The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, these effects were seen as constituting a barrier, making faster speeds very difficult or impossible.sonic barrier . thefreedictionary.com.sound barrier . oxforddictionaries.com. The term sound barrier is still sometimes used today to refer to aircraft approaching supersonic flight in this high drag regime. Flying faster than sound produces a sonic boom.
In dry air at 20 °C (68 °F), the speed of sound is 343 meters per second (about 767 mph, 1234 km/h or 1,125 ft/s). The term came into use during World War II when pilots of high-speed fighter aircraft experienced the effects of compressibility, a number of adverse aerodynamic effects that deterred further acceleration, seemingly impeding flight at speeds close to the speed of sound. These difficulties represented a barrier to flying at faster speeds. In 1947, American test pilot Chuck Yeager demonstrated that safe flight at the speed of sound was achievable in purpose-designed aircraft, thereby breaking the barrier. By the 1950s, new designs of fighter aircraft routinely reached the speed of sound, and faster.
History Some common whips such as the bullwhip or stockwhip are able to move faster than sound: the tip of the whip exceeds this speed and causes a sharp crack—literally a sonic boom.May, Mike. "Crackin' good mathematics" . American Scientist, Volume 90, Issue 5, September–October 2002. p. 1. Firearms made after the 19th century generally have a supersonic muzzle velocity.
The sound barrier may have been first breached by living beings about 150 million years ago. Some paleobiologists report that computer models of their biomechanical capabilities suggest that certain long-tailed dinosaurs such as Brontosaurus, Apatosaurus, and Diplodocus could flick their tails at supersonic speeds, creating a cracking sound. This finding is theoretical and disputed by others in the field.Wilford, John Noble. "Did Dinosaurs Break the Sound Barrier?" The New York Times, December 2, 1997. Retrieved: January 15, 2009. Meteorites in the Earth's upper atmosphere usually travel at higher than Earth's escape velocity, which is much faster than sound.
Early problems The existence of the sound barrier was evident to aerodynamicists before any direct in-aircraft evidence was available. In particular, the very simple theory of thin airfoils at supersonic speeds produced a curve that went to infinite drag at Mach 1, dropping with increasing speed. This could be seen in tests using projectiles fired from guns, a common method for checking the stability of various projective shapes. As the projectile slowed from its initial speed and began to approach the speed of sound, it would undergo a rapid increase in drag and slow much more rapidly. It was understood that the drag did not go infinite, or it would be impossible for the projectile to get above Mach 1 in the first place, but there was no better theory, and data was matching theory to some degree. At the same time, ever-increasing wind tunnel speeds were showing a similar effect as one approached Mach 1 from below. In this case, however, there was no theoretical development that suggested why this might be. What was noticed was that the increase in drag was not smooth, it had a distinct "corner" where it began to suddenly rise. This speed was different for different wing planforms and cross sections, and became known as the "critical Mach".
According to British aerodynamicist W. F. Hilton, of Armstrong Whitworth Aircraft, the term itself was created accidentally. He was giving demonstrations at the annual show day at the National Physical Laboratory in 1935 where he demonstrated a chart of wind tunnel measurements comparing the drag of a wing to the velocity of the air. During these explanations he would state "See how the resistance of a wing shoots up like a barrier against higher speed, as we approach the speed of sound." The next day, the London newspapers were filled with statements about a "sound barrier." Whether or not this is the first use of the term is debatable, but by the 1940s use within the industry was already common.
By the late 1930s, one practical outcome of this was becoming clear. Although aircraft were still operating well below Mach 1, generally half that at best, their engines were rapidly pushing past 1,000 hp. At these power levels, the traditional two-bladed propellers were clearly showing rapid increases in drag. The tip speed of a propeller blade is a function of the rotational speed and the length of the blade. As the engine power increased, longer blades were needed to apply this power to the air while operating at the most efficient RPM of the engine. The velocity of the air is also a function of the forward speed of the aircraft. When the aircraft speed is high enough, the tips reach transonic speeds. Shock waves form at the blade tips and sap the shaft power driving the propeller. To maintain thrust, the engine power must replace this loss, and must also match the aircraft drag as it increases with speed. The required power is so great that the size and weight of the engine becomes prohibitive. This speed limitation led to research into jet engines, notably by Frank Whittle in England and Hans von Ohain in Germany. This also led to propellers with ever-increasing numbers of blades: three, four and then five were seen during the war. As the problem became better understood, it also led to "paddle bladed" propellers with increased chord, as seen (for example) on late-war models of the Republic P-47 Thunderbolt.
Nevertheless, propeller aircraft were able to approach their critical Mach number, different for each aircraft, in a dive. Doing so led to numerous crashes for a variety of reasons. Flying the Mitsubishi Zero, pilots sometimes flew at full power into terrain because the rapidly increasing forces acting on the control surfaces of their aircraft overpowered them.Yoshimura, Akira, translated by Retsu Kaiho and Michael Gregson (1996). Zero! Fighter. Westport, Connecticut, USA: Praeger Publishers. p. 108. . In this case, several attempts to fix it only made the problem worse. Likewise, the flexing caused by the low torsional stiffness of the Supermarine Spitfire's wings caused them, in turn, to counteract aileron control inputs, leading to a condition known as control reversal. This was solved in later models with changes to the wing. Worse still, a particularly dangerous interaction of the airflow between the wings and tail surfaces of diving Lockheed P-38 Lightnings made "pulling out" of dives difficult; in one 1941 test flight, test pilot Ralph Virde was killed when the plane flew into the ground at high speed. The problem was later solved by the addition of a "dive flap" that upset the airflow under these circumstances. Flutter due to the formation of shock waves on curved surfaces was another major problem, which led most famously to the breakup of a de Havilland Swallow and death of its pilot Geoffrey de Havilland, Jr. on 27 September 1946. A similar problem is thought to have been the cause of the 1943 crash of the BI-1 rocket aircraft in the Soviet Union.
All of these effects, although unrelated in most ways, led to the concept of a "barrier" making it difficult for an aircraft to exceed the speed of sound.Portway, Donald (1940). Military Science Today. London: Oxford University Press. p. 18: "For various reasons it is fairly certain that the maximum attainable speed under self-propelled conditions will be that of sound in air", i.e., . Erroneous news reports caused most people to envision the sound barrier as a physical "wall," which supersonic aircraft needed to "break" with a sharp needle nose on the front of the fuselage. Rocketry and artillery experts' products routinely exceeded Mach 1, but aircraft designers and aerodynamicists during and after World War II discussed Mach 0.7 as a limit dangerous to exceed.
Early claims During WWII and immediately thereafter, a number of claims were made that the sound barrier had been broken in a dive. The majority of these purported events can be dismissed as instrumentation errors. The typical airspeed indicator (ASI) uses air pressure differences between two or more points on the aircraft, typically near the nose and at the side of the fuselage, to produce a speed figure. At high speed, the various compression effects that lead to the sound barrier also cause the ASI to go non-linear and produce inaccurately high or low readings, depending on the specifics of the installation. This effect became known as "Mach jump".Jordan, Corey C. "The Amazing George Welch, Part Two, First Through the Sonic Wall" . Planes and Pilots Of World War Two, 1998–2000. Retrieved: June 12, 2011. Before the introduction of Mach meters, accurate measurements of supersonic speeds could only be made remotely, normally using ground-based instruments. Many claims of supersonic speeds were found to be far below this speed when measured in this fashion.
In 1942, Republic Aviation issued a press release stating that Lts. Harold E. Comstock and Roger Dyar had exceeded the speed of sound during test dives in a Republic P-47 Thunderbolt. It is widely agreed that this was due to inaccurate ASI readings. In similar tests, the North American P-51 Mustang demonstrated limits at Mach 0.85, with every flight over Mach 0.84 causing the aircraft to be damaged by vibration.
One of the highest recorded instrumented Mach numbers attained for a propeller aircraft is the Mach 0.891 for a Spitfire PR XI, flown during dive tests at the Royal Aircraft Establishment, Farnborough in April 1944. The Spitfire, a photo-reconnaissance variant, the Mark XI, fitted with an extended "rake type" multiple pitot system, was flown by Squadron Leader J. R. Tobin to this speed, corresponding to a corrected true airspeed (TAS) of 606 mph.Spitfire – Typical high speed dive . spitfireperformance.com. In a subsequent flight, Squadron Leader Anthony Martindale achieved Mach 0.92, but it ended in a forced landing after over-revving damaged the engine.
Hans Guido Mutke claimed to have broken the sound barrier on 9 April 1945 in the Messerschmitt Me 262 jet aircraft. He states that his ASI pegged itself at . Mutke reported not just transonic buffeting, but the resumption of normal control once a certain speed was exceeded, then a resumption of severe buffeting once the Me 262 slowed again. He also reported engine flame-out.
This claim is widely disputed, even by pilots in his unit. All of the effects he reported are known to occur on the Me 262 at much lower speeds, and the ASI reading is simply not reliable in the transonic. Further, a series of tests made by Karl Doetsch at the behest of Willy Messerschmitt found that the plane became uncontrollable above Mach 0.86, and at Mach 0.9 would nose over into a dive that could not be recovered from. Post-war tests by the RAF confirmed these results, with the slight modification that the maximum speed using new instruments was found to be Mach 0.84, rather than Mach 0.86."Me 262 and the Sound Barrier". aerospaceweb.org. Retrieved: August 30, 2010.
In 1999, Mutke enlisted the help of Professor Otto Wagner of the Munich Technical University to run computational tests to determine whether the aircraft could break the sound barrier. These tests do not rule out the possibility, but are lacking accurate data on the coefficient of drag that would be needed to make accurate simulations. Wagner stated: "I don't want to exclude the possibility, but I can imagine he may also have been just below the speed of sound and felt the buffeting, but did not go above Mach-1."