Shocks

Shocks actually form before an aircraft reaches the speed of sound (Mach 1), as the air travelling over the top of the wing (and various other parts of the aircraft such as the canopy) is accelerated above the speed of the aircraft. Thus, air travelling over parts of the top of the wings might be travelling at, say, Mach 1.2 when the aircraft itself is only travelling at, say, Mach 0.9. This situation is categorized as transonic flow and is the flow type in which most military and large commercial jets cruise. (In fact, if you're flying on a large commerical jet like a Boeing 747 and the conditions are right, you just might be able to look out the window over the wings and see distortions in the air that are due to shocks.) Transonic flow is much more complicated than supersonic flow (where the flow everywhere over the aircraft is supersonic) and subsonic flow (where the flow everywhere over the aircraft is subsonic) as it consists of behavior found in both of the two extreme categories.

The following photographs show military aircraft traveling at transonic speeds where condensation has formed in regions of low pressure (corresponding to regions of high velocity) on a humid day. However, as Professor Mark Cramer of Virginia Tech points out, the condensation is actually helping to visualize the Prandtl-Glauert singularity instead of any shock structure or region where the flow has gone supersonic. Unfortunately, this phenomenon is more complicated to explain and understand than shocks and requires some study of compressible flow. However, since the aircraft in these pictures are indeed traveling at transonic speeds, the condensation does give you an approximate idea of what the shock structure surrounding an aircraft in transonic flight might look like.


Navy F-18 in transonic flight.
Navy F-18 in transonic flight


Blue Angel F-18 in transonic flight during Fleet Week in San Francisco.
Blue Angel F-18 in transonic flight during Fleet Week in San Francisco


Navy F-14 in transonic flight.
Navy F-14 in transonic flight


If you would like more than a single image of an aircraft in transonic flight, check out this short video (1.32 Mb mpeg) of an F-14 performing a low flyby for an aircraft carrier.





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