First, some history. In June of 1989, SIG Mfg. introduced the FourStar 40 to the modeling world. No, it wasn't an overnight success, but its reputation quickly spread. Novice flyers suddenly looked like pros and experts couldn't believe how slow it could land without stalling. Instructors began calling us with stories about training people to fly R/C with the FourStar 40, even though it's never been promoted as a trainer. Clubs even began using it for one-design pylon races!


Modelers were also amazed at how fast and easy it was to build. A couple of weeks of spare time was sufficient for most builders. We've heard from people who can frame one up in twelve, ten, even six hours. Another group of modelers actually built and flew a Four-Star 40 in a day! The design has become so popular in some clubs that it's practically the official "club plane".

It became fairly obvious that the Four-Star 40 was like a good movie it deserved a sequel. The reason that it was enlarged to a 1.20-sized airplane was simple - that's what we wanted for ourselves, so that's what we built! Besides, large sport models have become very popular the last few years and SIG needed an airplane that could introduce sport modelers to the world of "big birds". With its 81" wing span, the Four-Star 120 is big enough to qualify for I.M.A.A. fly-ins, but is still fairly easy to transport.

The Four-Star 120 isn't a simple blow-up of the "40"-size. Some elements of aircraft design don't conform well to enlargement. For instance, the engines for this design are proportionally heavier than those for the smaller Four-Star, which meant a shorter nose moment was necessary for the "120". In effect, the "120" is an entirely new design, but it still incorporates the aerodynamic and structural features that made the Four-Star 40 such a success. What are they? There's no secret to it, really - the simple answer is "lightness and wing area".

The two main goals driving the design of the Four-Star 120 were the same as for the "40". First, it had to have outstanding flight performance which includes smooth aerobatic capabilities and solid slow-speed handling characteristics. You won't believe how this airplane goes from a rip-snortin' aerobatic dynamo in the sky to a smooth-as-silk floater in the landing pattern! Second, it had to be very easy to build, allowing the modeler to whip through construction without a bunch of sanding and shaping. Several prototypes were built and flown to make sure the design lived up to the goals just described. We believe the final design presented in this kit represents the finest blend of building and flying characteristics to be found in a sport model of this size.

Personally, I've had more fun with the Four-Star 120 than any model I can remember. There's no doubt in my mind that you'll enjoy it just as much!

Engines, Propellers, and Mufflers

There is a tremendous variety of engines available in the size range specified for the Four-Star 120. A good 1.20-size 4-stroke will TRULY fly this airplane well - that'swhat it was designed for! However, 2-stroke and 4-stroke engines work equally well in this model, so choose your favorite type, keeping in mind the type of performance that you wish the model to have. If you want maximum aerobatic capability and vertical performance, use an engine towards the upper end of the recommended size range. Engines at the bottom of the range will still allow for exciting flights with slightly reduced vertical performance. Remember, this is an airplane that likes to fly through aerobatic maneuvers with its wing rather than by the brute force of its engine.
.90 - 1.202-Stroke
- 1.20 - 1.604-Stroke

Use only those propellers recommended in the instructions supplied with your engine. If you use a very high-powered engine in your model, we recommend using a prop with a relatively large diameter and low pitch. This will give you loads of pulling power during maneuvers without a lot of excess speed. An 18" diameter propeller is about as big as you can use on this model and still have adequate ground clearance. Paint the tips of your propeller with a bright-colored paint so that you can easily see the prop arc when the engine is running. Be sure to balance your propellers to keep vibration to a minimum.

Use a muffler! A loud engine may cost you (and possibly your club) the use of your flying field if it annoys a non-flying neighbor. Use the muffler that came with your engine or one of the many after-market mufflers that are available.

Radio Requirements

A four-channel radio system is required for the Four-Star 120 to operate the ailerons, elevator, throttle, and rudder. Each aileron uses its own servo, so you'll need a total of five servos. For positive control (and peace-of-mind) we recommend using heavy-duty servos with at least 55 oz.lin. of torque for all of the flight controls.


A standard servo may be used for the throttle. A "Y" harness is necessary to plug the two aileron servos into the receiver. We also recommend using at least a 1000 mAH battery pack in the model. Be certain that your radio system frequency is approved for use in R/C model aircraft.


There are many different glues available today for model construction that it can be confusing to even the experienced modeler. To simplify matters, most glues can be classified as one of four basic types:
  1. Fast cyanoacrylate adhesives (abbreviated in this book as "CA") such as SIG CA, Hot Stuff, Jet, etc ...
  2. Easy-to-use water-based glues such as SIG-BOND (yellow) and SIG SUPER-WELD (white).
  3. Super strong (but heavier) two-part epoxy glues such as SIG KWIK-SET (5-minute cure) and SIG EPOXY (3-hour cure).
  4. Traditional solvent-based model cements such as SIG-MENT.

Each of these types has different characteristics and advantages. Often times, the choice of which type to use is strictly a matter of personal preference based on your experience with a previous model. However, because of the vast use of Lite-Ply and hardwoods in the FOUR-STAR 120, we have found that the CA glues seem to work the best for general construction. In fact, the construction sequence of the fuselage is designed with the use of CA glue in mind. Other glues could be used, but CA is recommended as our first choice because of its ability to penetrate an already assembled joint. In other words, the fuse parts can first be assembled dry (without glue), the alignment checked and adjusted, and then the glue can be applied to the joints. You should also have on hand some epoxy (both 5-minute and slow dry) and SIG-BOND because these glues are called out in several of the steps in these instructions.

SIG CA, like most brands of cyanoacrylates, comes in three viscosities thin, medium, and thick. Odorless CA's are also available from several manufacturers. Odorless CA is generally more expensive, but is ideal for people who can't tolerate the fumes of normal CA. An accelerator spray and debonder are also available and are described below.
  • Sig CA Thin - Watery in consistency, thin CA should only be used when the two parts to be joined are in perfect contact with zero gap. Capillary action pulls this glue deep into the wood resulting in a very strong bond and it dries in just a few seconds. Thin CA can be used to tack assemblies together, but these joints should be glued again later with medium or thick CA. Thin CA is also necessary for installing EASY HINGES.
  • Sig CA Medium - Our medium thickness CA is excellent for almost any step during construction, and is particularly recommended for gluing the plywood fuselage parts. The extra thickness allows the glue to fill small gaps, but it dries a little slower than thin CA. If you want only one type of CA, use medium thickness.
  • Sig CA Slow - This thickest formula is good for filling large gaps and building up strong fillets at joints requiring extra strength. It also dries slow enough to allow you to apply it to one part and position it on another before it dries. (With the thin and medium CA's, the parts must be in contact and positioned correctly before glue application.) This feature is useful when laminating large sheeted areas like a fuselage side and a fuselage doubler.
  • Sig Kwik-Shot Accelerator - Spraying accelerator on CA (any thickness) will cure it almost instantly. Although CA is fast, it's sometimes nice to speed it up even more.
  • Debonder - This can be used to separate parts, but you'll probably use it for unsticking your fingers more than anything else!

CAUTION: Some people have experienced allergic reactions when exposed to epoxy or cyanoacrylate glues. This is very rare. However, it is always important that such glues, and also paints, thinners and solvents, be used with adequate ventilation to carry fumes away.

You'll Need a Good Sanding Block

An assortment of different size sanding blocks are indispensable tools for model construction. A good general purpose block can be made by wrapping a 9"x11" sheet of sandpaper around a piece of hardwood or plywood. Use three screws along one edge to hold the overlapped ends of the sandpaper. Put 80-grit paper on the block during general construction. Switch to 220-grit paper for final finish sanding just before covering.

In addition to the large block, there are places where a smaller one is handy. Also, a sandpaper "file" can be made by gluing sandpaper to a flat spruce stick or around a hardwood dowel for working in tight places.


About The Building Sequence

The quickest and most efficient way to complete a model is to work on several pieces at the same time. While the glue is drying on one section, you can start on or proceed with another part. Keep in mind that the number, sequence used in this book was chosen as the best way to explain the building of each major component and is not intended to be followed in exact one-two-three fashion. Start on the wing at No.1 and after doing as many steps as is convenient, flip over to "FUSELAGE CONSTRUCTION" and do a step or two there, then back to "WING CONSTRUCTION" and so forth. You will arrive at points where you can go no farther until another component is available. Plan ahead! Read the book completely and study the full size plans before beginning construction.

Refer to "The Basics of Radio Control"

In addition to these instructions you are reading now, the booklet "The Basics of Radio Control" has been included with this kit as a reference for installing the engine, fuel tank, and radio in the Four-Star 120. It also contains very important information for preparing your model for flight. Modelers of all experience levels are encouraged to read this book and follow its guidelines for success. Highly experienced modelers may want to pass the booklet on to their club or student RIC pilots so that they can benefit from the information as well.

Notes Before Beginning Construction

Any references to right or left refers to your right or left as if you were seated in the cockpit.

To build good flying models, you need a good straight building board. Crooked models don't fly well! The building board can be a table, a workbench, a reject "door core" from the lumber yard, or whatever - as long as it is perfectly flat and untwisted. Cover the top surface of the building board with a piece of celotex-type wall board or foam board, into which pins can be easily pushed. Don't hesitate to use plenty of pins during assembly to hold drying parts in their correct position.

When pinning and gluing parts directly over the full-size plans, cover the plan with wax paper to prevent gluing the parts to the plans.

Don't use a ball point pen for making marks on the model during construction. If not sanded off, these ink marks will show through the model's final finish. Use a pencil instead of a pen.

Leave all die-cut parts in the sheets until needed in construction. Then remove the pieces from the sheets carefully. If difficulty is encountered, do not force the part from the sheet - use a modeling knife to cut it free. The die-cut balsa and plywood parts can be identified using the plans and the "KEY TO DIE-CUT PARTS". Mark the identification numbers on the corresponding parts before removing them from the die-cut sheets. All of the other parts can be identified by the "COMPLETE KIT PARTS LIST". Sort the different sizes of sticks and sheets into individual piles to avoid confusion during building.

Die-Cut Balsa
83/32"x3"x 8" W-1 and W-3 Wing Ribs 43/32"x3"x18" W-2 and W-3 Wing Ribs 11/8"x6"x21-1/2" Fuselage Top Deck
Sheet Balsa
41/16"x4"x30" Stabilizer and Fin Sheeting 43/32"x2-1/2"x42" Trailing Edge Sheeting 13/32"x3"x36" Wing Center Sheeting 23/32"x4"x36" Wing Center Sheeting
11/4"x2"x12" Stabilizer Center Block, Fin Base
Balsa Shear Webs
123/32"x4"x1-11/16" SW-1 Shear Webs (2 packs of 6) 63/32"x4"x1-3/16" SW-2 Shear Webs (1 pack of 6) 221/16"x4"x11/16" SW-3 Shear Webs (2 packs of 11)
Stick Balsa
53/16"x3/8"x24" Fuselage Stringers, Fin Spacer 141/4"x1/4"x42" Wing Spars (12), Stabilizer/Fin Frames (2) 31/4"x3/4"x30" Stabilizer/Fin Frame 13/8"x3/4"x2" Spacer for Tail Fairing Blocks
11/2"x3/4"x5" Trailing Edge Fill-In 13/4"x30" Triangle - Braces, Tail Fairing Blocks
Special-Cut Balsa
25/16"x1/2"x42" Trailing Edge (tapered) 23/8"x3/8"x42" Leading Edge (quarter round) 23/8"x2-3/4"x38" Pre-Cut Ailerons 23/8"x3"x14" Pre-Cut Elevators
13/8"x4"x12" Pre-Cut Rudder


Die-Cut poplar Plywood (Lite-Ply)
21/8"x6-3/4"x48" Rear Fuselage Side, Wingtip, Wing Dowel Support 21/8"x6-3/4"x48" Front Fuselage Side, Fuselage Doubler, Nose Tripier, Aileron Servo Mount, SRS 11/8"x5"x48" FT, TWM, T-2, T-3, Stab Support 11/8"x5"x38" FBR, HR, APG
11/8"x5"x24" FBF, Tank Floor, F-2, F-2D 11/8"x6"x24" F-3 thru F-6, F-3S, F-4S, F-4T, F-5T, T-1
11/4"x1/4"x9" SPRUCE: Stab Brace 41/4"x1/2"x16" SPRUCE: Spar Doublers 41/4"x1/2"x42" SPRUCE: Main Wing Spars 11/4"x1/2"x12" BASSWOOD: Grooved Hatch Rails
13/8"x3/8"x27" BASSWOOD: Servo Rail Material 23/4"x3/4"x1-1/2" BASSWOOD: Wing Hold-Down Blocks 25/16"dia.x2" BIRCH: Wing Dowels
Die-Cut Birch Plywood
11/16"x6"x10" Hatches, Hold-Down Plates, EHS, RHS, AHS, BWP 15/32"x2"x11" Dihedral Brace 25/32"x5-1/2"x8-1/4" F-1; Landing Gear Mount
Formed Wire
13/32" dia.x7" Tailwheel Wire with Molded Nylon Tailwheel Bracket 11/8" dia.x7" Elevator Joiner Wire
1Clear Plastic Canopy 1Pre-Bent Aluminum Landing Gear 12"x30" Fiberglass Tape (Wing Center and Tailwheel Wire) 11.20 4-stroke size Reinforced Nylon Engine Mount
28-1/2"x14" White Paper (for Aileron Servo Lead Tubes) 2Full Size Printed Plans 1Photo Illustrated Instruction Booklet 1"The Basics of Radio Control" Booklet
310"x27" Color Decals
2#2 Flat Washers; (for tailwheel wire) 10#2x3/8" Sheet Metal Screws (8 for hatches, 2 for tail brace wires) 8#2x3/4" Sheet Metal Screws (for control horns) 2#4x1/2" Sheet Metal Screws (for tailwheel bracket)
4#8x1" Sheet Metal Screws (for engine) 32-56x1/2" Mounting Bolts (for tail brace wires) 38-32x3/4" Mounting Bolts (for landing gear) 46-32x3/4" Socket Head Bolts (for engine mount)
21/4-20x2-1/4" Steel Bolts (for wheel axles) 72-56 Hex Nuts (for tail brace wires) 44-40 Hex Nuts (for jam nuts) 41/4-20 Hex Nuts (2 per axle)
46-32 Blind Nuts (for engine mount) 38-32 Blind Nuts (for landing gear) 21/4-20x1-1/2" Nylon Wing Bolts 4Molded Nylon Control Horns
2.200" o.d.x36" Nylon Inner Push rod Tubing (yellow) 2.270" o.d.x36" Nylon Outer Push rod Tubing (black) 1.130" o.d.x20" Nylon Tubing (throttle pushrod housing) 11/16" dia.x20" Flexible Steel Cable (throttle push rod)
42-56x10" Threaded Rods (tail brace wire material) 42-56 Metal Threaded R/C links (for tail brace wires) 42-56 Solder Clevis (for tail brace wires) 64-40x8" Threaded Rods (2/ailerons, 2/rudder, 2/elevator)
44-40 Metal Threaded R/C Links (2/ailerons, 1/rudder, 1/elevator) 44-40 Solder Clevis (2/ailerons, 1/rudder, 1/elevator) 24Easy Hinges 1Push rod Connector Assembly



Building The Wing Panels


Construct four main spar assemblies by gluing the four 1/4"x1/2"x16" hardwood spar doublers to the four 1/4"x1/2"x42" hardwood main spars. Use epoxy for this step, and make sure the spars are kept straight while drying. Any bends or twists built in now are there to stay!


  1. Pin one.of the main spar assemblies in place on the plan, being certain that the outboard end of the spar doubler is positioned correctly.
  2. The 3/32" balsa trailing edge sheeting is provided extra wide so that the forward edge can be cut perfectly straight (use a long straightedge). Pin the bottom T.E. sheeting in place, aligning the front edge with the plans.
  3. Use three or four wing ribs to accurately position the 1/4"x1/4"x42" balsa bottom rear spar, then pin the spar in place.


  1. Place a scrap of 3/32" balsa near the main spar to accurately space the W-1 wing ribs up from the building board. (The spacing is required for the center sheeting which will be added later.)
  2. Use the dihedral gauge side of the die-cut plywood Dual Tool to set the root W-1 wing rib at the proper dihedral angle (20),then glue it to the spars and the trailing edge sheeting.
  3. Add the remaining wing ribs (three W-1, two W-2, and six W-3) to the wing panel.


  1. Use the "SHEAR WEB IDENTIFICATION" diagram on sheet two of the plans to identify the three types of pre-cut balsa shear webs (SW-1, SW-2, SW-3) that are used in the wing panel. The vertical wood grain is important for maximum wing strength, but it makes the webs somewhat fragile before installation. If one should break, simply glue it back together and install it normally.
  2. Install six SW-1 and three SW-2 shear webs as shown on the plan. Trial fit each web before gluing, sanding the ends as necessary to make them fit snugly between the ribs on either side. The bottom of the shear webs should be centered on the bottom main wirig spar; the top of the shear webs should be centered on the notch for the top main wing spar. The two inboard rib bays do not receive shear webs because that is where the dihedral brace will be installed later.
  3. Install an SW-3shear web in each rib bay (11 total), at the front edge of the trailing edge sheeting. These webs provide support for the sheeting and help stiffen the wing torsionally.
    NOTE: The shear webs between the W-1 wing ribs will need to be shortened significantly to fit.


  1. Glue the pre-shaped 5/16"x1/2"x42" balsa trailing edge to the top of the trailing edge sheeting and the ends of the wing ribs.
  2. Cut a trailing edge fill-in block from the 1/2"x3/4"x5" piece of balsa included in the kit, and glue it in place. Carve and sand the top of the fill-in block to match the W-1 wing ribs.


  1. Temporarily remove any pins in the bottom trailing edge sheeting, then add the 3/32" balsa top trailing edge sheeting (trim the front edge straight before gluing). For this step it is recommended that you apply Sig-Bond (aliphatic resin) to the top of the trailing edge, the ribs, and the SW-3 shear webs before setting the top sheeting in place. Re-pin the trailing edge to the building board to keep it straight while it dries.
  2. Trial fit a main spar assembly in the upper rib notches. If any of the shear webs are too tall, they should be trimmed to allow the spar assembly to sit all the way down in the rib notches. When satisfied with the fit, glue the spar assembly in place. Check the root W-1 rib again with the Dual Tool to be sure it is still at the correct angle.
  3. Glue the pre-shaped 3/8"x3/8"x42" balsa leading edge to the front of the ribs.
  4. Glue the two 1/4" sq. x42" balsa top forward spars in place.
  5. Glue the forward-most 1/4" sq. x42" balsa bottom forward spar in place. (The final forward spar will be added later.)
  6. Add the 1/4" sq. x42" balsa top rear spar.


Install a die-cut lite-ply wing dowel support (WDS) in the innermost rib bay between the two 1/4" sq. spars behind the leading edge. Notice that one end of the support is pre-cut to match the dihedral angle of the inboard W-1 rib.


Form a 14" long aileron servo lead tube by wrapping one of the supplied pieces of 8-1/2" x14" plain white paper around a broom handle. Temporarily tape the tube together, slide it off the broom handle and into the 1" holes in the wing panel. Position the outboard end of the tube flush with the outer W-2 wing rib. Remove .the tape and allow the tube to unwind, then glue it to the ribs.
NOTE: Remove any pins from the structure that are located under the area where the top center sheeting will be installed (in the next step). Otherwise, you may find it difficult to remove your wing from the board later!


  1. Cut the top center sheeting piece that fits between the main spar and the rear spar from the 3/32"x3" balsa sheet provided in the kit. Trim and sand the sheet to fit, then glue it in place. Sig-Bond is recommended for gluing all of the center sheeting to the ribs and spars because it will be easier to sand the joints smooth later.
  2. The rest of the top center sheeting should be cut from 3/32"x4" balsa, and glued in place.


When the glue has dried, unpin the wing half from the board and install the remaining 1/4" sq. x42" balsa bottom forward spar.

IMPORTANT! If you have been using thin or medium CA glue, now is the time to go back over every joint using medium or thick CA. Don't be stingy here - the integrity of your wing depends upon strong glue joints. Glue BOTH sides of EVERY joint, even the aileron servo lead tube. Make certain the shear webs are bonded to the spars AND the wing ribs on each side. Double check the leading edge stick and all of the 1/4" sq. spars for complete bonding to the ribs.


11. Install the bottom center sheeting except for the two areas shown in the photo. Cut pieces from the 3/32" x 4" balsa that you used earlier.


  1. Cut off and sand the spars, leading edge, trailing edge, and sheeting at both ends of the wing, flush with the end ribs.
  2. Although the leading edge is pre-shaped, it should be smoothed now with a long sanding block. Also, trim and sand the overhanging portion of the trailing edge sheeting flush with the trailing edge.
  3. Sand the top center sheeting smooth. (The bottom center sheeting is sanded later, in step 19.) Cut a 3/4" sq. access hole in the top center sheeting as shown on the plan.


Glue the die-cut lite-ply wingtip to the outboard end of the wing panel.

OPTIONAL: If you prefer rounded wingtips over the square ones, simply add a soft balsa block to the wingtip (instead of the lite-ply part), then carve and shape it as desired.


The two center W-1 wing ribs have two vertical slits between the main spar notches to help locate the dihedral brace. Carefully remove the material between the slits, then extend the openings to the spars as shown in the diagram. A small sanding stick made from scrap 5/32" ply with sandpaper glued to the edge is a handy tool for this step.


Repeat steps 2 through 14 to build the opposite wing half.

Joining The Wing Panels


  1. Trial fit the two wing panelswith the die-cut 5/32" plywood dihedral brace installed between the main wing spars. Be certain that the dihedral brace is not preventing the panelsfrom making solid contact with each other at the center. If necessary, trim or sand the dihedral brace for a snug fit. The dihedral angle of 20 per wing panel will be automatically built-in by the dihedral brace. If you want to check the angle, place the wing on a table so that one side sits flat, and the other side is raised. The distance from the table to the bottom of the wingtip should be about 2-3/4", but a variation of up to 1/2" either way is acceptable and will not affect the flight performance. The most important thing is to have a tight joint at the wing center with no gaps.
  2. Epoxy glue the dihedral brace into ONE of the wing panels, and allow to dry.


Use epoxy (either 5-minute or slow-dry) to join the two wing panels. Generously apply glue to end ribs and the exposed edges of the dihedral brace, then carefully slide the other wing panel into place. Wipe away any excess epoxy that oozes from the center joint (a rag dampened with alcohol works well). Before the glue dries, make sure that the leading and trailing edges of both panels are perfectly aligned.


Glue the die-cut 1/16" plywood wing hold-down plates to the bottom of the wing, flush with the trailing edge.


Finishing The Wing

NOTE: Complete the steps in "Mounting The Wing To The Fuselage" before proceeding.


  1. While you still have access through the bottom of the wing, check the glue joints around the wing hold-down dowels and the dihedral brace. If necessary, apply another coating of epoxy to the joints.
  2. Finish off the bottom center sheeting, again using 3/32" balsa.
  3. If you haven't done so already, give the wing a final sanding. Sand just enough to take off any prominent high spots or bumps. Excessive sanding may distort the airfoil shape.


The 2" wide fiberglass tape can be applied to the wing center joint (top and bottom) using one of the following two methods:

  1. Coat the wing center joint with slow-drying epoxy glue.
  2. Lay the tape on top of the glue.
  3. Holding one end of the tape so it won't slip, "squeegee" the glue through the tape with a small paddle of scrap balsa. Scrape over the tape several times with the paddle to smooth the tape and remove any excess glue.
  4. When dry, sand lightly to remove any rough spots. Try not to sand into the fiberglass tape itself.
  1. Cut the tape to length, then lightly spray one side with a spray adhesive (such as 3M "77").
  2. Position the tape on the wing center joint.
  3. Soak the tape with thin CA. The spray adhesive simply holds the tape in place - it won't affect the strength of the CA. A second coat of CA will help fill in the weave of the fiberglass, resulting in a smoother surface. Rub the second coat with your finger (protected with plastic wrap - keep it moving!) to smooth out the glue. Use a fan to keep the CA fumes away from your face.
  4. When dry, sand lightly to remove any rough spots. Try not to sand into the fiberglass tape itself.



Sand the trailing edge of the aileron round and bevel the front using a sanding block. A pencil line drawn down the center will help keep the bevel symmetrical.


Imbed the die-cut 1/16" plywood horn support in the bottom surface of the aileron by carefully cutting a 1/16" deep relief as shown in the diagram below and the "Control Horn Assembly" diagram on Sheet 2 of the plans. Firmly glue the support into the relief.


  1. Center a nylon control horn on the plywood support with the five horn holes lined-up vertically with the point of the leading edge bevel. Mark the position of the two flange holes, then drill at the marks with a 3/32" drill bit.
  2. Reinforce the control horn area by soaking the wood around the two holes with thin CA.This will help keep the nylon horn plate from crushing the balsa when the control horn is installed later.


Temporarily tape the aileron to the back of the wing with its inboard end spaced 2-1/2" from the wing center joint. Sand the outboard end of the aileron flush with the wingtip. Leave the aileron taped in place and temporarily mount the nylon control horn to aid in the next section.


Aileron Servo Installation

The ailerons on the Four-Star 120 are operated by two servos mounted separately in each wing panel (see "RADIO REQUIREMENTS" earlier on in these instructions).


Cut two hatch rails from the special-cut 1/4"x1/2"x12" basswood stick provided in the kit. Glue the hatch rails in place, flush with the bottom edge of the W-2 and W-3 wing ribs. The hatch rails provide a seat for the aileron hatch and a flat surface for attaching the covering material.


  1. The die-cut 1/16" plywood hatch may require some trimming or sanding for a perfect fit on the hatch rails. With the hatch in place, drill four holes through both the hatch and the hatch rails (at each corner of the hatch) with a 1/16" drill bit.
  2. Remove the hatch, then redrill the four holes in the hatch with a 3/32" drill bit. The hatch is held in place with four #2 x 3/8" sheet metal screws. Be sure to mark each hatch so you can tell later which hatch goes with which wing panel!


  1. Since servos come in many sizes, you need to make a custom servo tray from lite-ply and basswood to fit your particular servo. Cut a hole in the die-cut lite-ply aileron servo mount to fit your particular servo. The forelaft dimension of the hole should be about 1/16" larger than the length of your aileron servo.
  2. Cut two 3/8" sq. x4" basswood servo rails (from tile 27" long piece provided in the kit) and glue them to the aileron servo mount flush with the front and rear edges of the hole.
  3. If necessary, trim or sand the edges of the servo mount until it fits snugly in position between the ribs. Trial fit the mount, but don't glue it in yet.


Carefully position the servo on the mount so that the outer hole of the servo arm is lined-up with the control horn on the aileron. The goal here is to keep the aileron push rod wire parallel to the wing ribs (and the airflow). Mark the 4 servo mounting holes, then fas.ten the servo to the aileron servo mount using either the screws provided with the servo or long wood screws (#2 x3/4", not provided in kit).



  1. Make an aileron pushrod as shown on the plan from a 4-40 x8" threaded rod, a 4-40 solder clevis, and a 4-40 R/C link.
    The actual length of your pushrod may have to be altered slightly depending on the shape of your aileron servo.

  2. Temporarily connect the aileron pushrod to the servo arm (with the solder clevis) and the aileron control horn (with the R/C link). Adjust the position of the servo mount in the wing as necessary to keep the pushrod from hitting the bottom spar and the servo arm from hitting the hatch.
    Also make certain that the covering material on the top of the wing won't make contact with the servo.


When satisfied with its position, glue the servo mount assembly to the wing ribs.

Reinforce the joints at the servo mount and the ribs with a couple of scrap pieces of stick balsa.


The plywood hatch will need an opening cut into it for the aileron pushrod.
Again, the exact-size and shape of this opening will depend on the shape and position of your pushrod. You can make a nice-looking slot by drilling two holes, then connecting the edges with knife cuts.

When you are done with the hatch, remove the aileron, the aileron control horn, and the aileron pushrod; The servo can be left in place since it won't interfere with covering.


You may prefer to install your aileron servo as shown in the diagram below. The advantages are that the push rod can be run directly to the control horn with no bends, it's lighter, less complex, and provides easier servo access. The disadvantages are a slight increase in drag and a less "finished" look to the bottom of the wing. Simply replace the hatch rails and plywood hatch with some 3/32" balsa sheet (trim the bottom of the ribs to fit), and cut an opening that's just large enough to install and remove the servo.