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Sound barrier
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===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 [[airfoil]]s 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".<ref name=Vincenti>{{cite journal |first=Walter |last=Vincenti |date=1997 |title=Engineering Theory in the Making: Aerodynamic Calculation "Breaks the Sound Barrier" |journal=Technology and Culture |volume=38 |number=4 |pages=819β851 |jstor=310695 }}</ref> 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 (United Kingdom)|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.<ref name=Vincenti/> 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 [[Propeller (aircraft)|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 engine]]s, 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.<ref>Yoshimura, Akira, translated by Retsu Kaiho and Michael Gregson (1996). ''Zero! Fighter''. Westport, Connecticut, USA: Praeger Publishers. p. 108. {{ISBN|0-275-95355-6}}.</ref> 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 Lightning]]s 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.<ref name=Vincenti/> The problem was later solved by the addition of a "dive flap" that upset the airflow under these circumstances. [[Aeroelasticity#Flutter|Flutter]] due to the formation of [[shock wave]]s 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.<ref>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., {{convert|750|mi/h|km/h|abbr=on}}.</ref> 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.<ref name="ley194811">{{Cite magazine |last=Ley |first=Willy |date=November 1948 |title=The 'Brickwall' in the Sky |url=https://archive.org/stream/Astounding_v42n03_1948-11_cape1736#page/n77/mode/2up |magazine=Astounding Science Fiction |pages=78β99}}</ref>
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