Balsa USA J-3 Cub presentation and reference image

Active Giant-Scale Build · Aircraft Bay

1/4-Scale
J-3 Cub

A Balsa USA Cub being shaped around the qualities that made the type worth loving in the first place: floaty handling, grass-field usefulness, a big prop working at modest speed, long useful flights, and enough authority to fly with confidence without turning the airplane into something it is not.

Follow the Build See What Changed Current Status

The Build

A Cub that grew into a much better question.

The kit began as a return to giant-scale building and a logical step beyond the Carbon Cub. As the project matured, the question stopped being “what motor fits?” and became “what kind of Cub do I actually want to own, fly, service and understand for years?”

Learn the airplane

Build the kit, improve the work, rebuild pieces when needed, and let construction skill return through the project rather than pretending it never went away.

Construction before cosmetics

Define the mission

Long flights, big wheels, low-speed confidence and conventional Cub aerobatics matter more than headline speed or unlimited vertical performance.

Power for authority, not speed

Keep improving the questions

Old radio, motor, ESC, covering and finish “locks” remain useful history when they explain how the design evolved. They are not allowed to overrule later decisions simply because the word LOCKED appears in an old file.

Evolution is part of the story

Aircraft Mission

Power for authority.
Not speed.

The goal is a large Cub that feels light on its feet, gets off grass confidently, carries useful reserve for climb and go-around, cruises comfortably at partial throttle, and still has the control and power for hammerheads, stall turns, large loops and ordinary sport-scale aerobatics.

Floaty handling

Large / tundra wheels

Long useful flight duration

Strong takeoff and climb authority

Basic aerobatics without a 3D mission

Moderate extra weight accepted when it buys reliability or capability

Physical power-system study

The BadAss mock engine belongs to the J-3 study.

The 3D-printed BadAss BA-6230-205Kv mock engine was made to support the quarter-scale J-3's power-system planning. It was briefly placed on the larger L-4 only to judge its physical size; that comparison did not make it an L-4 propulsion choice.

Close view of the red and black BadAss BA-6230-205Kv motor mockup for the quarter-scale J-3 Cub — J-3 design-basis motor mockupThe close crop keeps attention on the printed motor study; final selection and installation remain open.

Decision Evolution

The power system did not “keep changing.” The question kept getting better.

Several technically reasonable systems were explored. Some were later superseded by mission clarity, availability, economics, telemetry needs or the desire for more useful margin. Each study helped sharpen the current direction.

Early Scorpion8S-primary / 10S-capable studies optimized around long flights and Cub-appropriate power.

Premium ScorpionLater larger Scorpion combinations remained technically strong and attractive.

12S PNP StudyA-5025 / Tribunus direction showed a high-quality matched-system path.

Economics changedImport/tariff cost changed the value equation even though Scorpion still worked technically.

Current design basisBadAss BA-6230-205Kv became the serious performance/value basis while the Castle HV160 retained useful margin and logging.

Current direction: BadAss BA-6230-205Kv with the Castle HV160 as the performance and value basis. The final motor, propeller and battery combination will follow the complete system match and testing.

Systems Architecture

Contain failures. Keep control.

Even as radio hardware changed from early Spektrum ideas through the RadioMaster/ELRS “Fort Knox RF” phase and finally into the FrSky Tandem ecosystem, the durable engineering ideas survived.

Control stays independent

Propulsion loss must not remove flight controls. Accessory failure must not brown out the flight-control system.

Current ecosystem

FrSky Tandem X20R, ETHOS and RB25S are the current foundation. Exact receiver combination remains open.

Independent surfaces

Left/right elevators, ailerons and flaps are treated independently; no flap Y-harness is part of the current architecture.

Purchased servos

Hitec D646WP ×6 for elevator/aileron/flap roles, D845WP ×1 for rudder, plus Savox SH-0255MG ×1 for tow/auxiliary service.

Telemetry

Used for awareness, diagnostics and post-flight engineering — not as primary flight authority.

FlightVision

Recording and spectator/post-flight storytelling are accessory functions. They do not get authority over the control or RF system.

Covering, Lighting and Presentation

Restrained on purpose.

The Cub should look believable at a glance and reward a closer look. The same philosophy applies to fabric, lighting and the developed USA 250th concept: thoughtful detail without turning the airplane into a visual stunt.

KoverallCurrent covering direction. Final working procedure must follow the current project record rather than older Stewart-only assumptions.

Traditional finish chemistryLater work moved toward nitrate/butyrate dope with Koverall; actual inventory and final procedure still require current-source reconciliation before application.

We the People / USA 250thHighly developed preferred visual concept — not a guaranteed final physical finish. Execution remains open if the scheme cannot be achieved to the desired standard.

UniLightCurrent lighting family, including 15 mm / 4W aluminum landing lights with lenses.

Dusk realismLighting is intended to feel restrained and believable, not like a modern night-flying light show.

Image provenanceThe hero above is presentation/reference imagery. It is not a photograph proving Jon's completed current Cub.

Before the covering closes

If it fits but cannot be understood later, it is not finished.

Hidden systems are treated as future maintenance problems unless access, labeling, strain relief, photographs and replacement paths are designed before the fabric closes the structure.

Control linkage access

Servo hatches and wing service bays

Lighting disconnects

Receiver and antenna routing

Pitot / telemetry routing

Battery restraint and cooling

FlightVision access

Photograph before covering

Current Truth

What is proven, what is the design basis, and what remains open.

The site deliberately refuses to turn recommendations, purchases or design studies into installation claims.

Purchased / currentFrSky X20R, RB25S, Castle Phoenix Edge HV160, purchased servo set, UniLight family and FlightVision program direction.

Current design basisBadAss BA-6230-205Kv; 10S/12S-capable propulsion architecture; large efficient propeller subject to testing.

Still openExact receiver combination, final motor purchase/install, final prop, battery arrangement/capacity/connectors/CG placement, Castle-to-FrSky live telemetry path, tow-release safety classification, final USA250 physical execution.

“The result is not a history of bad decisions. It is a record of progressively better questions.”

The project record preserves the older RadioMaster/ELRS, Scorpion, SeaKing, 8S/10S and covering/finish studies as real history without allowing them to outrank the current governed BUSA decisions.