I have been building control-line models pretty much the same way for a long time. Over the years I have pretty much ignored sport models, but only pretty much. I try to look at what everyone is doing for ideas on how to simplify and improve how I build models. I have recently been contemplating building a few easy-to-build control-line sport models with foam wings and external controls. The new ideas have come from Pat Johnston, Will Naemura, Rob Costido and many others. Bear in mind that this model is simply a prototype to prove building methods and flight characteristic changes caused by mufflers. It was never intended to be reproduced, thus no plans were ever drawn.

Cutting Foam Wings

Cutting the wing core with a hot wire.

I have no idea how many ways there are to cut foam wing blanks. Clearly the best way is to use a CNC foam cutter, but I’ve never even actually seen one of them. I use three different methods, the most common of which I will briefly describe here. The first thing you will need is a power source. I used an electric train transformer from Goodwill for decades. If you don’t want to put a lot of time into a power source, an HO electric train transformer from the local hobby shop should do the job. My bow is made from PVC water pipe with .015" 0r .018" multi-strand flying line for the actual hot wire. I make templates out of Formica if I am planning to use them repeatedly. Otherwise, 1/8” plywood works fine. It is very important either way that the template surface be sanded very smooth.


When creating the airfoil, assume that the hot wire will cut the core about 1/32” per side smaller than the profile of the template. A word on templates: I use a heavy straight pin sticking out of the front of the template to start the cut and make the trailing edge of the template ¼” wide where the airfoil ends with a straight ¼” wide extension extending about 2” beyond that. The first foam cores I ever had were from a Vampire kit and they came to a point at the trailing edge. Over the years I have made the cores thicker and thicker at this point until I settled on ¼” for the template which leaves the trailing edge about 3/16” thick. Again, no one, not even me, is likely to tell you any of this is the best method, but it’s cheap, easy, and works fine.

A Bit About Foam Blanks

First, even when cutting tapered wings, I always cut the leading edge of the blank square to the root of the wing. This allows me to use this edge as a reference for cutting spar notches, bladder tube openings, etc. I have settled on cutting all my spar notches with a table saw. I have used a hot wire with templates or a router table in the past and both will work fine. If I’m cutting tapered wings, I always cut the slots on a table saw so that the wide section of the spar is vertical. This creates a tapered slot for a tapered spar which saves about ½ ounce of weight without sacrificing strength and rigidity unnecessarily. I always cut the wing cores long enough to be able to create shaped wing tips in the foam if you are using them. For this model I cut a vertical slot with my bandsaw 1/8” wide from the root to 2” from the tip to utilize a 1/8” balsa vertical web spar. From my experience, any weight that you save is insignificant. The rigidity, repairability and damage resistance is significant. If for some reason you are planning to use internal controls, cut out only the minimum required to install the control system in the inboard wing only.

The actual structure for the wing is dirt simple. I cut a 1/8” slot in the wing blank before cutting the airfoil. The spar is a full span piece of 1/8” sheet balsa. Once the wing blank is cut and sanded, I used Titebond glue to glue the spars into the cores and at the same time glue the wing halves together. The full length spar does the alignment for you.  After sanding I wrapped drywall joint tape around the center joint. I then applied two coats of Titebond thinned 50/50 with water to the drywall joint tape. For this model I used 6” wide drywall tape but I believe the 2” would work fine.

A note on wing tips: I prefer to cut the tip outline into the foam wing. I then wrap a piece of masking tape around the wing where the tip rib would be and a piece of ¼” wide masking tape around the centerline of the tip. The foam is then sanded straight between those two pieces of tape. The tape is then removed, and the final shape is sanded.

Controls

I use external controls for several reasons. The obvious reason is that it’s easier,which it absolutely is. The reason I have gotten over feeling guilty about using external controls with buttons on the bellcrank is that it is simply a better thing to do once you get over being embarrassed by it. You now don’t need to use leadouts and I hate to make leadouts.

You can and should put both lines through the same hole in the leadout guide. And the main reason, you will never have to worry about a line clip being twisted causing the model to crash about 20 feet out of launch.

I mounted the bellcrank to a piece of plywood 1-½” x 2-5/8” with the screw head and washer under the platform with the screw sticking up and the nut on top (photo at right). I then secured the screw head and washer to the plywood with C/A so the screw would not turn in the plywood. I next attached the platform to the center of the model with 3/8” on the outboard side and the rest on the inboard side with lots of epoxy and put the whole thing aside to cure. Once the adhesive has thoroughly cured, remove the nut and bellcrank.

Airplane Parts

The fuselage, stabilizer, elevator and rudder are all whatever you want the model to look like. I strongly recommend that the 1/8” plywood doublers extend about 1” past the wing spar to help spread the load and better tie the fuselage to the bellcrank mount, but that is the only thing I would do different from using a wooden wing. Also, assuming you will be finishing the model with plastic film, I find it easier to cover all the parts before the model is assembled. If desired, the wood parts can be finished with paint if desired (I can’t do paint). Another thing I always do is to coat the engine mount area of the fuselage with slow-cure epoxy to prevent fuel from soaking into the parts in that area.


The leadout guide is piece of 3/32” plywood with a brass grommet pressed in place.

All the sub-assemblies finished and ready to be put together.

Final Assembly

The only practical way to finish a bare foam wing is with low temperature plastic film. The easiest thing to use is low temperature model specific film. I’m old so the only one I am familiar with that may still be available is Top Flite Econokote. I’m sure some of the new films will work just fine at low temperatures but I’ve not used any of them. I tend to use either laminating film or cellophane gift wrap, both of which require foam safe spray adhesive for use on foam wings. On this model I used both laminating film and cellophane gift wrap with some Sunday newspaper comics thrown in just for fun.

Getting Serious

I used slow-cure epoxy for the wing to fuselage joint and 5-minute epoxy for the tail parts, then slow cure again to install the leadout guide.  Once the bellcrank, pushrod and horn are all set up, be sure you either use a nylock nut on the bellcrank bolt or if using a standard nut put a drop of thin C/A on the nut threads. For the button bellcrank I used a standard 3” nylon bellcrank with 4-40 brass thumb nuts (McMaster Carr # 92741A100) and 4-40 flat-head screws 3/8” long (McMaster Carr # 91253A108)

Lines

To make up the line ends for the button bellcrank, I start by bending the bare line end in half 2-1/8” from the end and crimping tightly.


I then kink the doubled line 1-1/8“ from the end.


Final assembly is done by using the proper size ferrule crimped in place with a 1/8” tail hanging out. The line end is finished by using shrink tubing to identify the lines as up or down.


Below is a much-used set of lanes attached to a bellcrank.

Flying

The first flight was, not surprisingly, rather spooky. The model rolled in on launch which resulted in an inside loop before line tension was such that the model could be controlled. The model then flew outboard-wing-high the whole flight, much reducing line tension on inside maneuvers. When the flight was over, John Thompson and I discussed what to do about the problem, which might have resulted from the heavy muffler acting as a counterweight. The choices of things we thought could be done at the field were either more tip weight or a trim tab. I wasn’t happy with either choice, but the trim tab seemed to be the better of two not-perfect answers. With the trim tab installed, the wing leveled out and line tension was now consistent during both inside and outside maneuvers. The trim tab, however, did not fix the low line tension on launch resulting in another inside loop from launch on the next flight.

Conclusions

As a general statement, the project was highly successful. Some thinking led me to believe that the roll might be easily cured by simply moving the location of the leadout guide from the top of the wing to the bottom.  I did this by inserting a piece of drip irrigation tubing through the wing with the outer end directly below the original leadout guide and the upper end protruding through the top of the wing about 4 inches inboard of the original leadout guide. This seemed to cure the problem and the model now flies great!  At the same flying session Russ  Hester was flying one of his 80 mph Combat models with a muffler simply added to the engine with no problem with the roll I had just fixed on this model.  Hmm?

Stay tuned to Flying Lines. This project has spawned some other ideas for future projects.