The Return of the TigerKitten (9)

This time around I want to work on the cowl enclosure. Once upon a time we would have called this the “engine cowl”…can we still say that? Whatever you call it, the cowl on the TigerKitten is supplied as a one-piece plastic molding consisting of the cowl shape itself surrounded by a margin of extra plastic left over from the molding process. Premier could have trimmed this off neatly for you, but they would have to charge for the time necessary to do that…and, by trimming to a predetermined dimension they would have eliminated the option you have to fit your cowl to the exact dimensions that have resulted from your building process on your fuselage. It’s better this way.

Back when, to get a cowl like this you would have had to do plenty of carving and sanding of balsa blocks, or even make your own master plug, then a mold, and then lay up a glass -cloth-and-resin cowl. That is exactly how I produced the cowls for the original TigerKitten prototypes over twenty years ago. You can still do that, but you don’t have to. Premier used my original female mold to cast a new master, or plug, to match the original design and then used it to produce the molded plastic cowls that are included in today’s kits. If there’s enough interest, I might be able to address that process sometime in the future, but for now let’s be happy that we can get neat, light, accurate molded cowls like this and talk about the best way to turn it into part of a really well built airplane.

I am going to add a feature that does not appear on the Premier kit plans or in the instructions. It’ll be a judgment call whether you want to do it or not on your airplane…have a look at what I decided to do and make your own decision.

This is the "raw" cowl molding after I trimmed away most of the overhang, or extra plastic from the original matrix. This particular "pull" did not result in a clean, definitive transition from the matrix and so leaves some question as to exactly where I need to cut to get a cowl that will fit the rest of the airplane. Rather than go looking for another part, I'm going to show you how to fix this on your own, as this is a problem you are going to run into a lot using the molded parts that are supplied in many different kits. By measuring back - on the plans - from the front of the cowl (where the back of the prop spinner meets the cowl) to the front of F-1, I can determine how long the actual trimmed cowl needs to be. I marked this cut-off line with some masking tape. BTW...that's the Cobra motor I'm going to use sitting on the plan.

Trimming off the remaining excess plastic requires some care. If you cut too far, you can probably fix the damage, but that's lot's of needless extra work. I'm using a pair of short bladed, curved shears to snip the extra material away a bit at a time. Don't try to finish the cut tight up against the tape this way...leave some material for the next step.

The magic tool for this part of the job is my good old sanding block. This time I'm using 80-grit production paper to get a fast, clean cut without having to "scrub" at the workpiece. When I'm finished I will have sanded right up tight against the rear edge of the tape. When you are sanding along the edge of a plastic molding like this one, it's a lot easier to move the abrasive back and forth along the edge rather than across it...that would invite grabbing and tearing of the plastic.

I'm going to add a feature to this model that isn't mentioned on the plans. Because the molded plastic cowl is more flexible than the fiberglass units I got used to on my prototypes, I decided to add a backplate that functions as a reinforcement to keep the rear edge of the cowl more rigid. I'm using 1/16" aircraft plywood for this, and here you can see how I simply traced the outline of the rear cowl face onto the wood with a nice sharp pencil.

I want the plywood plate to fit inside the rear of the cowl, flush with the back edge of the plastic, so I am marking the insert to relieve the outside edge by the thickness of the cowl...about 1/32" . With that done I trimmed the plywood back to the inner line.

This is a test fit of the insert into the cowl to ensure that it goes into place without either gaps or bumps that would stretch the plastic cowl out of shape and spoil the fit with the rest of the nose structure.

With the fit of the outside edge of the insert confirmed, I marked the cut line to remove the inner portion of the plywood plate so the completed cowl will slip into place around the motor installation.

I slipped the plywood insert into place inside the cowl molding and glued it using ZAP-A-GAP. Here I have a little sanding left to do to clean up the fit. As we go on you'll see what this completed assembly should look like from the outside.

There's more going on here than meets the eye. Inside the cowl the entire motor, mount assembly, and spinner are assembled in place, held in alignment with that masking tape. Since the motor mount assembly provided in the kit mounts against the F-1 former with adhesive, I decided to use an unusual method of aligning the motor/spinner assembly with the cowl. After checking that the required thrust offset is correctly built into the back face of the motor mount and that the F-1-to-spinner distance is correct, I put a bit of slow curing epoxy onto the back face of the mount and aligned the entire assembly as you see here. By default the thrust angles are correct AND the spinner aligns perfectly with the cowl. This may seem backwards, but it works fine...think about it.

Once the epoxy that is tacking the mount assembly in place has cured, I gently remove all that tape and slip the cowl off. At this point the motor mount assembly is in exactly the right place but it is still somewhat delicately attached.

A few short strips of lightweight fiberglass cloth and some ZAP-A-GAP do a fine job of creating a reliable bond between the mount assembly and F-1. Make sure you use enough adhesive in a place like this to saturate the glass cloth thoroughly. You'll know that's happened when the weave of the cloth appears clear rather than white.

There's a good reason for having several different grades of ZAP. I want to reinforce the attachment of the motor mount to F-1 everywhere possible. Around the top radius I pressed a couple more short strips of glass cloth into place tight up against the wood, and then saturated the whole deal with thin ZAP, which wicked into every pore and crevice to turn the whole assembly into a single unit of structure. I did the same thing along the bottom side of the mount.

As soon as the ZAP hardened, I used a sanding block to clean off all the rough edges and smooth the exposed surfaces. (I also covered the Cobra motor with a bunch of masking tape while I was sanding the fiberglass.)

Here I am using a razor saw to cut openings in the reinforcing insert at the rear face of the cowl. You'll see in a moment what these are for.

The modified cowl with the backplate insert I added fits snugly against F-1, but it still needs something to hold it in place. I have added two tabs of 1/8" plywood glued to F-1 and recessed just far enough from the outer face of the fuselage side to let the cowl slip over them and fit flush. Here I'm adding reinforcement in the form of some more strips of fiberglass cloth bonded with fast ZAP

Now you can see how the cutouts I made in the cowl backplate insert are going to fit over the attachment tabs.

Now I have assembled the cowl in place, properly lined up with the rest of the fuselage and held in place with some masking tape. I previously marked the tab locations and the cowl to line up so the marks for the screw holes will center over each tab.

Drilling through the cowl wall into each of the tabs is easy.

I'm using 3/8" socket head sheet metal screws to retain the cowl. These are just long enough to reach all the way through the tabs inside without being needlessly long or heavy. I'll remove the tape once all the drilling is done and the screws are tightened in place.

This step is an easy one to forget. No matter how carefully you have lined up everything during the assembly so for, there are going to be minor misalignments bewteen the cowl and the rest of the nose structure. This is the time to get aggressive where the sanding block meets those last little imperfections...but...be sure you don't sand through any thin spots. As before, I'm using 80-grit production paper here to cut cleanly without any extra pressure.

There's one spot on the nose of the TigerKitten that may need extra attention during this final shaping with the sanding block. The lower front corners of the fuselage where they meet the cowl may insist on sticking out and creating bumps, so don't be afraid to let the sanding block show that extra wood who's in charge. There's a backup piece of structure inside here to give you a little extra something to sand on.

The last step in the job of fitting the cowl is cutting and trimming the air intake hole. I began by rough-cutting this opening with the cowl removed. Now I'm using one of my favorite shop tricks...the custom sanding tool made from whatever is at hand that fits the job, wrapped with an odd piece of the right grade of sandpaper...to clean up the outline of the air intake.