SIG Mfg. Co., Inc...401-7 South Front Street...PO Box 520....Montezuma, Iowa 50171-0520

Congratulations on your purchase of the SIG FOUR-STAR 20EP. For many years the SIG Four-Star series of low-wing sport airplanes have proven to be among the most popular and versatile at the flying field. Originally designed for 2-stroke glow engines, Four-Stars have also excelled with 4-stroke, gasoline, and now electric motors. The secret to their popularity is their light flying weight and low wing loading, combined with excellent aerobatic performance. The maneuverability and smooth handling of the Four-Star will make it the sport model you'll want to take to the flying field every time.

The FOUR-STAR 20EP is the first Four-Star kit designed primarily for electric power (although the structure is configured to easily accommodate a glow engine if you wish to go that way). This use of a modern electric power system in the legendary Four-Star has produced a marvelous little R/C ship that is a pure joy to fly. It's small enough to carry around in your vehicle completely assembled and ready to fly almost anywhere, anytime!

Assembly of the FOUR-STAR 20EP is quick and easy, following the detailed instructions in this manual. All parts are CAD-drawn and the majority are laser-cut, so that everything fits the way it should! We strongly suggest that you read through the manual first to get familiar with the various parts and the assembly sequences.


Additional Items Needed

The following items are not supplied in this kit but are needed to complete the airplane. Because of the wide variety of brands available and the influence of local preferences, the final choice of these items is left to you. In most cases, we will list what worked well for us in our prototype models. You should select equivalent items.
    200-300 watts with a kv specification suitable for turning a 9x6E propeller. Motor should also include prop adapter hardware and a radial-style motor mount.

    Matched to your motor - typically 35 amp.

    * We use and recommend the Maxx Products Co. Combo 19A (, which is a packaged motor system that includes their Himax 3510-1100 Brushless Outrunner Motor, a Castle Creations Thunderbird 36 amp ESC, a radial motor mount, an APC 9x6E propeller, a prop adapter, and mounting screws.
    Himax 3510-1100 Motor specs:
    Diameter35.2mm (1.39")
    Length32.2mm (1.27")
    Shaft Diameter4.0mm
    Weight89g (3.1 oz.) motor only
    Max. Power250W
    Kv=1100 Rm=.055 lo=1.2
    Any other brand of outrunner motor and speed control of equivalent size and specifications would also work.

    1800-2400 mAh

    For maximum performance from your battery pack, you MUST use a proper battery charger! Make sure your charger is designed for the type of cells in your battery pack. Using the wrong battery charger can be very dangerous! We recommend that you use a quality peak detection lithium-polymer battery charger with cell balancing.

  • SPINNER (if desired)
    2" to 2-1/4" diameter spinner is optional. You could also use an "acorn" style prop nut alone. Your choice.

  • 2-1/4" dia. MAIN WHEELS

    "Standard" size radio gear is not recommended for an airplane as small as the Four-Star 20EP. The Four-Star gets its great flight performance from many factors, but two of the most important factors are its light weight and generous wing area. The lighter the final weight is, the better this airplane will fly! The radio equipment you install will have a great effect on the flight performance of your airplane. Maximum performance can only be achieved with light weight components.

    RECEIVER a small or micro size receiver should be used in this model. In our flight tests we have used the Hitec Electron 6 (weight: 19 grams) and the FMA M5 (weight: 11 grams). Both are dual conversion receivers.

    SERVOS standard size servos, the ones that come with most sport radio systems, are too big and too heavy for the Four-Star 20EP. You will need smaller servos. In our prototypes airplanes, we used Hitec HS-81 Micro Servos, which have a torque rating of 36 oz/in and weigh .58 oz each. Any other brand of servo that is in the same size and torque range will also work just fine.

    SERVO CHORDS You will need one Y-Harness chord to connect the two aileron servos together inside the wing. Every radio manufacturer has Y-Harness chords available for their radio systems. If they have an option for a short Y-Harness or a long Y-Harness, you can probably use the short one due to the small size of the airplane. In addition, you will also need a short standard Servo Extension Chord to plug into the receiver and connect to the aileron Y-Harness.

    Any brand of quality iron-on plastic covering material is suitable for the Four-Star 20EP. If the manufacturer has a "lite" version, use it to save a little weight. The color choice is entirely up to you. It will take approximately 1 to 1-1/2 rolls of covering material to cover the Four-Star 20EP, depending upon how many colors you elect to use.


    Choice depends a lot on you personal preference. We used Thin, Medium, and Thick SIG CA; CA Accelerator; Fine point CA applicator tips; and SIG Epoxy Glue (5-minute and 30-minute).

    Screwdrivers, Pliers (regular, needle nose, flat nose), Wire Cutters, Scissors, Ruler, Pencil, Metal Straight Edge, Triangle, Flat Building Board, Hobby Knife with #11 Blades, Single-Edge Razor Blades, Wax Paper, T-Pins, Paper Towels, & Rubbing Alcohol (for cleaning off excess epoxy glue).

  • POWER DRILL with Assorted DRILL BITS




    Use 80-100 grit to rough sand and shape parts; 220-300 grit for finish sanding. A good general purpose sanding block can be made by wrapping a full 9"x11" sheet of sandpaper around a piece of hardwood or plywood. Use screws or thumbtacks along one edge to hold the overlapped ends of the sandpaper in place. In addition to that block, make a small sandpaper "file" by gluing a strip of 80 grit sandpaper onto a scrap plywood stick. Sandpaper glued or taped to different size wood dowels are also handy to have around.

Glow Power Option

The Four-Star 20 can be powered with a 2 or 4-stroke glow engine instead of an electric motor, if you prefer. We recommend glow engines in the .15 to .20 range. No major structural modifications are necessary to accommodate a glow engine.

The checklist of changes if using a glow engine is short:
  1. Leave the 3 small circles near the bottom of the F-1A and F-1B plywood formers in place. Do not punch them out. Work some glue into the cut lines to keep them permanently in place. The purpose of those 3 holes was to cool the electric motor battery. Obviously not needed in a glow version. In fact, they need to be sealed up so that glow fuel exhaust residue does not enter the nose of the airplane.
  2. Do not install the plywood BT (Battery Tray) in the nose. You will need that clear space in the nose for a fuel tank.
  3. In addition to the glow engine, you will need to purchase a suitable engine mount and a plastic 4 oz. fuel tank. You will also need to buy a 5th servo for the throttle, and a suitable throttle pushrod.

Notes Before Beginning Construction

Any references to right or left, refer to your right or left as if you were seated in the cockpit of the airplane.

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 hollow wood door from the lumber yard, or anything else that is pefectly flat and untwisted. Cover the top surface of the building board with a piece of celotex-type wall board or dense foam board, into which pins can be easily pushed.

When building directly on top of the full-size plans, cover the plan with wax paper to prevent gluing the parts to the plan.

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


The laser cut balsa and plywood parts can be identified using the "KEY TO LASER CUT PARTS". Mark the identification numbers on the corresponding parts before removing them from the laser cut sheets. Leave all the laser cut parts in the sheets until needed in construction. Then remove the pieces from the sheets carefully. Use a sharp #11 hobby knife to cut through any "tabs" that are holding the parts in the sheets. Lightly sand off any remainder of the tabs, flush with the actual edge of the part.

All of the other parts can be identified by the "KIT INVENTORY LIST" that follows. Sort the different sizes of sticks and sheets into individual piles to avoid confusion during building.

Laser Cut Balsa Wood Parts
21/16"x3"x36" Sheet No.1: wing ribs W-2, W-3, W-4, W-4A 13/32"x3"x36" Sheet No.2: Fuselage Top 13/32"x3"x36" Sheet No.3: fuselage parts FBT-2, FD-1 13/32"x3"x36" Sheet No.4: W-1 wing ribs, Spar Webs
23/32"x4"x36" Sheet No.5: Fuselage Sides 13/16"x3"x36" Sheet No.6: tail parts Tail Parts; S-1, S-2, S-3, S-4, S-5, S-6, S-7, S-8, S-9, E-1, E-2, E-3, E-4, E-5, E-6, E-7, R-1, R-2, R-3, R-4, R-5, R-6, R-7, R-8, F-1, F-2, F-3, F-4, F-5, F-6, F-7 13/16"x3"x36" Sheet No.7: W-1A, Ailerons, Wingtips
Laser Cut Plywood Parts
11/8"x6"x24" Sheet No.8: APG, F-1A, F-1B, F-2, F-2B, F-3, F-4, F-4T, F-5, F-5T, F-6, F-6T, FBT-1, BT, DB, FHR, HT-1, HT-2, LG-1, LG-2, P-1, P-2, SM-1 (4), T-1, T-2, TWM, WBP 21/4" Motor Spacers
Balsa Wood Sticks
33/32"x3/16"x36" Stringers 11/16"x1/4"x18" Capstrips 81/8"x1/8"x24" Sub Spars 41/8"x3/8"x36" Main Spars, Main Spar Doublers, Servo Bay
41/4"x1/4"x24" Leading & Trailing Edges
Balsa Wood Sheets
21/16"x1-1/4"x24" Trailing Edge Top Sheet 21/16"x1-1/2"x24" Trailing Edge Bottom Sheet 21/16"x2"x24" Wing Center Section Sheeting 13/32"x3"x13" Fuselage Top Sheeting
21/4"x1/4"x2-9/16" Servo Mount Rails 11/4" dia.x1-1/2" Birch Dowel
Music Wire
11/16"x4-3/8" formed Tailwheel Wire 11/16"x4-1/4" formed Elevator Joiner 21/32"x24" Elevator & Rudder Pushrods
Plastic Parts
1Molded Plastic Canopy 1Molded Plastic Cowling
22-56x10" Threaded Rods; for aileron pushrods 13/4" dia. Tailwheel 4Nylon RC Links; for rud(1), ail(2), ele(1) 2Right Nylon Control Horns; for ele(1), left ail(1)
2Left Nylon Control Horns; for rud(1), right ail(1) 1Nylon Landing Gear Clip 1Nylon Tailwheel Bracket 8#2x3/8" Sheet Metal Screws; for cowling, hatch, tailwheel bracket
8#2x1/2 Sheet Metal Screws; for control horns 12-56x3/8" Bolt; for tailwheel 12-56 Hex Nut; for tailwheel 24-40 Blind Nuts; for main landing gear
24-40x3/8" Bolts; for main landing gear 26-32x1-1/2" Bolts; for axles 26-32 Lock Nuts; for axles 46-32 Hex Nuts; for axles
14Hinges; for ail(6), ele(4), rud(3), tailwheel(1) 110-32x1" Nylon Wing Bolt 110-32 Blind Nut; for wing hold down 2Threaded Couplers; for elevator & rudder pushrods
11/16" id Wheel Collar & Set Screw; for tailwheel 1Hex Key Wrench
1formed Aluminum Landing Gear 2.130 odx19 Nylon Pushrod Tubes 1Decal Sheet SIGDKM106A 1Decal Sheet SIGDKM106B




We will build the wing in separate right and left wing panels, and then join the panels together after they are assembled. Let's start with the LEFT WING PANEL. To get started, tape or pin the wing plan to your building board. Cover the plan with a layer of waxed paper to keep glue from sticking to the plan.


Cut four 1/8"x3/8"x36" balsa sticks down to 24" long for the Main Spars. Cut the four pieces of leftover stick to 9" long. These will be the Spar Doublers. Glue one Spar Doubler to one end of each Main Spar. Let dry.


Pin one of the main spar assemblies in place on the plan. Align the center end of the spar assembly (the end with the doubler on it) exactly with the line on the plan that indicates the center of the wing. Let the excess spar length extend past the wing tip.


Remove all the balsa wing ribs from laser-cut sheets no. 1, 4, and 7. Make sure you have correctly marked the ribs with their part numbers according to the KEY TO LASER-CUT PARTS diagrams. You should have: (2) W-1 ribs, (2) W-1A, (2) W-2, (2) W-3, (12) W-4, (2) W-4A, and (2) Wingtips.


Glue W-1A to the front of each W-1 rib, being very careful to get the edges exactly aligned. Be sure to make a right and left W-1 rib assembly.


Carefully remove one of the Spar Webs from laser cut sheet no. 4. Hold the Spar Web against the plan and familiarize yourself with it's exact orientation. Note that the open rib notches should be along the top of the Spar Web when it's installed in the wing, not on the bottom.


Slide ribs W-2, W-3, and W-4 into their notches in the Spar Web. Pin this assembly in place on top of the Main Spar. Pin down only the Spar Web at this time, not the ribs - the ribs must remain loose until the trailing edge pieces are added in a few minutes.


Add rib W-4A.


Add the W-1/W-1A rib assembly, pinning it in place on the end of the Spar Web. Note that W-1 will not sit perfectly vertical - it should be tipped very slightly with the top edge towards the wingtip to account for the wing's dihedral angle. The correct angle is pre-cut into the end of the Spar Web. Just make sure you pin W-1 firmly against the end angle of the Spar Web - that will insure that you are within tolerance for the dihedral angle later.


4a. Slip the 1/16"x1-1/2"x24" Balsa Trailing Edge Bottom Sheet in place under the back ends of the wing ribs. Slide it forward against the notches in the bottom of the ribs. Now pin the ends of all the wing ribs to the Trailing Edge Bottom Sheet and firmly to the plan.


Pin the 1/14"x1/4"x24" Balsa Trailing Edge in place on top of the Trailing Edge Bottom Sheeting and up against the ends of the wing ribs.


Check one more time that all the wing parts are properly aligned and pinned together and pinned down to the building board. Then glue all the joints with Thin CA adhesive. Let dry.


Fit one of the Main Spar assemblies (from step 1a.) in place in the notches in top of all the ribs. Glue securely and let dry.


Glue the 1/16"x1-1/4"x24" Balsa Trailing Edge Top Sheet in place.


Cut a piece of 1/8"x3/8" balsa stick to fit between ribs W-4 and W-4A, where shown on the plan at the rear of the aileron servo bay. Glue it in flush with the bottom of the ribs.


Cut two pieces of 1/16"x1/4" balsa stick for the Capstrips that go along the sides of the W-4 and W-4A ribs, as shown on the plan. Glue the capstrips in place flush with the bottom of the ribs.


Glue the 1/4"x1/4"x24" Balsa Leading Edge in place.


Glue two 1/8"x1/8"x24" Balsa Sub Spars in place in the top rib notches.


1/16"x2"x24" balsa is provided for sheeting the center section of the wing. Start by cutting the stock into seven 3-1/4" long pieces.


Glue one of the 3-1/4" long pieces in place up against the trailing edge top sheet. Trim the width of another 3-1/4" long piece of sheeting stock to fit between the first piece and the back of the top main spar. Glue in place.


Trim more pieces of the sheeting stock to fit between the top Sub Spars, as shown. Glue in place and let dry.


When all the glue joints are dry, take out all the pins and remove the wing panel from the building board.


Glue two 1/8"x1/8"x24" Balsa Sub Spars in place in the bottom rib notches.
Note: Do not install any 1/16" balsa sheeting to the bottom of the center section at this time.


Now is a good time to fit your aileron servo in the wing. Remove two SM-1 pieces from the laser-cut plywood sheet. Be sure to sand off any excess part of the "tabs" that held the parts in the big sheet (the fit of SM-1 between the W-4 wing ribs is tight).


Glue the front SM-1 part in place.


Measure the length of your servo case to determine exactly where the rear SM-1 part should be located. Then glue it in place.


Drill pilot holes for your servo mounting screws and then mount the servo. When done, remove the servo and set aside.


14. Cut a 1/2" dia. hole in the top sheeting just behind the main spar. This is for the aileron servo chord to exit the wing panel.


Use a large sanding block to sand any excess spar ends and sheeting flush with the W-1 rib. Use a sanding block large enough to sand the entire part at the same time. Work slowly to insure that the end rib remains flat and straight.


Use a large sanding block to sand any excess spar ends or sheeting flush with the last W-4 tip rib.


Glue the 3/16" balsa Wingtip in place on the end of the wing panel. Make sure to match up the edges perfectly with rib W-4.


Wrap a small piece of 80 or 100 grit sandpaper around the 1/4" hardwood dowel provided. Use this to sand a groove in the end of the 1/4" balsa leading edge where the dowel will be installed later.

The LEFT WING PANEL is now basically done. Repeat steps 1 through 17 to build a RIGHT WING PANEL.

Joining The Wing Panels


Note the dashed lines that are cut into the W-1 end ribs to indicate the location for the laser-cut plywood DB Dihedral Brace. Use a sharp #11 hobby knife to cut through the rib between the dashes to create an opening for the Dihedral Brace. Do the same for both the W-1 end ribs of BOTH wing panels.


Trial fit both wing panels together with the Dihedral Brace installed. Make sure the panels come together tightly with no gaps. Then take the wings back apart.


Glue the DB Dihedral Brace halfway into one of the wing panels. Epoxy glue is recommended.


Coat the end rib and the Dihedral Brace with glue and then slide the other wing panel in place. Push the panels together tight! Match up the edges of the wing panels perfectly. Use pins or clamps to hold the wing panels tight together until the glue dries.


Install 1/16" balsa center section sheeting to the bottom of the wing panels, just as you did in step 11 for the top of the wing.


Sand the wing to final shape. Round the 1/4" sq. balsa leading edge as shown in the cross-section drawings on the plan. Trim and sand down the top surface of the 1/4" sq. balsa trailing edge to blend with the shape of the wing.


22. Drill the center of the leading edge with a 1/4" dia. drill bit until the 1/4"x1-1/2" hardwood dowel can be slid into place in the notches in the W-1 ribs. The dowel should end up protruding about 5/16" out front of the leading edge. When satisfied with the fit, glue the dowel in place.


Locate the laser-cut plywood WBP Wing Bolt Plate. Glue it in position on the bottom of the wing. Make sure it is centered over the joint of the wing panels, and that the back edge of WBP is flush with the back edge of the wing.


Use a 3/16" dia. drill bit to drill completely through the wing in the center of WBP. Be sure to use a "backing block" of some kind to support the top wing sheeting as you drill through. Cosmetic Note: for better final appearance, and to make it a little easier to apply the covering material, we like to bevel the edges of WBP as shown here. Bevel the edges before gluing WBP on the wing.


Remove the 3/16" balsa Ailerons from laser-cut sheet no. 7. Shape the leading and trailing edges of the Ailerons to the shape shown on the CROSS-SECTION AT W-4 drawing on the plan - round the trailing edges, and "v" shape the leading edges. When finished sanding, set the Ailerons aside for now. We will hinge them to the back of the wing after all the parts are covered.


Begin by taping or pinning the Stabilizer and Elevator plan to your building board. Cover the plan with a layer of waxed paper to keep glue from sticking to the plan.


Assemble stabilizer parts S-1, S-2, S-3, S-4, S-5, and S-9 together over the plan. Glue the parts together where they join.


Add the stabilizer ribs S-6, S-7, and S-8.


When dry, unpin the stabilizer from plan and sand it to final shape. Round the leading edges. Leave the trailing edge and the tips square.


Assemble and glue the elevator parts together over the plan, just like you did the stabilizer.


When dry, remove the elevators from the plan and make a couple light passes on the top and bottom surfaces with your large sanding block to smooth them out.


Sand the trailing edges of both elevators round, as shown in the cross-section drawing of the elevator on the fuselage side view plan. Leave the tips of the elevators square.


Draw a hinge line all the way down the middle of the leading edge of each elevator. Make sure it is centered.


Lay the elevators in position up against the trailing edge of the stabilizer, making sure to have the tips lined up. Then lay the 1/16" Wire Joiner in position and mark its location on the elevators.


Use a 1/16" dia. drill bit to drill a hole in the leading edge of the elevators to accept the leg of the Wire Joiner.


d. Use a sharp #11 hobby knife to cut a groove in the leading edge to accept the Wire Joiner.


Wipe the Wire Joiner clean with some type of solvent (paint thinner, mineral spirits, rubbing alcohol, etc.), and then glue it into the elevators with epoxy. Be sure to keep the leading edges of the two elevators aligned straight while the glue dries.


Use a sanding block to bevel the leading edge of the elevators to a "v" shape, using the hinge line as a guide. Refer to the cross-section drawing of the elevator on the plan.


Assemble all the fin and rudder parts over the plan. When dry, remove them from the plan and sand them to final shape. Fin - round the leading edge, leave the trailing edge square. Rudder - round the trailing edge, "v" shape the leading edge.


Temporarily tape the elevators to the back of the stabilizer. Then use a sanding block to sand the ends of both pieces together until they match perfectly. The tips can be left square or sanded round if you prefer.

The tail surfaces are now complete and can be set aside until covering. They will be hinged after covering is done.