FF was the ancestor of all model-flying sports, and all sorts of models were successfully flown for years before radio control systems were perfected, but today only a tiny minority of the population even knows that FF exists.

Basically, it's the building and flying of model airplanes with no external guidance of any kind, control-line or radio control.

For such a tiny group, free-flighters produce, fly, and compete with a bewildering variety of models. They include:

Gliders, both hand-launched and tow-line. These manage to ride thermal updrafts up and out-of-sight with some regularity.

Rubber-powered. People see these go up hundreds of feet in the air, and stay up for many minutes, and are astonished to learn they're powered by a rubber-band.

Motorized. These can be powered by miniature internal-combustion engines, electric motors, or even compressed air motors. Competion classes are typically limited to a motor-run of a few seconds (as few as 5 or 7), during which time they must gain as much altitude as possible by spiralling up almost vertically. A mechanical timer will then shut the engine off, at which point the model enters a glide. Whenever possible, a flier will attempt to augment the duration by launching into a thermal, which the model will exploit by gliding in a circle. This can work too well, resulting in the loss of the model as it flies away. These planes are therefore always equipped with a dethermalizer device, which, activated by timer after a few minutes of flight, changes the balance point or deflects a flying surface in such a way as to spoil the aerodynamics and cause the plane to descend safely.

Indoor duration. These can be gliders or rubber-powered, much less frequently motorized, and are flown in gymnasia, hangars, arenas, or even deep underground in salt-mines. The rubber-powered ones include amazing ultra-lightweight gossamer creations, built of whisker-thin balsa wood, covered with microfilm, and braced with hair thin wire. Weighing around a gram with wingspans of up to 30 inches, their dragonfly-wing propellers revolve at about 1 revolution per second, and the models float through the air at walking speed. They tend to be so fragile that the turbulence of a person jogging by can crumple them, but a long, skinny rubber loop wound 2-3 thousand turns will keep them aloft many, many minutes. The record is now over 1-hour aloft!

Free-flight scale. Unlike the prior categories, an FF scale model is patterned after a full-scale prototype. They can be rubber powered or motorized, the best ones can stay aloft over 2 minutes outdoors (longer if thermals are encounterd), and competition typically involves static judging of fidelity to scale and workmanship in addition to duration aloft.

Spectators seeing FF models fly for the first time are typically mystified that they can fly with no guidance. This should come as no surprise to the pilots of full scale airplanes, however, as every student pilot is taught early-on that an airplane, if properly trimmed, will quite cheerfully fly hands-off. And this is the secret of FF: The careful balance, adjustment, and trimming of the model so that flight forces in opposition combine to give the plane stability.

Oh, and, to be successful, you also need to obsess to a degree approaching mental illness about weight reduction.

With the proliferation of cheap radio-control systems and pre-fabricated 'ready-to-fly' model airplanes, FF has become more and more obscure. FF is more challenging than other aspects of the model-aviation hobby; it is simply difficult to get one of these things to fly well. New participants therefore have to be motivated to persist until they acquire the skills to succeed. Very much the antithesis of instant gratification. Kits tend not to be readily available, (at least, not good ones) so you therefore have to order plans and materials from cottage industries. The practitioners also tend to be older, many having gotten their start in the years surrounding WWII. I note with sadness that I'm often one of the 2 or 3 youngest entrants at competitions (I'm 45), and the numbers keep getting smaller...

On the bright-side, however, FF models do tend to be inexpensive, at least in terms of materials. As a free-flight scale modeller, I typically have less than 10 US dollars worth of balsa wood and tissue paper in my models, and I also use a lot of found materials such as cellophane, styrofoam, soda straws, and so forth. On the other hand, I have upwards of 100 hours of labor in some of my contest jobs!

Finally, FF has had a disproportianately large influence on the technology of aviation. Just about all of the distinguished designers, engineers, and pilots in aviation history prior to, say, 1970 cut their teeth, and got their first inspiration, from free-flight model airplanes.