Larry Kruse's Super Cub

01. Even in light wind, this model is a stable platform. It was popular with pilots in the author’s club.

The creation of this little aircraft was inspired by the work of a magazine columnist to resurrect a Top Flite Jigtime Free Flight (FF) model, and by my CO2-powered FF design published in 1987. I thought it would be interesting to meld a Jigtime-type model with my original design—and so the Super Cub was born!

To speed up this construction project, gather the necessary materials. Start by acquiring 2mm foam. I got mine in the three-sheet 11.5 × 11.5 × 2mm size, which is plenty for this model. It also comes in larger sheet sizes, so why not have enough on hand to do your own experimentation? The power system and controls that I used are also off-the-shelf units.

Other components that you need to gather include a 24-inch length of .025-inch diameter music wire for the pushrods, at least a 15-inch piece of .030-inch music wire for the landing gear, and some lightweight, 1-inch plastic wheels, such as those available from Guillow’s. Scrap 1/16-inch balsa for the landing gear lamination, Du-Bro micro control horns, and four bamboo cooking skewers (more about them later) will complete your materials list.

The model is held together primarily with Bob Smith Industries (bSi) Super-Gold foam-safe CA glue. I used hot glue to mount the ESC/servo brick on its mounting rails and the motor and gearbox into their mounting slots in the firewall. I recommend using hot glue for both of these purposes before any other adhesive.

(Editor’s note: The author used a Flyzone brick [receiver/ESC/servo combo PC board] for his Super Cub, which has since been discontinued. A UMX brick from Horizon Hobby would be a good substitute.)

The wheels are held in place with a blob of Duco Cement, which is widely available, as is Pacer Formula 560 canopy glue, which will prove useful in the later stages of construction.

02. When the author built the Super Cub, its powertrain was composed of Flyzone products that have since been discontinued. Pictured are the motor, gearbox, receiver/servo brick, battery, and propeller that he used. Similar units/components would also work.

03. All of the pieces are cut from 2mm foam unless otherwise noted. Check the plans to determine which fuselage formers need to be laminated. The windshield piece is a pattern for the actual celluloid windshield.

At a Glance

Specifications

Type: Semiscale sport model

Wingspan: 20 inches

Length: 13.25 inches

Weight: 1.23 ounces (without battery)

Motor: 7mm brushed

Gearbox: Flyzone 6:25:1 ratio 1.0mm

Propeller: 117 × 64mm two-blade

Battery: 1S 3.7-volt, 140 mAh LiPo

Radio receiver: 2.4 GHz four-channel receiver/ESC/servo combo board or suitable replacement

Radio transmitter: Six-channel

Flight duration: 5 to 6 minutes

Examining the plans reveals that there aren’t many parts, and they can all be cut out before assembly. The easiest method to cut out the parts is to make a copy of the plans and reinforce them with lightweight poster board. Cutting out each part provides a template that can be used to trace the parts onto the 2mm foam using a sharp, soft-lead pencil. A disposable scalpel or sharp hobby knife with a #11 blade works nicely to cut out the parts.

Some lamination is required, particularly in the front of the fuselage and in the motor/nose block area. Glue those parts ahead of time. The nose block’s multiple layers can wait until the motor and gearbox are mounted. Some adjustments and sanding are required and are easiest to do when the motor/firewall assembly is in place.

Lightly mark the former locations on the inside of the fuselage sides. Build the traditional fuselage box by gluing laminated formers F-3 and F-4 in place, followed by F-5 and F-6. Pull the tail section together. Ensure that the assembly is square and has no twist in it before gluing the sides together at the tail.

The laminated scrap mounting rails for the control brick are next. Using hot glue generously applied with a toothpick, glue the corners of the unit in place (servo arms facing the bottom of the fuselage), and then glue former F-2, tilted forward as shown on the plans.

Mount the motor and gearbox in the precut slotted hole in the firewall, pointed down approximately 5° and 3° to the right of the centerline. Allow the propeller shaft to protrude enough to clear the laminated nose block. Toothpick-applied hot glue is the best adhesive for the task. Hold the motor/gearbox unit in place until it sets up. Glue the motor mount into position with CA adhesive in the front of the fuselage (inside the fuselage sides), with an additional bead of CA applied on the backside.

Carefully cut slots in the fuselage sides just aft of former F-5 to allow the elevator and rudder pushrods to exit. A U-bend at the rear of each pushrod allows length adjustments. Thread the pushrods through the fuselage from the rear and attach them to the servo arms by first removing the arms, threading the pushrods into their respective outside mounting holes, then screwing them back into place. Keep the servo output shafts centered.

Moving back to the nose area, install the top sheeting between the firewall and F-2. You need to make the sheeting curve as needed. This can best be accomplished by rolling the sheeting over some type of mandrel. I used a 3/4-inch diameter aluminum wing tube. Experiment a little with scrap pieces and you’ll soon get the hang of it.

The foam must be rolled according to its grain. (Yes, foam does have a grain!) The grain usually runs lengthwise on a stock rectangular sheet. It has one shiny side and one dull side. Place the shiny side face down as you roll the foam across the mandrel. If you make a mistake and keep it shiny side up, wrinkles will form lengthwise and you’ll have to start over! Again, experiment with scrap pieces.

04. The control horns are slipped onto the pushrods and mounted on the elevator and rudder using foam-safe CA adhesive. The elevator requires a horizontal Z-bend and the rudder needs one that is vertical. Place both pushrods in the outside holes in the control horns.

When the nose’s top sheeting is in place, laminate several layers of the nose block. If the propeller is on the motor shaft, it must be carefully removed so that the nose block can slide over the propeller shaft. I cut the three rear laminations with large holes in their centers and the front two laminations with only the engine crankshaft hole and the side air intake openings.

After it is laid up, glued with CA, and cured, glue the block to the front of the firewall and sand to the contours shown on the plans and in the photos. At that point, put the propeller back on the propeller shaft while you can still reach inside of the fuselage from the bottom to provide leverage for the required finger pressure.

Bend the landing gear wire to shape, make up and install the balsa "sandwich" landing gear mount as shown, and then get ready to install the wing halves.

If you cut the wing blank out as one piece, separate the two panels from the center section. All three pieces require undercamber, so simply place them individually on the previously mentioned mandrel and roll each one spanwise until a definite curve is evident.

Remove all rings from your fingers to avoid denting the foam as you roll it over the mandrel. When the undercamber looks acceptable, glue one wing rib to the center joint of each panel and the two shorter center ribs to the outside edges of the center section, as shown on the plans.

To prepare the wing panels to be joined to the center section, prop up each panel 1 inch at the tip and sand the panel rib as you would a hand-launched glider wing blank. Be gentle because you’re only working with a 2mm rib thickness. When satisfied with the angles you sanded into both panels, glue them to the center section, again propping each up 1 inch at the tip until the glue dries.

When the panels are dry, glue the 1/8-inch leading edge (LE) doublers to each panel and install the outboard notched camber ribs. Carefully sand the LEs to an acceptable rounded shape and taper them to nothing where the wingtip curve begins. Add the outboard notched camber ribs. Ensure that no twist is built into either wing panel when the ribs are glued in place. Gently round all of the remaining edges.

Glue the wing to the top of the fuselage. Position it carefully and squarely before applying any glue. Add the top fuselage deck. It will look nicer if you taper the bottom of the sheet so that it fairs in as much as possible into the top of the wing, as shown on the plans.

Both tail surfaces can be cut out as single units. The stabilizer requires a round toothpick as an elevator joiner that can be fitted into a slot cut just aft of the line separating the stabilizer and the elevator. Make sure that only the back portion of the slot receives the glue. When the glue is dry, carefully cut the elevator free using a straightedge to keep the dividing line straight.

Sand the rear of the stabilizer to a 45° chisel point on the bottom so that the unit can be hinged on the top with no joint showing. I used 1/2-inch wide Blenderm tape for the hinge and applied it only to the top of the stabilizer/elevator joint. The fin and rudder are treated in the same fashion, with a 45° flex angle sanded into the joint line on the rudder and Blenderm reattaching the two pieces.

Neutralize the rudder and elevator servos, and then put a Z-bend in both pushrods precisely parallel with the end of the fuselage. You need a horizontal bend for the stabilizer horn and a vertical bend for the rudder horn. Clip off any excess and ensure that the pushrods can be slipped into the pushrod holes. If that is a problem, file down the ends of the pushrods with a fine file or slightly enlarge the control horn holes.

Place the stabilizer unit on its mounting notch and lightly sand either side of the mount to ensure that the stabilizer is parallel and square with the wing when it’s viewed from the rear and the top. If it is, glue it in place then adhere the pushrod with the control horn attached in position. Do the same with the rudder and make sure that the elevator and rudder are neutral when the radio is turned on. Make any necessary adjustments using the U-shaped adjustment kinks of the respective pushrods.

If everything looks good, close the bottom of the fuselage—all except for the tail wheel laminate, which should be added only after it is determined that the center of gravity (CG) is in the correct position and no tail weight is needed. That should not be a problem because the prototype required 3 grams of weight in the nose. The bottom of the fuselage between the motor mount and the landing gear is also laminate and, like the tail wheel laminate, it should be left open until the proper CG is determined.

The struts aren’t necessary, but a Cub without struts doesn’t look right. Because I decided to include them, I made them functional.

The struts are constructed from bamboo skewers available at most grocery stores. I planed four of them down on both sides to a 1/32-inch thickness using my David razor plane, sanded them smooth, and glued them together over the plans. They are overly long and should be trimmed to a length that fits in the fuselage and the camber rib slots that doesn’t twist or change the dihedral of the wing panels. White Testors Spray Enamel makes the struts blend in nicely.

The lightweight plastic wheels, landing gear fairings, and windshield complete the "dress up" part of the assembly. I cut an oversize piece of light celluloid as a windshield blank, sprayed it with Testors gloss black enamel, and mounted the painted side to the inside with Pacer Formula 560 canopy glue. That gave the windshield a nice, glossy finish that matched the side windows cut from black MonoKote trim sheet. The landing gear fairings "float" above the landing gear wire and are attached to the fuselage using Pacer Formula 560 canopy glue.

Callie Graphics created the numbers and letters. The red fuselage stripe and the Piper Aircraft rudder markings were cut from red MonoKote. I used standard sticky-back notebook reinforcement circles for the white wheel centers. It beats masking and trimming something that small! The wheels are secured with drops of Duco Cement.

05. This threequarter front view creates a nice scalelike illusion for the semiscale airplane. The markings and lettering are from Callie Graphics. A steel-edge ruler and a ballpoint pen were used to make the aileron control surfaces.

Balance the model on the CG that is noted on the plans by shifting the battery as needed on its hook-and-loop strip on the bottom of the fuselage. If this doesn’t balance it, add weight either fore or aft. The prototype required 3 grams of weight in the nose.

The best way to hand launch the Super Cub is to apply approximately half power and toss it forward and level. It should fly straight and allow enough time for you to find both radio sticks to control it.

The first flight on the little Cub required two clicks of down-elevator and two clicks of right rudder to get it to a hands-off flight mode. From that point, rise-off-ground takeoffs—either indoors or outdoors—required merely bringing it up to speed until it lifted off on its own.

Because of its lightweight wing loading of 35 grams without the battery, it is a stable model to fly. It’s not intended for wind stronger than 6 to 7 mph and struggles to fly into the wind in such breezes because of the strut and landing gear drag.

Indoor landings are uneventful if you incrementally reduce the throttle and flare the airplane just above the floor.

When outdoors, point the aircraft into the breeze and keep a little power on all the way to the ground, using the elevator to level it as it approaches. The music wire landing gear protects everything if a landing is less than optimal.

The little Super Cub has fulfilled its goals of mimicking the old Jigtime kits with its ease of construction and finish. The Super Cub proved to be as satisfying of a flyer as its original sheet balsa ancestor. I hope you enjoy it!

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