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Control line (also called U-Control in some countries) is a simple and light way of controlling a flying model aircraft. The aircraft is connected to the operator by a pair of wires, that works the elevator of the model. This allows the model to be controlled in the pitch axis. It is constrained to fly on the surface of a hemisphere by the control wires.
Sewing thread may be used in stead of wires to control small models, when safety does not allow the use of wires, but its air resistance is greater. An unusual variation is a single wire that controls by twisting. A third wire is sometimes used to control the engine or other functions. Electrical signals sent over the wires can be used to control still more functions. It is sometimes possible to keep a model in flight after its engine stops by leading it around in a circle. Control line models are often somewhat, or even extremely, asymmetrical to help keep the lines tight, even overhead or in winds.
Competitions
Competitions for control line aircraft are held in various classes. These include speed, precision aerobatics (aka stunt), team racing, combat, naval carrier, and scale.
Speed
Speed is divided up into different engine capacity classes. As the name suggests, the idea is to have the model go as fast as possible. The model is timed over a number of laps, and the pilot must hold the handle controlling his model in a yoke on top of a pole in the middle of the circle. This is in order to stop the pilot from assisting the model to go faster by increasing the line tension and leading the model (known as whipping).Precision aerobatics
Precision aerobatics consists of flying a set of maneuvers (all based on the loop) which are judged by a panel of judges for accuracy, bottoms height, and other factors. "Stunt" models tend to be among the larger Control Line models, wings spanning around 45 inches (1.1 m) from right wingtip to left wingtip. They are often powered by a two-stroke engine in the 6-10 cc (0.40-0.60 cu) range. The total range of engine size is from (0.15-0.90 cu). The aircraft is almost always fitted with flaps, which work in conjunction with the elevator to increase the pitch maneuverability. When up-elevator is applied, the flaps on the wings go down, and vice versa. "Stunt" models are often beautifully painted, since the judging of the maneuvers is partly aesthetic, and a "pretty" model might help improve the score.Team racing
Team racing is an event for two-person teams—the pilot and the pit crew. There are various classes of team race: A and B (for different engine sizes), Goodyear (certain restrictions on the shape of the model), and so on. However, the basic idea of all the events is that a number of models (often three or four) fly together, aiming to complete a given number of laps before any of the others. The model is also required to make a pit stop during the race, where it is refuelled, the engine restarted, and the model re-launched. This is the job of the pit crew. There are rules about how the pilots must walk around each other, and how to pass (harder than one might think, as each model is on the end of a pair of control lines).Combat
Combat is an event where highly maneuverable flying-wing aircraft are flown two at a time. Each model tows a paper streamer, and the aim is to cut the opponent's streamer as many times as possible with one's propellor. Points are scored for each cut and taken away, when on the ground or as an infringement penalty . As combat models are fast and twitchy, and are chasing each other around, the mortality rate of the models is high. Deliberate collisions are not allowed, but plenty of accidental ones happen. Combat is a very exciting event, particularly for the pilots but also for spectators.Navy carrier
Navy carrier is an event where semi-scale models of real naval aircraft are flown. Takeoff and landing are from a simulated aircraft carrier deck, with arrestor wires. The aim of the flight is to complete a number of fast laps, flown as quickly as possible, followed by a number of slow laps, flown as slowly as possible. This is followed by the carrier deck landing, attempting to snag the arrestor wire. Carrier models usually have a third control line, worked by a finger trigger in the handle. This line allows the throttle setting of the engine to be controlled. Often, a carrier model will have flaps. However, unlike a "stunt" model, these flaps are worked separately from the elevator. A large amount of flap is usually applied during the "slow" laps.Scale
Scale is an event where an accurate scale model of a real aircraft is flown. Scoring is based partly on static judging, partly on how closely the aircraft resembles the full-size and how well it is finished, and partly on the flight performance. Extra points are often awarded for "working" features of the model, such as a retracting undercarriage, droppable bombs, and so on. Certain more complex scale models use a fly-by-wire approach to allow a multitude of extra working features. A radio-control transmitter can be adapted to send its signal along the elevator control lines, instead of broadcasting them. Standard servos can then be used in the model.Weapons
Various weapons such as the US Army's TOW (Tube launched, Optically tracked, Wire guided) anti-tank missile and some German WW II missiles are controlled by a sort of telegraph through wires that they trail out behind them. This has the advantage over radio control that it is not easily jammed.
See also
"Control line"