Question: What is happening to the airplane in this picture? (U.S. Navy photo by Ensign John Gay)
A) It is entering a well-defined warm front.
B) This picture was taken at the instant the pilot applied afterburners.
C) It was taken just as the airplane approached Mach 1.
D) This is what happens when someone throws a water balloon at a jet doing 500 knots.
E) It’s nothing special actually. This jet is simply flying through a (very) small cloud.
Answer: It’s choice C. This is quite a striking photograph. In it, an F/A-18 Hornet assigned to Strike Fighter Squadron One Five One, USS Constellation battle group, is shown ‘breaking the sound barrier’ (or at least approaching it) in the skies over the Pacific Ocean on July 7, 1999. Under the right conditions of temperature and humidity (as well as altitude, wind speed, and the shape and trajectory of the plane) a ‘vapor cone’ effect results. Reaching Mach 1 (or even flight at transonic speeds) can result in similarly impressive displays of condensation clouds. (A smaller shock wave can be seen forming on top of the canopy.) These are caused by something known as the Prandtl-Glauert singularity, in which temperature and pressure variations become exaggerated (and theoretically approach infinity) as flight speed approaches the ambient sound speed. As a result, temperature perturbations in portions of the flow can become large enough to cause condensation of ambient water vapor. If condensation does occur, then the resultant cloud is referred to as a Prandtl-Glauert condensation cloud. Note that it does not define the edge of the sonic shock wave in cases of flight at or just above the speed of sound. These occur where the local flow increases pressure and reduces velocity (that is, compressing and thus heating), not where it decreases pressure and increases velocity (i.e., expanding and therefore cooling). This type of condensation is typically symmetrical above and below the aircraft, which distinguishes it from condensation clouds (which are lift-induced). In this photograph and others like it that you may come across, what we actually see isn’t the shock wave (which is in front of the cloud) but the manifestation of a partial configuration or state change as the flow jumps back to subsonic. If you’d like to see a brief but phenomenal video of this in action (in this case, during a supersonic low-level F-14 fly-by) you can find it at several web sites, such as:
http://www.galleryoffluidmechanics.com
http://www.navy.mil/homepages/cv64/noflash/media/video/video.htm
http://chamorrobible.org/gpw/gpw-20040817.htm
