Grumman A-6 Intruder

Specifications

Manufacturer

Grumman

Date in service

A-6A . . . . . . . . . . . . . . 1963

A-6E . . . . . . . . . . . . . . .1972

Type

Attack

Crew

Two

Engine

Pratt & Whitney J52-P8B

Users

U.S. Navy and U.S. Marine Corps (retired in 1997)

Dimensions

Wingspan . . . . . . . . . . . .53.0 ft

Length . . . . . . . . . . . . . . 54.7 ft

Height . . . . . . . . . . . . . . 15.5 ft

Wing area . . . . . . . . . . 529 sq ft

Weight

Empty . . . . . . . . . . . .28,000 lb

Gross . . . . . . . . . . . . .58,600 lb

Performance

Max speed . . . . . . . . 563 knots

Highlights of Research by Langley for the A-6

1. Wind-tunnel studies of aerodynamic and thermal characteristics in the Langley 16-Foot Transonic Tunnel provided data for selection of the configuration for the fuselage-mounted speed brakes..
2. Tests of A-6 configurations in the Langley 20-Foot Vertical Spin Tunnel determined that spin recovery characteristics were significantly enhanced by enlarging the rudder and increasing the deflections of the horizontal tail and rudder.
3. Flutter clearance tests for a composite wing for the A-6E were successfully conducted in the Langley 16-Foot Transonic Dynamics Tunnel.

The Grumman (now Northrop Grumman) A-6 Intruder was an all-weather, two seat, subsonic, carrier-based attack aircraft. Designed in the late 1950’s, the Intruder played a critical role in the Vietnam War with over 35,000 combat sorties by 1973. The last version of the aircraft, the A-6E, was widely regarded as the best all-weather precision bomber in the world. As an example of its effectiveness, during the strike on Libyan terrorist-related targets in 1986, A-6E Intruders penetrated sophisticated Libyan air defense systems, which had been alerted by the high level of diplomatic tension and rumors of impending attacks. Evading over 100 guided missiles, the strike force flew at low altitude in complete darkness and accurately delivered laser-guided and other ordnance on target. Composite wing replacements and upgraded systems and weapons improvement programs maintained the A-6’s combat capability until its retirement from the fleet in early 1997.

Langley’s contributions to the A-6 Program began in 1959. Studies of the aerodynamic and thermal characteristics of candidate designs for the fuselage-mounted speed brakes, which were located adjacent to the hot efflux of the turbojet engines, were conducted in the Langley 16-Foot Transonic Tunnel. Tests were also conducted in the 20-Foot Vertical Spin Tunnel to determine the spin and recovery characteristics of the aircraft and to determine the size of parachute required for the spin test aircraft. Initial Langley tests indicated that spin recoveries were significantly improved with an enlarged rudder and more deflection angles for the horizontal tail and rudder. The rudder was enlarged, and the control system was modified to provide the extended deflections. Follow-on spin tests were also conducted over the years to determine the effects of new external stores and new weapons on spin recovery characteristics.

Arguably, the most important contribution of Langley to the A-6 Program was a series of two test entries in the Langley 16-Foot Transonic Dynamics Tunnel (TDT) to ensure flutter clearance for an advanced composite wing that was incorporated into the A-6E fleet to extend the fatigue life and capabilities of the aircraft. Initial tests in the TDT ended in flutter failure of the model wing, but a revised wing design passed flutter demonstration requirements and permitted the fleet to utilize the advanced wing.

Langley Contributions to the A-6

Speed Brake Studies

In May 1959, a team from Langley and Grumman led by Langley researcher Charles E. Mercer conducted powered-model tests in the Langley 16-Foot Transonic Tunnel to determine the optimum configuration for fuselage-mounted speed brakes–thrust spoilers for the A-6 (then designated the A2F-1). The speed brake panels were located immediately behind the engine exhaust nozzles on the rear fuselage, which caused concern over the aerodynamic loads and the thermal environment of the speed brakes during high-power conditions. To further complicate the engineering issues, the original aircraft also had a tilting tailpipe to increase lift at low speeds and thereby reduce approach speeds. The test examined several candidate configurations, including a perforated panel design, which was subsequently incorporated into the early A-6A and EA-6A. Later variants of the A-6 used symmetrically deflected split ailerons at the wingtips for speed brakes, and the original speed brake panels were inactive or deleted.

The A-6 prototype made its first test flight in April 1960. The Navy was satisfied with the 90-knot approach speed, so the deflectable nozzle design feature was eliminated in production aircraft. The Langley staff, however, continued research on thrust vectoring, which later contributed to more advanced fighter designs.

Powered-model tests in the Langley 16-Foot Transonic Tunnel evaluated several speed brake configurations.

Spin Tunnel Tests

Initial evaluations of the spin and spin recovery characteristics of the A-6 by Henry A. Lee in the Langley 20-Foot Vertical Spin Tunnel indicated that spin recoveries could be significantly enhanced by an enlarged rudder and by additional deflections of the rudder and horizontal tail. The original A-6 design had incorporated extended throws on the rudder and horizontal stabilizer for the power approach configuration, so an assist spin recovery switch was installed so the pilot could extended deflections for the cruise configuration. The Langley tests determined that the greater horizontal-tail deflection angle (trailing-edge up) provided a better flow field on the vertical tail during spins, thereby increasing the effectiveness of the rudder to terminate the spin. The rudder of the production aircraft was enlarged as a result of these tests.

The Composite Wing

By the mid-1980’s, the A-6 was beginning to show its age. The accumulated stress of high-g catapulted takeoffs and arrested landings on carriers and the long exposure to salt water were beginning to take a toll on the life of airframe components. Studies were underway for a new wing design, and inspections of the A-6 structure revealed major corrosion problems. In January 1987, a fatal accident resulted in an investigation of the structural health and projected lifetime of the A-6 fleet (ref. 5).

At the time of the accident, U.S. Marine Corps First Lieutenant Bob Pandis and his bombardier/navigator, Lieutenant Colonel John Cavin, were practicing dive-bombing missions at the El Centro Naval Base in California. During a 40-deg diving run at about 500 knots, the left wing of their A-6E broke off the aircraft at an altitude of about 8,000 ft and the aircraft began to spin wildly out of control. When the wing separated from the aircraft, fuel from the severed fuel cells in the aircraft immediately ignited in a giant fireball, adding to the severity of the rolling motion. The violence of the roll caused the empennage to separate from the aircraft, which started cartwheeling end-over-end. Pandis ejected from the out-of-control aircraft and suffered major injuries, while Cavin died in the crash.

Pandis later recalled that the two other A-6’s in his flight had noticed fuel venting from his left wing during the run for the dive. About halfway into to the dive, the crews of the two aircraft saw a dramatic increase in fuel venting and were shocked to see the left wing tear off the aircraft.

Prior to this terrible accident, 72 A-6 aircraft had been temporarily grounded and another 109 were operating under flight restrictions. The Boeing Company had begun to design a new wing under a Navy contract that was awarded in 1985. With a carbon fiber–epoxy resin torsion box, light alloy control surfaces, and some titanium components, the new wing was much lighter and designed for four times the fatigue life of the existing wing. In addition to becoming a retrofit for the A-6E fleet, the wing was also intended for a new, advanced version of the A-6 to be known as the A-6F. (The A-6F was later canceled in the prototype stage when the Navy decided to replace the A-6 fleet with the stealth A-12 aircraft).

Flutter tests of the new A-6 composite wing in the Langley 16-Foot
Transonic Dynamics Tunnel included numerous external store loadings.

At the request of the Navy, tests were conducted in the Langley 16-Foot Transonic Dynamics Tunnel (TDT) in February 1986, to ensure that the new wing would not exhibit flutter within the flight envelope. The preflight flutter clearance tests required two separate entries in the TDT under the leadership of Langley researchers Stanley R. Cole and Jose A. Rivera, Jr. The first composite-wing model was lost in a catastrophic flutter event while testing a “pencil” store configuration that represented the fuel tanks. Pencil stores are slender metal rods that simulate the moments of inertia of the real fuel tanks while minimizing aerodynamic effects. The test of the pencil store configuration is part of the flutter clearance test; however, the result was much more severe flutter than expected and the model was destroyed. Based in part on these results, Boeing modified the structural design of the A-6 composite wing. A second model incorporating the new wing design was fabricated and tested in the TDT in June 1987. This second model demonstrated no flutter incidents within the scaled flight envelope plus a safety margin.

A unique aspect of both models was that they were semispan models (only one half of the aircraft was modeled); however, Boeing designed a unique root constraint to simulate the carry-through structure of the wing at the fuselage. The semispan model was larger than a full-span model, thereby permitting more accurate representation of the structural characteristics at model scale. Also of interest with regard to model design, the first model had a very clean wing surface. However, the decision was made to include the bumps, fences, and other wing features that were present in the actual flight hardware on the second model.

Subsequent to the Langley tests, the composite wing was retrofitted to about 200 A-6E aircraft, which significantly increased the aircraft’s capability, safety, and operational life.

By 1988, the team of McDonnell Douglas and General Dynamics began to build the A-6 replacement called the A-12 Avenger. Unfortunately, Defense Secretary Richard Cheney cancelled the $57 billion project after cost overruns exceeded $2.7 billion dollars in the development phase. Nonetheless, the decision to retire the A-6 was unchanged, and the F/A-18 Hornet became the replacement aircraft for the Intruder. The last flight of the A-6 Intruder occurred in early 1997.

NASA Official
Gail S. Langevin

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Peggy Overbey

Last Updated
October 17, 2003