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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.)

Mach 1: Assaulting the Barrier

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

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.”

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.

Even more fascinated was NACA engineer Robert Gilruth, who realized that some of the local airflow over the wing of Cooper’s Mustang was going nicely, controllably, predictably supersonic. The NACA had been trying to do transonic research by dragging high-speed models to extreme altitudes aboard a B-29 and an F-82 Twin Mustang and then dropped them straight down onto a bombing range near Langley, Virginia—getting brief bits of data by tracking the plunging models with radar or by laboriously digging them out of the mud and reading the instrument recording that had survived.

Putting a tiny model atop a strut or “sting” on a P-51’s wing, right in the airflow that in places had accelerated to speeds of Mach 1.4, seemed much neater. From this experience, engineers realized they could reproduce the same effect right in their transonic wind tunnels by mounting models atop wing-shaped “bumps,” where they would also encounter supersonic air.

WHEN THE U.S. Army Air Forces imported Frank Whittle’s jet engine from England in 1942, the Jet Age did not arrive with it. The Bell P-59, the Air Force’s first operational jet-fighter, was still too slow to get itself in trouble. The next jet, the Lockheed P-80, was fast enough to suffer from aileron buzz caused by the capricious dance of shock waves on its control surfaces. But again the problem was encountered only in steep dives and again was counteracted by fortifying the control surfaces.

Not until after World War II did aircraft design take the radical turn toward supersonic flight that the jet invited, producing airplanes that looked strikingly different from their subsonic forebears. The changes derived mainly from two sources of information: NACA high-speed flight research and the scientific war spoils from Germany. The first easy answer to going faster, which turned out to be swept wings, came from both.

The NACA’s Robert Jones had been quietly studying the effect of sweep-back on the lift of large-span wings at the research lab in Langley, Virginia. He completed a formal report in April 1945, which the NACA issued on June 21 to military services and companies with security clearances. That May, Boeing engineer George Schairer accompanied von Kármán, then the Army Air Forces’ chief scientist, on an intelligence gathering mission to a once-secret aeronautics research installation in Braunschweig, Germany.

Poking into various offices in the laboratory, Schairer and von Kármán came upon a small model of an airplane with swept wings. Both had been closely following Jones’ studies and were anxious to get their hands on anything relating to the design of the model, but the sullen German aerodynamicists in Braunschweig shrugged off their questions. Von Kármán decided to play good cop/bad cop. Though the Soviets were nowhere nearby, he turned to his assistant and loudly said, “We’re through here. I think now it’s time to notify Russian intelligence to take over.” Terrified by the thought of a Soviet debriefing, the German director of engineering took von Kármán’s assistant to a nearby drywell and showed him where they had dumped all of their best research, including considerable wind tunnel data on the behavior of swept-back wings on the transonic regime.

Schairer immediately wrote Boeing headquarters, telling the company to stop work on a straight-wing dodo of a Mach 1 design that was already well under way. He returned to Seattle with microfilmed German data that resulted in the Boeing B-47, progenitor of the 707 and the B-52. The NACA’s equivalent data from Jones’ report convinced North American to put swept wings on a somewhat refined Air Force version of the FJ-1 Fury—a slow, tubby, straight-wing Navy jet that had already gone into limited production. The result was the F-86 Sabre, the first operational fighter in the world routinely capable of flying faster than sound (though it took a slight dive to do it) and one of the most aesthetically pleasing and operationally successful aircraft ever built.

Why the Bell X-1 challenged the “sound barrier” with straight wings when those of airplanes all around it, including its sibling rival, the Douglas D-558-2 Skyrocket, were swept has been a matter for interpretation since 1945. (The wings of the Douglas D-558-1 Skystreak were also straight.) When the Army Air technical Service Command awarded Bell the contract in March 1945, its engineers had already been briefed by wing sweep champion Robert Jones. But when word came back from Braunschweig, Army Air Forces General Alden R. Crawford accused the NACA of incompetence for not insisting on swept wings. The NACA responded by claiming prudence and pointed to the dearth of experimental evidence, especially at low speeds. A few who witnessed the petty spats between the two organizations later in the program have suggested that the NACA knew better but was determined to give the Army exactly what it asked for, which was a straight-wing rocket-propelled airplane, instead of the turbojet-powered design that the NACA favored. Whatever the reason for them, the straight, thin wings that carried the X-1 blithely past Mach 1 proved there was more than one way to skin a cat (see “Don’t Make Waves,” below)

One advantage straight wings had over swept-wing designs was rigidity: the more radically a wing was swept, the less torsionally rigid it became. This is why early swept-wing jets sometimes suffered “aileron reversal,” which was probably one source of the common misconception—helped along by Hollywood and the mid-1950s English classic film Breaking the Barrier—that “the controls reversed” as an airplane approached the speed of sound.

When the right aileron, say, on a weak-winged fighter such as the McDonnell F3H Demon was deflected upward at subsonic speeds, it would, as expected, command a roll to the right. But at near-supersonic speeds air pressure against the raised right aileron instead warped the trailing edge of that wing down, thus turning the entire right wing into an enormous aileron that commanded a left roll, to the pilot’s bafflement. Some airplanes could literally perform aileron rolls in the direction opposite full stick deflection, and F3Hs were known to return from combat practice maneuvers with permanently warped wings.

The solution to the problem was to design wings with greater rigidity. The English Electric Lightning, although sounding like a household appliance, somehow achieved adequate rigidity despite an enormous amount of wing sweep: a 60-degree angle between the leading edge and a line perpendicular to the fuselage centerline. Only a single Soviet fighter—the Sukhoi Su-7—and delta-wing aircraft such as the Concorde ever equaled or exceeded the Lightning’s arrow-shaped sweep.

Even fewer have equaled the Lightning’s ability to go supersonic in level flight without using afterburner, a talent so rare, in fact, that it wasn’t until September 1989, 35 years after the Lightning first flew, that an experimental F-14 Tomcat with a special engine demonstrated “supercruise”—supersonic cruise without afterburners—for the first time. And that happened only after the aircraft was boosted past Mach 1 on full burner.

In 1953, one year before the English Electric Lightning did it, the North American F-100 Super Sabre became the first fighter in the world to fly faster than sound in level flight, though by benefit of an on-or-off afterburner that in a single unthrottleable torrent of kerosene raised the engine’s thrust by half. The pride of the U.S. fighter fleet in the mid-’50s, the F-100 demonstrated how little could even then be taken for granted about supersonic flight.

North American was desperate to get the F-100 into production, so when Air Force test pilot Pete Everest turned down the Super Sabre as having some unacceptable handling qualities, North American put the brute in the hands of a group of young tigers from the Tactical Air Command. They all thought it was neater than a wet T-shirt contest and far more exciting than the F-86s they’d been flying. The Air Force’s claque outvoted Everest, and F-100s started coming off the production line.

In 1954 they also started coming apart, killing five pilots, including the North American factor test pilot who’d okayed the airplane in the first place. It turned out the original F-100A Super Sabre had such a long, heavy fuselage atop short, heavily loaded wings that it wanted to go sideways or tumble—or preferably to do both at once, called “yaw coupling.” And when it inevitably did, the resulting force tore off the vertical tail. Later models were given a larger and considerably stronger tailfin.

THE TRANSOCEANIC regime was still a mysterious one in the 1950s. Airplane builders confronted it by equipping their machines with enormous engines and afterburners, and designers expanded wings into full deltas or shrank them into short, skinny vestiges of wings like those on the F-104 Starfighter, Lockheed’s missile-with-a-man-in-it. But sometimes even this combination of brute force and creative extremes was not enough to wrest speed from an unwilling atmosphere, and in 1952 Richard Whitcomb discovered why.

Whitcomb, one of the most productive transonic aerodynamicists in the world, would go on to develop Whitcomb winglets—the vertical wingtip extensions seen on advanced transonic aircraft from the 747-400 to recent Learjets—and would play a major part in the development of the supercritical airfoil for more economical transonic cruise. As Adolf Busemann, the German inventor of the swept-wing concept, once said of Whitcomb, “Some people come up with half-baked ideas and call them theories. Whitcomb comes up with a brilliant idea and calls it a rule of thumb.”

Shock stall—the effective loss of lift caused by supersonic perturbations on the wing—is one of the two Katies barring the door to supersonic flight. The other is wave drag—the increase in air pressure, or resistance, against the entire airframe shouldering its way through transonically compressed air. Vintage aeronautical engineering had concerned itself almost entirely with airfoil cross-section, as though the wing was the only significant part of an airplane and was simply a three-dimensional extension of a two-dimensional curve. As airplanes began to probe the transonic region, designers realized the wing had to be studied as a whole—that its sweep, shape, structure, and aeroelasticity all had to be considered together.

And now, as true supersonic flight revealed a whole host of unexpected control, flow, and drag problems, designers discovered that every external component of the airframe—wings, fuselage, tail, engine pods, intake ducts—affected the others so strongly that the entire vehicle had to be studied as a single entity.

Whitcomb’s Area Rule was the first major outgrowth of this revelation, and the need for it was demonstrated by one of the great slips ’twixt aviation’s cup and lip, the Convair F-102. The F-102, the first U.S. pure-delta design put into production, was based on some German World War II research and wing tunnel data on components. Unfortunately, the wind tunnel data was incomplete—“perhaps the most outstanding case in the history of aviation of full-scale drag proving to bear little relation to drag measured in the wind tunnel,” says writer Bill Gunston. There was no way the supposedly supersonic F-102 could get anywhere near Mach 1.

What was creating the unexpected drag, it turned out, was a considerable mismatch between fuselage and wing. Whitcomb’s brand-new Area Rule said that if you graphed the area that an airplane presented head-on to the air, along an axis extending from the tip of its nose to the end of its tail, the resulting line, a measure of square footage at each point, should come out as a smooth curve. Spikes in the curve, created by the sudden addition of a wing to the cross-section just where the fuselage was fattest, say, meant enormous drag.

The easiest way to flatten such spikes was to locally decrease the fuselage cross-section if that’s where the wing needed some breathing room. The result was the grotesque wasp-waistedness of the hastily modified production F-102 and its far faster successor the F-106. The similarly Coke bottle-shaped Grumman F11F Tiger was the first airplane to be designed from inception to take advantage of the Area Rule and as a result was the first Navy line aircraft to pass Mach 1 in level flight.

IN AN ERA WHEN prosperous airline passengers can fly at twice the speed of sound, aerodynamicists have learned a lot about supersonic flight. But even if the sound barrier is down, the lift spoiler remains. It limits the practical flight speeds nearly as firmly as did that imaginary sonic wall. To fly faster than sound—to achieve lift despite shock spikes and to shove aside wave drag—requires massive doses of power or fuel or money. Or, more likely, all three.

In the 1960s, brute force and extreme airframes vastly boosted fighter speeds, and soon everybody—French Mirages, Swedish Viggens, Soviet MiGs, U.S. Century Series jets, even bombers—was routinely doubling the Mach. But after it was discovered that airplanes could fly at two, three, even four times the speed of sound, a strange thing happened: For the first time in the history of flight, designers applied the brakes. Today we are flying slower than we were 20 years ago.

The cream of our military crop, although capable of much higher speeds, clusters on the classic transonic band, cruising and maneuvering at between Mach .8 and 1.2. For commercial aviation, the infrastructure is already far behind the airplane. The complexity of air traffic control, the congestion of runways, the limited access to airports, and the economics of what is becoming a 21st century mass-transit system have made supersonic flight with foreseeable technology irrelevant for all but the most limited and premium applications.

The need for speed seems to have been satisfied. Perhaps. But maybe this is just a Mach .8 plateau, where we rest and await the development of aircraft shapes that create comfy little low-drag shock ripples rather than waves; of airfoil curves that produce lift without sonic drag; of airplanes that reflect all their booming shock-created energy up, toward noiseless space.

A well-known aerodynamicist and aviation entrepreneur who is either laughably optimistic or an unrecognized visionary recently insisted all this was possible. Then he told the story of an orphan he adopted some years ago. When he first met the child, he asked what the boy wanted to be when he grew up. “Oh, I can’t tell you that,” the boy said. “It might not have been invented yet.”

#####

What’sAckeret 1?

Ernst Mach, the man whose name has become synonymous with high-speed flight, never saw an airplane that traveled much more than one-tenth the speed of sound, for he died in 1926. Mach published the work that resulted in the concept of “Mach number” in 1887, 15 years before the airplane was invented. He hadn’t the slightest interest in aircraft and was actually studying the flight of artillery rounds when he did his pioneering work on quantifying the speed of sound—and was doing it largely as an outgrowth of some photographic laboratory techniques he had developed to study sound wave propagation from meteorites, explosions, and projectiles.

How odd that two of the most significant technological achievements of our lifetimes both devolved in part from the arcane science of ballistics: not only Mach’s work on supersonics but the development of computers in the 1940s to create precise artillery firing charts for gun-laying.

Nonetheless, Mach wouldn’t be pleased that he’s universally remembered as a result of that virtually inconsequential experimental dalliance rather than for his work as a psychophysiologist, his criticism of classical physics and mechanics, or his contributions to Einstein’s theory of general relativity. Who remembers Mach’s band? (Not a ragtime group but a phenomenon that relates the physiological effect of spatially distributed light stimuli to visual perception.) Anybody you know been quoting the Mach principle? (Einstein’s term for Mach’s claim that the inertia of an isolated body can have no meaning.) Who gets any crossword puzzle mileage out of Mach angle? (The actual object of his supersonics research—the angle between a shock wave and the direction of motion of the object creating the wave.

No, its “Mach number” that will go down through the ages as the legacy of this stubborn, brilliant, and multi-talented Czech scientist-philosopher.

But don’t feel sorry for him. Save your laments for Jacob Ackeret. Remember him? He was the director of the Institute of Aerodynamics at the Swiss Federal Institute of Technology, and in 1929 he suggested the term “Mach number” for the ratio of the speed of an object to the speed of sound in the medium within which the object is traveling. If he’d had a good PR guy, we might today be quoting Ackeret numbers, discussing Ackeret-2 Concordes and determining critical Ackeret.

 

The Sonic Boom

They don’t call them “booms” for nothing. If you’re imagining rolling thunder, think instead of a cherry bomb in a trash can. Given the right combination of atmospheric conditions, altitude, and airplane—particularly its weight, shape, and maneuvering configuration—the sudden arrival of a shock wave can sound like trains colliding. Oddly enough, one parameter that matters little is speed: Above a certain Mach number, a sonic boom can actually weaken as the airplane goes faster.

The first human-made sonic booms (supersonic bullets and artillery shells aren’t heavy enough to create the phenomenon) were trailed by German V-2 rockets reentering the atmosphere, rattling English windows far below as the missiles headed London-ward to do far worse.

In 1949 sonic booms were still rare enough that the San Francisco sheriff’s department dashed about trying to find the source of mysterious “explosions” reported by worried suburban residents. Turns out that the NACA’s nearby Ames Laboratory had just acquired an early F-86 Sabrejet, the first production aircraft that could go supersonic, and two NACA test pilots were routinely doing just that.

The British soon discovered that sonic booms were so spectacular you could build entire airshow routines around them. The Brits had experimented with using booms as weapons, maneuvering an aircraft to “throw” the thunder at structures. Concluding that breaking the enemy’s dishes really wouldn’t change the course of the battle, Royal Air Force pilots nonetheless perfected the technique of pitching the sound at the crowd during the then annual Farnborough Show, and it got so the size of your boom was a greater measure of pilothood than wristwatch complexity.

This all came to a tragic end in September 1952, when de Havilland test pilot John Derry pulled apart a D.H. 110—the prototype Sea Vixen—trying to make a yet-bigger boom. One of the 100’s two engines somersaulted lazily into the Farnborough crowd, killing 28 and injuring 63, forever making anathema at Farnborough, and throughout the United States as well, the act of flying directly toward an airshow crowd.

Air Force pilots boomed for the fun of it during the ’50s. “We used to hear sonic booms all the time when I was teaching at Texas A&M,” reminisces Howard Wolko, a curator at the National Air and Space Museum. “Whenever a graduate joined the Air Force and finished training, he’d come back and buzz the campus supersonically. You’d hear it and just think, Oh, yeah, another Aggie got his wings.”

That tradition ended when one fighter job centerpunched his own field with a particularly powerful shock wave created by a dive to 8,000 feet. He broke windows, loosened doorframes, and cracked ceilings all over the base, ending the era of casual sound-barrier breaking, although isolated incidents continued to cause damage and injuries.

Strain gauges installed in houses that have been subjected to sonic booms show that the increase in pressure distorts a small building by a fraction of an inch. Two shock waves travel with an airplane flying faster than sound, one at the nose and one at the tail. The passage of the first almost instantly raises the air pressure slightly. In the 0.3 seconds or so before the second shock, the pressure rebounds to slightly less than ambient. The arrival of the second shock restabilizes it. This doubly whammy not only breaks windows but creates that characteristic boom-boom of a low-altitude supersonic flight.

The intensity of sonic booms is measured by the increase of pressure, or “overpressure,” and it can be doubled and tripled by maneuvers such as pull-ups and steep banks. Since the shock wave trails from the airplane at an angle, it’s thought that the ultimate sonic booms are produced by dives just steep enough to cause the entire “face” of the shock wave to arrive on the ground at once.

The all-time Glass-Makers’ Appreciation Award, however, should go to the U.S. Air Force F-104 pilot who in November 1959 buzzed an uncompleted terminal at Ottawa’s Uplands Airport at 500 feet and did $300,000 worth of damage to windows and the roof.

Little more was known about sonic booms until the mid-1960s when the Federal Aviation Administration, assigned to determine the social acceptability of supersonic transports, did what it claimed was more research on noise than had been done in the whole of human history.

During 1964 the FAA had the Air Force wallop Oklahoma City with eight sonic booms every flyable day for six months, using the area’s 700,000-odd inhabitants as unwitting and unwilling subjects in an experiment to determine society’s threshold of auditory pain. Century-series fighters and B-58 Hustlers played the city like a drum, which doubtless is why the project was code-named Operation Bongo.

The effects of the booms ranged from the inevitable to the incredible. One victim claimed that her bra strap snapped whenever the Air Force lowered a particularly loud boom. (“The engineers had a strain gauge all fixed up for her, but she wouldn’t give her name,” said one FAA spokesman.) Another Sooner said the daily 7:00 a.m. blast scooted her bed across the floor. A third claimed her furniture was shrinking. But some effects were serious. A high school student was beaned by a light fixture jogged loose during history class. An Oklahoman with hypertension was ordered by his doctor to leave the city for the duration of the tests, for the sound of the booms made him so furious that he quite literally could have died.

In all, the FAA got 15,116 complaints during the Oke City tests, sometimes as many as 500 a day. Many of them were simple noise gripes, but legitimate suits for property damage ended up costing the government $123,070 in the five years following the experiment.

Dismayed by the negativity of the Oklahomans and curious about how much of the claimed damage was boom-caused, the FAA built a small mock village on the White Sands Missile Range in New Mexico and bombarded it in January 1965 with F-104 thunderclaps producing as much as 10 pounds of overpressure per square foot—far more than the Oklahoma test’s levels.

Nothing broke, so the FAA proudly called in the press and did the demo for the TV cameras. Asked to make one more pass (“and get it real low so we can get a good shot…”), the Starfighter pilot got carried away: his 39-pounds-per-square-foot blast turned the town into what could have passed as the set for a spaghetti-Western barfight, with broken glass everywhere. All the FAA could salvage from the PR shambles was the claim that 2,000 chickens hatched from eggs that had been subjected to the sonic booms had a higher fertility rate than those developed in reverent silence.

In 1965 the Air Force made a series of supersonic mock attacks on Chicago with B-58 Hustlers. Among other damages, the entire plaster ceiling of a large conference room in an Evanston church collapsed during one run, and during another a 14-year-old boy took an 11-stitch cut from an exploding pane of glass in his high school classroom. Though sonic booms were sometimes referred to in those days as “the sound of freedom,” 2,520 unimpressed Chicagoans filed damage claims and were paid over $65,000, just over $1,000 per bombing run.

Mach 1 With a Propeller?

“There I was at 40,000 feet,” the young Army Air Forces pilot begins—really—“in the AAF’s latest P-47N with a very specific purpose in mind, mischievous as it was.”

Lieutenant Raymond Hurtienne’s purpose, it turns out, was to break the speed of sound in a propeller-driven aircraft, and he swears he did it, in the spring of 1945, in the skies above Long Island. “I rolled her over, pointed her straight down, retarded throttle, full left trim and full forward stick,” he wrote in a letter to a P-47 pilot’s association. “As the speed increased, control responses became more and more rigid. The airspeed indicator became stuck against the peg at 575 mph. Vapor trails were forming at both wingtips. The stick seemed like concrete. The altimeter was unwinding at a terrific rate. This was it: I had hit Mach 1. There wasn’t another plane in the skies that could touch me.”

That sort of thing seriously griped Herbert O. Fisher, and mischief had nothing to do with it. Herb Fisher, who died last July at the age of 81, made a living diving Republic P-47 Thunderbolts to their absolute maximum controllable airspeed while he was a test pilot for Curtiss-Writght’s propeller division after World War II. He would have been the first to tell you that neither he nor anyone else ever put a World War II piston engine aircraft through the sound barrier. Or, as fellow test pilot Tony LeVier once put it, “Anyone who did ain’t here to tell about it.”

Fisher wore the high-belted pants and tucked-in tie of a man in his 80s, but his eyes were clear, his voice strong, and he had an easy laugh. He chuckled about the radar detector in his big station wagon (New Jersey plates P40P47, for the two airplanes that were his specialty), saying he never drove faster than 60 mph anyway. But once he routinely did 10 times that speed in a P-47—an airplane built for a real-world maximum of a little over 400 mph. Fisher made over 100 high-speed descents from altitudes as high as 38,000 feet and achieved instrument-verified airspeeds of Mach .83 (about 600 mph at that altitude)—but no higher.

“The first time I heard this sort of thing was an Air Force pilot who came out with publicity that he went Mach 1 in a Thunderbolt over Europe,” Fisher groaned. “There’s no way he could have gone Mach 1, but he still believes it. He’s still out there preaching it.”

Other U.S. and British pilots have claimed to have done it as well, recalling flights in Spitfires, P-38s, P-47s, and P-51s. But there is one basic, irrefutable reason why their claims are, as Fisher might have put it, malarkey. A propeller—even one designed to current state-of-the-art standard for maximum efficiency—continues to create thrust up to a point somewhere short of supersonic. At that instant, it suddenly loses efficiency and begins to create not thrust but enormous drag. “It becomes a flat plant,” Fisher said; “a big brake.” One Spitfire pilot, who attained the highest verified speed, Mach .9, achieved by a World War II propeller-driven aircraft, discovered this in a big way when the sudden braking forces became so powerful during a dive that the entire propeller and most of the engine cowling broke off.

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.

Sweeping the wing back has the same effect as reducing the ratio of thickness to chord and therefore also requires less work from the air. Since a swept wing meets the air at an angle, the distance the air must travel from the leading edge to the trailing edge of the wing is longer, and its acceleration is more gradual.

The supercritical airfoil is a much more recent invention, again by Richard Whitcomb, for delaying the formation of shock waves. The airfoil gets its name from the term “critical Mach number,” the speed, usually around Mach .7, at which the first shock waves appear on an airplane’s wing. Whitcomb found that by flattening the upper surface of the wing, he could both weaken the shock waves and push them aft to a point closer to the wing’s trailing edge. Reducing the camber, or curvature, of the wing also sacrifices lift, however. Whitcomb counteracted this effect by increasing the camber of the trailing edge. With a supercritical airfoil, the airplane can boost its critical Mach number; that is, it can fly faster without the loss of lift and increase in drag caused by the formation of shock waves close to its wing’s leading edge.

SIDEBAR: Not So Fast

Until the Navy develops supersonic submarines, Mach number will inevitably apply to objects zipping through the atmosphere. (A space vehicle might travel seventy-seven thousand miles an hour, but it’ll never break Mach 1 Out There: there is no sound, ergo, no speed of sound, in the vacuum of space.) But strictly speaking, Mach number is the ratio of the speed of an object to the speed of sound in whatever medium it’s traveling through. So Mach 1 for a bullet going through a bar of soap or a nail being hammered into a two-by-four by a particularly powerful carpenter will be quite different from Mach 1 for Tom Cruise buzzing the Miramar Tower.

Sound’s speed also changes with the temperature of the air, not simply its density, and though it’s typically about 742 mph at sea level, the strongest shout will generally travel only 661 mph anywhere between 26,000 and 60,000 feet, the band of pre-stratospheric tropopause at which air temperature normally remains constant.

To further complicate matters, it now turns out that H.C. Hardy, the physicist who established those specifics in 1942, based them on a dose of…well, maybe not bad air, but some pretty ordinary atmosphere literally picked up from a breeze through his lab window. Recent work by George S.K. Wong of the National Research Council of Canada has determined that the speed of sound at standard sea level conditions is not 741.5 mph, but 741.1. “You know physicists,” Wong said. ‘They always calculate things assuming this, assuming that.” Unfortunately, he did not know how reliable his corrections were.

 

Piling On

If you’ve ever seen the circles rippling outward from a pebble tossed into a pond you can imagine the invisible disturbance waves that radiate in three dimensions from an airplane—or any other disturbance—in the atmosphere. Disturbance or pressure waves are the very phenomena that transmit sound; the speed at which they pulse outward, therefore, is also the speed at which sound travels.

An airplane moving slower than the speed of sound stays comfortably behind the forward-moving wave fronts, which perform a certain service for the airplane by preparing the air for its arrival. At this speed the airplane politely bumps aside the air molecules in its path. Each molecule has time to move out of the way and to nudge its neighbor out of the way as well. Pick up the pace, however, and the wing (and/or the tail, cockpit canopy, radome, or any other airframe component) of an airplane flying close to or faster than the speed of sound arrives before the air knows it’s coming.

As an airplane increases speed, the wave fronts ahead of it get closer and closer together. When the airplane reaches Mach 1, the wavefronts overtake one another and pile up in a concentrated front, a “shock wave.” A shock wave marks an instantaneous change in air pressure, temperature, and density.

Past Mach 1, the combined motions of the airplane and the pressure waves still radiating outward form a conical front, which moves continuously with the airplane and which engineers call a “Mach cone.”


This article first appeared in the December 1990/January 1991 issue of Air & Space.

 

 

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