The essence of free-flight is that the aircraft have no need for any form of external control, for instance by radio. Aircraft of this type have been flown for over two centuries. They are designed to be inherently stable in flight; if disturbed by a gust of wind or a thermal up-current they will return automatically to stable flight. For this reason most free flight aircraft are not replicas of man-carrying ones, for they are designed for a quite different purpose. Their stability is achieved by a combination of design and trim, - the relationship between centre of gravity, wing and tailplane incidence and rudder setting.
Because they are much lighter with respect to their wing area, free-flight aircraft fly much slower than the engine-powered radio-controlled models that many people first think of when ‘model aircraft’ is mentioned. .
Usually the sole object in free-flight is flight duration, and one of the sport’s fascinations and challenges is to design the most efficient aircraft within the various competition limits on such parameters as minimum weight, maximum wing area and motive power. It is solely a stopwatch, rather than any judge’s opinion, that decides the result.
When flown competitively, the usual aim is maximum flight duration. In the case of models flown outdoors, the modeler attempts to launch the model into rising column of air, a thermal. These outdoor free flight models tend to be designed for two very different flying modes: climbing rapidly under power or tow, and gliding slowly while circling with minimum fall rate. Much of the challenge in designing and flying these models is to maintain aerodynamic stability in both modes and to make a smooth transition between them. Modern models use mechanical or electronical timers to move control surfaces at preset times. Detecting the thermal into which to launch is vital and can involve several methods, ranging from radio telemetered temperature and windspeed measurements plotted on a chart recorder to Mylar streamers or soap bubbles to visualize the rising air.
