As part of the USA’s experimental flight program, the X-29 first took to the air from Edwards Air Force Base on 14 December 1984. A total of two aircraft, based on Northrop F-5A Freedom Fighter airframes, were produced and they flew in various tests up till 1991. The Grumman X-29’s key characteristics were forward-swept wings (at more than 33 degree angle), canard control surfaces and composite materials. The design was inherently unstable to increase its agility, so it utilized computerized fly-by-wire controls that could make 40 corrections per second while flying at a maximum speed of Mach 1.8. The carbon-fiber composite materials were necessary to manage the twisting that results from forward-swept wings. This aeroelastic divergent twisting results from aerodynamic forces that rotate the wings’ leading edges upwards, and which could easily result in structural failure. Interestingly, this was only the world’s second jet-powered aircraft to have forward-swept wings, the first being the Junkers Ju 287 of 1944 vintage!