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Convair 990 CV-990

The four-engine Convair 990 jet transport was an attempt by the Convair Corporation to compete with Boeing and Douglas in the highly competitive jet transport marketplace of the late 1950s. Unfortunately for Convair, Boeing and Douglas had captured the early market with sales of the 707 and DC-8, respectively, whereas Convair's initial attempt to enter the rapidly growing industry was marred by massive losses of over $425 million on its Convair 880 transport. When Boeing marketed their new 720 transport it threatened to eliminate Convair from the competition; Convair responded with a new design designated the Convair 990, which would be marketed on speed and luxury. The aircraft would differ from the earlier 880 in having a stretched fuselage for increased capacity, a larger wing, and the first turbofans ever used by a civil transport. The new turbofans were supplied by the General Electric Company. The addition of a fan to the rear of the CJ805 to create the first turbofan engine for commercial service for the Convair 990.

The four-engine Convair 990 was nearly the same in configuration as the Boeing 707 a the McDonnell Douglas DC-8. In fact, when seen at the airport, the Convair 990 was often confused with one or the other of the more familiar 707 or DC-8 aircraft. But the 990 was somewhat smaller and lighter in weight than the 707 and the DC-8. The gross weight of the 990 was 253 000 pounds. The maximum cruising Mach number of the 990 is 0.89, which is the highest of any of the subsonic jet transports. The high cruising Mach number of the aircraft is due in part to the Whitcomb bumps on the trailing edge of the wing. The two pods mounted on each wing at the trailing edge make the aircraft readily identifiable and are used to increase the critical Mach number as well as to augment fuel volume.

The range the aircraft was not intercontinental, and the payloads was lower than those of the Boeing and Douglas aircraft. For these reasons, perhaps, and because the aircraft became available to the airlines somewhat later than the 707 and the DC-8, only a relatively small number of Convair jet transports were built. The first flight of the more advanced Convair 990 was in 1961. Only 37 examples of the 990 were built.

Richard T. Whitcomb spent most of his working life doing research in wind tunnels at NASA's Langley Research Center in Hampton, VA. Inspired by a presentation on transonic flows made at Langley by Adolph Busemann, a world famous German aerodynamicist who had come to Langley following World War II, Whitcomb realized that the transonic disturbances and shock waves produced by aircraft were a function of the longitudinal variation of cross-sectional area. As a result of this phenomenon, the drag near the speed of sound for a wing-body combination was the same as that of a body of revolution with the same longitudinal distribution of cross-sectional area. For most airplane configurations, adding the cross-sectional area of the wing to that of the fuselage results in an abrupt increase, or bump, in the overall longitudinal area distribution.

Whitcomb's discovery was initially highly classified, but the aircraft industry was immediately notified and briefed on the results of wind-tunnel tests that verified his hypothesis. Whitcomb was subsequently awarded the coveted Collier Trophy for his discovery and the development of the area rule, and history has recorded numerous applications to military aircraft beginning with the U.S. Navy's F11F Tiger, which almost flew faster than speed of sound without an afterburner in August 1954. Perhaps the most dramatic application of the area rule was for the U.S. Air Force's delta-winged F-102 aircraft.

One of the more significant examples of the application of the area rule for local flow problems involved Convair 990. During briefings with Convair engineers, Whitcomb advised them to incorporate his concept of concial wing-mounted antishock bodies for local area ruling of the wing and enhanced high-speed performance. Impressed with the potential of the antishock body concept, Convair designed the 990 with the wing-mounted bodies. The first flight of the new aircraft occurred on January 24, 1961, and even with the beneficial effects of the bodies, high-speed drag problems were immediately noted during the flight tests. The top speed was limited to 580 mph (40 mph less than the guarantee) and a serious range deficit was also noted that would prevent coast-to-coast operations. An extensive drag reduction program was initiated that led to modifications that resulted in the achievement of cruise performance in excess of the original guarantees. The modifications included a sharper, less-drooped wing leading edge; a nacelle afterbody extension; a wing-fuselage fillet redesign; and the addition of engine nacelle and pylon fairings.

During the drag reduction program, General Electric representatives requested the assistance of Whitcomb in minimizing an extremely large nacelle-wing-pylon interference drag problem that had been identified in flight tests. Pressure measurements made around the nacelle afterbody, pylon, and wing indicated the presence of a strong shock wave with significant wave drag for aircraft Mach numbers from 0.80 to 0.90. In addition, shock-induced separation contributed to the drag problem. The new turbofan engines had the fan located toward the rear of the engine; this location resulted in a sudden increase in area distribution near the wing trailing edge. Essentially, flow encountering the convergent-divergent channel between the nacelle, pylon, and lower wing surface was being accelerated to supersonic conditions, which resulted in a standing shock. The level of drag rise for the entire aircraft with increasing Mach number above 0.80 was approximately equal to the nacelle afterbody pressure drag.

Whitcomb analyzed the problem using the principles of his area rule on a local basis. In particular, the area contained by the nacelle upper surface, pylon side surface, and wing lower surface was analyzed for each nacelle in terms of smoothness of the area distribution and found to have abrupt changes in area distribution (due to the pylon and fan location) along the length ranging from the nacelle intake to the trailing edge of the wing for both the inboard and outboard nacelles. Any fixes for the problem could not change the wing or the nacelle basic lines, but auxiliary fairings could be added to the pylons and nacelles.

Following applications of the local area rule, several pylon, nacelle, and wing fairings were proposed to smooth out the area distribution, and the most effective configurations, consisting of forward and aft pylon fairings, were adopted for production aircraft. This configuration resulted in a significant increase in the drag-rise Mach number for the aircraft, from about 0.80 for the basic configuration to about 0.89 for the modified aircraft.

NASA later acquired a Convair 990 aircraft for use in its research programs at the Dryden Flight and Ames Research Centers for activities ranging from evaluating new landing gear and brake designs for the space shuttle to direct lift control and medium-altitude research missions. The CV-990, built in 1962 by the Convair Division of General Dynamics Corp., Ft. Worth, Texas, served as a research aircraft at Ames Research Center, Moffett Field, California, before it came to Dryden.

A Convair 990 (CV-990) was used as a Landing Systems Research Aircraft (LSRA) at NASA's Dryden Flight Research Center, Edwards, California, to test space shuttle landing gear and braking systems as part of NASA's effort to upgrade and improve space shuttle capabilities. The first flight at Dryden of the CV-990 with shuttle test components occurred in April 1993, and tests continued into August 1995, when this photo shows a test of the shuttle tires. The purpose of this series of tests was to determine the performance parameters and failure limits of the tires. This particular landing was on the dry lakebed at Edwards, but other tests occurred on the main runway there.

Wingspan36.58 m120 ft 0 in
Length42.43 m139 ft 2 in
Height12.04 m39 ft 6 in
Wing area209 m22250 sq ft
Take-off weight114760 kg253000 lb
Empty weight54840 kg120900 lb
ENGINE4 x 7280kg General Electric CJ-805-23C turbofans
Max. speed990 km/h615 mph
Cruise speed895 km/h556 mph
Range6115 km3800 miles

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Page last modified: 07-07-2011 02:29:21 ZULU