An experimental aircraft
manufactured by Grumman
to explore and test new technologies in aircraft design, the most easily-identifiable of these being the forward-swept wing
(FSW) design. The X-29 was tested in a program that ran from 1984
and the results provided an engineering
database for the development of future
FSW concepts had been tested by both Germany and the United States during World War 2. The NACA Langley Memorial Aeronautical Laboratory used wind tunnels to test models utilizing FSW, and Germany built an pilot-less aircraft with FSW. Unfortunately, material technology was not sufficiently advanced to be able to produce aircraft that used them reliably. The problem was that the wings could not be manufactured stiff enough to prevent them flexing dangerously at high speed, without making them too heavy and thick to be sufficiently manoeuvrable. Work had to be suspended until material technology caught up with FSW theory.
The idea behind FSW is that the air flowing over the wings tends to flow inwards towards the wing root rather than outwards towards the wingtip, as would occur on a backward-swept wing. The effect of this improved airflow is that the ailerons do not stall at high angles of attack. Although appearances of the X-29 might suggest otherwise to the casual observer, it was also theorized that FSW would actually reduce drag by 20%-40%, along with resultant fuel savings.
In the 1970s composite materials were introduced that were far stronger than existing materials, yet lightweight, which made FSW aircraft possible. Wings made from composite materials would be able to bend to a degree but would limit twisting and should not deform or break off in flight. Presumably as a result, in 1977 the Defence Advanced Research Projects Agency (DARPA) and the Air Force Flight Dynamics Laboratory (now the Wright Laboratory) issued proposals for a research aircraft to explore FSW concepts. Studies prior to this suggested that using FSW would improve control and lift, and reduce drag. The aircraft was intended to validate this.
Grumman won the contract from the US Air Force to develop an aircraft with forward-swept wings in 1980, with a budget of $87m. The program was jointly funded by the USAF, DARPA and NASA. The first X-29 (a total of two were built) was ready in 1982, the second following in 1989. To cut costs, Grumman used parts from other aircraft wherever possible in the X-29's construction. This included the fuselage and nose wheel from the F-5, an engine as used in the F/A-18, and undercarriage from the F-16.
Employing FSW, the X-29 also had forward-mounted canards which shared the lifting load with the wings and controlled the aircraft’s pitch. The flaperons on the wings altered the wing shape to act as flaps used together, and as ailerons discretely. The configuration of the X-29 made it extremely unstable in flight and like several aircraft now in service (such as the F-16, the F-117 and the F-22) was impossible to fly manually - that is, without computer assistance. The control surfaces were controlled by a triple-redundant digital fly-by-wire system that made up to 40 adjustments to the flight configuration every second, to maintain an artificial stability. Analogue backups existed for each of the three flight control computers so if a single digital system failed, the other two would compensate. Any more than that and the system would switch to analogue mode. This made the X-29 about as risky (as far as system failure was concerned) to fly as an aircraft equipped with a 'normal' flight control system.
The two aircraft had their individual areas of research - No. 1 aircraft demonstrated and validated the aforementioned principles of FSW - the wings were tested at moderate angles of attack without stalling, and the flight control system did indeed provide adequate stability for the aircraft. In addition, the materials used satisfactorily prevented the wing from warping or breaking/cracking in flight. As theorized, the X-29’s design gave it great improvements in manoeuvrability over other aircraft, and test pilots reportedly found the aircraft “exciting” to fly.(4) The ‘supercritical’ airfoil design the X-29 employed used a flatter upper surface than standard aerofoils and lessened the build-up of shock waves on the wing surface at high speed, which reduced drag.
No.2 aircraft tested the same principles as No.1 aircraft but under more extreme flight conditions, also testing the viability of the X-29 as a military aircraft. It was tested under high angles of attack, and was found to give excellent control response at angles of attack up to 45 degrees. Even at 67 degrees it performed better even than the simulations had predicted and pilots reported it was still controllable. No.1 aircraft was not tested above 21 degrees. No.2 aircraft was also used in research by the US Air Force into the effectiveness of vortex flow control (VFC) for controlling aircraft at high angles of attack when the normal flight control surfaces were ineffective. VFC involves altering the vortices that flow over the aircraft and in doing so alter its attitude. In this case, two high-pressure nitrogen tanks and control valves were installed in the upper portion of the nose of the X-29, which would inject gas into the vortices that flowed over its nose at high angles of attack.
Wind tunnel testing at Wright Laboratory and 60 research flights in 1992 showed that as theorised, VFC was effective at generating yaw at high angles of attack when the rudder was ineffective.
By the time the X-29 was retired in 1994, it had become the first FSW aircraft to exceed the speed of sound in level flight and had completed a total of 422 flights: 242 by No.1 aircraft, and 180 by No.2 aircraft (120 in its own test program, and 60 as part of the VFC research). An X-29 also appeared at Dayton (Ohio) International Airshow and the Experimental Aircraft Association's (EAA) International Convention and Sport Aviation Exhibition at Oshkosh, Wisconsin.
The X-29 is now on display at Wright-Patterson AFB museum.
X-Planes Data Site; "Grumman X-29A";
- NASA; "X-29 Fact Sheet";
- (Author unknown); "X-29";
- Wright-Patterson AFB; "GRUMMAN X-29A";
- Roman Chess; "Grumman X-29;
- K-12 Internet On-Ramp; "The Grumman X-29A";
via Google cache (currently dead);
High, Edward E.; "Grumman X-29A";