Control Line - Controls

Controls

The aircraft is typically controlled by a set of 20-70-foot lines usually of multi strand stainless steel, single strands of music wire, Or G.S.U.M.P. ( Gel Spun Ultra high Molecular weight Polyethylene, made by Dupont, and called "Spectra" Tm). For competition the lines are tested before flight with a "pull test" that varies with the model weight and category to verify that the lines and control system (primarily the bellcrank and its attachment to the rest of the model) will withstand the line tension during flight. For sport flying, non-metallic lines of kevlar, dacron, Spectra (tm) or other low-stretch 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 bell-crank, 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 (and flaps, if used) 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 towards and away from the pivot of the scroll, and has a pushrod attached. The 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 obviate the need to pay royalties on the "U-Control" patent, including systems with the lines connected directly to the elevator with pulleys to change the direction, methods that connected the lines directly to the pushrod through screw eyes, but most worked very poorly compared to conventional 2-line control.