The major difference between free-flight and radio-controlled scale models is that all the adjustments to the flight path of the former must be made before release. Free flight models must be capable of stable flight, whether under power or on the glide, without the need or the opportunity to carry out in-flight alterations. This is a challenge most surely met by lightly loaded, slow-flying models based on full size machines having those characteristics, best typified by early bi-planes.