Sound barrier

The sound barrier or sonic barrier is the sudden 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 began to be able to reach close to the speed of sound, these effects were seen as constituting a barrier making faster speeds very difficult or impossible. The term sound barrier is still sometimes used today to refer to aircraft reaching supersonic flight.Speeds of 950 km/h (590 mph) are reported to have been attained in a shallow dive 20° to 30° from the horizontal. No vertical dives were made. At speeds of 950 to 1,000 km/h (590 to 620 mph) the air flow around the aircraft reaches the speed of sound, and it is reported that the control surfaces no longer affect the direction of flight. The results vary with different airplanes: some wing over and dive while others dive gradually. It is also reported that once the speed of sound is exceeded, this condition disappears and normal control is restored. The sound barrier or sonic barrier is the sudden 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 began to be able to reach close to the speed of sound, these effects were seen as constituting a barrier making faster speeds very difficult or impossible. The term sound barrier is still sometimes used today to refer to aircraft reaching supersonic flight. In dry air at 20 °C (68 °F), the speed of sound is 343 metres 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 it was 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. 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. Firearms made after the 19th century have generally had a supersonic muzzle velocity. The sound barrier may have been first breached by living beings some 150 million years ago. Some paleobiologists report that, based on computer models of their biomechanical capabilities, certain long-tailed dinosaurs such as Brontosaurus, Apatosaurus, and Diplodocus may have been able to flick their tails at supersonic speeds, creating a cracking sound. This finding is theoretical and disputed by others in the field.Meteors entering the Earth's atmosphere usually, if not always, descend faster than sound. The tip of the propeller on many early aircraft may reach supersonic speeds, producing a noticeable buzz that differentiates such aircraft. This is undesirable, as the transonic air movement creates disruptive shock waves and turbulence. It is due to these effects that propellers are known to suffer from dramatically decreased performance as they approach the speed of sound. It is easy to demonstrate that the power needed to improve performance is so great that the weight of the required engine grows faster than the power output of the propeller can compensate. This problem was one that led to early research into jet engines, notably by Frank Whittle in England and Hans von Ohain in Germany, who were led to their research specifically in order to avoid these problems in high-speed flight. Nevertheless, propeller aircraft were able to approach the critical Mach number in a dive. Unfortunately, doing so led to numerous crashes for a variety of reasons. Most infamously, in the Mitsubishi Zero, pilots flew at full power into the terrain because the rapidly increasing forces acting on the control surfaces of their aircraft overpowered them. 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; however, 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 de Havilland Swallow and death of its pilot, Geoffrey de Havilland, Jr. 27 sept 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. 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 aerodynamic engineers during and after World War II discussed Mach 0.7 as a limit dangerous to exceed. 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'. Before the introduction of Mach meters, accurate measurements of supersonic speeds could only be made externally, 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 the P-47 Thunderbolt. It is widely agreed that this was due to inaccurate ASI readings. In similar tests, the North American P-51 Mustang, a higher performance aircraft, demonstrated limits at Mach 0.85, with every flight over M0.84 causing the aircraft to be damaged by vibration.