Mach Crit and Supercritical Wings

F-18-mach-crit

This F-18 is showing off critical Mach.

Sometime around 1962 aerodynamicists realized there was more that could be done.

More efficiency to gain at speeds near the speed of sound.

But most importantly, the phenomena so iconic in this picture wasn’t fully understood until the mid 1960s.  Sure, wings had been swept and shock waves studied.  And it is no coincidence that the area rule was being noodled around this time.

But supersonic flow, over a wing, when the aircraft itself was not yet supersonic, caused a lot of consternation to problem solvers in defense and civilian aviation.  With buffeting in maneuvering combat aircraft (defense) and thirstier airliners at higher speeds (civilian), both groups needed a solution.

The last frontier was in the curves and thickness of airfoils.  To get bet better “high speed cruise” efficiency, some rethinking of airfoil shape was needed to deal with the pesky shock wave.  Much like the tipsy and late arriving uncle at Thanksgiving, you don’t want the shockwave to show up without having a plan to deal with it.

The history and technological drivers of the supercritical wing (or one that could overcome the “critical” Mach problem) is tied to powerplant evolution (please push it faster through the air), defense needs (let me turn, aim and shoot with out worrying about the Uncles binge drinking) and the insatiable appetite for cheaper travel all conspired to give us something that looks like this:

supercritical_wing_diagram

The end game of this design? In the F-18 image the result of pushing that shockwave back is a visible success.  The longer the delay in shockwave onset, smart people say, the better for everyone.  The smoother you can glide into super sonic flow (as fighters of today do) and the less drag you overcome as you approach the speed of sound the better for maneuverability (at speed) as well as economy of fuel burn.

This in turn led to “super cruise” where machines like the F-22 Raptor (and the Concorde) can comfortably hang around at Mach 2.0 speeds and above, in an efficient and relatively trouble free regime.

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