Mountain Models J-3 Cub User Manual
J-3 Cub
Fuselage Kit
J-3 Cub Specifications
Wingspan: 48 in.
Length: 30 in.
Wing Area: 332 sq. in. (Standard Wing)
Weight (Ready to Fly): 13.4 - 17 oz. (Depends on motor & battery)
Wing Loading: 5.8 – 7.6 oz. / sq. ft. (Standard Wing)
Revision History
Date Revision Notes/Comments
11/28/2006 1.0 Document initial release.
Thank you for purchasing the Mountain Models 1/9 Scale J-3 Cub. This plane is a scale
aileron/elevator/rudder (full house) setup, designed for the low experience pilot on up who
wants a super easy flying scale plane.
If you have not flown ailerons before, this plane could make a great aileron trainer for you. If
you have no tail dragger experience, this plane also makes a great first tail dragger.
We HIGHLY recommend resisting the temptation to overpower the model. In the thin air at
6000’ where it was designed, it flies beautifully and has plenty of extra power, even with the
cheap 80 watt brushless out-runner motors.
Please let us know you experiences building and flying this model. We look forward to customer
feedback and want to know how you’re doing with our products!
Thank you,
Brian Eberwein
Mountain Models
PO Box 6815
Colorado Springs, CO 80934
www.mountainmodels.com
Phone: 719.630.3186
Before You Begin
Check to make sure that all of your parts are there and in good shape.
Parts List
Number
in Kit
Bundled Parts
1 1/8” Lt. Ply Sheet
1 1/16” Ply Sheet
1 1/32” Ply Sheet
3 1/8” Balsa Sheets
4 3/32” Balsa Sheets
1 1/16” Balsa Sheet
1 1/32” Balsa Sheet
2 1/8” Balsa Sticks
2 Pushrod Housings x 18”
1 Windows (2 Sides, 1 Front)
1 Plan Sheet
1 These instructions of course!
Metal
4 1/16” x 12” Wire
2 0.032” x 18” Wire
1 0.039” x 9” Wire
Plastic
1 1 ea. Cowling and 2 ea. Engine Parts
Description of Part
2
Number
in Kit
Bagged Parts
1 1/16” x 3/4” x 5.75” Ply Battery Stick
1 1/4” Balsa Sheet
1 3/16” Balsa Sheet
1 1/8” x 4.25” Dowel
2 Micro EZ Servo Connector
2 DuBro Micro EZ Link
5 4-40 T-Nut
2 4-40 x 1” Nylon Bolt
2 2” DuBro Wheels
1 3/4” DuBro Tail Wheel
1 #2 Motor Screw
4 Cowling Screws
2 Magnets
2 #4 Washer (Magnetic)
5 #4 Washer (Non-Magnetic)
2 EZ Hinges
2 3/32” x 3/4” Aluminum Tube (Wheel Bushings)
2 1/8” x 5/8” Aluminum Tube (Wheel Bushings)
3 4-40 x 3/8” Socket Cap Screw
1 Allen Wrench, 3/32”
2 Wheel Retainers
1 1/2” Double Sided Velcro
1 3/4” Sticky Backed Velcro (Pile and Loop Sides)
1 Spider Wire, 12 feet
Description of Part
Building Materials You Will Need
• Smooth and flat work surface
• Wax paper or clear plastic wrap to protect the work surface
• Thin and thick Cyanoacrylate (CA) glue
• 5 Minute Epoxy
• Hobby knife with #11 blades
• Needle nose pliers
• Wire bender or pliers for bending landing gear wire
• Wire cutters
• Screwdrivers
• Sanding block, 320 to 400 grit sandpaper
Finishing Materials You Will Need
• Covering material (So-Lite or similar)
• Sealing iron for applying the covering
• Paint for cowling and engine plastics
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Electronics You Will Need
• 4 channel radio minimum
• 4 channel receiver minimum
• 2 ea. aileron servos (we recommend either the GWS Picos or Naros)
• 2 ea. tail servos (we recommend the Hitec HS55s, or GWS Picos/Naros)
• 2 ea. 12” servo wire extensions
• 1 ea. Y-Connector for ailerons (or 5+ channel RX with 2 aileron servo mixing on TX)
• Brushless Motor of 80 to 100 watts or Geared brushed GWS350C motor
• Electronic Speed Control
• 2 cell 1200 mAh LiPo pack
General Building Tips
• READ THE INSTRUCTIONS all the way through and study the plans BEFORE starting any
work on the model.
• Tape the plans to your nice clean work surface and cover it with wax paper or plastic wrap.
You want to keep your work surface clean and not glue the parts to the plans, right?
• Balsa is a lightweight and fragile wood, so you do need to be careful with it; however, you
will also need to use a little bit of force to make everything fit properly, so don’t be too
timid.
• Do not remove any pieces from the balsa sheets until they’re ready to be used. That way,
parts won’t get mixed up or disappear.
• Do NOT glue anything until told to do so.
• Join all of your pieces using thin CA (Cyanoacrylate) glue, unless we tell you otherwise. In
general, only a small amount of CA is necessary to glue parts together.
• Don’t over force your pieces together. If they aren’t fitting together properly, make sure you
have the right pieces and that they are oriented correctly. If needed, you can lightly sand
the part to fit after making sure it is the correct part and oriented correctly.
• If you want to remove the charred edges caused by the laser cutting process, lightly
dampen a cloth with bleach and gently rub the affected areas. Removing the char will not
increase the strength but will make it look better. It also keeps that dark edge from showing
under the lightweight coverings.
Assembly Instructions
against the normal building conventions. We have found that fewer pieces get mangled this
way, since you are building the stronger pieces first. For example, the tail feathers are at the
end of the building process instead of the beginning, so that they are less likely to get damaged
and are being built right before they are attached to the fuselage and each other.
Experienced builders may notice that this building order goes
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Step 1: Assembling the Fuselage
The first thing you are going to assemble is the fuselage of your airplane. This involves
assembling the fuselage sides, formers, stringers, firewall, etc.
1. Glue the 3/32” balsa fuselage side parts together with thin CA. Ensure that they are seated
properly with no gaps.
To remove the pieces, gently flex the balsa sheets until the pieces fall out.
You may find that you need to carefully trim the extra pieces of wood that
originally held the piece to the sheet. You can also use a #11 X-Acto blade to
trim them out of the sheets.
2. Glue the 3/32” balsa reinforcement piece F9A to F9 and F10A to F10 with the engraved text
forward on both parts. Make sure the parts are flush with the bottom and lower sides of the
parts as shown in the left image below.
3. Press the 3/32” balsa part C8 into the 1/8” balsa part F8. The text on F8 should be facing
TOWARDS C8 and the text on C8 should be facing UP. Reference the above right image. Do
NOT glue the parts together yet.
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4. Insert a blind nut on each of the 3/32” balsa C9 triangles as shown below. They go on the
“lasered” side of the parts. MAKE SURE the blind nuts are flat against the parts. Secure with a
bit of CA around the outside of the blind nut flange. Make SURE you do NOT get CA on the
inside threads.
5. Insert the reinforcing triangles as shown in the below left image.
6. Slide the 1/8” Lt. Ply F5 onto C8 as shown in the right image above. Make sure the text is
facing away from the other parts. Do NOT glue yet.
7. Press the 3/32” balsa F12 into F11. The text on F12 should be facing UP and the text on F11
should be facing forward (away from F12) as shown in the image below.
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8. Press the 3/32” balsa F6, with the text facing FORWARD into the RIGHT fuselage side. Insert
the 3/32” balsa F4 into the right side with the text facing FORWARD, as shown in the image
below left. Make sure they are fully inserted and tack glue them in place, making sure they are
at a 90 degree angle to the fuselage side. Press the assembly from step 5 into place on the
right fuselage side as shown in the right image above. Tack glue the formers into place,
making sure they are at a 90 degree angle to the fuselage side.
9. Press the left fuselage side into place on the formers from the previous step, as shown in the
above right image. Make sure the formers are at a 90 degree angle to the side and tack glue
them to the left fuselage side. Make sure the C8 part is all the way in F8 and F5 and tack glue
them together.
10. Insert F9 and F10, with the text facing FORWARD into the right fuselage side. Place the
F11/F12 assembly from step 6 into the right fuselage side. Bend the left side in and press it
onto F9, F10, F11, and F12. Do NOT glue anything yet. Lightly sand the rear inside of the
fuselage parts so they come together nicely as shown below. There are engraved lines
showing about how far back you need to sand and the rear should be about 1/8” thick when
you are done.
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11. Insert the 1/8” balsa F16 part into the tops of the formers as shown below. There will be some
space between the tops of the middle formers and the top of F16. This is so the formers do
not touch the covering when you’re done. Line up the fuselage on the top view of the plans.
The top view on the plans might be a bit long, this is OK, just make sure the back end lined up
on the center line. Weigh the fuselage down to the plans as shown below. After you are sure
all the formers are fully seated and everything is straight, glue the formers and the top
stringer in place with thin CA.
12. Glue the 3/32” balsa cabin sides together over the plans. (Make sure you have wax paper or
plastic wrap over the plans of course!) Make sure that the bottom sides of the parts, with
marks from the laser, are facing the same direction. Each cabin side is on one fuselage sheet.
13. Fit the cabin sides to the fuselage. Make the “lasered” sides face to the inside. Glue them in
place.
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14. Glue the 3/32” cabin top reinforcements in place as shown in the image below. These go
inside the cabin and seat against the cabin top (C8) part.
15. Insert the 3/32” balsa reinforcements as shown in the image below. They are on the same
sheet as F10. Make sure they are fully inserted before gluing them in place.
16. Insert the 3/32” balsa F3 and F7 parts as shown below, with the text facing FORWARD. Secure
with thin CA.
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