// BEN SAMPSON
THE IMPOSSIBLE AIRCRAFT
Devised during the height of the cold war, the X-29 helped develop
several key features of 21st century fighter jets
Commonly known by those outside of the aviation
industry as the “Impossible Aircraft” because of its
back-to-front appearance, DARPA contracted
US aerospace and defense conglomerate Grumman to
develop the X-29 in 1981.
Engineers at the company built two aircraft. The first
X-29 made its maiden flight in December 1984. Grumman
operated the first four flights, after which NASA took over
the program, operating test flights from the Ames-Dryden
Flight Research Facility, now known as the Dryden Flight
Research Center until 1992.
The X-29’s forward-swept wings were mounted on the
back of the fuselage, with canards in front of the wings.
The design aimed to provide improved maneuverability,
better supersonic performance and a lighter aircraft
structure. The inward airflow over the wings travelled
towards the root of the wing instead of outwards towards
the tip, preventing the wing tips from stalling at high
angles of attack.
Engineers had to innovate in several areas to make the
X-29 possible. Advanced composites capable of dealing
with the stresses on the wings were produced.
A computerized fly-by-wire flight control system to
maintain control of the otherwise unstable aircraft was
developed. This had triple redundancy to ensure safety.
Design innovations in the aircraft included
the forward-swept wing and its thin supercritical airfoil,
the strake flaps and the close-coupled canards.
The researchers found that the combination of
forward-swept wings and movable canards provided
excellent control response at up to a 45° angle of attack,
higher than comparable fighter aircraft. In addition,
during the eight years of the flight testing program, X-29
No.2 was equipped with a spin recovery parachute, so
high angle of attack maneuvers could be conducted. X-29
No.2 was maneuverable up to an angle of attack of about
25°, with a maximum angle of 67° reached in a
momentary pitch-up maneuver.
When the X-29 program was cancelled in May 1992,
research using No.2 X-29 continued until August. The
follow-on study conducted by the US Air Force and
Grumman investigated the use of vortex flow control to
provide increased control for aircraft at high angles of
attack when normal control systems are ineffective.
Wind tunnel tests at the US Air Force’s Wright
Laboratory and at the Grumman Corporation showed that
injection of air into the vortices would change the
direction of vortex flow and create corresponding forces
on the nose of the aircraft to change or control the nose
heading. Some 60 test flights then successfully
demonstrated the phenomenon.
The two X-29s are now on display at Wright-Patterson
Air Force Base and the NASA Armstrong Flight Research
Center in the USA.
Despite its unusual appearance, the X-29 proved one of
the most successful of the X-planes, flying a total of 436
flights. It devised several new technologies and
techniques found in modern fighter jets, including the use
of aeroelastic tailoring to control structural divergence
and the computerized control systems used to deal with
the aircraft’s instability. \\
114 SEPTEMBER \\ AEROSPACETESTINGINTERNATIONAL.COM
1 //The X-29 was the first
forward-swept wing aircraft to
break the sound barrier during
level flight, a record it set in
Max takeoff weight
From a single GE
2 // The only significant difference between the two X-29s
was an emergency spin chute deployment system
mounted at the base of the rudder on X-29 No. 2