1/4-scale Piper Cub: Completed and Flown!

VINTAGE SIG 1/4-SCALE PIPER J-3 CUB BUILD-ALONG

By Dick Pettit

Contrary to popular belief, I did not teach the Wright brothers how to fly.  In all honesty, I have been building and flying R/C aircraft since 1981 or so.  Since that time a lot of kits and ARFs have crossed the threshold of my workshop, in both directions.  Recently, I began to slow down a bit, resolving to only build models when the mood struck me, and that mood struck quite hard a short time ago.

 I was in the process of loading a model I had sold into his truck when he asked me if I would be interested in building an unbuilt SIG quarter scale J-3 Cub kit. (Photo 1) He had purchased the first run kit way back in 1981 and only opened the box to see what was inside.  He offered it to me under the pretense that I only could have the kit if I wrote about it in the form of a review article.  I agreed with that and got the go-ahead from the editorial staff here at Model Airplane News. 

I was also told by the original owner of the kit that it was destined to be decorated in the same was as his fathers Cub was lettered and was used to give the kit’s owner his tail dragger certificate for is private pilot’s license.  He even had the correct registration numbers for the model made up and those were included with the kit.  This was going to be a really great project, considering the background of the lit.

But it does not end there.  When I began investigating the hobby or R/C models, my mentor, a fellow named Jim Letchworth showed me the ropes, allowing me to take the sticks on several of his models before I began to build and fly my own planes.  Jim then came into work one day and told me that he had gotten one of the first kits of a “huge” quarter scale Piper Cub and he was already starting to cut out the print wood balsa parts (Photo 2) using his Dremel jig saw.  When it was completed and flying, the sight of that gigantic (for its time) Cub flying across the field convinced me that giant models were going to be the avenue I was going to follow from than on.

Jim passed away a few years later and one of his wished was to give me that Cub because I had loved it so much.  I also purchased a few of Jim’s tools at the estate sale, including that Dremel jig saw.  When I started cutting out the print wood parts with that jig saw, (Photo 3) it seemed like just yesterday that Jim and I were at the field flying his Cub.  This is going to be a very inspirational project for me and I hope you enjoy what I write about it.

I plan to build it in the same way as SIG describes, using as many “period” tools and materials that were available at that point in time.  I located an OS 120 four stroke engine (not a Surpass, those came later) for the project, and that was the same power plant that was used on Jim’s Cub.   Did some investigating and fount that SolarTex was introduced in the early 1980’s and I plan to cover the Cub using that material.  I could have used SIG Koverall and dope, but decided on an iron-on pre-colored material instead.  I probably will use modern radio equipment for the sake of safety rather than 75 MHZ (remember blue and white, red and white, etc?)  The Pacer brand of adhesives have been around for quite a while and despite the fact that they didn’t introduce ZAP CA products until the 1990’s, I’ll still use them on the Cub.  DuBro quarter scale Cub wheels and fuel system components will be used to complete the model.  With all that out of the way, I’ll open the box and start building my “Retro Cub”.

Inside the kit box, I found a little slip of paper (Photo 4) that told me that this kit was “especially packed for me” by a lady named Hazel J.  Hey, wasn’t one of the former owners of SIG named Hazel?

The instruction manual had turned brownish (Photo 5) but it follows along with the present day manual with the exception of the photographs (Photo 6).  The plan sheets were missing but I had ordered a set from SIG and they still had to be cut and taped together considering the “huge” size of this
model (Photo 7).  The wing is still built in two pieces as originally designed, with the center section attached to one panel (Photo 8).

RETROCUB Installment 2

Back in 1981 when the SIG quarter scale Cub kit was introduced, many modelers were using “model airplane glue” to put their wooden kits together.  They worked well, despite their slow drying time and horrendous odor.  Others were using white or yellow wood glue, which provided easier clean-up, no odor and strong joints.   However in the early 1980’s, that new-fangled “super glue” that only seemed to stick your fingers to one another wasn’t being used all that much.  

This installment finds the builder in the early 1990’s, using tools and materials introduced about that time.  It was also the time when the Academy of Model Aeronautics (AMA), in conjunction with the Federal Communications Commission (FCC), worked out a plan whereby R/C modelers would have the use of 50 separate frequencies rather than the 6 or 8 available previously.  They also mandated the use of “narrow band” radios to minimize cross-channel interference, and these radios were affectionately known as “1991 Stickered”, because of the gold sticker that had to be on each transmitter.  What a breakthrough in radio technology that was, or so we thought.

Let’s get back to building.  Fuselage formers are fabricated using several die cut parts and several pieces of plywood and balsa.

 The Cub fuselage sides are constructed using several of the printwood parts we cut out a while back along with many sticks of 5/16” square balsa, seen here.

The parts are pinned to the board, and glued together using adhesive products that I have been using for many years.  ZAP brand instant adhesives, distributed by Pacer Products, have been around since the 1990’s and I still prefer them to this day.   I prefer ZAP-A-GAP for wood to wood joints, since it fills minor gaps and cures quickly, and that’s what I used for these initial construction steps.   

When the first side is complete, the pins are removed and the fuselage side is sanded smooth.  Next, a sheet of wax paper is placed over the first side and the second side is built over it.  

By aligning each part exactly, the two sides are identical.

The two sides are then checked and sanded where necessary and plywood doublers are joined to each side, making sure to make a left and a right.

I used Thick ZAP, rather than epoxy, to add the plywood doublers, and after weighting them down and waiting a few minutes, the sides were strong and flex-free. 

With the two lower sides completed, the upper front sides are added, starting with the lower right fuselage side.  This is pinned to the board over the fuselage side view covered with wax paper.  The already assembled formers are then added to the right fuselage side after marking their positions and making sure they are at a 90 degree angle to the doubler.  I used ZAP-A-GAP to hold the formers in place.

The second side is now Zapped in place to the formers after remembering to mark the doublers with the former locations. The top fuselage rail is attached to the top of the formers and the fuselage subassembly can be unpinned from the board.

The back end of the fuselage sides are tapered and balsa upper and lower fuselage spacers are made up using the templates on the plans and glued in place using the fuselage top view as a reference.  Care must be taken to prevent building a “banana fuselage”.

The main landing gear blocks are then epoxied into the notches in the fuselage sides and I may pin them with wooden dowels for extra strength.  The front part of the fuselage is made from laminated firewall parts and the side pieces we made earlier.  A top spacer sets the correct angle.  The assembly is then epoxied to the front of the fuselage structure,  Pre-bent wing brace wires are then epoxied into the slots in the front sides of the fuselage after drilling holes in to top wing former.

Outer front fuselage panels are now epoxied to the nose and formers and sheeting are installed to make up the top of the front fuselage.

 Thick balsa is then installed between the main landing gear blocks and sanded smooth .

Rear fuselage formers and stringers are now added making sure to keep everything straight. 

The upper stringer requires notching and bending to form it to the correct curve.

Next time, I’ll make preparations for installing the engine and fuel tank, along with assembling the cowl and building up the main landing gear.  See y’all then!

In this installment we move from the early 1990’s into the late 1990’s and pick up kit construction with the main landing gear assembly and the tailwheel.  I jumped ahead in the manual because the engine I had purchased turned out to be something it was not advertised to be, which was “almost new”. 

So onward we go, beginning at the rear of the fuselage with the tail wheel mount.  The photo below shows the plywood mounting plate with the leaf spring tail wheel assembly attached to it.  No wheel comes with the kit, so I rummaged through my stock of wheels, finding this one that looked to be about the correct size.  I also needed a few more wheel collars to keep the wheel and the steering arm from binding.  Once the rudder is completed, the tail wheel arm will be connected via a wire rod, and the rudder controlled with a pull-pull system. 

The main landing gear is made from formed music wire pieces that have to be wrapped with copper wire and soldered together.  The SIG kit comes with that copper wire but I felt it was very small in diameter and it was supplemented with wire that was a bit sturdier.  After making a few “adjustments” to the fit of all the wires, the joints were wrapped with copper wire  and soldered together using a propane torch and plumbers solder.  This type of solder has no lead content and can be used with water soluble soldering paste.  It is perfectly strong enough to be used in applications such as this and the joints I have made in the past have never failed. 

When the solder has cooled, it can be cleaned with an old tooth brush and some hot running water and dishwashing soap.  I found that the upper cross brace was too large to fit as described, seen below, and a new piece was bent to the correct size and soldered to the main gear legs.  The upper V part has a small piece of music wire through which a rubber band is wrapped to provide a small amount of flexibility in the landing gear during taxiing and landing. 

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Once the gear legs are completed, pieces of balsa filler is epoxied between the front and rear legs as fillers, seen below, and are then covered with thin fiberglass cloth and more thinned epoxy for added strength.  Rather than glass cloth, I used SIG Koverall applied with water base polyurethane, which should be strong enough for this application.

A pair of beautiful DuBro ¼ Scale Cub wheels were chosen to keep the front of the Cub up off the ground, seen below.  Traditional wheel collars are used behind and in front of each wheel, making sure to grind a flat spot on the outer end of each axle to keep the collars in place.  The Cub hub cap will hide the outer collar and the inner collar will not be seen at al
l.

A suitable engine finally arrived and I then had to provide some sort of engine mount with which to attach it to the firewall.  I originally planned to use a set of DuBro Vibration Isolation engine mounts but the firewall on the Sig Cub is not large enough to accommodate their large footprint.  I resorted to one of the tried and true motor mount solutions, a glass filled plastic mount from Dave Brown Products.  Their footprint is considerably smaller and will fit on the Cub firewall very nicely.

I had to decide on the orientation of the cylinder before mounting the engine.  In years past, an inverted cylinder on a four cycle engine was not the most desirable installation.  However, side mounting may require a large amount of cutting and trimming of the cowl and dummy engine to fit, and with the advent of modern onboard glow igniters, a downward facing cylinder may be the best possible mounting orientation.

Holes were drilled in the firewall after locating the thrust and center lines of the firewall and blind nuts were used with the mounting bolts to keep everything in place.  The OS 120 Surpass engine has a choke assembly on the carburetor hub is placed at the correct position that barely touches the firewall when the prop hub is at the correct position.  I cut a small relief groove in the firewall, seen in below, to provide necessary clearance and drilled the engine mounting holes in the mount.  I tapped the holes for the mounting bolts, but a bit of carpentry had to be done on the fuselage.  Who can guess what these holes are for?  The throttle control rod, the fuel lines and the onboard ignition system holes will be drilled after all those parts have been chosen.

I decided to mount the fuel tank, a 16 ounce DuBro model, to the underside of the front fuselage former, almost in line with the center of the carburetor.  I used a three line system, one for the carburetor, one for the vent and the third for a fill and drain line with a second clunk, seen in below.  This drain and fill line will be connected to a DuBro Fill-It fitting, allowing the tank to be filled or drained without using any of those traditionally leaky filler valves.

The fuel tank itself will be installed on a plywood plate along with sponge material to isolate it from vibration.  The onboard ignition system with battery will also occupy the space in the front of the fuselage. 

That’s it for this installment.  Next time I’ll be starting to assemble the tail surfaces. 

Stay tuned…

Over the Holiday weekend I made significant progress on the J-3 Cub, but only after taking a step or two backwards.  I decided that since the instructions told me to assemble the plastic cowl before sanding the front of the fuselage, I probably should put the cowl together.  When I first looked at the plastic parts, they appeared to be OK for use on the plane.  But under close scrutiny under a strong lamp, I saw a number of “pock marks” in the cowl sides, probably caused by problems during vacuum forming.  You may be able to see one or two of them in the photo below, but there are more than 20 of them in each cowl side, too many to risk filling, sanding and filling again.

Rather than take a chance, I looked around the internet and found that Fibertech N More (http://www.fibertechnmore.com/store/) had just the fiberglass cowl with molded engine cylinders I needed for the Cub.  I contacted them and made arrangements to get their model S-62 cowl.  When it arrives, I’ll get back to sanding the front of the fuselage.

Something else I had overlooked was the balsa filler pieces between the man landing gear legs and the reinforcement fiberglass cloth applied with water base polyurethane.  The photo below shows how the back side of the balsa filler has been reinforced with ¾ ounce glass cloth and several coats of polyurethane.  When dry, the edges will be sanded smooth and glass cloth will be applied to the front sides, sanded smooth and made ready for final covering.

In the mean time, progress is still being made in the workshop.  I took out the plan sheet for the horizontal stabilizer, located the printed wood parts I had cut out earlier and after a bit of sanding, the stab outline was Zapped together. Speaking of sanding the edges of parts, two of my invaluable tools I use to cut wood strips squarely and sand the edges perfectly are my For
emost Miter Saw and Miter Sander, seen below.  A razor saw cuts quickly and accurately at any angle when used with the Foremost Miter Saw fixture, and the sanding blocks on the Miter Sander cleans up the edges of any piece of woof for perfect fitting parts.  These are two of my favorite tools!

Once the stab was complete, the elevator halves, rudder and fin were assembled in a similar fashion, sanded flat and made ready for hinging.  SIG supplies flat pinned hinges with the kit, but I used point style hinges just because I feel more comfortable installing them.  Since the point hinges are longer than the flat plate hinges, extra wood has to be installed through which holes are drilled for the points.  Robart Manufacturing makes a handy drill fixture for small or large point style hinges and they allow the holes to be drilled perfectly.  I then use a countersinking bit to enlarge the opening just a bit to allow room for the pivot point of each hinge.  The photo below shows the hinges, drilled holes and the drill fixture.

Since the full scale J-3 Cub’s major airframe parts are made with round metal tubing, the edges of the model’s tail surfaces have to be sanded round, including the leading and trailing edges of the stab and rudder.  Using a Master Airscrew razor plane, a sanding block and a piece of sandpaper folded in my hand, I made each perimeter surface nice and round, as seen below.  These pieces are now ready for final sanding and covering later in the build proves.

I had a bit more time before the holiday weekend was over, so I started on the wing assembly, beginning with the center section.  The two piece wing is built with the center section permanently joined to the left wing panel, and the right panel is removable.  The center section is framed with ribs and sheeting, adding the open skylight during the process.  The dihedral brace and rear joiner are added and everything is allowed to dry, as seen below.  No sanding is done on the center section until both wing panels are completed since they are used to establish the correct shape of the entire wing structure.

Wing construction starts out by covering the right wing plan with wax paper, cutting to length and laying down the spruce spars and installing all the wing ribs except for the root rib, which is made from a balsa and a plywood rib.  The wing center section is then plugged into the root rib and that assembly is Zapped into place to set the proper dihedral angle.  The rib and spar assembly is removed from the plan and front and rear lower sheeting is pinned to the plans.  The rib/spar assembly along with the center section is then aligned to the sheeting and Zapped to it.   You will notice that the aileron is built as a part of the wing structure.  It will be cut loose once the entire wing is complete.

The instructions say to start on the second wing panel, but I elected to add the wing tip structure to the first wing panel before continuing.  The upper leading and trailing edge sheering is Zapped into place and the leading edge is attached, secured by masking tape until the glue dries.   In the underside of the wing, hard points for the wing strut and jury strut attachment points are added. The die cut ply and balsa wing tip parts are then Zapped into place to complete most of the wing structure.

I’ll let the wing pieces dry for the time being and since the holiday weekend is just about over, I’ll close for now and start next week with the second wing panel, which is built just about the same way, except that it is permanently connected to the center section.

The month of January was very “eventful” with several moderate snow storms, an ice storm or two and the beginning of hardcore basketball season.  I realize that the harsh weather we have down here in North Carolina is nothing compared to what our friends in the Midwest and Northeast have experienced, but schools and work were closed here for a few days so they could salt and sand the roads.  Anyway, I have had the time to do a number of building steps on the 30 year old SIG Quarter Scale J-3 Cub and here’s what has happened.

I said I was going to build the second wing panel just like the first one?  Well, that’s exactly what I did, and the photo below shows the two panels bolted together with preliminary sanding completed.  But, if you look very closely, you may notice that there are a few pieces of wood missing.  Yes, the two panels are
exactly the same, with the same pieces missing on each side.  Somewhere in the instructions is a line that says “…add the upper and lower aileron sheeting…” and I took this to mean the trailing edge sheeting.  There is LEADING EDGE aileron sheeting parts too!  It took about an hour to remove the cap stripping and replace them with the balsa upper and lower sheeting. 

Once this was complete, the ailerons were cut out and the ribs were trimmed to install the wing aileron trailing edge and the aileron leading edge.  A bit of sanding and filling and the ailerons were ready to be hinged to the wings.

I decided to replace the pin type hinges in the kit with some Klett hinges I had in the shop.  They are larger, thicker and have holes drilled in them to allow the hinge glue to bond better.  The photo directly below hows a pair of 1/16” holes drilled downward at an angle through the wing trailing edge a bit farther apart than the width of the hinge. 

 A hobby knife with a sharp blade was used to cut out the wood between the holes and the hinges fit in the slots snugly, seen in the photo below.  The same technique is used on the aileron leading edge and the hinges will be glued in place after the wing and ailerons are covered.

The plans give two options for connecting the ailerons to servos, one of which is a pair of pushrods connected to bellcranks and driven by a single servo in the wing center section.  You have to consider that the kit was designed in 1980 and that was a popular way to connect ailerons to a servo.  Today it is thought to be “Dark Ages technology”.  The other option is to mount a pair if servos in the outboard part of each wing panel with a cover plate on the bottom, as shown below.  This means mounting the servo using a removable mounting bracket, and bending the pushrod with a severe Z bend to allow it to exit the wing.

I used the cover plate to mount the servo using some hardwood blocks to keep them in place.  It was a straight shot from the servo arm to the aileron horn, as seen here. 

 Speaking of aileron horns, the plans show the use of a modified control horn glued to a plywood rib in each aileron.  I initially used this method until I bumped the plastic horn against the work surface, resulting in the horn cracking completely off the aileron.  I was able to break the other horn from its aileron just as easily and replaced them both with inset plywood plates and standard plastic horns. 

With the wings needing only final sanding, I began work on mounting the new cowl I got from Fibertech N More.  You may remember that the supplied plastic cowl had a number of defects in it, and I placed an order for a replacement fiberglass cowl with the dummy engine cylinders already installed.  The cowl arrived in a few weeks (they make up each cowl at the time the order is received) and the only things needed to finish it was sanding the center joint, cutting out holes for the air cleaner and cylinder head and mounting it on the nose of the Cub.

The photo above shows the hardwood blocks glued to the firewall using ZAP-A-GAP from Pacer Products.  Just to make sure they stay in place, I have added dowel pegs glued through each one into the firewall.  I had to cut a small clearance hole for the cylinder head on the OS 120 Surpass and while I was at it, I cut an air exit hole on the bottom.  The photo below shows the cowl screwed in place with the engine sticking out where it is supposed to stick out.  Once everything is done, I’ll cut the clearance hole for the muffler and the needle valve extension and prepare the cowl for painting.

The wing can now be fitted to the fuselage and once made square, bolt holes are drilled through the wing and bolt plate in the fuselage.   

 They are then tapped for ¼-20 nylon bolts and wood dowels are installed at the front.  

The photo above shows the results of cutting, planing and sanding the inner wing fairing blocks that are mounted to the wing center section.  These will certainly be a challenge to cover with fabric.

The next step in the manual is the fitting of the wooden wing struts using aluminum brackets at each end.  I used a sanding disk to round the ends of each aluminum fitting and drilled several holes in each one to allow epoxy to flow through, making for a solid joint to the strut.  The plans show the proper bends needed for each metal fitting and where to drill the mounting holes.

The wing struts are made from hardwood and are slotted at each end for the metal fittings.  They are actually cut to the correct length to fit between the fuselage and the hardpoints at the outboard ends of each wing panel.  The larger front strut has to be cut at an angle to mate with the rear strut and this joint is secured by the inner metal fitting and some slow curing epoxy.  The plans are used to set the correct angle and paper clamps were used to keep the parts in place.

Once the joints have cured, the inner fitting is bolted to the fuselage and the outer fittings are used to mark the locations of the bolt holes and blind nuts. The blind nuts are pulled tight and secured with a small amount of epoxy and the struts are ready to be shaped.  I used a router bit on my high speed rotary tool to rough the shape and finish sanded the struts with a long sanding block.

The strut fitting joints and locations of the jury strut mounting bolt holes are then reinforced with a layer of thin fiberglass cloth and epoxy.  

These areas were then sanded smooth and the jury struts were built up from brass tubes and 4-40 hardware.  Care must be taken to place the bends at the right places and in the right direction.  

The manual wants you to permanently attach the jury struts to the main struts, but I felt that this would hinder the covering application.  I labeled all the parts and I’ll wait until the covering is in place before installing the jury struts.

I felt it was a good time to install the rudder, elevator and throttle servos and started by building a set of mounting rails in the middle of the cabin area, below the “belt line”.  No materials are provided but a quick visit to the leftover wood bins resulted in some hardwood rail materials that worked perfectly.  I used the servos to locate the mounting holes, screwed all the servos in place and then glued the completed assembly inside the fuselage. 

I used a single servo for the elevator, and I fabricated a music wire joiner rod that connects the two elevator halves together.  Had this been a high speed heavy aircraft, I would have opted to use a pair of elevator servos, each driving a single half.  The rudder will be connected to its servo with a pull-pull system, not specified in the instructions, but still a proper method for a 30 year old design.

The stab has to be glued to the fuselage after aligning it to the wing and center of the fuselage.

The elevator halves are then hinged temporarily and the pushrod is made up from a fiberglass arrow shaft with 4-40 rods at each end.  

The elevator horn is screwed into place and the location at which the rod exits the fuselage is marked for a piece of sheet balsa.  A slot is cut in this piece of wood through which the pushrod passes. 

The pull-pull cables have to run from the rudder servo arm to the rudder horns, passing through the fuselage.  More sheet balsa is glued into place at these locations and the exit holes are lines with short sections of plastic tube to prevent the wires from wearing holes in the wood.  The tail wheel will connect to the rudder using a plastic horn and a pair of coil springs after the plane is covered.  I added some balsa cross braces to keep the elevator pushrod from flexing under load.

Next time I’ll take everything outside and start the process of “making square things round”. 

It’s getting down to the home stretch in the building process for the 1980 Vintage SIG Quarter Scale Cub kit.  Like I always say, “…it’s 90% complete now…only the other 75% left to go…” seems to always hold true when I build any kit plane.  All of the major assemblies are completed and they all seem to fit together pretty well, so it’s time to do the last two steps in the build process, sanding and covering.

I like to call the sanding process “making square things round”, but I have to do most of the sandpapering outdoors because I would rather not get sanding dust all over my other planes and my amateur radio equipment.  A breezy day allows the sanding dust to be blown away and after each piece is completed it is cleaned off and made ready for covering.   

The photo above shows one of the wing panels prior to finish sanding.  The sanding bar has 120 grit sandpaper on it which will be replaced with 220 grit after all the lumps and bumps are smoothed out.  I’ll use a 320 grit sanding pad to wipe over each part to give a fine finish to all the wooden surfaces.

 The photo above shows what’s left on the ground after sanding all the parts of the Cub kit.  At least it seems that I took that much off all the parts.  Actually, my rule for sanding is that after you think you’re done, you should do it once again.  And then do it again.

I brushed off all the sanded parts with a bristle brush, followed by a tack rag.  I make sure the covering table is clean also and cover the surface with a clean bath towel.  The Cub will be covered with SolarTex Cub Yellow that I bought from Balsa USA.  A 5 meter roll was just not going to be enough and an extra 2 meter roll would not give me the required lengths needed to finish the wings, so a 10 meter roll was purchased.  I pre-cut the covering sections for the wing panels which gave me strips of covering to be used on the ailerons, elevators, rudder and other smaller parts.  The rest of the roll will be used to cover the fuselage.

This photo shows the first part I covered, an elevator half, starting with the bottom.  Those single edge razor blades in the photo are indispensible when cutting covering, but they do have to be replaced quite often.  Fabric covering really dulls the edges, and once I have trouble getting a blade to start cutting, it is put in an old pill bottle to dispose of safely.  The new one really cuts well, and I may use as many as 30 blades during typical covering process.

I use my trusty covering iron set to about 275 degrees to stick the covering down to one end, then the other and finally the rest of the perimeter, pulling the fabric tight at each place I stick down.  The iron is then turned up a bit and the edges of the covering are rolled around the structure, pulling tight once again to minimize wrinkles.  Heat and pulling tight will always result in a good covering job around corners.  The other side of the part is then covered using the same technique and only then is the covering shrunk down tight with the iron set to a higher setting.  The photo below shows a completed elevator half.

Here’s a little trick I use when covering flat parts like servo hatches and similar structures.  The covering is cut a bit oversized and ironed down to the part.  It is then trimmed at the corners as seen on the left side of the photo below and ironed around the edges and finally to the bottom of the part.  If done correctly, there will be no overlaps at the corners and the edges will be covered perfectly.

The other parts along with the wing panels have now been covered using the same heating and pulling techniques used to cover the smaller parts.  I usually let the parts rest a few minutes before shrinking tight, but I sometimes find that a day outdoors in the sunshine just may bring out a small bubble or wrinkle.  If the sun isn’t out or if it’s chilly, I use my heat gun to look for these minor defects.  The covering iron is then run over each and every square inch of covering to make sure it is all stuck down tight. The photo below shows all the parts completely covered.  In case you’re wondering, I’ll be getting to the fuselage later on because there are a few different covering steps needed to reproduce the scale covering methods used on the full scale Cub.

While I had some time, I began to open up the holes and slots in all the parts that needed to be opened, such as strut bolt holes, hinge slots and servo hatches.  Rather than use a razor blade to cut these holes followed by a hot covering iron to seal the remaining covering back down, I do both steps at the same time.  I use a small tip on a hot soldering iron to do both, cut the hole and seal the edge, as seen here.

  You will have to remember not to use this soldering iron tip to solder wires ever again.  Bolt holes, hinge slots and any other openings needed in covering, either fabric or film, can be made using this technique. 

The photo above shows the completed hinge slot and the sealed edges of the covering.  Screw holes on the servo plates can also be made by piercing the covering with the tip of the hot iron as seen here.

Now that the ailerons and the wing panels are both covered and the hinge slots cut open, it would be a good time to install the hinges.  Many modelers use epoxy to join the hinges to the wings and ailerons but I have had extremely good luck using Pacer Hinge Glue to do this.  Yes, I can hear all of you saying that water base glue cannot possibly adhere to plastic hinges and I agree with you 100%.   That’s not how Hinge Glue works.  When the Hinge Glue touches the wooden structure, the water in it starts to expand the wood fibers.  The glue is applied inside the hinge slot and the hinge is then inserted quickly.  After just a few minutes the wood has expanded enough to hold the hinge in place even before the glue has had a chance to cure completely.  If you try to pull the hinge out at this time, it will be extremely difficult.  The wood has expanded through the holes in the hinges, holding them in place.  When the glue cures completely after 24 hours, the joint is as strong as the wood itself.  You will also notice that if the hinge is flexed, the joint will make a cracking sound.  This is the glue cracking since it does not stick to the hinge joint and can be cleaned out with the point of a hobby knife,  You wind up with an easily flexing hinge joint that will stay in place for a long time.  The photo below shows the aileron hinges in place and curing.

The same technique holds true for point style hinges, like the ones I used in the elevators and rudder.  Pacer Hinge Glue is applied to each hinge hole, the point hinge is inserted and aligned and the part is then positioned such that the glue cannot drip inside the covering.  This can be done by standing the part with the hinge line facing down until the Hinge Glue starts to cure.  The wood around the hinge point expands and conforms to the ridges on the hinge points, hereby holding them tightly until the glue cures.  

The photo above hows the rudder and elevator hinges being test fit into the stab and fin.  No, I did not glue them in place at this time.  I wanted to make sure they aligned perfectly before the glue cured.  The photo below shows the aileron servo plate screwed into place with the aileron linkage and control horn installed.  You can also see where I cut the exit holes in the covering with the soldering iron tip.  The servo leads were pulled through holes in the ribs using sections of string I installed prior to applying the covering.

All that is left to cover is the fuselage, but before we tackle that part, I have to do a few minor assembly steps.  Starting at the rear, there needs to be a set of tail brace wires made from 2-56 wire rods and clevises, which are included in the kit.  The 2-56 clevises, both a threaded one and a solder type, have to be modified prior to installation. 

I snapped off the part of the clevis with the pin in it leaving the part with the hole, as seen in here.

  This hole was then drilled out with a 3/32” drill bit through which a 2-56 bolt is inserted to bolt the clevis to the tail pieces.  A small bend is made in each clevis and the 2-56 rods are then trimmed to length to have the clevis holes line up with the drilled holes in the structure.  Plumbing solder and water base flux is used to solder the clevis in place and a jam nut is used to keep the threaded clevis secure once the brace wires are adjusted.  All that is necessary is to clean the brace wires, followed by priming and painting. 

The photo above shows the brace wires installed on the uncovered tail pieces.  The 2-56 bolts will be trimmed to length and secures with elastic stop nuts (AKA, safety nuts)

The side windows and windshield have to be cut out and trimmed to fit using a template on one of the plan sheets. 

The windshield is cut from a large piece of clear plastic, taped into place and mounting holes are drilled in the fuselage sides and the top center section, as seen in below.  Care must be taken to avoid making the windshield too small. 

The side windows are supposed to be cut out and glued to the fuselage sides after covering using CA adhesive, but I felt this would “fog” the clear plastic.  It would also look un-scale-like with the windows just stuck to the fuselage sides.  I used a tiny router bit in my high speed rotary tool to cut a recessed groove in the outer edges of the window openings (PHOTO 83) into which the plastic window material can be cut to size and glued into these recesses.  The router bit can be adjusted for the correct depth of cut to match the thickness of the plastic window material.  I experimented with some scrap balsa stock to get the depth correct, and after the dust settled, the windows were cut to size and made ready to install later, as seen in PHOTO 84.

Inside the fuselage, I didn’t plan to install a fully scale interior, but I did want to at least have a pilot figure, and he needed something to sit on.  I cut out some balsa “shelves” that fit between the fuselage formers and sides, covered them with silver gray material and glued the Great Planes pilot figure to the rear shelf, as seen in PHOTO 85.  When flying alone, the Cub pilot always sits in the rear seat for correct balance.  I’ll have to figure out a way to hold these shelves in place but still allow them to be removed for access to the servos and other electronic gear.

There’s only the fuselage left to cover now, and that will have to be done using several pieces of SolarTex as I’ll explain later.  The bottom of the fuselage was covered first, followed by one side, securing it to the structure with a medium heat iron and shrinking later with the iron set to a hotter setting.  The window frame area was covered with strips of SolarTex and the side covering material was then ironed into the window grooves, as seen here.

  Next is the front of the upper fuselage with those rounded wing fairings that had to be broken loose and covered separately. 

This area also has the wing brace wires that block the easy application of covering and a small trim iron had to be used.  Once both fairing blocks were covered, they were Zapped back into place and a strip of black trim stripe material was used to cover the seam.

I just love trim stripe tape, don’t you?

The fin to fuselage covering has to be applied in a special way to make the fairing look correct.  The covering only touches the front of the fin, the upper fuselage stringer and the top of the stabilizer, and must be applied carefully so none of the edges pull loose during shrinking.  I don’t like a lot of seams in covering, but this time there had to be a few. The stab was then covered on the bottom, followed by the other side fin and fuselage top covering and finally the rest of the stab.  I had to be extremely careful when shrinking the covering with the heat gun, but in the end, it came out looking pretty good.

Along with the Cub kit, the person who gave it to me also included a set of side stripes and registration numbers that represented the markings used on his full scale Cub back in the 1970’s.  These were vinyl graphics, but they were 30 year old graphics, meaning that the backing was really stuck to the lettering.  I had to use a blast from my heat gun to separate the backing from the markings before applying them to the Cub. 

The photo above shows the trademark Cub lightning bolt on one side of the fuselage.  I had heard that no two Cubs ever had the exact same shape lightning bolt on the fuselage when they left the Lock Haven plant, so I was convinced that the lightning bolts on this Cub had to be correct. 

Another trademark of a Piper J-3 Cub was the little bear cub on the fin, and I used the 30 year old water slide decals that were in the kit box.  Softening the adhesive in warm water first, the decals were transferred to the tail quite easily, as seen here.

  The rudder registration numbers were also applied using vinyl graphics.  Finally the wing registration numbers were applied as seen here.

The photo below shows a ground photo of the full scale Cub taken in the early 1970’s, and the registration numbers can be seen clearly.

That’s about it for this installment.  Next time I’ll add some epoxy around the firewall for fuel proofing, do a little more detail work and install the engine and fuel system.  Then the receiver and battery can be installed wherever it needs to be for the best balance.  Cowl and trim painting will follow and then we head to the flying field to put the Cub in the air. 

See y’all later…

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It’s getting to be the time in the construction process on the RetroCub where we have to tie up all the loose ends.  I like to say, “It’s 90% completed and we only have the other 75% left to complete”.   I’ll start by mounting the engine, an OS 120 Surpass model, to the motor mount, after installing the fuel tank under the front of the fuselage.  I made up a throttle linkage using a piece of music wire between the servo and the throttle arm, because it was a straight shot between the two points.  A small hardwood piece braces the center of this wire to prevent unnecessary motion when the engine is running.  I also installed a SwitchGlo Pro onboard ignition system, running the plug and ground wires out the firewall and running the control lead to where I suspect the receiver and battery will be placed.  This is all seen below.

It is now a perfect time to mount the wing, tail wheel, cowl and anything else that will be flying with the Cub to check where the balance point is located when compared to the location shown on the plans.  With everything in place, I marked the suggested balance point on the underside of the wing and used my Great Planes Balancer
to see if any relocation of heavy components would be needed. 

To my amazement, the Cub leveled out perfectly at the recommended balance point, meaning that the battery and receiver could be placed right under the CG and would not upset the balance.  I calculated the recommended balance point to be 27% of the Mean Aerodynamic Chord, a very good place to start when flying a plane for the first time.

I then made arrangements to mount my battery, a 2 cell A123 Lithium Nanophosphate type, on the fuselage floor, secured with rubber bands and cushioned with foam.  The receiver will be mounted on a removable plate directly over the battery and the ignition control box will be a bit forward of the battery. 

  I mounted a heavy duty charge switch on the side of the fuselage, in the middle of the black lightning stripe, making it practically invisible. 

 After bundling all the servo and battery ires together, I installed the windshield and side windows permanently.  Pacer Canopy Glue was used on the sides and small wood screws hold the windshield in place.  I also used small wood screws to hold the fake fuselage “floor” in place. 

The cowl, a product of Fibertech N’More, was already trimmed and cut to clear the cylinder head and muffler exit.  I wet sanded the seams and applied a light coat of auto body primer to locate any minor defects.  A number of tiny but visible pin holes showed up and were immediately filled with lightweight filler and re-sanded.  Another light coat of primer was applied and there were no more pin holes to be seen.  Several more light coats of primer were applied and wet sanded, followed by a coat of white primer, seen here.

 Why white?  Yellow paint is very transparent and will not cover gray primer very well, but it will cover white primer.  Why didn’t I start with white primer?  Mainly because I like to use gray primer first because you can see defects a lot easier.  I guess it’s just personal preference.  I then applied several coats of Cub Yellow paint,  allowed it to cure several days and found the color match was close but not perfect.

Once the yellow was dry and I could no loner smell any paint odor, I masked off the dummy engine area with 3M fine line masking tape and applied a coat of flat black to this area.  

  Again, letting the black dry thoroughly before removing the masking, I applied a coat of matte finish clear polyurethane that darkened the yellow enough to match the Cub Yellow SolarTex covering. 

 I carved simulated fuel tank filler from a section of wooden dowel and added a wire “fuel gauge” to the top of it, seen here.

  It was given a goat of epoxy and painted gloss black and attached to the front of the fuselage.  A piece of 4-40 control linkage rod was bent to shape and inserted into drilled holes on the right side of the fuselage to simulate a step for the pilot and passenger seen here.

  Remember that the pilot sits in the back seat when flying solo, as shown below.

FLYING 
I guess everything is complete except for a final balance check, battery charging and planning to meet my flying buddy, Rick Cawley, at our club field.  The Cub fit nicely inside the car with the fuselage in the passenger seat and both wing panels lying across the back seat.  It took less than 10 minutes to put the wings together, fuel up the tank and make a radio range check.  Rick was manning the video camera while I made the first few flights, and I would use the still camera later on while Rick flew the Cub for the photo shots. 

The O.S. 120 Surpass started easily after priming and a few flips of the prop.  The high speed mixture was leaned just a bit and the Cub was taxied to the far end of our long grass runway.  Those trees in the background may appear to be close but they are a least 150 feet from the center of the runway.  With the video camera rolling, the throttle was advanced slowly and the Cub lifted its tail and left the ground in short order.  I made a left turn and found that both down elevator and right aileron was needed to maintain straight end level flight.  After those adjustments were added, the Cub flew very nicely at half throttle or less.

I made a few low passes for the camera and then did a set of opposing stall turns.  Those looked beautiful to me, so I did a few more just to be sure.  Next was a slow right hand barrel roll, which also looked pretty nice.  A few more low passes from the opposite direction would show that the Cub indeed had a left side for the camera.  A rather large loop was completed needing only half throttle to gain altitude.  I then made a touch-and-go, which was a bit on the bouncy side but the tail stayed up and once power was added, the Cub was off and flying again.
I then made a practice landing which showed both me and Rick that I was the one that needed “practice” in minimizing the number of bounces the plane performed.  The next takeoff was a bit wobbly, but I must have gotten overly enthusiastic on the rudder stick.  More rolls and loops were done and I set up for a landing.  This time it was a perfect Cub landing, with the main wheels planted firmly in the grass and the tail held high until most of the forward motion had stopped.  The first series of flights were a success.

I refueled the tank, checked the battery and handed the transmitter over to Rick to get his opinion of how the Cub performed.  His takeoff was a bit straighter than mine, and he began to get that grin on his face every time he likes the plane he is flying.  He asked the Cub to perform a number of scale-like maneuvers and the Cub always answered back with grace and beauty.  He even tried a stall turn but stopped at full power with then nose pointed straight up.  The Cub just hung in mid-air, the prop grabbing on to each and every molecule of air it could find, before it slid backwards and started flying again.  The OS 120 Surpass seems to have plenty of power for even non-scale maneuvers.

 Rick did a few touch-and-gos and then brought it in for a landing and he still lad that grin on his face.  I asked him how he thought the plane flew and he said it was fun to fly and very gentle on the controls.  I had to agree with him on that comment.

Test flying for the day was over and we packed up and headed for home.  Back at the shop I found out why the right aileron trim was needed.  The right wing panel had a 1 degree twist in it which was easily removed with a repositioning of the rear wing strut mounting bolt.  The down elevator needed to keep it level in flight may be the result of not enough down thrust, but I won’t make any adjustments until I fly the Cub some more.  For now, I am very satisfied with its performance.
That’s about it for the kit review of a 30 year old SIG J-3 Cub.  I had fun and since this kit is still available from SIG, I’d recommend it to anyone who wants to build a quality kit plane that qualifies for giant scale flying events.

 Plus, it’s a Cub, and we all know that Cubs fly great!