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The Speed of Sound

by John M. White |

Sometimes we find a video that is both entertaining and informative at the same time. The one that follows does exactly that: Here is a transcript of the narrative: Sound travels at about 760 miles per hour, or 340 meters per second and about 661 knots on an average day at sea level. And sometimes, you can almost see it. Going close to that speed through air can cause some unusual visual effects. This compiled footage includes F-14s, standard and Blue Angels F-18s, plus the SR-71 and an Atlas Rocket launch. AVweb contacted sources at NASA to research the phenomena. One of the photos shows an SR-71 bending light waves from the pressure differential it produces. Air and temperature both drop as air speed increases. Rapid changes in density can be enough to bend light like a lens. How does it happen? Airspeed increases as it flows over the form of an aircraft pushing its way towards Mach 1, the speed of sound. That means that aircraft even slower than the speed of sound can accelerate the air flowing over them beyond the speed of sound. As pressure all around the aircraft drops, that air cools, and as air cools its ability to hold moisture drops, and if there is enough moisture you will see it form in clouds that appear to be attached to the aircraft itself. Those cone shaped clouds you see are not, in fact, travelling with the aircraft. Each section of air is spontaneously reacting to the temperature and pressure change induced by the aircraft’s body sliding past. Then there are the shockwaves. Shockwaves are formed by the air that can’t get out of the way fast enough, forming a pressure wave. We perceive that pressure wave as sound. Aircraft actually usually produce two booms. They just reach us so quickly in succession that we can’t distinguish the two. All these factors are quite normal and have come to be expected in images that capture supersonic, transonic and even near supersonic flight. But on rare occasions we do get a glimpse of something special. At Kennedy Space Center on February 11th, 2010, an Atlas rocket launched into ideal conditions for showing off shockwave. You can see the result in the video. I trust that you found this as interesting and entertaining as I did. Until next time keep your wings straight and level Hersch! JetAviator7 At 42,000' in approximately level flight, a third cylinder was turned on. Acceleration was rapid and speed increased to .98 Mach. The needle of the machmeter fluctuated at this reading momentarily, then passed off the scale. Assuming that the off-scale reading remained linear, it is estimated that 1.05 Mach was attained at this time. — (then) Captain Charles E. Yeager, Air Corps, formal typewritten test flight report on first supersonic flight, 14 October 1947. NACA tracking data and the XS-1's own oscillograph instrumentation later showed 'Glamorous Glennis' had attained Mach 1.06 at about 43,000 feet. ps: Don't forget to sign up for our newsletter "All Things Aviation" here!

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