Mach 1: Assaulting the Barrier

In 1947, no airplane had ever gone faster than the speed of sound.

The Douglas D-558-2 Skyrocket (shown here at Edwards Air Force Base circa May 1949) pushed past Mach 2 on November 20, 1953, beating an advanced X-1 to the record. (US Navy via National Air and Space Museum. Photo SI A-5168-C.)
Air & Space Magazine

Ralph Virden was the first to fall. Virden, a Lockheed test pilot was flying his P-38 through a dive test in November 1941 when the airplane pitched manically and became nearly uncontrollable because of what later came to be called “Mach tuck.” The twin-engine Lightning, gaining speed in the dive, was still well below the speed of sound, but the air accelerating over its wing was moving faster than the airplane itself. When Virden hit Mach .675, the airflow over the wings became supersonic. A shock wave leapt to life over the wing stubs between the fighter’s lozenge-like cockpit cab and its engine nacelles. The inboard wings suddenly stalled; the airplane slumped. The usually strong airstream that the wings guided back and down onto the fighter’s horizontal tail ceased, no longer counterbalancing the weight of the engines and forward structure. The nose rotated down—“tucked.”

This wouldn’t have come as a surprise to Virden. P-38 designer Kelly Johnson had been one of the first to postulate the effects of compressibility, the baffling behavior of air moving at supersonic speeds. So the P-38 that Virden was flying, one of the first of the twin-boom fighters to be built, had a raised tail, which had already been fitted with special devices to give it more muscle in the inevitable struggle to regain balanced flight.

What Johnson and Virden didn’t know, because Lockheed’s wind tunnel couldn’t simulate speeds as high as its P-38 could reach, were the exact locations and various strengths of the pressures working on the aircraft. So when Virden activated the spring-loaded servo tabs on the elevator, he thought they would help him wrench the tail back down. They worked too well: the forces of the dive recovery pulled the airplane’s tail off and Virden died in the ensuing crash.

The supersonic era truly had begun for the United States. Flying faster than sound had moved from theory and wind tunnels to real airplanes carrying real pilots and acting in ways nobody yet understood. In less than 20 years, airplanes had progressed from speeds that can be surpassed today by most Toyotas to velocities at which 2,000-horsepower metal monoplanes could knock on a door that even now isn’t fully open.

U.S. aviation came late to high-speed flight. The Germans were experimenting with quadrupling and quintupling airspeeds in 1922 at Göttingen, while the Jenny was still the hot setup for U.S. pilots. Fritz Opel flew the first rocketplane in 1929, and in 1935 Europeans organized an entire scientific congress devoted to supersonic flight. They met in Italy, whose air force had already established the world’s only high-speed-flight research squadron. Three years later a high-level study by the U.S. Navy stopped research on jet propulsion, concluding that gas-turbine engines would forever be too big to power anything smaller than ships.

Still, U.S. pilots couldn’t help nibbling at big-time Mach percentages, for even their piston-engine airplanes had become so sleek and heavy that gravity could pull them to speeds where they butted up against the phenomenon called compressibility. “We knew about Mach 1 going clear back to the P-36 and the P-40,” said the late Herbert O. Fisher, the former chief production test pilot of the Curtiss-Wright Corporation, which manufactured those early Hawk fighters—the retractable-gear successors to the big biplanes. “Nothing could go 600 mph in level flight, but pilots were beginning to dive fighters. We ran into compressibility back in ’38.”

The mystery of compressibility had already created one of those say-it-all catch phrases—like ‘”cold fusion” and “computer virus”—that reporters love because it characterizes something that they lack either the space or the understanding to explain. Not many people remember W.F. Hilton, a British aerodynamicist, or the reporter who in 1935 asked him about the purpose of the National Physical Laboratory’s new high-speed wind tunnel. Everybody remembers what Hilton said, though. He displayed a graph plotting the abrupt increase in airfoil drag as its speed nears Mach 1. “See how the resistance of a wing shoots up like a barrier against higher speed as we approach the speed of sound?” he explained. Barrier…speed…sound…Sound Barrier!

The imagery took hold. Twenty years later, Douglas D-558 test pilot William Bridgeman described flying on “the reef of the sound barrier, where compressibility lurked to shake a plane to pieces or suck it out of control straight down into a hole in the ground. As a result of combat demands, aircraft had to be flown right into this monster.”

To those unfamiliar with the science behind the buzzword, “sound barrier” may have the same effect as “time warp,” conjuring up some kind of boundary between safe, understood reality and a mystical zone of perverse forces. Hilton brought up the subject of sound for a very good reason, however, because of the way molecules of air respond to a disturbance in their midst. A molecule at the point of disturbance, which could be an airplane beginning to move, a lightning bolt rending the air, or a human voice, bumps into the next molecule, and that molecule into the next, and so on, like a line of falling dominos. This is exactly how sound is transmitted. Excite those air molecules too fast, however, and the molecules don’t just nudge the next ones on, they bunch up like commuters in a Tokyo subway. They compress and form a shock wave (see “Piling On,” below).

“The pressure of an oncoming aircraft is transmitted to the air,” explains Howard Wolko, special advisor for technology at the National Air and Space Museum. “As the airplane goes faster and faster, it gives a shorter and shorter signal, and the air can’t prepare itself. And when that happens, Bernoulli’s Principle goes to hell in a handbasket.”

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