Was Chuck Yeager the First to Break the Sound Barrier?

by Brian Dunning
May 19, 2009

We all know the story of how Captain Chuck Yeager opened the throttles of the Bell X-1 Glamorous Glennis in October, 1947. Breaking the sound barrier was to aviation what Neil Armstrong's first step was to the space program: No matter how many others went higher or faster later, it will always be that seminal, unassailable "first" that can never be topped. Yeager's name will always sit atop every list of record-breaking pilots, up there by himself in his own special stratosphere. But: Was he really the first pilot to fly faster than sound?

Plenty of stories out there say Yeager wasn't the first. How do we know what to believe? Do we accept the popular official story, or do we give credibility to the other claimants with good evidence of their own? Today we're going to point our skeptical eye at some of these other claims, and see who really deserves the credit.

There are certainly many pilots who approached the sound barrier but didn't live to tell about it. The years preceding Yeager's flight were among the most exciting in aviation history, as World War II drove aeronautic advancement like never before. Planes that had been shot down often entered the transonic realm as they plummeted, and were torn apart by the resulting shockwaves. Dive bombers had to have special air brakes developed to prevent them from breaking up, which sometimes happened anyway. Of the many pilots who toyed with the sound barrier in WWII — all unintentionally, of course — most never survived the adventure.

During WWII, engineers didn't yet have any flight test experience that taught us how to design aircraft capable of supersonic speed. Even in 1947, Yeager's X-1 was designed after a 50 caliber bullet, known to be stable at supersonic speeds. WWII had seen widespread use of the German V-2 rockets, which were supersonic, so we knew such flight was possible. But the V-2 was ballistic, it didn't require a controllable airframe; and designing a supersonic controllable airframe was the problem for aeronautical engineers. The main issue is called shock stall, and it's what happens when a control surface approaches the speed of sound. A shockwave forms around the control surface, rendering it useless, and the pilot has no way to control the aircraft.

Propeller aircraft can never reach the sound barrier, since the tips of propeller blades hit the sound barrier before the rest of the plane does. The propeller blades go into shock stall, and the plane can no longer accelerate. There are many claims of propeller driven dive bombers breaking the sound barrier during WWII, but these have to all be considered implausible. Approaching the sound barrier, an airplane is already well above its terminal velocity, the speed at which drag matches the acceleration imparted by gravity. Propellers are shock stalled, and there is neither thrust nor gravity available to accelerate a diving airplane past a certain point. As any aircraft approaches the speed of sound, airflow over some parts of the plane will exceed Mach 1 and create shockwaves. These shockwaves cause intense buffeting. Many propeller driven WWII fighter planes, including the Supermarine Spitfire, the Lockheed P-38 Lightning, and the North American P-51 Mustang, experienced these effects at Mach 0.85. Similarly, jet engines of the day were not designed to work with supersonic airflow entering through the compressor vanes; such engines would flame out.

However, one particular fighter plane of WWII was not driven by either propellers or jets: The German rocket powered Messerschmitt 163 Komet. The Komet was designed by the great Alexander Lippisch, a pioneer of delta wings and ramjets. By the end of WWII, Lippisch had a test glider of a supersonic ramjet powered aircraft actually undergoing flight tests. He understood the requirements of supersonic flight. The Komet was designed to fly as fast as possible while staying under the critical limits at which trouble happens. The Komet's delta wing was exceptionally thin. This delays the onset of shock stall over the primary airfoil, and allowed the Komet to remain stable up to Mach .85. However, Lippisch had no answer for shock stall at the control surfaces on the trailing edge of the delta wing, and so the Komet was destined to remain subsonic. In combination with the relatively low 3,800-lb thrust of the Komet's Walter rocket engine (about half the power of Yeager's X-1), the Komet had little expectation of going supersonic, except in an uncontrollable, powered dive which would probably be unrecoverable.

Nevertheless, stories persist of Komet pilots breaking the sound barrier, years before Yeager did. Komet test pilot Heini Dittmar, flying an early prototype in 1941, reached an officially measured speed of 1,004 kph in level flight. This was probably around Mach .95 but we don't know for sure since the flight was classified until after the war and the altitude is unknown. However, Dittmar made the flight at partial throttle to avoid buffeting, using an engine only half as powerful as that which went into production. But just because later versions were more powerful doesn't mean they wouldn't run into exactly the same limitations at the same top speed. One unofficial report claims that Dittmar hit 1,130 kph in 1944 (Mach 1.06), and another states that in a steep dive he created sonic booms that were heard on the ground. However, these stories first appeared in 1990 book written by Dittmar's friend Mano Ziegler, and do not have contemporary corroboration or documentation. But the best evidence against Komets breaking the sound barrier is the fact that the Allies did capture all of the program's classified data, and no supersonic flights were ever recorded, even in secret.

Claims you'll find on the Internet that "Komet pilots routinely broke the sound barrier" cannot be given much weight, given the aircraft's limitations well understood by Alexander Lippisch. As an aircraft approaches the speed of sound, the shockwave over the wing moves the center of lift backwards and the plane noses down. This is a condition called Mach tuck. Normally you'd pull back on the stick to correct this, but conventional elevator controls on the trailing edge of the tailplane would be unable to get any bite, since the elevators would be shock stalled. The only way out of Mach tuck is to use an all-moving tailplane to trim back to level. With its delta wing, the Komet had no tailplane at all, let alone an all-moving tailplane. It had fabric-covered elevons on the trailing edge of the delta wing, which would always be shock stalled. Even if a pilot opened his Komet's rocket engine to full throttle to muscle his way past the sound barrier, Mach tuck would send him tumbling out of control irrecoverably, and probably destroy the airframe.

It's also important to be aware of a limitation of early airspeed indicators. One built for subsonic speed is probably going to give unreliable readings in the presence of shockwaves. A phenomenon called compressibility error gives inaccurately high airspeed readings as the aircraft approaches the speed of sound. This error is called Mach jump. To counter this, supersonic aircraft use a Mach indicator instead of an airspeed indicator. The speed of sound at any given pressure and altitude is determined primarily by temperature. A Mach indicator is essentially an airspeed indicator mounted on an aneroid diaphragm to correct for static air pressure. Since Mach and airspeed are both dependent on temperature, they cancel each other out and no temperature diaphragm is needed. Komets had airspeed indicators, not Mach indicators; and so even the speeds logged by the German test pilots are probably incorrectly high.

One of the best known claims to the sound barrier comes from German WWII fighter pilot Hans Guido Mutke, flying perhaps the most devastating fighter of the war, the Messerschmitt Me-262. The 262 was the first true operational jet powered fighter plane in the world, sporting twin BMW 003 turbojet engines mounted below the swept wings. Although the 262 entered the war too late to have any real impact, it boasted a 5:1 kill ratio against allied fighters. Mutke was cruising at 36,000 feet when he began a steep dive under full power. With his airspeed indicator pegged at its limit of 1,100 kph (just over the speed of sound, but remember the airspeed indicator problem), Mutke reported severe buffeting and loss of control. Suddenly the buffeting stopped and he regained control, with the airspeed indicator still pegged; and it's this that could indicate he had broken the sound barrier. Unfortunately his engines flamed out, not being designed for supersonic speeds, and he slowed, and the severe buffeting returned. Finally his speed dropped enough that he regained control again and was able to restart his engines. He returned to base, and it was found that his aircraft had lost many rivets, and its wings had become so distorted that the plane had to be scrapped.

Mutke never understood what had happened until Chuck Yeager's flight was declassified and the supersonic flight profile became known: Severe buffeting while approaching Mach 1, then the shaking stops above Mach 1, and then resumes upon deceleration below Mach 1. But unfortunately for Mutke, there was not, and could not have been, any independent verification of his speed or of the period of smooth supersonic flight. Nobody denies the damage done to his plane during the buffeting period, but supersonic flight was not necessary for this to happen.

The designer of the Me-262, Willy Messerschmitt, always stated emphatically that the 262 was incapable of supersonic flight. In flight tests, he found that at Mach 0.86, the 262 experienced Mach tuck: It lost control and assumed a nose-down attitude that could not be corrected by the pilot, and throttling down was the only way to resume control. The 262 only had conventional elevators on the trailing edge of its tailplane, like all aircraft of the day, so these would have shock stalled and not been able to correct the Mach tuck. But the 262 also had an additional feature: The tailplane was actually all-moving for trim purposes. This was a separate electrically operated control, and it was normally used to keep the plane level as its fuel supply was consumed. Mutke reported that he actually employed this all-moving trim control in order to get out of the nose-down state, a technique which may not have been considered in Messerschmitt's own tests. Mutke's report was given additional credibility in 1999, when computer modeling and scale model wind tunnel testing conducted at Munich Technical University found that the 262 was capable of reaching, and passing, Mach 1.

So, while we cannot prove or disprove Mutke's claim, it is possible that he did reach supersonic flight. However, like in sports, it's not what happened, it's what the referee says happened that matters. Mutke's feat was unverified and unofficial, and certainly unintentional; so even if he did break the sound barrier before Yeager, it "doesn't count."

There are two other flights that don't count, both accomplished by George Welch, a civilian test pilot for North American Aviation. On October 1, 1947, just 13 days before Yeager broke the sound barrier in the X-1, Welch took the new XP-86 fighter prototype up for its maiden flight. In a powered dive from 35,000 feet, Welch reported Mach jump on his airspeed indicator, showing that he was traveling supersonic. Anecdotal stories say that a sonic boom was heard on the ground. Welch's airspeed was not being officially recorded, and no official record states that he broke the sound barrier. If he did, it was either unverified or classified. Welch believed that he did, and to hammer the point home, he gave a repeat performance. While Yeager was strapped into the X-1 still attached to its B-29 mother ship, just before the historic flight, Welch again put his XP-86 into a steep dive. Some stories say that he buzzed the B-29 close enough for those onboard, Yeager included, to hear his sonic boom. He made a 4g pullout from his dive, and those same stories say that his sonic boom was louder than Yeager's just 20 minutes later.

There is no engineering reason to doubt Welch's claim. The history books credit George Welch with breaking the sound barrier in the XP-86 in a dive six months later on April 26, 1948, with official measurements and a proper Mach indicator on board. Did he do the same thing before Yeager's flight? He may well have, and a lot of people say he did. But there's a significant difference between Yeager's flight and those of Welch, Mutke, Dittmar, and probably others. Their claims to the sound barrier were all in dives, and were transient at best. Yeager's X-1 Glamorous Glennis, on the other hand, was the first aircraft capable of sustained supersonic level flight. Sure, being the first to break the sound barrier becomes less glitzy when you have to pile on qualifications. But every aviation milestone has been an incremental one; few are truly revolutionary. Yeager, Welch, Mutke, and Dittmar all made real contributions to the science of aviation. All had "the right stuff". At some point, any lines you draw to separate their achievements come down to semantics; yet you still have to draw those lines somewhere. Flights have to be official, they have to be verifiable, and they should demonstrate a deliberate capability. And so, while it's a virtual certainty that the sound barrier was broken by someone somewhere in some circumstance, Chuck Yeager's flight of the Bell X-1 is the only flight to meet all the criteria of a true aviation first.

Correction: An earlier version of this gave the mother ship type on Yeager's October 14, 1947 flight as a B-50, but it is confirmed from the radio transcript on that day to have been B-29 tail number 512800. The confusion stems from that both B-29s and B-50s were used in the program, and various articles on both the NASA and USAF websites give conflicting accounts. (The B-50 was a re-engined version of the B-29 with a few outwardly visible differences; in most ways they were identical.) —BD

By Brian Dunning
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