Control line

Lore & Background

Early versions of control line merely constrained the model to fly in a circle with no control, known as round-the-pole flying. The first recognizable system that manipulated control surfaces is generally attributed to Oba St. Clair in June 1936 near Gresham, Oregon. St. Clair's system used a large apparatus with many lines, called 'the Full House,' and was used only on his model Miss Shirley; no evidence exists that anyone else built a plane for it. The name most associated with the invention and promotion of control line is Nevilles E. 'Jim' Walker, who invented the patented 'U-Control' system used on virtually all two-line models today. His American Junior company was the largest producer of models and held numerous patents until they were overturned in a 1955 patent infringement suit against Leroy M Cox, based on prior art from St. Clair. A perpetual trophy named for Walker is awarded to the winner of a flyoff between US Junior, Senior, and Open age class champions in AMA-sanctioned control-line aerobatics competition.

Control-line models are built from materials such as balsa wood, plywood, paper, plastic, spruce, polystyrene foam, and occasionally modern composites. Construction varies by category: aerobatic and combat models are lightly built for maneuverability; speed models are sturdy with aluminum or magnesium pans; racing models balance lightness and strength. The lines must remain in tension, usually provided by centripetal acceleration, but during a loop the model traverses a planar path and other factors like engine offset or lead-out rake provide tension. Many models have adjustable features such as tip weight boxes, rudder offset, line sweep, and elevator/flap controls. Some aerobatics models use a variable rudder system (Rabe rudder) to vary rudder offset during flight. Profile fuselages (cut from a single sheet) are simple and common on trainers; built-up fuselages are more complex but offer better engine runs.

Reader's Guide

Control line represents a significant chapter in model aircraft history, bridging free flight and radio control. Its development by Oba St. Clair and later Jim Walker established a practical method for precise pitch control using simple mechanical linkages. The U-Control system became the dominant standard, enabling aerobatic maneuvers, combat competitions, and speed records. The system's constraints—flying on a hemisphere, maintaining line tension—shaped unique design features such as longer inboard wings, tip weights, and adjustable rudder offsets. The legacy of control line is preserved in ongoing AMA-sanctioned competitions, including the Walker Trophy flyoff. Its influence extends to specialized variants like Monoline for speed models and multi-line systems for throttle and other functions. The simplicity and low cost of control line have made it accessible for decades, while its technical demands continue to challenge builders and pilots.

Did You Know?

• Control line is also called U-Control, a trademarked system invented by Nevilles E. 'Jim' Walker.
• Oba St. Clair's 1936 system, the Full House, used a large apparatus with many lines and was only used on one model, Miss Shirley.
• During a loop, a control line model flies on the edge of a cone, not a hemisphere, and centripetal acceleration does not provide line tension.
• Approximately two-thirds of the aerodynamic drag in a control line system comes from the lines and connectors.