Variations on this theme providing control by one or three lines also exist, but are nowhere near as common as the two-line form described here:
Diagram adapted from one in the Aeromodeller Data Book published by Model & Allied Publications.
Aerobatic models are normally fitted with large wing flaps which move in the opposite direction from elevators. There are a few ways of achieving this, some providing a differential action that applies more flap to the wing nearest the flying circle centre, but a common arrangement is shown in the following diagram:
Diagram from the Control Line Manual by R.G. Moulton, published by Model Aeronautical Press Ltd.
The control lines themselves are usually made of a twisted cable about 0.3mm diameter consisting of three or more fine strands of wire soldered together,
stainless steel cable, as is commonly used for "toothy critter" fishing line,
high quality solid steel wire commonly known as piano wire.
Cable has a several advantages over piano wire: it is less prone to kinking (hence the trade name Laystrate); its soldered construction protects its steel strands from rusting; and it does not have the tendency for lines to cling together magnetically, occasionally a source of sticky controls when using piano wire.
Very small models have also been flown successfully on cotton button thread or carpet thread, but modern threads containing lots of polyester are more flexible and result in rather vague control.
The factors that led to the popularity of control line for many years are:
- The interest and wonder which many people once felt for aviation contributed to the popularity of all forms of aeromodelling.
- Ability to operate control line in relatively small spaces makes flying possible in towns and cities where wide open spaces are scarce.
- Opportunity to develop piloting skills capable of amazing aerobatics adds a dimension not possible with free flight models.
- Simple mechanical controls are inexpensive and can be made by almost anybody.
- Simple mechanical controls were more reliable than early attempts at radio control (jokingly referred to as radio semi-control by control line enthusiasts).
- Ability to fly several models together in one circle makes some interesting competitions possible.
- Commercial availability at affordable prices of suitable engines, kits, materials, and accessories plus good coverage in several monthly aeromodelling magazines contributed to making control line for several years the most successful form of aeromodelling in many areas.