Editing Grumman X-29 (section)

==Design and development==
Two X-29As were built by [[Grumman]] after the proposal had been chosen over a competing one involving a [[General Dynamics F-16 Fighting Falcon]]. The X-29 design made use of the forward fuselage and nose landing gear from two existing [[Northrop F-5|F-5A Freedom Fighter]] airframes (63-8372 became 82-0003 and 65-10573 became 82-0049).<ref name=AIS_X-Planes>{{cite web |url=http://www.ais.org/~schnars/aero/x-planes.htm |archive-url=https://web.archive.org/web/20010506050622/http://www.ais.org/~schnars/aero/x-planes.htm |url-status=dead |archive-date=6 May 2001 |title=The X-Planes: From X-1 to X-34 |website=AIS.org |editor-first=Andreas |editor-last=Gehrs-Pahl |year=1995 |access-date=1 September 2009}}</ref> The control surface actuators and main landing gear were from the F-16. The technological advancement that made the X-29 a plausible design was the use of [[carbon-fiber]] composites. The wings of the X-29, made partially of [[graphite epoxy]], were swept forward at more than 33 degrees; forward-swept wings were first trialed 40 years earlier on the experimental [[Junkers Ju 287]] and [[OKB-1 EF 131]]. The Grumman internal designation for the X-29 was "Grumman Model 712" or "G-712".{{sfn|Donald|1997|p=483}}

[[File:Grumman X-29 Cockpit.jpg|thumb|right|X-29 [[cockpit]]|alt=Aircraft cockpit with numerous old circular dials and gauges. In front of the controls is a black stick control column.]]

===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}}

===Aeroelastic considerations===
[[File:X-29 at High Angle of Attack with Smoke Generators.jpg|thumb|left|X-29 with aft control surfaces deflected]]

In a forward swept wing configuration, the aerodynamic lift produces a twisting force which rotates the wing leading edge upward. This results in a higher angle of attack, which increases lift, twisting the wing further. This [[Aeroelasticity#Divergence|aeroelastic divergence]] can quickly lead to structural failure. With conventional metallic construction, a torsionally very stiff wing would be required to resist twisting; stiffening the wing adds weight, which may make the design unfeasible.{{sfn|Pamadi|2004}}

The X-29 design made use of the [[isotropic|anisotropic]] elastic coupling between bending and twisting of the carbon fiber composite material to address this aeroelastic effect. Rather than using a very stiff wing, which would carry a weight penalty even with the relatively light-weight composite, the X-29 used a laminate which produced coupling between bending and torsion. As lift increases, bending loads force the wing tips to bend upward. Torsion loads attempt to twist the wing to higher angles of attack, but the coupling resists the loads, twisting the leading edge downward reducing wing angle of attack and lift. With lift reduced, the loads are reduced and divergence is avoided.{{sfn|Pamadi|2004}}