Control line
Control line is a simple and light way of controlling a flying model aircraft. The aircraft is typically connected to the operator by a pair of lines, attached to a handle, that work 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 lines.
The control lines are usually either stranded stainless steel cable or solid metal wires of anywhere from to. Sewing thread or braided fishing line may be used instead of wires, but air resistance is greater. A third line is sometimes used to control the engine throttle, and more lines may be added to control other functions. Electrical signals sent over the wires are sometimes used in scale models to control functions such as retracting undercarriage and flaps.
There is also a control system that uses a single solid wire, this is called Monoline. When the pilot twists the wire around its axis, a spiral inside the airplane spins to move the elevator. While it can be used with some success on any type of model, it is best for speed models where the reduced aerodynamic drag of the single line is a significant advantage. The control provided is not as precise as the two-line control system.
Almost all control-line models are powered with conventional model aircraft engines of various types. It is possible to fly control-line models that do not use on-board propulsion, in a mode called "whip-powered", where the pilot "leading" the model, whose lines are attached to a fishing or similar pole, supplying the necessary energy to keep the airplane aloft, in a fashion similar to kite-flying.
History
Early versions merely constrained the model to fly in a circle but offered no control. This is known as round-the-pole flying. The origins of control-line flight are obscure, but the first person to use a recognizable system that manipulated the control surfaces on the model is generally considered to be Oba St. Clair, in June 1936, near Gresham, Oregon. St. Clair's system used a rather large apparatus similar to a television antenna, onto which many lines were attached. This system is very different from those currently in use on modern control line models. It is of interest to note that St. Clair only produced one model, the Miss Shirley, that used this system, which he called "the Full House." To date, there is no evidence to show anyone else ever built a plane to use the Full House system.The name most associated with the inventions and promotion of control line, and the inventor of the formerly patented system known as "U-Control" was Nevilles E. " Jim" Walker. His "American Junior" company was by far the biggest producer of models and held numerous patents on the two-line system until overturned during a patent infringement suit by Walker against Leroy M Cox, based on "prior art" from St. Clair in the 1955 trial. One of the most coveted prizes in control-line aerobatics competition sanctioned by the AMA, awarded to the winner of a flyoff between the US Junior, Senior, and Open age class Champions, was originally provided by and is named for Walker. This is one of the oldest perpetual trophies in modeling that is still awarded.
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 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 required, as once at speed the model's altitude is maintained by centripetal Acceleration. Racing models need to be both relatively light for good acceleration from the start, or after a pit stop, and to reduce the pitch of the airfoil required to maintain lift. Race Aircraft also be fairly strong to withstand the pit man catching the model after landing.
To control the airplane, the lines must remain in tension. Centripetal Acceleration is generally sufficient to maintain line tension if the airplane is properly "trimmed", but sometimes additional features such as rudder offset and engine offset are added to provide extra tension. It is of interest to note, that when a control line model does a loop, it no longer flies on the edge of a hemisphere, but traverses the edge of a cone, a planar path, and the motion of the model produces no centripetal acceleration. In the condition of flying a loop, other factors must therefore provide the line tension, such as engine offset, or lead-out rake. 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 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 only an inboard wing, which eliminates the drag of the outboard wing completely. In general 2/3rds of the aerodynamic drag of the entire control line model systems is created by the lines/connectors.
In general there are two types of fuselage construction that are used in control line: "profile" 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.
Controls
The aircraft is typically controlled by a set of 20–70-foot lines usually of multi strand stainless steel, single strands of piano wire, or G.S.U.M.P.. For sport flying, non-metallic lines of kevlar, dacron, or other low-stretch fiber materials are commonly used. This type of control was originally trademarked as "U-Control" and is by far the most common control method.The controls of a conventional 2-line/"U-Control" system consist of lead-out cables, a bellcrank, push rods and control horns. These are connected so that differential motion of the lines rotates the bellcrank, causing a pushrod to move either forward or aft. The pushrod is connected to the control surface with a control horn that moves the elevator up and down. The pilot holds a handle to which the lines are attached. Tilting the handle with the fingers, wrist, and/or elbow motion causes the differential movement in the lines. By convention, tilting the hand so the top is closer to the pilot than the bottom results in "up" elevator, much like pulling back on a full-scale airplane control stick. Also by convention, most airplanes are flown nominally counter-clockwise as viewed from above, with the leadout cables exiting the left wing. This is not universal and some pilots fly in the opposite direction. Flying clockwise has a slight advantage in some situations because most engines run so that the torque will roll the airplane away from the pilot, increasing line tension in upright level flight.
The controls can be expanded by adding a third line that controls the throttle. The most common system for throttle control is that devised by J. Robert Smurthwait, of Baker Oregon, and is widely available. The throttle is usually a conventional carburetor as used on radio control models schemes that couple limited rudder and/or aileron, and variable leadout position are often found on carrier planes as well as elevator and flaps/
Monoline control works by twisting the single line. The pilot holds a handle with a twisted flat piece of metal on bearings in one hand, and a "bobbin" in the other. Moving bobbin towards or away from the handle twists the line. Inside the airplane, the rotating line rotates a spiral scroll with a follower. The follower moves toward and away from the pivot of the scroll, and has a pushrod attached. Then, as the scroll rotates, the pushrod moves fore and aft. The rest of the system is like the two-line system. The control of a monoline system is much less precise than a two-line system because the line itself tends to twist up before it moves the scroll, leading to a somewhat vague control response with considerable lag. It does however have the advantage of not requiring as much line tension to move the controls, and the single line has less drag than the two slightly smaller lines used in conventional two-line control.
Other control methods were devised early on to avoid having to pay royalties on the "U-Control" patent, including systems with the lines connected directly to the elevator with pulleys to change the pitch, methods that connected the lines directly to the pushrod through screw eyes, but most worked very poorly compared to conventional 2-line control.