ACCUEIL | UK VERSION ACTUALITE | LIVRE D'OR
 

Accueil > Technique > Spécificité






18. DEFINITION OF MACH NUMBER

©inside story

Aircraft speed, especially supersonic, is conveniently expressed as Mach number, which is the ratio of the speed of a body and the speed of the sound. It was first conceived by Ernst Mach (1833-1916) and later coined by Jacob Ackeret, the Swiss aerodynamicist of renown. The speed of the sound varies with altitude and temperature, being approximately 760 mph at sea level, while as altitude increases and temperature decreases at the standard rate of 1.98°C per thousand feet, it becomes 660mph at 36000ft, the start of the stratosphere. Once in the stratosphere the temperature and thus the speed of sound remain constant up to about 65,000ft.

19. SUPERSONIC FLIGHT

The first aircraft to fly at supersonic speeds was a Bell XS-1 rocket-powered research plane piloted by Major Charles E. Yeager of the U.S. Air Force on Oct. 14, 1947. After being dropped from the belly of a Boeing B-29 mother ship, the XS-1 broke the (local) sound barrier at 1,066 km (662 miles) per hour and attained a top speed of 1,126 km (700 miles) per hour, or Mach 1.06. The first supersonic, passenger-carrying, commercial airplane (or supersonic transport, SST), the Concorde, was built jointly by aircraft manufacturers in Great Britain and France and entered regular service in 1976.

The first land-traveling vehicle to break the sound barrier was the ThrustSSC, a British-made car powered by two jet engines from an F-4 Phantom jet fighter. Driven by Andy Green, the ThrustSSC broke the sound barrier for the first time on Oct. 13, 1997, and set an official world land-speed record on October 15 with an average (supersonic) speed of 1,228 km (763 miles) per hour on the Black Rock Desert, Nevada.



20. HTTP://WWW.POURLASCIENCE.COM/



RÉPONSE DE FRANçOIS COULOUVRAT

Contrairement à l'opinion courante, le phénomène du bang sonique ne se produit pas uniquement lorsque l'avion vole exactement à la vitesse du son (lorsqu'il franchit le "mur du son"), mais continûment pendant toute la phase de vol supersonique. Comme il est expliqué dans l'article, en vol supersonique, le son émis par l'avion est contenu à l'intérieur du cône de Mach. Un observateur fixe, au sol par exemple, subit, lorsqu'il passe à travers le bord du cône de Mach, une brusque variation de pression, appelée "onde de choc", qui produit la sensation auditive de "bang" sonique, la plus gênante du point de vue de la perception auditive. Pour un avion de forme compliquée, il se produit plusieurs ondes de chocs successives, mais, comme je l'ai expliqué dans l'article, elles se propagent à des vitesses légèrement différentes et ont tendance à fusionner.



A l'intérieur du cône de Mach, derrière l'onde de choc, le son total résulte d'un processus d'interaction complexe entre les bruits émis par tous les points de l'avion à différents instants. Si l'on se place suffisamment loin de l'avion (à quelques fois sa longueur ou son envergure), seules les contributions se propageant dans les directions normales au cône de Mach interfèrent de manière constructive. Ceci fait que l'ensemble du bruit significatif se propage le long des mêmes rayons normaux au cône de Mach que les ondes de chocs.

21. SONIC BOOM

Sonic boom is an impulsive noise similar to thunder. It is caused by an object moving faster than sound -- about 750 miles per hour at sea level. An aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship's bow. When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves which travel forward from the generation or "release" point.



As an aircraft flies at supersonic speeds it is continually generating shock waves, dropping sonic boom along its flight path, similar to someone dropping objects from a moving vehicle. From the perspective of the aircraft, the boom appears to be swept backwards as it travels away from the aircraft. If the plane makes a sharp turn or pulls up, the boom will hit the ground in front of the aircraft.

Characteristics

Duration of sonic boom is brief; less than a second -- 100 milliseconds (.100 seconds) for most fighter-sized aircraft and 500 milliseconds for the space shuttle or Concorde jetliner .



Ground width of the boom exposure area is approximately one mile for each 1,000 feet of altitude; that is, an aircraft flying supersonic at 30,000 feet will create a lateral boom spread of about 30 miles. For steady supersonic flight, the boom is described as a carpet boom since it moves with the aircraft as it maintains supersonic speed and altitude.

22. THE WAVE OF CRASH



What is a Sonic Boom?

http://www.sky-flash.com/index.php



SHOCK WAVES ==> http://www.pbs.org/wgbh/nova/barrier/boom/

Informations Générales




©2006 D. TREBOSC. Tous droits réeacute;servés
Conditions d'utilisation | Contact | Crédits
Toute reproduction, même partielle, du contenu de ce site est soumise à autorisation.

 Ce site respecte la loi Informatique et Libertés.
Pour en savoir plus sur la protection des données personnelles, cliquez ici.