Editing Grumman X-29 (section)

===Three-surface design and inherent instability===
The X-29 is described as a [[three surface aircraft]], with [[canard (aeronautics)|canards]], [[forward-swept wing]]s, and aft [[strake (aviation)|strake]] control surfaces,{{sfn|Roskam|1985|pp=85–87}} using three-surface longitudinal control.<ref name=NASA_factsheet/> The canards and wings result in reduced trim [[aerodynamic drag|drag]] and reduced wave drag, while using the strakes for trim in situations where the [[center of gravity of an aircraft|center of gravity]] is off provides less trim drag than relying on the canard to compensate.{{sfn|Roskam|1985|pp=85–87}}

The configuration, combined with a [[center of mass|center of gravity]] well aft of the [[aerodynamic center]], made the craft inherently [[relaxed stability|unstable]]. Stability was provided by the computerized flight control system making 40 corrections per second. The flight control system was made up of three redundant digital computers backed up by three redundant [[analog computer]]s; any of the three could fly it on its own, but the redundancy allowed them to check for errors. Each of the three would "vote" on their measurements, so that if any one was malfunctioning it could be detected. It was estimated that a total failure of the system was as unlikely as a mechanical failure in an airplane with a conventional arrangement.<ref name=NASA_factsheet>{{cite web |url=http://www.nasa.gov/centers/dryden/news/FactSheets/FS-008-DFRC.html |title=Fact Sheet: X-29 Advanced Technology Demonstrator Aircraft |publisher=NASA Armstrong Flight Research Center |date=28 February 2014 |access-date=24 August 2014}}</ref> If all of the flight computers failed mid-flight, the aircraft would have disintegrated due to aeroelastic forces before the pilot could keep it stable or even eject.<ref name="X-29: NASA’s ambitious 1980s fighter jet with inverted wings">{{cite news |last1=Prisco |first1=Jacopo |title=X-29: NASA's ambitious 1980s fighter jet with inverted wings |url=https://www.cnn.com/style/article/grumman-x-29-nasa-darpa-fighter-plane/index.html |access-date=15 October 2024 |work=CNN |agency=CNN Style |date=12 July 2019 |language=en}}</ref>

The high pitch instability of the airframe led to wide predictions of extreme maneuverability. This perception has held up in the years following the end of flight tests.  Air Force tests did not support this expectation.{{sfn|Butts|Hoover|1989}} For the flight control system to keep the whole system stable, the ability to initiate a maneuver easily needed to be moderated.  This was programmed into the flight control system to preserve the ability to stop the pitching rotation and keep the aircraft from departing out of control.  As a result, the whole system as flown (with the flight control system in the loop as well) could not be characterized as having any special increased agility.  It was concluded that the X-29 could have had increased agility if it had faster control surface actuators and/or larger control surfaces.{{sfn|Butts|Hoover|1989}}