Aircraft | X-29 (Grumman, Darpa, NASA, USAF) |
Type | Experimental fighter |
Year | 1984 |
Engine | General Electric F404-GE-400 : 16,000 pounds of thrust |
Wingspan | 27ft 2in |
Length | 48ft 1in |
Height | 14ft 10in |
Weight | 17,600 pounds MTOW |
Max. speed | Mach 1.6 |
Ceiling | 50,000 feet |
AoA | 67 degrees angle of attack |
Crew | 1 |
The X-29 almost looked like it was flying backward. Its forward swept
wings were mounted well back on the fuselage, while
its canards horizontal stabilizers to control pitch were
in front of the wings instead of on the tail. The complex geometries of
the wings and canards combined to provide exceptional maneuverability,
supersonic performance, and a light structure. Air
moving over the forward-swept wings tended to flow inward toward the
root of the wing instead of outward toward the wing
tip as occurs on an aft swept wing. This reverse air flow did not allow
the wing tips and their ailerons to stall (lose lift) at high
angles of attack (direction of the fuselage relative to the air flow).
The concepts and technologies the fighter-size X-29 explored were the
use of advanced composites in aircraft construction;
variable camber wing surfaces; the unique forward-swept wing and its
thin supercritical airfoil; strake flaps; close-coupled
canards; and a computerized fly-by-wire flight control system to maintain
control of the otherwise unstable aircraft.
Research results showed that the configuration of forward swept wings,
coupled with movable canards, gave pilots excellent
control response at up to 45 degrees angle of attack. During its flight
history, the X-29's were flown on 422 research missions
242 by aircraft No. 1 in the Phase 1 portion of the program; 120 flights
by aircraft No. 2 in Phase 2; and 60 flights in a
follow-on "vortex control" phase. An additional 12 non-research flights
with X29 No. 1 and 2 non-research flights with X-29
No. 2 raised the total number of flights with the two aircraft to 436.
Program History
Before World War II, there were some gliders with forward-swept wings,
and the NACA Langley Memorial Aeronautical Laboratory did some wind-tunnel
work on the concept in 1931. Germany developed a motor-driven aircraft
with forward-swept wings during the war known as the Ju-287. The concept,
however, was not successful because the technology and materials did not
exist then to construct the wing rigid enough to overcome bending and twisting
forces without making the aircraft too heavy.
The introduction of composite materials in the 1970's opened a new field
of aircraft construction, making it possible to design rugged airframes
and structures stronger than those made of conventional materials, yet
lightweight and able to withstand tremendous aerodynamic forces.
Construction of the X-29's thin supercritical wing was made possible
because of its composite construction. State-of-the-art composites permit
aeroelastic tailoring, which allows the wing some bending but limits twisting
and eliminates structural divergence within the flight envelope (i.e.,
deformation of the wing or breaking off in flight).
In 1977, the Defense Advanced Research Projects Agency (DARPA) and
the Air Force Flight Dynamics Laboratory (now the
Wright Laboratory), Wright-Patterson AFB, Ohio, issued proposals for
a research aircraft designed to explore the forward
swept wing concept. The aircraft was also intended to validate studies
that said it should provide better control and lift qualities
in extreme maneuvers, and possibly reduce aerodynamic drag as well
as fly more efficiently at cruise speeds.
From several proposals, Grumman Aircraft Corporation was chosen in December
1981 to receive an $87 million contract to
build two X-29 aircraft. They were to become the first new X-series
aircraft in more than a decade. First flight of the No. 1
X-29 was Dec. 14, 1984, while the No. 2 aircraft first flew on May
23, 1989. Both first flights were from the NASA
Ames-Dryden Flight Research Facility soon to be renamed the Dryden
Flight Research Center.
Flight-Control System
The flight control surfaces on the X-29 were the forward-mounted canards,
which shared the lifting load with the wings and
provided primary pitch control; the wing flaperons (combination flaps
and ailerons), used to change wing camber and function
as ailerons for roll control when used asymmetrically; and the strake
flaps on each side of the rudder that augmented the
canards with pitch control. The control surfaces were linked electronically
to a triple-redundant digital fly-by-wire flight control
system (with analog back up) that provided an artificial stability.
The particular forward swept wing, close-coupled canard design used
on the X-29 was unstable. The X-29's flight control
system compensated for this instability by sensing flight conditions
such as attitude and speed, and through computer
processing, continually adjusted the control surfaces with up to 40
commands each second. This arrangement was made to
reduce drag. Conventionally configured aircraft achieved stability
by balancing lift loads on the wing with opposing downward
loads on the tail at the cost of drag. The X-29 avoided this drag penalty
through its relaxed static stability.
Each of the three digital flight control computers had an analog backup.
If one of the digital computers failed, the remaining two
took over. If two of the digital computers failed, the flight control
system switched to the analog mode. If one of the analog
computers failed, the two remaining analog computers took over. The
risk of total systems failure was equivalent in the X-29 to
the risk of mechanical failure in a conventional system.
Phase 1 Flights
The No. 1 aircraft demonstrated in 242 research flights that, because
the air moving over the forward-swept wing flowed inward, rather than outward
as it does on a rearward-swept wing, the wing tips remained unstalled at
the moderate angles of attack flown by X-29 No. 1. Phase 1 flights also
demonstrated that the aeroelastic tailored wing did, in fact, prevent structural
divergence of the wing within the flight envelope, and that the control
laws and control surface effectiveness were adequate to provide artificial
stability for this otherwise extremely unstable aircraft and provided good
handling qualities for the pilots.
The aircraft's supercritical airfoil also enhanced maneuvering and cruise
capabilities in the transonic regime. Developed by NASA and originally
tested on an F-8 at Dryden in the 1970s, supercritical airfoils flatter
on the upper wing surface than conventional airfoils delayed and softened
the onset of shock waves on the upper wing surface, reducing drag. The
phase 1 flights also demonstrated that the aircraft could fly safely and
reliably, even in tight turns.
Phase 2 Flights
The No. 2 X-29 investigated the aircraft's high angle of attack characteristics
and the military utility of its forward-swept wing/canard configuration
during 120 research flights. In Phase 2, flying at up to 67 degrees angle
of attack (also called high alpha), the aircraft demonstrated much better
control and maneuvering qualities than computational methods and simulation
models had predicted. The No. 1 X-29 was limited to 21 degrees angle of
attack maneuvering.
During Phase 2 flights, NASA, Air Force, and Grumman project pilots
reported the X-29 aircraft had excellent control response to 45 degrees
angle of attack and still had limited controllability at 67 degrees angle
of attack. This controllability at high angles of attack can be attributed
to the aircraft's unique forward-swept wing- canard design. The NASA/Air
Force-designed high-gain flight control laws also contributed to the good
flying qualities.
Flight control law concepts used in the program were developed from
radio-controlled flight tests of a 22-percent X-29 drop model at NASA's
Langley Research Center, Hampton, Va. The detail design was performed by
engineers at Dryden and the Air Force Flight Test Center at Edwards. The
X-29 achieved its high alpha controllability without leading edge flaps
on the wings for additional lift, and without moveable vanes on the engine's
exhaust nozzle to change or "vector" the direction of thrust, such as those
used on the X-31 and the F-18 High Angle-of-Attack Research Vehicle. Researchers
documented the aerodynamic characteristics of the aircraft at high angles
of attack during this phase using a combination of pressure measurements
and flow visualization. Flight test data from the high-angle-of-attack/military-utility
phase of the X-29 program satisfied the primary objective of the X-29 program
to evaluate the ability of X-29 technologies to improve future fighter
aircraft mission performance.
Vortex Flow Control
In 1992 the U.S. Air Force initiated a program to study the use of vortex
flow control as a means of providing increased aircraft control at high
angles of attack when the normal flight control systems are ineffective.
The No. 2 X-29 was modified with the installation of two high-pressure
nitrogen tanks and control valves with two small nozzle jets located on
the forward upper portion of the nose. The purpose of the modifications
was to inject air into the vortices that flow off the nose of the aircraft
at high angles of attack.
Wind tunnel tests at the 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.
From May to August 1992, 60 flights successfully demonstrated vortex flow control (VFC). VFC was more effective than expected in generating yaw (left-to-right) forces, especially at higher angles of attack where the rudder loses effectiveness. VFC was less successful in providing control when sideslip (relative wind pushing on the side of the aircraft) was present, and it did little to decrease rocking oscillation of the aircraft.
Summary
Overall, VFC, like the forward-swept wings, showed promise for the future
of aircraft design. The X-29 did not demonstrate the overall reduction
in aerodynamic drag that earlier studies had suggested, but this discovery
should not be interpreted to mean that a more optimized design with forward-swept
wings could not yield a reduction in drag. Overall, the X-29 program demonstrated
several new technologies as well as new uses of proven technologies. These
included: aeroelastic tailoring to control structural divergence; use of
a relatively large, close-coupled canard for longitudinal control; control
of an aircraft with extreme instability while still providing good handling
qualities; use of three-surface longitudinal control; use of a double-hinged
trailing-edge flaperon at supersonic speeds; control effectiveness at high
angle of attack; vortex control; and military utility of the overall design.
The Aircraft
The X-29 is a single-engine aircraft 48.1 feet long. Its forward-swept
wing has a span of 27.2 feet. Each X-29 was powered by a General Electric
F404-GE-400 engine producing 16,000 pounds of thrust. Empty weight was
13,600 pounds, while takeoff weight was 17,600 pounds.
The aircraft had a maximum operating altitude of 50,000 feet, a maximum
speed of Mach 1.6, and a flight endurance time of approximately one hour.
The only significant difference between the two aircraft was an emergency
spin chute deployment system mounted at the base of the rudder on aircraft
No. 2. External wing structure is primarily composite materials incorporated
into precise patterns to develop strength and avoid structural divergence.
The wing substructure and the basic airframe itself is aluminum and titanium.
Wing trailing edge actuators controlling camber are mounted externally
in streamlined fairings because of the thinness of the supercritical airfoil.
Program Management
The X-29 program was funded initially by the Department of Defense Advanced
Research Projects Agency. The program was managed by the Air Force's Wright
Laboratory, Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson
AFB, Ohio.
The flight research program was conducted by the Dryden Flight Research Center, and included the Air Force Flight Test Center and the Grumman Corporation as participating organizations.
Reverse airflow-forward-swept wing vs aft
swept wing. On the forward-swept wing, ailerons remained unstalled at high
angles of attack because the air over the forward swept wing tended to
flow inward toward the root of the wing rather than outward toward the
wing tip as on an aft-swept wing. This provided better airflow over the
ailerons and prevented stalling (loss of lift) at high angles of attack.
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