At sea level sound travels at 760 mph.  Above 4,000 feet the speed of sound is about 660 mph.  The term ‘Mach Number’ is named after Ernst Mach. It was devised to describe the ratio of aircraft speed to the speed of sound and uses this formual: M=V/ Vs (mach number =vehicle velocity / velocity of sound)

Ernst Mach in 1902.

As an airplane moves through the air at high speed, it builds up a pile of air ahead of it.  At about 450 mph, the accelerated flow above the wing may become locally supersonic, moving as fast as sound, or even faster.  At that point, the plane is in the transonic region, an area of mixed subsonic and supersonic flow.  If the speed is increased further, the complete flow around the airoplane becomes supersonic, and the plane is in the supersonic region, moving faster than sound.  During transonic flight, shock waves form on an aircraft, move back and forth and if the plane is not designed specifically for the sonic environment–as was the case with World War II fighters–violently disrupt the airflow. Critical changes in pressure and loading may occur and the aircraft can undergo severe structural stress.  The total airframe drag may rise sharply.  The controls sometimes lose their effectiveness, thanks to the shifting position of shockwaves and the distortions in airflow around the airplane. 

An F/A-18 breaking the sound barrier and its associated shockwaves.

It was for these reasons that, just prior to and during World War II, ‘compressibility’ became such a serious problem to designers of high-speed military aircraft that an increasingly public debate raged whether or not in fact a piloted airplane could ever safely exceed what was luridly called the ‘sound barrier’. 

Ralph Virden next to a Boeing Model 80.

In November of 1941, test pilot Ralph Virden took off from Lockheed’s Burbank plant in the first of Lockheed’s pre-production YP-38 Lightnings.  The test card for the flight called for a high-speed dive to evaluate some new spring-loaded elevator tabs intended to improve the Lightning’s ability to recover from a dive, followed by a low-altitude demonstration before watching AAF officials. 

Lockheed YP-38 in flight.

Observers on the ground heard the snarling whine of a P-38 at full power; the noise built up to a strident crescendo suddenly broken by the awful thump of an airplane striking the earth.  The subsequent accident investigation showed that when Virden desperately attempted to pull out of its dive-at about 535 mph (Mach 0.7) at 3,000 feet-the structural loads exceeded the strength of the tail, ripping it from the plane.  The compressibility problem would have to be solved if higher aircraft speeds were to be achieved.

Geoffrey Jr and the Swallow he met his fate in.

In 1946, Geoffrey de Havilland Jr. planned to establish a new speed record, breaking Captain E.M. ‘Teddy’ Donaldson’s record of 615.778 mph in a de Havilland DH 108 Swallow. At 5:30 pm on the afternoon of September 27, 1946 Geoffrey de Havilland taxied the DH 108 out on the runway at the company’s Hatfield Aerodrome, ran up the Goblin turbojet and with the engine shrieking at full power, rolled ever faster down the runway and then into the air.  The first reports of disaster reached the company 30 minutes after the swift DH 108 lifted away from Hatfield.  The Swallow had crashed and Geoffrey de Havilland was missing.  Ten days later the pilot’s body washed ashore at Whitstable.  The son of famed aircraft designer Sir Geoffrey de Havilland was the second son to perish testing company aircraft.  The accident lent credence to the then-current dire predictions about the existence of an impenetrable sonic barrier through which no aircraft could fly and reinforced the British government’s determination to avoid manned supersonic research. 

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