand paper is in the picture but it is and it is course 40 grit paper. 

 

This is a wing for the F86 Saber jet. First sand the spar pockets to fit the spars perfectly and snugly. Then pre-trim the spars and sand the out-board ends to match the wing tip curvature. Install the spars using Urethane glue, Yellow wood workers glue, or Epoxy - our favorite is the Polyurethane Gorilla glue. 

If you use Gorilla glue, squeeze a small bead along the bottom of the spar groove and drag the bottle tip back through it to spread the glue along the bottom and up the sides. Then dampen the spars with water (Gorilla glue only) and insert into the spar pocket.

Cover with 1 strip strapping tape and 1 strip colored tape over that as per kit instructions.

Fit the wing panel into the jacket - in this case, top side down - locate it in precisely the position it was when cut by the hot wire.

Lay the cut off part of the wing jacket over the wing and put a board over it to spread the force and put some heavy weight on top.  

Here I have both wing panels laid up and curing at the same time. Unless you used 5 min Epoxy, leave them over night to cure. When they are removed from the jackets, you will find that the spars glued in with no bumps and the Polyurethane glue will not leak out from the tape and expand all over the place. Some of our wings have washout cut into them and this will insure they come out perfectly true and straight.
Here is a cool little tool for making perfect spar pockets of any size in foam

here I have a piece of sand paper - 40 to 80 grit automotive paper - a piece of spar wood of the size you are installing in your foam wing or fuselage  -  a thin flat piece of wood, in this case a paint stir stick. 

bevel the ends of the spar stick and glue it to the edge of the sand paper

use a razor blade to score the sand paper along the edge of the spar and cut it off - notice the paper is glued to the bevel and over the top - this keeps the end of the sander from digging into the foam

glue the spar to the paint stir stick - cut the stick off so the spar is a little longer than the stir stick.

glue a suitable handle on top of the stir stick and presto you have a quick, cheap, slot sander that will give perfect spar slots every time and will only sand as deep as the spar. Make several sizes and you will be able to sink a spar of any size in any foam thing you make.

 

Below I am using an adjustable spar sander which is very handy also - you decide which is best.

 
 
The basic idea for installing spars is to sand a groove into the top and bottom of the wing at the thickest part of airfoil for the purpose of imbedding a strip of wood, wood dowels, fiberglass or carbon fiber rod, or fiberglass or carbon fiber cloth. We use bamboo Shish Kabob skewers for lots of our wing spars because bamboo is very strong and light and cheap. You will notice that the skewer is only 12" long and does not extend to the wing tip. this is OK since the main stress on a wing is in the inner third of the wing panel. On a short wing the tip is strong enough with just the 12" length of the skewer. I have also used 1/8 and 3/16 diameter wood dowels for a super strong wing. Some customers use 1/4" x 1/4" balsa for the spar. Most guys run the spar all the way to the tip which is fine also. Not covered here but certainly an alternative that we and customers have used is iron on carbon fiber tape. Sand a divot into the surface of the wing from root to tip so that when the fiber tape is laid in it and the epoxy resin has coated it, the finished surface will blend nicely with the contour of the airfoil with very little sanding.

 

 Note: This is a little tool designed to cut a groove into a foam wing (or fuselage for that matter) of a specific width and depth. I used a thin piece of wood and wrapped and glued 80 grit sand paper around it. Then, with some micro C-clamps, clamped a length of aluminum angle to it as a stop so the groove will only cut so deep. Pick your sandpaper board so it will cut a groove wide enough for the spar you want to use and set the stop so it will cut just deep enough that the spar is flush with the top of the wing. 
Here the groove sander is being used on the bottom of the wing. On this wing I used 12" bamboo shish kabob skewers.

 

 Figure #3

Since this wing has two dihedral joints, I made a short piece for the center and used scarf joints to join the wing spars to the center. I only went up the wing the length of the skewers, about 12", and used strapping tape to the tip.
Spars can be glued with several different adhesives. 5 min epoxy works great. Elmers wood workers glue works good also but lately I have been using Probond urethane glue. This is an interesting adhesive in that it uses moisture in the wood as a "kicker" to start the curing process. As it cures, it will foam up and, if allowed to, expand about 5 times its original bulk. This ability to expand will fill the groove and lock the spar to the foam as the glue seeks out and grips microscopic pores in the foam and wood. After fitting the spar, pull it out and lay a small bead of urethane glue in the bottom of the groove. Then quickly dip the spar in water, shake and wipe off the excess liquid and press the spar into the groove.

If you use the urethane glue - follow the next instructions carefully or you will have a big mess!

 After embedding the spar into the wing groove, you will have to seal the groove with tape. The wing above has a layer of masking tape over the spare. A strip of packing tape would work better. Make sure it is sealed very well because remember what I said about the glue expanding? Well, it will seek out even the tiniest little hole and bubble out of it. Then you will have to sand it off in the morning and it is pretty tough to sand. This wing has seen lots of hard flying and combats and is still holding up very well. You could use Epoxy to attach the spars and it would work very well also but this is something new and is seems to work great.

Next let's tackle a repair job. This method can be used to repair most any foam wing.

This is my P51B and the wing was just taped up with strapping tape and covered with packing tape. It would flex a little going into a hard turn but was never a problem until I drove it flat out into a brush pile that should have been burned but wasn't yet. both wings were bent up at about 45 degree angle so I really needed do something about it. 

Clean the wings with paint thinner thoroughly to remove the oils.

The first thing to do is determine what you will use as a spar. I used 12" bamboo skewers again for this one but you could have used balsa or spruce or whatever else you have hangin' around. Next thing is to cut through the covering to get at the foam. Use a straight edge and a sharp razor knife to make two slices from the fuse out the length of the spar. Cut it only as wide as needed to fit the spar.
Expose the foam by peeling away the covering between the two slices. Notice that the skewer only extends about 12 inches out the wing panel so that is how far the wing is opened.  
You could make a little tool as in the previous spar article or you can simply double up sandpaper until it will sand the correct width groove in the foam for the spar. Don't sand any deeper than will be need to sink the spar in flush with the top surface of the wing. 
This repair used urethane glue again (I really like this stuff) but quick epoxy or wood glue is great also. Before gluing anything, make sure the grooves are all the way you want them. This plane got a set of spars on the bottom also.
Use the sharp point of the skewer to poke into the fuse a little ways and put a dab of glue in the hole.
This urethane glue is moisture cured so dip the spar (if wood) in water and wipe off the excess. I show here applying glue to the spar - that's stupid - instead run a tiny bead into the groove and imbed the spar into it. That is a much cleaner way to do it. Keep in mind that urathane glue expands about 5 times if not contained, so do not put very much into the groove or it will leak out and cover the wing. 
Wipe off the excess so tape will stick to the wing.
Tape over the spar and make sure it is sealed up thouroughly.
Use more tape if you have to.
I used some heavy steel blocks to weight the wing panel over night while the glue cured.
That about covers wing repair. After the glue cured I had to sand a little glue off that seeped under the tape then re-sprayed the wing with green paint. This plane went on to fly many more sorties before finally retiring it and after this repair, the wing withstood anything that thrown at it.