WIND TUNNELS OF NASA

 

Chapter 5 - The Era of High-Speed Flight

The Area Rule and the F-102 Story

 

[62] The Cold War and the Korean War were stark realities in the early 1950s. The U.S. Air Force urgently needed a supersonic fighter to maintain air superiority. In 1951 several purportedly supersonic aircraft were on the drawing boards. Engineers had sketched out smooth, sleek fuselages with thin wings and powerful jet engines. These craft looked supersonic, and the data from rocket-propelled models suggested that they would be supersonic in actual flight. In reality, the so-called transonic region from Mach 0.9 to Mach 1.1 had not yet been explored systematically in wind tunnels. Bulletlike aircraft, as it turned out, were not the sole answer to supersonic flight.

To remedy the acknowledged deficiency in transonic research, the NACA had begun operating its 8-foot high-speed wind tunnel with a slotted wall at Langley Field in early 1950. This newly modified tunnel, which attained transonic speeds, arrived on the scene at an opportune moment.

One of the hopefully supersonic fighters being built in 1951 was the Convair delta-wing YF-102, with the world's most powerful jet engine, the Pratt and Whitney J-57, and knife-edge delta wings. Convair aerodynamicists were sure their projectile-shaped plane would easily penetrate the Mach 1 barrier. By mid-1952 Convair and the Air Force were committed to the construction of two YF-102 prototypes. A production line was being set up in San Diego for the manufacture of hundreds more. The newly modified Langley 8-foot high-speed tunnel, however, was generating disturbing data suggesting that transonic drag (air resistance) for the YF-102 might be much higher than expected. In August 1952 a scale model of the YF-102 was mounted in the tunnel. To Convair's dismay, the model displayed such high drag in the vicinity of Mach 1 that there was serious doubt that even the powerful J-57 engine could push the YF-102 through the sound barrier.

Following the YF-102 model tests, NACA and Convair engineers went over the data together at Langley. At this time, NACA aerodynamicists described some of the surprising discoveries they had been making concerning transonic drag. Richard T. Whitcomb and his team at the 8-foot high- speed tunnel had been studying various aircraft configurations at transonic speeds in their slotted- wall tunnel. As the high-speed air flowed around the models, they expected to see shock waves forming near the noses of the models, but they were startled to find additional strong shock waves established behind the trailing edges of the wings. Obviously, the unexpected high drags being measured were [63] caused by the planes having to overcome the energy losses created by these extra shock waves. The YF-102s being built in San Diego would never go supersonic burdened with these aerodynamic anchors.

Happily, Whitcomb's tests also provided a way out that was almost as surprising as the original discovery of the extra set of shock waves. The YF-102's smooth, streamlined fuselage should be replaced with a wasplike waist and a bulging tail in such a way that the total cross-sectional area of wings, fuselage, and tail (not just the fuselage area) should be that of an ideal streamlined body. Thus the fuselage should be constricted where the wings were attached and then expanded at their trailing edges. Aircraft designed according to Whitcomb's Area Rule looked almost grotesque and were dubbed "flying coke bottles." Nevertheless, the wind tunnel data were convincing, and the Convair engineers went back to San Diego to incorporate the suggested changes into their YF-102 model.

Convair returned to Langley in May 1953 with a modified YF-102 model. New wind tunnel tests showed substantial drag reduction. Additional changes were suggested to follow the Area Rule more closely. The model was revised once again, and in October 1953, checked again in the high-speed tunnel. These tests promised that the YF-102 designed according to the Area Rule, would now meet Air Force supersonic requirements.

At this time, it was too late to change the YF-102 prototypes and the first aircraft on the production line. Besides, there was still some hope that the drag problem might not be as severe as the Langley wind tunnel tests had indicated. The first YF-102 prototype roared down the runway at Muroc Air Force Base, California, on October 24, 1953. Unfortunately, the J-57 engine flamed out on takeoff and the craft was damaged beyond repair on landing. On January 11, 1954, the second prototype flew successfully. But as flight tests proceeded, it became clear that the Langley wind tunnel data were indeed correct-the YF-102 would not go supersonic in level flight.

The Air Force was in a quandary; it needed the new aircraft in its inventory but it also wanted them to be supersonic. Hugh Dryden, Director of NACA, assured Air Force General Nathan Twining that NACA had the answer to transonic drag reduction and had already passed the information on to Convair and other aircraft companies. With this knowledge, the Air Force halted the Convair F-102 production line.

Convair had not been idle following the wind tunnel revelations at Langley. In just 117 working days during 1954 they redesigned the YF-102 according to the Area Rule and built a new prototype. The new aircraft, designated the YF-102A, had the prescribed wasp waist, bulbous fairings on the tail, a sharper nose and canopy, and a more powerful version of the J-57 jet engines. On December 20, 1954, at Lindbergh Field near San Diego, the prototype left the runway and, while still climbing, pierced the sound barrier. Using the Area Rule, the top speed of the YF-102A increased by about 25 percent. With flight success, the Air Force restarted the Convair production line, this time to build 870 F-102As and 340 "advanced" F-102As, designated F-106s. The F-106s have become the primary interceptors defending the continental United States into the early 1980s.

 


YF-102 and YF-102A

The YF-102 and YF-102A side by side. The narrowing of the YF-102A fuselage near the wings was dictated by the Area Rule and enabled the craft to become supersonic. Without the Area Rule, the YF-102 prototype never attained supersonic speeds in level flight.


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