Within the very high speed flows of air in an aircraft’s exhaust, you can set up a series of shock waves and expansion fans as a result of the differences between the pressure of the flow and that of the surrounding air.  When afterburner is engaged, the hot gases and the temperature changes these shocks and expansions cause, result in a diamond pattern forming in the exhaust plume.  In darker conditions, these diamonds are more conspicuous but they are visible even in normal daylight.

These diamond patterns are a function of the flow being symmetrical since most engines have round exhaust nozzles.  This isn’t the case for the F-22, though.  It has flattened nozzles with a pointed profile top and bottom.  This got me wondering what the effect is on the exhaust plume and whether the traditional diamonds are formed or whether the nozzle shape results in a different pattern of shock and expansions as they reflect within the plume.  I decided to dig in to some shots to see what I could find.

I don’t have a lot of F-22 afterburner shots.  While I have shot them a lot taking off, they often take off without afterburner.  Since they have plenty of power and burner use dramatically increases fuel consumption (and the F-22 is not over-endowed with range as it is), there is no point using burner if it isn’t needed.  Air shows are a time when they do give it plenty of burner, so that is the source of the shots.

The result of this is that there is definitely something unusual about the shock patterns.  I include some shots of F-16 and F/A-18 afterburner plumes and the normal shock patterns that create the hotspots known as the diamonds are very obvious and simple in shape.  For the F-22, things are very different with the patterns of hot zones being something more in line with the shape of the nozzle.  The way in which the patterns repeat is more complex than for an axisymmetric nozzle.  There is nothing much to conclude in these observations.  It is just something that appeals to an old aero guy like me.