The Return of the TigerKitten (4)

On this airplane, the primary structure of each wing panel consists of all the ribs, the leading and trailing edges, the upper and lower 3/16″ sq. spars and the spar web inserts, and the various pieces that make up the wing tip assembly. So far I have built the and right panels in turn flat on the board, over the kit plan, with the various components held in place using pins stuck into my soft-surfaced building board and clear plastic food wrap under the structure to keep my ZAP CA adhesives from sticking anywhere I don’t want them to. With that done I joined the panels using the three laser cut dihedral joiner/braces which lock in the correct automatically when aligned properly.

The wing of the TigerKitten is what is classically referred to as a “D-tube” structure, in which the ribs, spars and edges determine the shape and the working strength comes from the addition of rigid covering…in this case, 1/16″ sheet balsa…over all or part of the basic structure. Most commonly the leading edge from the main spar forward is sheet covered top and bottom, so that when you view the assembly from one end it resembles an elongated “D”. Usually the entire center section, two or three rib spaces out, is sheeted front-to-rear and capstrips are added to the outer edges of the ribs to form mini “I”-beam structures. That’s what we’re going to do here.

In the case of the TigerKitten, all the outer sheeting, as well as the capstrips and trailing edge caps, are cut from 1/16″ sheet balsa. If you are working from a semi-kit, you got all the ribs out of the box, but now you are going to have to pick out some more wood on your own. I could write a short book about the varieties and odd characteristics of balsa…suffice to say here that you want medium-to-soft balsa that is just slightly stiff when you bend it across the grain; that is, wood that you can bend around the curvature of the ribs but not so soft that you’re afraid of breaking it. To put it another way for those of you who don’t have a lot of experience picking balsa, the stuff on the hobby shop rack is almost always OK for airplanes like this one. Pick the lighter weight, lighter colored sheets, reject any that are twisted or that have obvious flaws, and if you have the opportunty, sort through the selection and choose pieces that are as much alike as possible.

Ready? Let’s get the wing finished.

This is the sheeting for the upper front of the left panel. It extends from the junction of the W-1 ribs all the way to the tip, but does not overhang W-5. The rear edge lines up with the rear face of the spar web inserts and the front edge is cut for assembly to extend just slightly beyond the 3/16" sq. balsa leading edge to provide enough material to "sand on" when it's time to create the leading edge radius. I have given the outer surface (the side you see) a light spray of warm water to induce a gentle bend along the grain. I want to begin assembly by using glue ONLY at the leading edge for the best possible control of the sheet, but I want to key it in relation to the top spar, so I have used several pins to hold it in place. So far I have not used any glue. Watch what happens next.

Because I sprayed the top/outside surface of the balsa sheet with water a few minutes ago I can easily bend it like this, up and away from the leading edge, to get the SLO ZAP exactly where I want it. (I am holding the sheet in the bent position with my other hand, out of the picture.) I'll apply a generous bead of SLO ZAP all along the 3/16" sq. balsa leading edge and up about 1/2 inch onto the front of each rib, then close up the joint.

With the rear edge of the sheet held at the spar with pins to keep it from moving, I have applied SLO ZAP all along the leading edge, where the sheet has been cut to overlap by a small margin. Now I am using a steel straightedge to roll and press the front edge of the sheet tightly against the leading edge until the ZAP grabs.I held the rear edge of the top sheet in place on the spar while I glued up the front edge. Now I can unpin the rear edge and roll it up to get access beneath it without disturbing the alignment. Here I am applying ZAP A GAP to the top edge of each rib as well as the upper surface of the the top spar. The sheet balsa is still wet and easy to bend at this point.

The top sheet is attached at the leading edge and ready for this part of the structure to be closed up. I am adding a generous bead of ZAP A GAP to the top edge of each rib where it will contact the inside surface of the balsa sheet and all along the top surface of the spar. The next step is to roll and press the top sheet into place. The sheet is still wet.

I have already added the 1/16" balsa sheet to the center section from the rear of the top spar back to the trailing edge. (Sorry, I was having so much fun I forgot to get a shot of that for you.) The capstrips come next. Th Kitten is a bit different from some airplanes in that I use a long spanwise capstrip along the top of the balsa trailing edge...this provides a 1/16" raised lip against which all the rib caps fit smoothly. Here I have a piece of 1/16" x 3/16" balsa that will be cut to fit the space between the leading edge sheet and the trailing edge cap.

With the individual rib cap cut to length I have added ZAP A GAP to the top edge of the rib and pressed the cap into place. The "medium speed" adhesive allows me to position the strip accurately and still cures quickly enough that holding it in place while it "sticks" is practical. No need for pins.

This is the top surface of the left wing with the leading edge sheet, the trailing edge cap, and all the rib capstrips in place. Notice the 3/16" sq. balsa spacer strip under the rear of the ribs, angled forward at the tip to provide washout as I explained earlier. Those are lead block building weights I use to keep the wing structure flat on the board without having to stick more pins through it.

I rocked the assembled wing over to rest the right panel on the building board and held it down with those building weights in preparation for adding the trailing edge and rib caps on that side. That part is really critical...RIGHT HERE is where adding those next few pieces of structure, the capstrips, is going to help lock the structure into its final shape...that is, flat, twisted badly, or washed out just the right amount.

The Kitten uses full span strip ailerons, in part because they work so well as flaperons (which we are going to use.) Taking my measurements from the plan, I cut four pieces of 1/4" balsa sheet to fit the profile of the center section trailing edge. These will be laminated to make two half-inch thick pieces, one for each wing panel. These will be trimmed to accept the DuBro No. 186 strip aileron horn assemblies.

This is the laminated trailing edge block for the left wing. I have taped it in place to mark where it needs to be tapered to match the airfoil section dictated by the rest of the wing structure.

I am using a small block plane to cut the left trailing edge block to the correct profile. This is a job I'll finish with a sanding block.

Both trailing edge blocks have been planed and sanded to final shape. I beveled the faces where they join at the center and have spot-glued them into place to check alignment and do any necessary final sanding to be sure they blend smoothly into the rest of the wing.

This is another job for the good old sanding block. When I'm done I want those trailing edge blocks to look like they are a seamless extension of the rest of the center section.

I have cut the trailing edge blocks loose from the wing after making sure they are sanded to fit as accurately as I can get them. This one is clamped in a vise between a couple of scraps of wood so I can use a curved chisel (a gouge) to rough out the slot into which the long shaft of the strip aileron horn will fit.

You won’t be able to see it when the structure is assembled, but I like having everything that is part of an airplane finished off and neat. Here I am using a round wood rasp to clean up the opening I made with the gouge to become a smooth slot.

: The DuBro strip aileron horn includes a molded nylon hinge tab slipped over the wire shaft to serve as a mounting device. Here I have the left horn in place in the trailing edge block assembly and the right horn ready to go into the slot I’ve cut for it. ZAP A GAP is my adhesive of choice here.

As assembled, the top and bottom 1/16" balsa leading edge sheets extend just a bit past the front of the 3/16" sq. balsa leading edge and stop. This is what they look like before any trimming or sanding.I like to use a block plane for the initial smoothing and shaping of structure like this. I'll cut off enough wood to produce a smooth, straight working surface while leaving a bit of material for the sanding block to take off.

The next step is to use the sanding block to get the entire length of the leading edge squared off and smooth. When that's done and I have a reliable reference for where the front of the leading edge radius is supposed to be, I'll finish rounding off the contour.

With the leading edge radius sanded in, I am continuing back around the top surface of the wing to blend in/smooth off all the structural joints that are not supposed to show through the covering when I'm done. This part of the job of building an airplane is one that MUST be done slowly and with care. Get it right and keep checking and going back until you are sure it's as smooth as you can get it.

Here's another look at using the sanding block to get the curvature of the leading edge JUST RIGHT before I go on to other things.

The stuctural trailing edge is actually the front surface of the aileron well. Here I am giving it a final sanding pass the ensure that it is squared off and free of bumps.

The first pass over the leading edge used 80-grit production paper to enable me to shape the structure cleanly without having to press down hard and risk distorting anything. I did an intermediate sanding with 120-grit and now I have switched to 320-grit for final smoothing of the surface. The trick is to work with each successively finer grit until all traces of the scratches left by the coarser paper are gone.

I am using a piece of 320 grit production paper to do a final smoothing and rounding of all those curved and rounded edges.

More of the same. I am using 320 grit production paper on a block to finish off the edges of all the capstrips smooth and even with each other. SANDING is not something you do for a few minutes and forget about...not sanding enough is perhaps the most common error new builders make that results in mediocre model airplanes.

This is the beginning of a corner insert...perhaps you could call it a gusset. I have cut "blanks" of 1/16" balsa sheet from the same stock I used for the surface sheeting in order to get similar stiffness, and cut triangular inserts with the grain at a 45 degree angle across the opening and glued them in place.

The sanding block comes out again to dress the surface of the insert even with the rest of the structure.

I used a template to mark the cutting line...in this case I found an odd bottle that had the right radius...and now I am using an ordinary No. 11 blade to make the cut.

Sanding an inside radius like this is hard to do without some help. I'm using an old can with 80-grit production paper taped around it to finish off the cut in a perfect arc.

Here I have a piece of 320 grit production paper on the same can to smooth of the radius to match the rest of the surface. "Noodely little stuff" like this DOES make a difference in building the kind of airplanes that other aeromodelers will go out of their way to look at.

I built the ailerons after the rest of the wing structure was complete so they would match up correctly. Here I have fixed the left wing panel to the building board over the plan and covered it with plastic wrap. The piece I am holding is more 1/16" sheet balsa and will become the bottom surface of the left aileron.

Sorry...I blew the focus on these shots. This is the best one I could get for you. This is the end-on view of the lower aileron surface showing how I beveled the rear edge so the the upper surface sheet will fit neatly against it.

With the bottom surface sheet in place, I am adding the laser cut aileron ribs using ZAP.

All the aileron ribs are in place on the lower surface sheet. I have made an insert from scrap 1/4" x 1/2" balsa, tapered it to fit, and glued it in place at the root of the aileron where the short end of the DuBro aileron horn will fit into it.

I have cut a top surface sheet using the plan as a pattern.

I assembled the top and bottom surfaces of the aileron, joined at the trailing edge, with more plastic wrap around the outside of the assembly and clamped everything in place using clothespins.

I added ZAP from the front, which is still open, to bond each of the aileron ribs and the surface sheets all along the rear edge. That plastic wrap keeps the clothespins from becoming part of the aileron.

The leading edge of the aileron is just another piece of 1/16" balsa sheet cut to fit. I used my sanding block to smooth and bevel the front of the assembly I glued up in the last step so the sheet would fit smoothly against it and then proceeded with this step.

This is the finished aileron with all the edges sanded smooth.