Grumman X-29

https://en.wikipedia.org/wiki/Grumman_X-29

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.

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.


Role
Experimental aircraft

National origin
United States

Manufacturer
Grumman

First flight
14 December 1984

Status
Retired

Primary user
DARPA

Number built
2

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.

Specifications (X-29)

General characteristics
Crew: one pilot
Payload: 4,000 lb (1,810 kg)
Length: 48 ft 1 in (14.7 m)
Wingspan: 27 ft 2 in (8.29 m)
Height: 14 ft 9 in (4.26 m)
Wing area: 188.8 ft² (17.54 m²)
Empty weight: 13,800 lb (6,260 kg)
Max. takeoff weight: 17,800 lb (8,070 kg)
Powerplant: 1 × General Electric F404 turbofan, 16,000 lbf (71.2 kN)

Performance
Maximum speed: Mach 1.8 (1,100 mph, 1,770 km/h at 33,000 ft (10,000 m))
Range: 350 mi (560 km)
Service ceiling: 55,000 ft (16,800 m)