Why are bow shocks angled slightly downward on airfoils? In this picture if you look closely at the very bottom airfoil diagram, you'll see the "subsonic airflow" part is more below the leading edge, or the blue part extends more down than up. Very slight, but it's visible. Other diagrams have this too, why is that?

(also little side question, why are there lines that look like a bow shock on the bottom airfoil at the trailing edge?)

enter image description here


1 Answer 1


I wouldn't take this diagram as being scientific or to-scale. It is an artists rendition of a diagram that has been copied and repeated many times, some small details get lost in translation.

In this diagram, in the first two charts, the blue region represents the area of supersonic flow. However, in the third diagram, they've switched meaning and the blue area is subsonic flow. Very tricky.

The only place a shock wave will be perpendicular to the flow is along the stagnation streamline. All the other streamlines, the shock helps to turn the flow, and it takes an angled shock to turn the flow.

There is a second set of shocks at the trailing edge of the airfoil. The bottom surface trailing edge flow is about horizontal, and the upper surface TE flow is down about 10deg. After the trailing edge, these two must be parallel (and the pressures equal). So, the top flow will turn a few degrees up, and the bottom flow will turn a few degrees down. Since the flow before the trailing edge is supersonic, it requires a shock wave to make this turn happen.

  • $\begingroup$ Ah okay that clears things up, thanks. So in the first 2 diagrams, do those shocks disappear at supersonic speeds, or do they happen along with the bow shock? Also, would the correct terminology for the trailing edge shock be an oblique shock? $\endgroup$
    – Wyatt
    Jan 30 at 22:57
  • $\begingroup$ They are the same shocks. The first weak shock appears at some subsonic Mach number. It moves aft to the trailing edge as the region of supersonic flow grows. Then, as you reach Mach 1, the bow shock forms. Yes, I would consider the trailing edge shocks to be oblique shocks. $\endgroup$ Jan 30 at 23:53
  • $\begingroup$ Oh okay, thanks a lot. So that would mean that the shocks in diagram 1 and 2 don't disappear, right? $\endgroup$
    – Wyatt
    Jan 30 at 23:58
  • $\begingroup$ Yes, they are the same shocks. They move aft until they reach the trailing edge. $\endgroup$ Jan 31 at 0:27
  • 1
    $\begingroup$ They happen when the region of supersonic flow above the wing reaches all the way back to the trailing edge. It is probably possible for that to happen at M<1, but in normal circumstances that would be something like M=0.98 or so. $\endgroup$ Jan 31 at 3:10

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