Sig J-3 1/4 Scale Cub Build Series – Part 4 – Truss work Fuselage Sides & Gluing Up Formers to Frame

Part 4 – Truss work Fuselage Sides & Gluing Up Formers to Frame

This is where things really start come together as they say! 

In part 4 of the J-3 Cub Quarter Scale build series we will look at framing up the truss work fuselage sides and also joining them to the formers we built in part 1.

Building the fuselage sides for this model J-3 Cub utilizes truss work style construction techniques. Truss work framing is one of the strongest ways to build a frame but still maintain a very light weight frame. Truss work construction has a high strength to weight ratio.

Begin with cutting out the appropriate sections of the plans and taping them together using the guides indicated on the plan.  Cover the plans with wax paper and secure in place as flat as possible over your building board with magnets. There are two printed balsa parts that will need to be cut out using a band saw or scroll saw. Cut out pieces MF-3 through MF-5 from the included printed balsa sheet. Take time to cut these out as accurately as possible.  Once the pieces are cut out make sure they are as equal to each other as possible – I like to stack them on top of one another and sand the larger piece until they are exactly the same.  Accuracy here will aid in the building process when it comes time to glue in the formers.

Before starting assembly of the fuselage, sort through the 5/16 square balsa sticks included with the kit and set aside the several pieces you will need to build the truss work frame. Try to sort the balsa according to grain structure and hardness.  There a few cutouts that need to be made on MF-4, use a razor saw and a square to make sure you cuts are straight and accurate for the cross grain cuts.  Next, just use a sharp no. 11 blade to easily cut along the grain to finish the cutout.

Begin with placing pieces MF-3, MF-4, and MF-5 over the plans.  Secure in place with magnets or pins. Also begin laying in the 5/16 square balsa sticks along the plans to see how everything lines up.  One thing to note, when building the truss work sides start with the longest pieces and work to the shorter ones.  This will be the most efficient way to utilize the balsa included with the kit.

Begin by gluing in the straight pieces of the truss work sides moving from the front of the fuselage toward the back.  Tack them in place with CA glue for now.  During the rest of the framing use a high quality wood glue.  Wood glue will take a little longer to dry but the joint will be very strong yet allow a little flex in the joint where needed.  Use clamps on the 5/16 square stock where it has to make a shallow curve according to the plans.  I used wood glue here and allowed it to dry over night.

Once the main pieces are dry you can begin working on the diagonal frame work to construct the rest of the truss style fuselage.  Again we will use a tip from Airfield Models to cut very accurate angles on the balsa sticks.

Once the fuselage truss work is completed for both fuselage sides (you can build both sides directly over the plans – no need to worry about a “right” and “left” at this point), lightly sand them to a nice smooth finish with a sanding block.  I made a sanding block from a piece of 2×4 that was run through an edge jointer on all four sides.  We want it to be very flat and each face square to each other.  I used 120 grit for the initial sanding over all the joints.  No need to  press hard, just let the weight of the block do the work for you.  Finish with 320 or higher grit for a smooth surface.

Another feature I decided to utilize was the use of gussets on the bottom of the truss fuselage sides.  Adding gussets will add a tremendous amount of strength with only a very small weight penalty.  Airfield Models has a template you can download for several common balsa sizes.  I used the 5/16 size for the Sig J-3 1/4 Scale Cub.

I used 1/16 plywood for mine and had several cut out on a CNC machine. Glue them in place as shown in the photos below.  Use of clamps while they dry will ensure they stay in position.

Once the truss work frames are finished and the gussets are in place, I glued in the die-cut 1/8 plywood sides to the fuselage.  At this point it’s important to pick a “left” and a “right” side.  Take extra caution here to ensure you don’t glue something to the wrong side of the fuselage!  I brushed on wood glue using a brush and stacked them together between pieces of wax paper while they dried.  Using clamps around the edges will help to make sure the edges are nice and tight with no gaps.  Add some weight over the fuselage sides while they dry.

With both fuselage sides full dry, it’s now time to begin gluing in the formers.  If you took your time to build the fuselage sides very accurately, they should come out to be nearly identical in overall dimension.  If not, sand them so they are equal as much as possible.

Begin with placing the right fuselage side over the plans and measure where the formers should be glued.  Drawing lines with a straight edge will help for lining them up when gluing them in place.  Glue the formers to the right half using epoxy or wood glue.  Use a square to make sure they dry perpendicular to the fuse side.  I cut small strips if 1/8 balsa and used them as a brace to hold the former in place while the glue set.  I also added weight as shown in the photo to ensure a very strong joint. With all three of the main formers in place, allow them to dry over night.  Gluing on the left side of the fuselage can be a little tricky.  I transferred the lines from the right fuselage side to the left to help use that as a guide.  See the photo below for how to use two triangle squares to accurately line up the front of the left fuselage side with the right side.  Also, this is one place where you can trust your eye – if everything looks good and it seems to measure up, then everything is most likely aligned.  Once all is dry, I added some additional triangle balsa as shown below.

In Part 5 we will take a look at joining the rear halves of the fuselage together and how to make sure you fuselage is perfectly square!

Sig J-3 1/4 Scale Cub Build Series – Part 3 – Stabilizer & Elevator

Part 3 – Horizontal Stabilizer and Elevator 

Building the horizontal stabilizer and elevator are pieces that can also be constructed quickly.

Most pieces for the horizontal stabilizer and elevator are included on printed 3/8 balsa sheets. Be sure to also set aside the included stick pieces you will need for this build and make sure they are of the proper thickness as shown on the plans.

I started by rough cutting the printed pieces on a band saw.  Since there are quite a number of pieces it best to do this while your mind is fresh.  Having to cut out all these pieces can be a little time consuming and start to  feel repetitive.

Elevator
Once all the parts were cut out, I set them aside and started with placing the 3/8 x 1/2 inch balsa sticks in place over the plans.  There is also a 3/8 spruce center piece.  I placed the spruce piece over the plans as accurately as possible and then cut the two 3/8 balsa outside sticks just a hair longer than needed.  Use a straight edge to make sure the sticks are as straight as possible along the hinge line.  This will help reduce any hinge gaps later during assembly.  I placed the E6 and E1 parts in place over the plans along with the spruce elevator horn block and glued this sub-assembly.

Next, I began to glue up the rest of the printed elevator parts.  I took time here to make sure each piece had a very solid and accurate glue joint – no gaps.  Use a disc sander to shape the edges of the parts accurately.  I glued in the parts one at a time and cleaned up any excess glue that would run out from the joint.  I used magnets here to hold the joints in their proper position as the glue set up.  This was somewhat of a slow process as I used a wood glue and would let each part setup fully (about 30 mins with the type of glue I was using) before going on to the next.    Using wood glue here over CA will make the joint very strong and still allow a little flex.  Once all the parts were in place I placed some boards with weights on top of the pieces and allowed it to dry over night.

Elevator Bracing
With the main elevator parts fully dry I was now ready to work on the internal bracing.  This was done with 1/4 x 3/8 balsa sticks.  Using a tip from airfield models allowed me to get very accurate and precise joints! See the tip here: Cutting accurate truss style balsa joints

Once all the bracing is in place, it best to sand the parts to a uniform thickness.  I made a sanding block from a 2×4 about 10-12 inches long.  I ran this board through an edge jointer to make the sides and bottom very flat and square to each other.  I used 120 grit sand paper to get the material to a uniform thickness and then finished with 320 or greater for a nice smooth surface.  When sanding balsa it’s not necessary to apply a lot of pressure, just let the weight of the block and sandpaper do the work for you.  Also rotate the piece often so you don’t over-sand one particular.  It is also a good idea to sand in alternate directions.  All in all I probably removed only about 1/32 of an inch of material!

Horizontal Stabilizer
The horizontal stabilizer was built much the same way as described above.  I started with gluing in the main center parts and working from the middle outward.  Make sure the hinge line is a straight as possible here too!

The internal bracing was done the same way – using a razor blade to accurately mark the proper angle on the balsa stick and then sanded smooth with a disc sander.

Once all the parts were in place I added some weight on top and let it fully cure before sanding it to a uniform thickness.

Once these parts were all fully dry I set them aside to wait their turn during final assembly!

Stay tuned for part 4!

Sig J-3 1/4 Scale Cub Build Series – Part 2 – Rudder

Part 2 – Rudder

The Sig 1/4 Scale Cub build series is well under way! In part two of this build series, we will focus on the rudder assembly. Last time in part one we built the main fuselage formers and also the rear formers F8 through F11. If you have not seen part one, click here to view part one of the Sig Quarter Scale Cub build series.

As with any build that I start, I like to begin with the simple aspects of the build. For example, the rudder is usually a piece that can be framed up fairly quickly which helps to give a sense of accomplishment right a way. This goes a long way in helping the builder stay motivated during the entire construction process!

For the rudder I also tile printed that portion of the plan and taped the pieces together and covered them in wax paper. In this case it’s a matter of finding the correct printed parts mentioned on the plans and utilizing some of the straight balsa stock included with the kit. For a few of the straight pieces in the rudder, you will have to cut and shape these by hand using the balsa stock provided. Most of the curved parts of the rudder are included on a printed sheet of 3/8” balsa. Be sure to make note of proper thickness and width of the balsa stock the plans call for.

The best way to cut out the printed parts is to use a small bench-top band saw and cut them to within 1/16^th inch of the printed black line. Then shape the pieces accurately to the line using a disc sander. Some shaping by hand will need to be done on some of the inside curves. Again, take your time here – a lot of test fitting over the plans to make sure everything is within the lines!

Once all the parts are cut out, lay them over the plans to test the fit. Once you are satisfied with the fit of the parts, I like to tack them together using thin CA. Use a magnetic building board or pins to hold the parts securely as they are glued in place. Once the main parts are in place, work on the inner framing. To ensure that the angles are accurate, you can use a bent T-pin to score the under side of the stock to mark the angle it needs to be cut at on both ends. Next, cut it close to the line using a razor saw and then finish for a nice snug fit with a disc sander or sanding block. A disc sander will help to ensure the angle is square along it’s entire face.

Once all the framing is completed, I like to use my favorite wood glue and brush a small amount into any gaps that need filling. Some final sanding will be needed to finish the part for final assembly.

Stay tuned for Part 3!

Sig J-3 1/4 Scale Cub Build Series – Part 1 – Formers

Part 1 Main Fuselage formers and Rear Formers F8-F11

The Sig J-3 Cub Build is now under way!

One of the first parts that are easy to build are the main fuselage formers and the rear balsa formers F8-F11. You will have to build the main fuselage formers from the balsa sticks provided – cutting them to size according to the plans from the 36” stock provided. Rear balsa formers F8-F11 are die cut balsa pieces you will have to glue together at the seams.

Before starting the build, I like to sort the balsa and included ply wood die cut parts into piles. With the Sig RC48 J-3 Cub kit, much of the balsa is supplied as die cut, printed balsa sheets, or in the form of balsa sticks. It helps to organize things; when the manual calls for the needed parts, they will be easier to find.  Most plywood parts are already die cut and are easily removed from their sheets.

Before starting this build I decided to have all four plates of the plans scanned into PDF. This allows me to print out certain parts without having to cut up the original plans. I print them out using the tile feature in Adobe’s Acrobat PDF viewer software. I also print them with a ½ inch overlap. This allows me to line up the tiled sheets much more easily. I tape them together using small pieces of Scotch tape. Using a large window as your background allows you to see through the pages allowing you to line things up perfectly as you tape them together!

I like to build off my magnetic building board – that I use for all my model building –  to ensure the parts are aligned as accurately as possible. Take extra time to make sure the main fuselage formers are square, will result in an air frame that is precisely built and accurate. Remember straight air frames fly better and helps to reduce any trimming issues!

Build the main fuselage formers as shown directly over the plans. Use a good wood glue or even epoxy here. Using slight pressure on the joints (using the magnets you can “wedge” the parts together as they dry) will result in a very strong and sturdy joint! Add some weight on the pieces that glue on top of the former sides to ensure a good bond.

Glue the rear former halves F8-F11 together using wood glue and tape on one side to ensure the seams are pressed tightly together while they dry. Place some wood glue in the seam, and then press the joint together by hand and wipe out any access glue that seeps out. Finally, place the formers in between to pieces of wax paper as they dry. Add some weight on top to ensure they dry straight and flat.  Use a piece of scrap board and a ten pound dumb-bell as weight on top of the formers.  The scrap wood board will help distribute the weight evenly over the formers.

Once the glue has dried the formers are now complete!  They may require some final sanding once mated to the fuselage sides.

Senior Telemaster With LED Strip Lights

The Senior Telemaster RC airplane makes a great trainer and is available many sizes: from mini all the way to a giant 12 foot wing span! With it’s slow flight characteristics and docile handling it’s truly a joy to fly – even for more experienced pilots!

Another primary advantage of the Telemaster design is its ability to be able to carry a lot of weight due to the low wing loading. This make it a great platform for modifications – as there is room to add some weight without effecting flight performance!

This particular Telemaster I acquired was not in very good shape and was in need of some repairs. I restored it earlier this summer: fixed the broken balsa stringers, recovered the wings, and shaped new ailerons for the wing. The fuselage was also twisted and needed to be straightened.  I was able to do this by the holding the correction (twisting it past the correct point since it will naturally spring back a little) in place while shrinking the covering with an iron. I added a set of flaps!

One other item I wanted to do was add some lighting to the wings and the fuse. Originally I had just added some LED strip lights to the main wing during my first round of repairs, but after my initial testing I decided I wanted to light the entire plane!

When I was recovering the wings with Ultracote, I drilled wholes in the ring ribs so I could run the LED strips through the wing. I ran a set in the front of the spare and also at the rear of the spar so I could light the entire wing. One thing I wanted to keep in mind when positioning the LED strips in the wing was to try to position the LED’s in such a way that the wing was “indirectly” lit – taking advantage of the internal reflection of the white Ultracote. This makes the lighting appear evenly distributed throughout the entire wing giving it a more natural “glow” effect.

The main colors I used for the wing and the horizontal stabilizer, were red and green – with the green LED’s on the right and red on the left. I used neutral white for the fuselage interior. I placed the fuselage LED strips on the bottom so they could shine up – again taking advantage of indirect lighting. Using this technique where possible really makes it glow and the light looks very even!

When it came time to install the LED strips into the horizontal stabilizer, I developed a new technique for placing the lights; perhaps one I wished I would have thought of instead of drilling holes in the ring ribs! Sometimes you “learn as you go”! For the horizontal stabilizer, I cut small slits in the rib and slid the LED strip in place as close as I could to the spar location. Once I was satisfied with the positioning, I added some thin CA to the small gap to restore the integrity of the ribs in the stab (the Telemaster uses an air foiled design for the horizontal stabilizer). Again, I ran a strip in front of the main spar and also at the rear – both strips shining outward from the center. This made for very even light distribution!

Installing the light in the fuselage, I decided the easiest method was to cut the covering down the very center using a straight edge so I could access the entire bottom of the fuselage. Of course hindsight would dictate to install the lights before I covered it the first time! For the LED strips in the fuselage, I stuck two strips of lights back to back (in this case I did want one set to show down and one set to shine upward into the inside of the fuselage). I ran two double strips on each side of the center line of the fuselage. I wired the back-to-back pieces together using jumpers I had made from some scrap servo lead wire.

Once I was satisfied with the placements of the lights, I secured the LED strips and wire leads with a hot glue gun. Hot glue works very well for this sort of thing!

I wanted to be able to control the LED lights from my transmitter. I decided to use the Turnigy Receiver Controlled Switch to do that. This switch simply gets wired into the positive lead of your battery, and the other end connects to an open channel in your receiver. Since this plane is a glow powered plane, I’m using two separate batteries, a normal NiMH 5 cell – 6.0 volt pack for the receiver and servos, and a 3S lithium polymer battery for the LED strips. One thing to note, is that for this circuit to work properly, the ground (black wires) of both batteries need to be connected together! Without doing this the switch will not operate! I’ve included a wiring diagram of the configuration for this type of setup.  Mode II is the diagram you should follow when using two separate batteries.

https://www.hobbyking.com/hobbycity/store/catalog/e-switch.jpg

Here’s a YouTube video that explains the switch modes in more detail: 

After all the lighting was installed it was just a matter of resealing the bottom covering and a very thin strip was needed to cover the bottom seem.

With all the LED’s in this setup, the entire system draws roughly 3-4 amps. I’m using a 3S 1300 mah lipo for this particular setup.

Good luck with your LED project!