The Grumman X-29 was an American experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies.  The X-29 was developed by Grumman, and the two built were flown by NASA and the United States Air Force. The aerodynamic instability of the X-29's airframe required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, and to reduce weight. The aircraft first flew in 1984, and two X-29s were flight tested through 1991.

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 F-5A Freedom Fighter airframes (63-8372 became 82-0003 and 65-10573 became 82-0049). 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".

Three-surface design and inherent instability
The X-29 is described as a three surface aircraft, with canards, forward-swept wings, and aft strake control surfaces, using three-surface longitudinal control. The canards and wings result in reduced trim drag and reduced wave drag, while using the strakes for trim in situations where the center of gravity is off provides less trim drag than relying on the canard to compensate.

The configuration, combined with a center of gravity well aft of the aerodynamic center, made the craft inherently 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 computers; 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.

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. 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.

Aeroelastic considerations

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 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.

The X-29 design made use of the 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.

Operational history
The first X-29 took its maiden flight on 14 December 1984 from Edwards AFB piloted by Grumman's Chief Test Pilot Chuck Sewell. The X-29 was the third forward-swept wing jet-powered aircraft design to fly; the other two were the German Junkers Ju 287 (1944) and the HFB-320 Hansa Jet (1964).  On 13 December 1985, an X-29 became the first forward-swept wing aircraft to fly at supersonic speed in level flight.

The X-29 began a NASA test program four months after its first flight. The X-29 proved reliable, and by August 1986 was flying research missions of over three hours involving multiple flights. The first X-29 was not equipped with a spin recovery parachute, as flight tests were planned to avoid maneuvers that could result in departure from controlled flight, such as a spin. The second X-29 was given such a parachute and was involved in high angle-of-attack testing.  X-29 number two was maneuverable up to an angle of attack of about 25 degrees with a maximum angle of 67° reached in a momentary pitch-up maneuver.

The two X-29 aircraft flew a total of 242 times from 1984 to 1991.  The NASA Dryden Flight Research Center reported that the X-29 demonstrated a number of new technologies and techniques, and new uses of existing technologies, including the use of "aeroelastic tailoring to control structural divergence", aircraft control and handling during extreme instability, three-surface longitudinal control, a "double-hinged trailing-edge flaperon at supersonic speeds", effective high angle of attack control, vortex control, and demonstration of military utility.

Aircraft on display
The first X-29, 82-003, is now on display in the Research and Development Gallery at the National Museum of the United States Air Force on Wright-Patterson Air Force Base near Dayton, Ohio. The other craft is on display at the Armstrong Flight Research Center on Edwards Air Force Base. A full-scale model was on display from 1989 to 2011 at the National Air and Space Museum's National Mall building in Washington, DC. The full-scale replica was moved to the Cradle of Aviation Museum in Garden City, New York in 2011.

Specifications (X-29)

Notable appearances in media 
The 1989 flight simulator game F29 Retaliator was based around the X-29 and imagined a future where it had been developed into a production fighter jet and fitted with various advanced weaponry.

See also

References

Notes

Bibliography

External links

 X-29 fact sheet at NASA.gov
 X-29 fact sheet by the National Museum of the United States Air Force
 X-29 photos, and X-29 videos at NASA.gov
 "X-29: Aircraft with Forward Swept Wings", part 1, part 2 at Military.com

X-029
X-29, Grumman
Single-engined jet aircraft
Forward-swept-wing aircraft
Canard aircraft
NASA aircraft
DARPA
Edwards Air Force Base
Relaxed-stability aircraft
Aircraft first flown in 1984
그러먼 X-29(Grumman X-29)는 전진익, 카나드 조종익면, 그 외 혁신적인 항공기 기술을 시험하기 위한 실험기였다. 공기역학적으로 불안정한 이 방식은 기동성을 증가시키지만 컴퓨터화된 유선비행 (플라이 바이 와이어) 제어를 필요로 한다. 무게를 줄임과 동시에 항공탄성학적 aeroelastic 발산 divergence때문에 전진익에서 일어나는 비틀림을 제어하기 위해 복합재가 쓰였다. X-29는 1984년 처음 비행했고 그 후 10년간 X-29 2대가 비행 시험을 했다.

설계와 개발 

제너럴 다이내믹스 F-16 파이팅 팰컨이 관련된 경쟁안을 물리친 후, X-29A 2대가 그러먼 우주항공사에 의해 기존의 노스롭 F-5 프리덤 파이터 골격을 기본으로 만들어졌다. (63-8372가 82-0003이 되고 65-10573이 82-0049가 되었다.) X-29 설계는 F-5A의 기수와 앞 착륙 장치, F-16의 조종익면과 주 착륙 장치를 이용해 만들어졌다. X-29 설계를 현실성 있게 만든 것은 탄소 섬유 복합재라는 기술적 진보였다. X-29의 주익은, 부분적으로 흑연 에폭시 수지로 만들어졌고, 33도 이상 앞으로 꺾여 있다. 그러먼 내부의 X-29 지칭은 "그러먼 712형" 약칭 "G-712"였다.

X-29A는 비행 시험에서 고기동성과 조종성을 보여주었다. 최대 받음각은 67° 에 달했다. 기체 형상과 항공역학적 중심에서 많이 벗어난 무게 중심 때문에, 이 기체는 태생적으로 불안정했다. 안정성은 초당 40회씩 보정되는 컴퓨터화된 비행 제어 체계에 의해 확보된다. 비행 제어 체계는 3중으로 예비되어 있는 디지털 컴퓨터에 다시 3중으로 예비된 아날로그 컴퓨터로 뒷받침되어 있다. 셋 중 하나만 있어도 비행할 수 있지만, 여유분 덕분에 오류를 확인할 수 있다. 셋은 각각 자신의 측정치에 "투표"를 하고, 어느 것이 오작동하면 그것을 발견할 수 있다. 이 체계의 총합적 실패는 통상적인 비행기의 기계적인 파손처럼 있을 법 하지 않을 것으로 여겨졌다.

항공탄성학적 고려 
전진익 형상에서, 항공역학적 양력은 날개 끝단을 위로 돌리려는 비틀림 힘이 생긴다. 이것은 높은 받음각을 만들어내고, 그로 인해 양력이 증가되고, 날개를 더 비틀게 된다. 이 항공탄성학적 분산은 곧바로 구조적인 파손으로 이어질 수 있다. 통상의 금속성 구조로는, 날개가 비틀림에 저항할 수 있을만큼 아주 뻣뻣해야 했다. 날개를 뻣뻣하게 만드는 것은 무게를 증가시키고, 설계 자체가 불가능할 수도 있었다.

X-29설계는 이 항공탄성학적 효과를 극복하기 위해 탄소 섬유 복합재의 휘어짐과 비틀림 사이의 비등방성 탄성 상쇄를 이용했다. 심지어 상대적으로 가벼운 복합재라도 무게에서 불리함을 짊어질 수 밖에 없는 아주 딱딱한 날개를 사용하는 대신, X-29는 휘어짐과 꼬임의 상쇄를 만들어내는 적층판을 이용했다. 양력이 증가하면, 휨 하중이 날개 끝을 위로 휘게 한다. 비틀림 하중은 날개를 높은 받음각으로 비틀려고 하지만, 상쇄에 의해 그 하중에 저항하게 되고, 끝단을 위로 비틀어 날개의 받음각과 양력을 감소시킨다. 양력이 감소되면, 하중은 줄어들고 발산을 피할 수 있다.

운용 역사 
첫 X-29는 1984년 12월 14일 에드워드 공군 기지에서 그러먼의 선임 시험 비행사 척 시웰 Chuck Sewell이 조종하여 하늘로 날아 올랐다. 1985년 12월 13일에, X-29 중 하나가, 초음속 수평 비행을 한 첫 전진익 항공기가 되었다. (전진익을 가진 첫 제트 항공기는 그 이전의 융커스 Ju 287이었다.)

X-29는 첫 비행 후 겨우 4개월만에 나사 NASA의 시험 프로그램이 되었다. X-29는 아주 신뢰성 있음이 증명되었고, 1986년 8월에 이르러서는 세 시간 이상 지속되는 연구 임무 비행에 참여하였다. 첫 X-29는 스핀 회복 낙하산을 장비하지 않았는데, 스핀에 들어갈 것을 예정하지 않았기 때문이었다. 두 번째 X-29는 그런 낙하산을 갖추고 고 받음각 실험 프로그램에 참여하였다. NASA의 시험 프로그램은 1984년부터 1991년까지 계속되었다. 그 때는 이미 첫 X-29가 1986년 퇴역하였고, 242회 비행하였다.

전시 중인 항공기 

처음 생산된 X-29는 현재 오하이오주 데이튼 근처의 라이트패터슨 공군 기지에 있는 미 공군 국립 박물관에 전시되어 있다. 다른 기체는 에드워드 공군 기지에 있는 드라이덴 비행 연구 센터에 전시되어 있다. 완전한 크기의 모형이 워싱턴DC에 있는 국립 항공 우주 박물관의 국립 몰 National Mall 관에 전시되어 있다.

제원 (X-29)

같이 보기

참고 
주석

문헌
 Thruelsen, Richard. The Grumman Story. New York: Praeger Publishers, Inc., 1976. .
 Treadwell, Terry. Ironworks: Grumman's Fighting Aeroplanes. Shrewsbury, UK: Airlife Publishers, 1990. .
 Winchester, Jim. "Grumman X-29". X-Planes and Prototypes. London: Amber Books Ltd., 2005. .

외부 링크 

 NASA's X-29 web page 
 NASA's X-29 fact sheet
 Grumman X-29 at fighter-planes.com 
비디오
 X-29 Aircraft with Forward Swept Wings military.com

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