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So Who Broke the Sound Barrier First?

July 24, 2018

The Bell X-1 flown by Capt. Chuck Yeager more than 70 years ago, on October 14, 1947, is widely accepted to be the first aircraft to fly supersonically.

 

Breaking the sound barrier was to aviation what Neil Armstrong’s first step on the moon was to space exploration: It will always be that unassailable ‘first’ that can never be topped. But let me perversely ask if Yeager really was the first to break the sound barrier?

Plenty of stories claim he wasn’t the first. The thing is, there were many pilots who tried, but didn’t live to speak about it.

The years leading up to Yeager’s flight were extraordinary, as World War II drove aeronautical development at literally break-neck speed. During WWII, engineers didn’t have any clear idea how to design aircraft capable of supersonic speed; the only thing they knew was that supersonic flight had to be possible, as a rifle bullet was supersonic. So, Yeager’s X-1 was designed like a .50 calibre bullet, as was Britain’s aborted Miles M.52.

Also, WWII had produced the supersonic German V-2 rockets, but the V-2 was ballistic and didn’t require a controllable airframe. The real challenge then was controllable supersonic flight.

As an aircraft approaches the speed of sound, the faster airflow over some parts of the airframe exceeds Mach 1 and creates shockwaves, which cause intense buffeting. Many propeller-driven WWII fighters, including the Spitfire, the P-38 Lightning and the P-51 Mustang, experienced these effects at Mach 0.85. Similarly, jet engines were not designed to work with supersonic airflow entering the compressor and so flamed out.

However, one particular fighter from WWII was not driven by either propellers or jets: The rocket powered Messerschmitt 163 Komet was designed by the still largely unrecognised genius, Alexander Lippisch, the pioneer of delta wings and ramjets. By the end of WWII, Lippisch had a test glider of a supersonic ramjet-powered aircraft undergoing flight tests. He already had a keen grasp of the requirements of supersonic flight.

The Komet’s thin swept-back wing delayed the onset of a shockwave over the wing. As an aircraft approaches the speed of sound, the shockwave over the wing moves the centre of lift rearwards and the nose pitches down. This is called ‘Mach tuck’. Normally you’d pull back on the stick to correct this, but conventional elevator controls on the trailing edge of the horizontal stabiliser are unable to get bite, since the elevators are stalled. The only way out of Mach tuck is to use an all-moving tail plane to trim back to level flight.

With its delta wing, the Komet had no tailplane, let alone an all-moving one. Even if the pilot opened his Komet’s rocket to full power to muscle his way through the sound barrier, Mach tuck would tumble him irrecoverably out of control, and probably destroy the airframe.

Nevertheless, credible stories persist of Komet pilots breaking the sound barrier, years before Yeager did. Test pilot Heini Dittmar, flying an early Komet 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 and temperature are unknown.

 

 

What we do know is that Dittmar made the flight at partial throttle to avoid buffeting, using an engine only half as powerful as that which went into production. However, just because later versions were more powerful doesn’t mean they wouldn’t hit the same transonic limitations. One unofficial report claims that Dittmar achieved 1,130 kph in 1944 (Mach 1.06), and another claims that in a steep dive he created sonic booms that were heard on the ground.

These stories first appeared in a 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 Brits captured all of the programme’s classified data, and no supersonic flights were ever recorded, even in secret.

It’s also important to understand the limitation of early airspeed indicators. A subsonic ASI gives unreliable readings in the presence of shockwaves. Compressibility error exaggerates airspeed as the aircraft approaches the speed of sound. This error is called ‘Mach jump’. 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, but perhaps least plausible, claims to breaking the sound barrier comes from German WWII fighter pilot Hans Guido Mutke, flying a Messerschmitt Me-262. 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. It’s this that could indicate he had broken the sound barrier. Unfortunately, not being designed for supersonic speeds, his engines flamed out. As he slowed, the severe buffeting returned. Finally, his speed dropped enough that he regained control and was able to restart his engines. When he landed, it was found that his wings were so distorted that the plane had to be scrapped.

 

 

Mutke didn’t understand 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 back down through Mach 1. 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.

And the clincher is that the designer of the Me-262, Willy Messerschmitt, always stated emphatically that the 262 was incapable of supersonic flight. In flight tests it was shown that at Mach 0.86, the Me-262 experienced Mach tuck. The nose-down pitch could not be corrected by the pilot; reducing power and thus speed was the only way to regain control.

The 262 had conventional elevators so these would have shock stalled and not been able to correct the Mach tuck. So why believe Mutke? Because the 262 also had an all-moving tailplane for trim. Mutke reported that he used the all-moving trim control to get out of the nose-down state, and his report was given additional credibility in 1999, when computer modelling 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’s possible that he did in fact reach supersonic flight. However, Mutke’s feat was unverified and unofficial, and certainly unintentional, so even if he did break the sound barrier before Yeager, it doesn’t really count.

Two other flights that don’t count were 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 flew the new XP-86 fighter prototype (which became the F-86 Sabre) on its maiden flight. In a powered dive from 35,000 feet, Welch reported Mach jump on his airspeed indicator, showing that he was traveling supersonically. Anecdotal stories confirm 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 George Welch’s claim. The Sabre really was capable of supersonic flight as Welch showed when he demonstrably broke 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 was the first to achieve controlled supersonic level flight.

 

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