Lockheed P-3 Orion

Specifications

Manufacturer

Lockheed

Date in service

July 1962

Type

Antisubmarine warfare

Crew

6 minimum
11 normal

Engine

Allison T56-A-14 turboprop

Users

U.S. Navy, New Zealand, Australia, Norway, Spain, Iran, Japan, Netherlands, and Portugal

Dimensions

Wingspan . . . . . . . . . . . .99.7 ft

Length . . . . . . . . . . . . . 116.8 ft

Height . . . . . . . . . . . . . . 33.8 ft

Wing area . . . . . . . . .1,300 sq ft

Weight

Empty . . . . . . . . . . . .67,486 lb

Gross . . . . . . . . . . . .139,760 lb

Performance

Max speed . . . . . . . . . 450 knot

Mission duration . . . over 14 hr

Highlights of Research by Langley for the P-3

1. Tests in the Langley 16-Foot Transonic Dynamics Tunnel (TDT) identified a catastrophic propeller-whirl flutter that had caused two fatal accidents of the Electra civil transport on which the P-3 was based.
2. Engine mount modifications suggested by the results of the TDT tests cured the problem and the Electra (and the derivative P-3) safely completed service without further flutter issues.

In late 1958, Lockheed began production of a new four-engine turboprop civil transport named the Electra. About 170 were subsequently built and supplied mainly to U.S. and South American airlines. The Navy called for design proposals for a new aircraft to replace the aging P-2 Neptune for maritime patrol and antisubmarine warfare (ASW) in August 1957. To save cost and permit service introduction as quickly as possible, the Navy suggested that a variant of an existing aircraft or one in an advanced design stage would receive favorable consideration if suitable for the missions. Lockheed proposed a militarized version of the Electra, and in April 1958, the Navy awarded Lockheed a contract to develop the aircraft. The P-3V Orion entered the inventory in August 1962, and over 35 years later it remains the Navy’s sole land-based antisubmarine warfare aircraft. It has gone through one designation change (P-3V to P-3) and three major models: P-3A, P-3B, and P-3C. The latter model is the only one now in active service. The last P-3 came off the production line at the Lockheed plant in April 1990.

Langley’s contribution to the P-3 program involved powered-model tests of the predecessor Electra configuration in the Langley 16-Foot Transonic Dynamics Tunnel (TDT). During 1959 and 1960, two catastrophic Electra accidents occurred with evidence that the aircraft had experienced violent flutter and the wings had been torn from the aircraft.

Working under great pressure (about 130 Electras were in service at the time), a NASA and industry team conducted tests in the TDT along with analytical calculations. The driving mechanism behind the catastrophic structural failure was discovered to be propeller-whirl flutter. The phenomenon of propeller-whirl flutter involves a complex interaction of engine mount stiffness, gyroscopic torques of the engine and propeller combination, and the natural flutter frequency of the wing structure. The results of the TDT tests very accurately simulated the Electra scenarios. Based on these results, the engine mount structure on all Electras was strengthened, and the aircraft (and the derivative P-3) continued in service thereafter with great success and safety.

Langley Contributions to the P-3

The Lockheed Electra

In the fall of 1958, Lockheed began deliveries of a new four-engine turboprop Model L.188 civil transport to U.S. airlines. The sleek airliner, known as the Electra, set a new pace for luxury and speed for propeller transports. Over 170 Electras were built by Lockheed and delivered to U.S. and South American airlines.

In August 1957, the U.S. Navy called for design proposals for a new advanced aircraft for maritime patrol and antisubmarine warfare (ASW) to replace the aging P-2 Neptune. The Navy strongly suggested that a variant of an existing aircraft or one in the advanced design stage be used to save cost and permit rapid introduction into fleet service. Accordingly, Lockheed proposed a military version of the Electra. In April 1958, the Navy announced that the Electra derivative had won the competition. Known initially as the P-3V, the aircraft was redesignated in 1962 as the P-3 Orion. The Orion retained the wings, tail unit, basic fuselage structure, power plant, and many subsystems of the Electra, although its fuselage was about 7 ft. shorter than the Electra’s.

Soon after introduction to the civil transport fleet, the Electra suffered two widely publicized fatal accidents with suspicious wreckage that raised concern over the structural integrity of the aircraft. On September 29, 1959, a Braniff Electra cruising near Buffalo, Texas, disintegrated without survivors. Investigation of the dispersal of aircraft wreckage revealed that the left wing had failed and separated from the aircraft in flight. On March 17, 1960, another Electra crashed in Indiana, with startling similarity to the Texas accident. Its right wing was found over 11,000 ft from the crash site, which indicated that it had also been torn from the aircraft. Over 130 Electras were operating in the civil fleet at the time, and authorities immediately reduced the cruise speed of the airliners while the investigation attempted to identify the cause of the fatal crashes.

Powered model of the Lockheed Electra mounted in the Langley 16-Foot Transonic Dynamics Tunnel for flutter tests.

Langley had recently completed the conversion of the Langley 19-Foot Pressure Tunnel into the world’s first aeroelastic test tunnel, known as the Langley 16-Foot Transonic Dynamics Tunnel (TDT). The TDT had just been calibrated, and a 0.125-scale powered model of the Electra was quickly prepared for flutter tests. Senior researchers I. Edward Garrick, Philip Donely, Robert W. Boswinkle, and D. William Conner led the planning for the investigation. Frank T. Abbott and Robert M. Bennett led the experimental work in the TDT. Wilmer H. Reed, Jr. and Samuel R. Bland led analytical studies to model and predict the phenomenon.

Lockheed and Langley flutter experts were concerned that the propeller driven Electra may have exhibited the phenomenon known as propeller-whirl flutter, in which the stiffness of the engine mounts interacts with the gyroscopic torques produced by the engine and propeller combination. This interaction results in an unstable wobbling motion that could resonate with natural frequencies of the wing structure and could cause catastrophic flutter of the wing.

The industry and NASA team reduced the stiffness of the outboard engine mounts on the model and found that the wobbling motion indeed coupled with the natural flutter frequency of the wings. The fatal resonance could build up and tear the aircraft apart in 30 sec. Based on these results, the engine mounts on all Electra aircraft were strengthened, and the Electra and its derivative P-3 aircraft have since operated safely.

NASA Official
Gail S. Langevin

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Peggy Overbey

Last Updated
October 17, 2003