Control Line Explained  #1

The following is a brief trip through the word of Control Line. I don't expect to convert any R/C people to flying Control Line, but it may dispel some on the mystery and misunderstanding that seems to cloud this branch of aeromodelling in these days when the only form of flying most people know is R/C

Part 1: Control Systems.

In the early 1940's in the USA when someone stuck lines on a model and attempted to control it's circular flight Control Line was born, it has always been know as U control in the USA. The history of Control Line is a fascinating subject in it's own right, Americans Victor Stanzel and Jim Walker feature large in it's development and introduction. It's worth noting that tethered flight was not new even then, as Indoor Round The Pole flying (RTP) with rubber powered models was popular in the UK before that time.

(A) The conventional bellcrank: elevator control only.

Control Line control systems evolved early on from various ideas into one of the most simple and elegant concepts, the humble bellcrank. It's action as illustrated below. it is so good for its purpose that there has been little need to use anything else.

Did you know that the term 'Bell Crank', originally came from the levers used in bell pulls and for changing the direction of bell wires.

bell crank

It even provides an automatic levelling action. In level flight if the arm is raised or lowered, the model will follow suite until it is level with the arm again. See below. Although this illustration is to show how to approach a first flight, the principal is the same, If the handle is held in a fixed position and the model rises, the bellcrank will start to feed in down elevator. The reverse applies if the model starts to loose altitude. The intensity of this effect depends on a number of factors, and for the beginner, may feel like it does not happen at all, but be assured, it does exist.


Another type of bellcrank commonly found on Control Line Team race models is the Circular Bellcrank and is more difficult to fabricate and fit for common usage.

circular bellcrank

(B) Extra controls

There are also a few variations on the theme. The most notable being the addition of a third line, an arrangement probably epitomised in the commercial J. Roberts Flight Control System, now sold under the Brodak brand name. This system allows the control of something else, in addition to the elevator, it's usually applied to throttle control.

The only downside to the three line system is the drag of the extra line. However, as the load is shared across three lines the cross section of the individual lines can be reduced to compensate.

roberts crank roberts handle

There are many variations on the mechanical three line bellcrank system. Probably the most well known are the original Mick Reeves bellcranks and handles, circa mid 1970's. Not commercially available, but not too difficult to make.

reeves bellcranks

reeves handes

These days there is an increasing use of electronic signalling down the lines to control servos as used in Radio Control models to achieve the same ends, ironically this  introduces a complexity that the three line system is pleasantly lacking. Each has it advantages and disadvantages

The electronic setup is probably epitomised by the Rolls Royce of electronic systems, the ScaleMaster handle, illustrated below. Mainly used for scale models where a large number of separate functions may be needed. Scalemaster went out of business circa 2006, but alternative's exist like the U/Tronics units made by Clancy Arnold in the USA. More info available here


Many people build their own systems using standard electronic and RC parts. At it's simplest a complete RC setup with the receiver and servos and batteries in the model and the pilot holding a transmitter have been used. Range is not a problem as the model will never be further away than the length of the lines, so any cheap kit could be utilized; but frequency and channel clashes with existing RC models could be a problem. You have been warned! The most common method is to send the encoded control signals down the lines and do away with the Radio Frequency signal altogether, which avoids the problems mentioned before.

One question that is bound to be asked is, "Why not just fly a Radio Controlled model?" This question misses the point. Even with all this electronic equipment the pilot is still physically connected to the model, and controls the elevator in the normal CL manner with all the tactile feedback that this gives, which is why people fly Control Line in the first place. The extra controls are an addition, not a replacement.

(C) Variations on a theme

Another system only uses one line, hence the name Monoline. The control is produced by twisting the wire, which in turn rotates a worm screw in the model, which in it's turn operates a lever connected to the elevator. There is one other feature which differentiates this system from the others, because the action is transmitted by twisting the wire, it is not dependant on line tension to maintain control. There does appear to be one drawback, there is no automatic compensation for attitude that is present in a conventional bellcrank set up which can make it counter intuitive to master for anyone used to a conventional bellcrank. It is however, ideally suited for speed models because of the reduction in drag of the normal extra line.

Examples of Handles and Actuators

hdl 2

mono crank

std handle

And finally, we can't leave the subject without mentioning Group Captain A C Dugeon's system of a rotating pylon on a concrete base that held the handle and allowed lines to be run to the outside of the circle where they where connected to a joystick, thus allowing the pilot to sit outside the circle and control the model in a civilised manner. Unfortunately no pictures are available. And also Spanish Dr. Jorge Prats Trian, who flew this model from behind whilst being driven along in a motor car.

car fly

  right Part 2