Control line

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.

The airframe

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.