So the only conceivable way a propeller-driven airplane could go supersonic might be with the prop stopped and feathered, in a terminal-velocity dive. But even that wouldn’t have worked in the 1940s, since airframe design was still taking baby steps through the transonic range. Immediately after World War II, Kelly Johnson, the legendary Lockheed Skunkworks engineer, built a six-foot-wingspan, 600-pound, solid-steel model of thee Lockheed P-80A Shooting Star (later designated F-80) and had it dropped from a P-38 at altitudes close to 40,000 feet. “In a vertical dive,” he wrote in a letter to Fisher, “the model would not exceed a true airspeed of higher than Mach .94. With the full scale model of the Lockheed F-80A, these results were confirmed, and there was no recorded case where this jet fighter, clean as it was, could ever exceed Mach .9.”
Leonard Greene, an engineer, ex-Grumman test pilot, and aviation entrepreneur who once developed important theories of high-speed aerodynamics at the Institute for Advanced Study in New Jersey, rolls his eyes and looks even wearier than usual when the possibility of World War II-type aircraft exceeding Mach 1 is broached. “We don’t have enough thrust today to put onto any World War II aircraft and make it fly at supersonic speeds,” he says. “Besides, it would come apart first.”
So were the P-47 pilots fibbing? Not at all, Fisher (and Johnson) explained. They were tricked by a simple phenomenon: airspeed indicators don’t function reliably in high-speed dives. The airplanes are falling so fast they can’t measure static air pressure quickly enough: while the instruments were down here, they were still measuring air from up there. Had neophyte Hurtienne’s indicator been accurate at an indicated 675 mph at 20,000 feet, for example, his true airspeed would indeed have been at least Mach 1.05 at typical temperatures. But it wasn’t. Because the airspeed calculation would have been based on an artificially high altitude reading, the airspeed indicator would show the airplane to be traveling faster than it really was.
Still, there are records to be set in flirting with Ernst Mach’s big One-Point-Oh in a Thunderbolt, and Herb Fisher helped set one few would dare try to top. Some 40 years ago—during an era that obviously predated corporate legal departments, liability suits, OSHA rules, and subparts of parts of Federal Aviation Regulations—Herb Fisher sat his three-year-old son on his lap, clamped an oxygen mask to the child’s tiny face, climbed to 30,000 feet, two-blocked the throttle, pushed over, and took Mrs. Fisher’s boy along on one of his Mach .8 dive tests, making Herbert Fisher Jr. “the fastest baby in the world.”
Don’t Make Waves
Paradoxically, many efforts at achieving supersonic flight have been directed at delaying it—at least in those instances in which the airflow goes supersonic in isolated areas over the airframe before the whole thing has reached Mach 1. In the 1940s airplane designers created new aerodynamic configurations all intended to take it easy on the air to enable a more gradual transition from subsonic to supersonic speed. Thin airfoils, swept wings, and, later, specially shaped airfoils labeled “supercritical” have been used on high-speed aircraft just to keep the airflow well adjusted.
The air’s gradual adjustment to an aircraft barreling through it is the key to Richard Whitcomb’s discovery of the Area Rule and also explains how airfoil shapes can delay the formation of shock waves. Studying the position of shock waves in Schlieren photographs of models in wind tunnels, Whitcomb saw that air flowing around an airplane was being violently shoved aside when it reached the intersection of the wings and fuselage. He realized that it was the abruptness of the increase in area at this intersection that caused the shock waves to form. By narrowing the fuselage at this point, Whitcomb was able to achieve a more gradual displacement of the air and therefore a decrease in its resistance. He formulated the Area Rule, which calls for only gradual changes in the area that an airplane presents head-on to the air.
Convair was the first to apply the Area Rule. In 1952 wind tunnel tests showed that the F-102, despite having a powerful engine and knife-edged delta wings, could not reach the supersonic speeds because of high drag. On Whitcomb’s advice, the Convair designers transformed the bullet-shaped YF-102 into the slim-waisted YF-102A, creating a faster airplane.
The shapes of wings also changed to keep the air from performing as much work at high speeds as it had been expected to perform at lower speeds. A thin airfoil, for example, decreases the distance that the air must flow over the wing compared with the distance that the air travels under it and therefore reduces the speed of the air over the wing. The idea behind a thin wing is to reduce the ratio of the wing’s thickness to its “chord”—the distance from the leading to the trailing edge.