Sound barrier

The sound barrier or sonic barrier is a popular term for the sudden increase in aerodynamic drag and other effects experienced by an aircraft or other object when it approaches supersonic speed. When aircraft first began to be able to reach close to supersonic speed, these effects were seen as constituting a barrier making supersonic speed very difficult or impossible.

In dry air at 20 °C (68 °F), the sound barrier is reached when an object moves at a speed of 343 metres per second (about 767 mph, 1234 km/h or 1,125 ft/s). The term came into use in this sense during World War II, when a number of aircraft started to encounter the effects of compressibility, a number of unrelated aerodynamic effects that "struck" their aircraft, seemingly impeding further acceleration. By the 1950s, new aircraft designs routinely "broke" the sound barrier.

Some common whips such as the bullwhip or are able to move faster than sound: the tip of the whip breaks the sound barrier 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 Apatosaurus and Diplodocus may have possessed the ability to flick their tails at supersonic speeds, possibly used to generate an intimidating booming sound. This finding is theoretical and disputed by others in the field. Meteorites 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 particularly noticeable on the Stearman, and noticeable on the North American T-6 Texan when it enters a sharp-breaking turn. 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.

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