It’s not that air forms a wall of any sort—a “sound barrier”—though it is indeed compressed to a greater density than the ambient atmosphere. The problem is that the shock wave that develops at some point on the airframe, almost invariably first atop the wing, acts like a spoiler, ruining the airflow and therefore the lift. But “Breaking the Lift Spoiler” would never sing as a movie title.
“There never was a sound barrier, and I don’t think any serious engineer ever thought there was one,” muses Wolko, who was on the engineering team of the supersonic Bell X-2 project. But as Kelly Johnson learned when he lost his test pilot, engineers had come up against some kind of barrier: a control barrier or a knowledge barrier or, as one engineer described it, “a wind tunnel techniques barrier.” As the frenzy of production to meet combat demands intensified in the early 1940s, one high-performance aircraft after another found the invisible enemy that killed Ralph Virden. Early models of the Republic P-47 Thunderbolt, the Curtiss SB2C Helldiver, and the Bell P-39 Airacobra all broke apart in dives.
“You can imagine their frustration,” says aerospace historian Richard Hallion, who has written several books about the U.S. engineers and pilots who pushed into supersonic flight. “Their best airplanes were falling out of the sky, and they didn’t have wind tunnels that could give them accurate data at the speeds where the airplanes were running into trouble. They had just solved the propulsion problems; they could see jet engines on the horizon. And now here was another altogether different obstacle they had to overcome. And they didn’t have the research tools to do it.”
After Ralph Virden crashed, Kelly Johnson, desperate to find a cure for the P-38’s woes, sent a model to the National Advisory Committee for Aeronautics for wind tunnel tests. The NACA suggested an elegant solution to the problem, an all-moving, trimmable horizontal stabilizer, one of the design features that allowed the Bell X-1 to maintain control as Chuck Yeager flew it “through the sound barrier” on October 14, 1947. But in the middle of a war, with almost 700 of the fighters on order, the company couldn’t afford the time for the redesign.
Instead, the NACA developed small, wedge-shaped “dive flaps” that were popped out of the underside of the wing at the first sign of Mach tuck. Many to this day assume the dive flaps simply slowed the airplanes below shock wave speed, but the truth is that they restored enough of the wing’s lost lift to enable the pilot to pull out despite the tail’s recalcitrance. They worked well enough to also be installed on some of the P-38’s contemporaries: P-47 fighters, A-26 attack bombers, and the two earlier U.S. jets, the P-59 and P-80.
Fixes like these merely delayed the control problems past Mach .675 into the troublesome speed band on either side of Mach 1, from about .8 to 1.2, a region that engineers call “transonic.” (NACA director Hugh Dryden and Theodore von Kármán of the California Institute of Technology coined the term. Dryden wanted to spell it “transonic,” which, strictly speaking, is correct—“across the speed of sound.” Von Kármán, who presumably would also have voted for crossection over cross section, prevailed.) Transonic denotes the range of speeds between formation of the first shock wave and the speed at which the entire wing has “gone supersonic” and is no longer encountering a troublesome mix of subsonic and supersonic airflow. At this point, an airplane has not only passed Mach 1 but also achieved stable, trimmed, controlled flight faster than sound.
Early experiments in transonic flight were dicey, intuitive affairs. In one experiment, for example, the NACA arranged to have a propeller-less P-51 towed aloft like a glider by a big twin-engine Northrop P-61 night fighter. The engineers were trying to get real-world figures exactly comparable to P-51 high-speed wind tunnel data in order to assess how accurate the wind tunnel was, so they needed to eliminate such factors as prop and even exhaust thrust.
Unfortunately, on one early flight in California the double-cable tow tether—like the pull rope on a child’s sled—came adrift from the P-61 before the Mustang could cast itself loose and begin the glide back to what was eventually to become Edwards Air Force Base. The metal lines snapped back and wrapped themselves firmly around the Mustang, quite complicating the already-necessary deadstick landing. Jimmy Nissen, the NACA pilot, bellied the P-51 in on the Muroc dry lakebed, but the flailing cables took out all of the base’s powerlines in the process. It was fortunate that Nissen didn’t break anything during the rough landing, for when he got to the hospital there was no current to run the X-ray machine.
The P-51 was of special interest to the pioneers of supersonics because among fast World War II fighters, the Mustang seemed the most resistant to high-speed controllability problems. Apparently its unique laminar-flow airfoil managed to keep the airflow attached despite shock wave-induced perturbations. The P-51 could dive faster, under control, than any other World War II fighter. In 1946 and ’47, Chuck Yeager in a P-51D with full instrumentation and cohort Bob Hoover in a P-47 dove “straight down,” wide open, from as high as we could go,” Yeager later wrote to a friend. Yeager reached Mach .81 in the Mustang and Hoover managed .805 in the bluff, radial engine Thunderbolt.
So it was a P-51 pilot—NACA engineering test pilot George Cooper—who first manipulated supersonic shock waves in flight. Cooper discovered real world evidence of the wind tunnel phenomenon called the Schlieren effect, created when light is refracted by the denser shock wave air. The phenomenon is visible either under controlled wind tunnel lighting or when the angle of sun and wing are just right. Cooper, fascinated, was able to make the shock wave move aft as he increased dive speed, move forward as he positioned the aircraft to increase lift, and dance back and forth—or “buzz”—at a specific Mach-versus-lift value. (His NACA test Mustang was amply equipped with instruments.) The buzzing coincided with the control buffet, for at that speed the unstable shock wave was disturbing airflow over the P-51’s control surfaces.