Sig J-3 1/4 Scale Cub Build Series – Part 7 – Nose Section Formers, Firewall & DLE 40 Twin Motor Mount

In part 7 we will be installing the nose formers and sheeting, the engine firewall and making a motor mount spacer for the DLE 40 twin cylinder gas engine.

Start by finding the nose formers from the die cut balsa supplied with the Sig kit.  Take measurements from the plans in order to properly place them on the nose section of the fuselage.  You will need formers F2, F3, and F4.

You will notice in the above photo I added some additional scrap pieces of balsa to help hold the formers in place as the glue dries.  In the photo below, I also added a 3/8 piece of basswood just behind former F2,  this will give the engine mount bolts a thicker piece of wood to bolt to.

Also add a block of wood behind the firewall under the nose area inside the cabin.

Glue together the two 1/8 inch thick die cut plywood firewall pieces together and allow to dry.  Make sure you use a enough glue so it squeezes out of the edges, we want a good tight bond here!   Wipe of any access as you add clamps around the edges.  Set aside and allow to dry overnight.

The DLE engine manual provides a drilling template that helps determine the placement of the DLE twin cylinder engine on the firewall.  Tape this in place on the firewall lining it up with the thrust lines you transferred from the plans.

Once the firewall is dry, you can now glue it to the main fuselage.  Test fit and sand as need for a nice easy-snug fit.  Once satisfied with the fit, use clamps and glue this in place.

Once the firewall has been installed, we will need to make a couple of spacer blocks for the DLE 40 twin engine to allow enough clearance for the mufflers.  Use the provided drill template to cut some spacer blocks and drill holes for the bolts you will be using large enough so they can slide through easily.  Now is a good time to drill the holes on the firewall and test fit the engine.  I ended up making two spacer blocks for just a little more clearance.

Once satisfied with the fitment of the engine, we can move on to sheeting the nose formers F2-F4.  The kit supplies some 1/8 thick balsa for this.  However, I found it easier to use two layers of 1/16 balsa.  Use wood glue in between the layers.  Use a number of rubber bands to hold the sheeting securely in place as it dries.

Once the sheeting is fully dry, trim the excess with a razor saw and sand smooth.

That’s it!

In part 8 we will be installing the rear formers!  Things are really starting to take shape!

Sig J-3 1/4 Scale Cub Build Series – Part 5 – Gluing Rear Fuselage Together

In part 5 we will look at gluing the rear parts of the fuselage together and completing the rest of the truss frame work.

Start with securing the top view portion of the plans to your work bench and cover with plastic.  Make sure it is as flat as possible.  Next, position the front of the fuselage over the plans matching up the positions of where the main formers are shown on the plans to where they are located in the fuselage.

Once the front of the fuselage is securely in position, you can begin to shape the tail end of each fuselage side as shown below.  To get a perfect angle while sanding, transfer the angle from the plans to each fuselage side.  I used a 2×4 sanding block that had been squared on all for sides and using a piece of wax paper, slowly sanded the angle to the line I had transferred from the plans.

Keep testing fitting the tail ends together as you sand to the line.  You may mind that one piece is slightly longer than the other.  The most important part of this process is that the tail ends of the fuselage are lined up as accurately as possible and match what is shown on the plans.

Glue the ends together using wood glue and small clamps.

Once the tail pieces are dry you can begin filling in the rest of the truss framework.  In the manual they recommend starting from the tail end and working forward toward the nose.  Since we installed gussets in part 4 of this series, we can simply cut and fit the 5/16 square stock pieces and slide them in place where the gussets are positioned.  I used thin CA to tack the pieces in place and then later brushed over the joints with wood glue. (One note, when cutting the bottom pieces that are placed into the gussets, cut a second piece at that time that’s exactly the same for use on the top portion of the fuselage side.)

For the top portion of the fuselage sides, the process is pretty much the same.    When gluing in the top portions of the square stock, I placed a construction square at each side of the fuselage at the glue point. This will help ensure that your fuselage remains square as you glue the pieces in place.!

You will also notice, I added in some additional 3/16 balsa filler in front of the 5/16 square balsa pieces.  This will be used for some additional bracing when we glue in the rear formers in the next step.  This won’t add very much weight but will help increase the gluing surface when installing the rear formers F8 – F13.

Once everything has been tacked in place with thin CA, brush over the joints with some wood glue and let dry overnight.

In Part 6 of this build series we will be installing the rear formers and top stringers!

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!