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Sig J-3 1/4 Scale Cub Build Series – Part 13 – Finishing and Covering.

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With the wings completed in part 13 of the Sig 1/4 Scale build series, we were left with an air frame that is now ready to be covered!

For this build, I opted to go with a color scheme other than yellow!  I know, I can’t believe it either!  Part of the reason for not sticking with the traditional Cub yellow color scheme was to avoid confusion in the air with the other yellow cubs in our club!

I decided on a rather simple three color scheme of bright red, white, and some gray for an accent color.  I covered the plane using the Hobbyking branded covering and I was quite pleased with how easy it was to work with and how well it formed around curved surfaces!

Another benefit of the hobby king covering is standard rolls around roughly 5 (15 feet) meters long and around $10 a roll!

https://hobbyking.com/en_us/hardware-accessories-1/covering-film.html

Just a couple of quick tips when applying covering film:

  • Start with the “bottoms”; this includes covering the bottom of the wings first, bottom of the fuselage, then work your way up to cover the sides and finally the top.
  • Use razor blades for cutting the excess covering instead of xacto blades as these blades seem a little thinner and sharper!
  • If you are overlapping colors, make sure the overlap is at least a 1/4 inch over the first color and preferably over a fully sheeted area.  This will ensure a nice tight seal.
  • Also keep in mind any overlap seams in relation to air flow.  You will want the leading edge of a wing to “overlap” the first color.  This prevents the airflow from eventually working the seam loose.

Electronics Used:

  • Futaba 6 channel RX
  • two 2-cell LiPo 2600 MAH (make sure your servos and RX are rated for 7.4 2S voltage)
  • Hitec HS5665MH Servos for control surfaces
  • Dubro Large Scale pull-pull rudder system.
  • Hitec Large Servo Horns.
  • Dubro 4-40 clevis and rods
  • Great Planes large scale control horns.
  • 2 Switches in combination with 2 cell LiPos above.  Each power system is separate.  One for the RX and servos, and one for the DLE 40 ignition.
  • Dubro 20oz Fuel Tank

 

Here are a handful of photos of the finished product. The clear windows were added after the maiden flight!  I’ve also included a few photos of the electronics setup for this plane as well.

A few parting notes:
You’ll notice the 20 oz gas tank is more centrally located as close to the CG as possible.  Sig recommends the tank be put far forward in the nose.  With the heavier DLE 40 engine this isn’t necessary. In fact, once all the electronics were in place, it was just a matter of moving the 2 RX packs around to locate the perfect CG!  I couldn’t believe how well it worked out!

Also the elevator servo is mounted in the rear of the plane instead of running a long control rod.  Take some precautions to seal this servo from water or use a water proof servo here.


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Sig J-3 1/4 Scale Cub Build Series – Part 8 – Rear Formers

In part 8 we will take a look at installing the rear formers along with the top stringers.

In the photo below you will notice I added some additional cross pieces, this will be used to add in some small reinforcement blocks when gluing the rear formers in place.

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The formers F8-F13 are included with the Sig kit as die-cut balsa. A couple of the larger formers are in two pieces; you will need to glue these together and add a small cross piece for extra strength. (shown in photo above)

Next, begin by gluing in the largest former and working your way back toward the tail.  One thing to note, it is important to line the fuselage up as described in the manual.  The formers will be glued in at a small angle relative to the ground.  What I did was prop up the fuselage at the required angle with some blocks. Use a small torpedo level to glue the formers in at a 90 degree angle.  Using this method will automatically account for the proper angle.

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Once all the formers are glued in place, I added some additional blocks behind each former for a some extra support.

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I let these dry overnight before adding the top stringers.  The top stringers require you to bend a curve into the 5/16 square stock in order to meet the rear of the cabin.  I soaked a paper towel in water and then wrapped the last 12 inches or so of the square stock with the wet towel and placed the ends into a plastic bag.  It only required about a 30 minute soak to make them soft enough to work with.  I used thick CA here and some kicker so I could quickly get them into position.

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I also added in some additional reinforcement blocks where the stringers meat the rear of the cabin.  This will help hold it more secure.

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This was a little shorter build article, but once you see the formers in place the fuselage really begins to take shape!

In part 9 we will be taking a look at building a few miscellaneous items: putting in the cabin floor, adding some triangle balsa in the nose, installing the fuselage side stringers and window fill-in posts.

 

 


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Sig J-3 1/4 Scale Cub Build Series – Part 6 – Nose Section & Securing to Fuselage

In part 6 of the Sig J-3 Cub 1/4 Scale Build series we will be taking a look at constructing the nose section and firewall.  Also we will be securing the nose section to the rest of the nearly completed fuselage.

Start by locating the printed balsa sheets containing parts MF-1 and MF-2.  You will need two of each part.  The framing for parts MF-1 and MF-2 will be constructed using some 5/16 square balsa stock included with the kit.  Build the framing directly over the plans and then fit in the pieces MF-1 and MF-2 as shown in the photos below.

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Set aside to dry.

Once the two sides of the nose are completed, make sure they are of equal dimension by squaring up any long sides using a disc sander or sanding block.   Also mark out for the location of the bottom of the 1/8 ply firewall that will be installed later.

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With the two sides of the nose of equal dimension, find the pieces of the firewall and the top portion of the nose section from the included die-cut 1/8 plywood sheet.  These pieces are not labeled but are easily identifiable by referring to the plans.  Glue up the pieces as shown in the following photo.

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The manual and the plans do not call for adding in the triangle balsa shown in the next photo but I think it’s a good idea and will add some extra strength in the nose without adding to much weight.

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Once the nose portion is fully dry, it’s time to begin securing it to the main fuselage.  See the photo below for how I used some plywood tabs (1/16th thickness) to help provide more gluing area instead of  using a butt joint.

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Once the nose has been glued on to the front of the main fuselage, you can now fit the pre-bent wire bracing that is included with the kit.  This will fit in between the pieces MF-1 and MF-2.  If the wire does not fit in the gap provided, simply place the wire in position over the nose section and draw a line on either side.  Remove the extra balsa with an Xacto.  This doesn’t have to be pretty as it will be covered with a 1/8 ply side and filled with epoxy.

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Rough up the ends of the pre-bent wire and wipe clean with alcohol.  This will help the epoxy stick better.

Inside the nose section I filled the slot for the pre-bent wire with epoxy and then covered the inside portion as shown below with a thin sheet of balsa.  This will help hold the epoxy in position as it dries.

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Before gluing the nose wire fully in place be sure to drill the two holes as shown on the plans.

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Once the wire has been glued in place it should look like the photo below: DSC_7442  The nose sides will get covered with 1/8 plywood.  If you have some epoxy already mixed up, now would be a good time to glue those sides in place as shown below:

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Once satisfied with the placement secure with rubber bands.

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Finally solder the main nose wire bracing as described in the manual and cover the joint with a blob of epoxy.

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