Prandtl Glauert Singularity or P.G. singularity is sometimes referred to as a “vapor cone”. The point at which a sudden drop in air pressure occurs is generally accepted as the cause of the visible condensation cloud that often surrounds an aircraft traveling at transonic speeds, though there remains some debate. Prandtl–Glauert singularity effects can be readily observed on a humid day by successfully cracking a whip. A visible cloud is produced at the point where the tip of the whip goes transonic.
Prandtl Glauert singularity
A Canadian Forces CF-18 Hornet showing a Prandtl-Glauert singularity (or cone). This is often mistakenly called breaking the sound barrier or a sonic boom. It has more to do with sudden change in air pressure and dew point. Speeds are restricted to sub-sonic over populated areas like here at the 08 Abby Airshow. To see it live seems pretty fast though.
The effect is also noticeable in modern super-high-bypass turbofan jet engines when operating at takeoff power, due to the low pressure and transonic fan blades in the engine inlet.
Check out this Prandtl Glauert Singularity video of Blue Angel #5 shot last week at Fleet Week in San Francisco. It’s a Boeing F/A-18 Hornet jet aircraft flying about 25 feet above the surface of the water, and creating what’s called a Prandtl-Glauert condensation cloud.
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This effect is commonly associated with “breaking the sound barrier” or more accurately, supersonic flight. However, this aircraft does not appear to be flying supersonic, which is faster than a mile in five seconds–the aircraft appears in frame at 2 seconds and is surely not a mile away when it begins its up and to the right pull-up at 8 seconds. In addition, are aircraft allowed to generate sonic boom over San Francisco Bay? I say the effect is dependent on air-frame generated local pressurel conditions and modified by local humidity conditions and can be generated at almost any flying speed; though higher speeds are more likely to produce the effect, mach 1+ is not necessary..
And, on second thought, has anyone ever seen similar effects generated in film of late WWII fighters?