Control Line - The Airframe

The Airframe

Control-line models are built of the same basic materials and construction methods as R/C and free flight models. Control-line model construction varies with the category of model. Aerobatics and combat models are relatively lightly built compare to R/C models as they need high maneuverability in the limited space offered by the control line "hemisphere". They are typically built with traditional materials like balsa wood, plywood, paper, plastic, spruce, and polystyrene foam, but modern composite and graphite/epoxy are occasionally used in high-load applications. Combat models must also be relatively easy and quick to build, as mid-air collisions and crashes are very common. Aerobatic model construction is typically quite complex and may require many hundreds of hours. Speed models must be very sturdy to withstand the forces of line tension and to permit a very rigid engine mount for maximum engine performance. Speed models are generally built around an aluminum or magnesium "pan" that forms about half the fuselage. Little or no maneuverability or even lift is required, as once at speed the model's altitude is maintained by centrifugal force. Racing models need to be both relatively light for good acceleration from the start, or pit stop, but must also be fairly strong to withstand the pit man catching the model at high speed. To control the airplane, the lines must remain in tension. Centrifugal force is generally sufficient to maintain line tension if the airplane is properly "trimmed" (adjusted), but sometimes additional features such as rudder offset and engine offset are added to provide extra tension. Weight in the outboard wing tip is usually used to balance the weight of the lines. Top aerobatics models typically have a large number of adjustable features like tip weight boxes, adjustable rudder offset, adjustable line sweep, and adjustable elevator and flap controls. Some aerobatics models use a variable rudder system (commonly called the Rabe rudder after its inventor, Al Rabe) to vary the rudder offset during flight. The adjustment of the various adjustable features on a modern stunt model can become quite complex. Many models also feature a longer inboard wing; aerobatics models use this to balance the lift from side-to-side, compensating for the difference in velocity from inboard to outboard wing, while some speed models use a very exaggerated longer inboard wing to provide streamlining of the flying lines. On small speed models the drag of the flying lines can be three times the drag of the rest of the model. In general there are two types of fuselage construction that are used in control line - "profile" (flat) and "built up". These are built with differing types of wings depending on the specific use of the aircraft. Profile models, where the fuselage is cut out from a single relatively thin sheet of wood with the "profile" of the airplane, are simple to build and repair, and are very common on trainer models. Sometimes the vibration of the engine causes poor engine runs on profile models. Built-up fuselages are much more difficult to build but generally look better and offer superior engine runs.